The search continues for my mysterious surging idle/stalling issue

[RustyDusty]

I had a long thread on here a while back on this issue when I was trying to get my truck to pass smog. You can check it out HERE If you'd like but I'm going to summarize the situation so I don't scare everyone off with multiple pages of comments to catch up on. My 1987 4x4 ford ranger with the 2.9 and A4LD has had a really frustrating issue where the idle goes up and down and sometimes stalls when the engines warmed up. On a cold start it sounds great and idles very smooth, I don't notice any fluctuation or surging idle but one it gets warmed up the idle goes up and down and up and down forever. If I stop the engine after its warmed up for instance if I go into a store when I come back out and start the truck while its still warm the truck RPM's will go up right at first then come back down so low it dies. I start it again and it will usually die again, sometimes even a third time until eventually it can keep idling but the idle is still going up and down over and over, then when I put it in gear sometimes it will even die again, its obviously very frustrating. If the truck sits long enough to cool down when I start it the truck it sounds great and the idle is smooth and eventually once it gets warm the cycle continues

I've learned to live with it but it still frustrates me because I've really tried to fix this issue. Its probably had 4 different IAC's, 3 different TPS, 3 different MAP sensors, ACT, ECT, Distributor, several TFI modules, spark plugs/wires, ECU, all new silicone vacuum lines, I've disconnected all harnesses, cleaned them and reconnected, I'e jiggled every wire, you name it I've probably tried it. I had it at a shop and it totally stumped everyone there. The ONLY thing I've found that seems to fix the issue and is how I passed smog is a ECU from a 1989 ford ranger 4x4 with a 2.9 and A4LD. I don't know why but this specific computer seems to run the engine properly. I opened my original computer and it looked pristine in there, I tested all the wires going to my original computer and everything checked out so I really didnt trust it would be a computer issue but the shop suggested it was the only thing they didnt test so I decided to throw an ECU in from a 1989 ranger even though I knew it wouldn't be a perfect fit since they use different solenoids in the transmission but to my amazement it seemed to have solved my issue. I drove around with this ECU in my truck for a week and couldn't recreate my issues. I got it smogged and passed with this ECU. Only issue was I couldn't put it into overdrive because of the solenoid issue so I was convinced I just needed to buy a new ECU that was an actual match for my truck and so I did and now with the new ECU my issue has returned just like with my original computer.

I guess I could keep throwing parts at it and buy another ECU but that seems insane to me. First, I'm wondering if anyone here has any ideas on this matter. Im trying to think of the things that could differ from this '89 computer and my '87 computer. The obvious difference is the way the trucks shift into overdrive but I cant figure out how that would create my issue. Another thing I've thought about is maybe the computer from the '89 ranger isn't one thats programmed to operate the EGR system if it was made for a truck that didnt have EGR.Aand thus maybe the issue with my truck is somewhere in the EGR system and because this '89 computer doesn't activate the EGR system then no problems come about. Is there anyone who could verify if something like this could be playing out here? Does anyone know how I can check if this computer is one that goes onto a truck with an EGR system? I'm attaching a photo of the part numbers of the ECU out of the 1989 ranger that seems to solve my issue. Can anyone think of anything else that differs about computers from these years? I'm stumped and I'm done throwing parts at this thing. Thanks if you made it this far!

 

[alwaysFlOoReD]

I'm interested in what people say. I had the same problem in my 86 4.0/a4ld. Never figured it out either.

 

[Shran]

So your truck has EGR and the truck that the new computer came out of did not...? Do I understand that correctly?

EGR can be a finicky thing... there are several little solenoids that trigger when the EGR valve opens among other things. I guess in my mind if the '89 computer is not programmed for EGR, it won't try to dump the EGR.

What happens if you run your original computer and unplug the vacuum line from the EGR valve?

 

[RonD]

Well EGR is not used at idle or until engine warms up
And lack of EGR system with EGR computer would be CEL(check engine light) and pinging/knocking while accelerating

1989 Automatic computer will try to activate the 3/4 shift solenoid which 1986/7(maybe 88) wouldn't have, but wouldn't cause stalling or idle issues

I would rule out or confirm a computer issue
On the throttle linkage is the Anti-diesel screw, looks like an idle screw, but everyone knows you can't have an idle screw on a fuel injected engine, lol, right

Any way, after engine is warmed up and idling unplug the IAC Valve
RPM should drop to 500 or so or engine may even stall, either is good, no vacuum leaks
If engine stalled turn Anti-diesel screw clockwise 1 full turn
Plug IAC Valve back in and restart
Unplug IAC valve again
Adjust anti-diesel screw until idle is about 750-800, needs to be high for automatics "in gear" test that
Drive it this way to see if you have surging or stalling with NO IAC Valve connected so no computer control of idle

Cold starts will be an issue, so you may want to plug IAC back in for that

Computer with IAC Valve should be +/- 4 rpm, its that fast to respond to higher or lower idle, which is why the surging or stalling is way out of character for the setup
Could be the TFI is sending incorrect RPM to computer, computer is "not seeing" the drop until its too late
Or its intermittent, it sees the drop which cause the surge, but drop was false, so just the surge
All computers, of any kind, have this one unchangeable rule, "Garbage IN = garbage OUT"
Not using the IAC Valve means computer has no control of RPM

TPS is there to give the computer a "heads up" to add more fuel quickly, or there would be a stumble when accelerating, it does what the Accelerator Pump did in a carburetor, not really part of idle control


PCV system, pull off the hose on PCV Valve and plug it, see if surging and stalling are gone
PCV Valve is a "controlled" vacuum leak, valve can stick open or closed randomly, just plug the hose and see if symptoms go away


Shift to Neutral before stopping, to see if the TCC(torque converter lockup) is the issue, unlikely but possible, would cause surging and stalling both

 

[RustyDusty]

So your truck has EGR and the truck that the new computer came out of did not...? Do I understand that correctly?

EGR can be a finicky thing... there are several little solenoids that trigger when the EGR valve opens among other things. I guess in my mind if the '89 computer is not programmed for EGR, it won't try to dump the EGR.

What happens if you run your original computer and unplug the vacuum line from the EGR valve?

I’m actually not sure if the 89 truck had EGR or not. I was wondering if anyone knows how to decipher the code on the 89 ECU, maybe the code says one way or the other?

when I disconnect the vacuum line to the EGR with the either the original ECU or the new ECU I still have the same symptoms unfortunately.

A friend of a friend who’s a long time mechanic said to not bother working on this truck anymore, the idle issue is a known problem on these old fords. He said I’ll never solve it. I don’t really believe it’s impossible to solve because it really seems like the computer is telling the truck to do nonsense things, mechanically the engine runs great, it’s this damn computer thats messing things up it would seem.

I actually had the exact same problem with an ‘89 econoline and it drove me nuts, it was less consistent than on this ranger and it would also do it on a cold start too. Sometimes jt would do it every day for a week then it wouldn’t give me any trouble for a month and then it would pop up again. I never figured it out but at the moment that van isn’t exhibiting those symptoms. Maybe it’s just some weird old fuel injected ford thing. I see tons and tons of posts on all the ford forums about symptoms like mine but either they never get it figured out or their fix doesn’t work for me.

 

[Paulos]

Sorry to see that you're still having to deal with this. Did you ever figure out what was causing your fuel pressure to drop so rapidly? I've read that bad injector(s), a bad check valve (built into the high pressure fuel pump), and maybe a bad fuel pressure regulator would cause the drop. And the bad FPR would cause a vacuum leak and a rich fuel condition. None of which is going to work in your favor with the surging problem.

Any trouble codes? Have you checked the timing when the engine is warm and surging, to see if it is stable (just a thought). Is the SPOUT connector in? Is the alternator voltage output in spec? I once had one that was putting out 18+ volts, and it was causing problems with sensors, etc. Another source of vacuum leaks can be the vacuum modulator on your A4LD. Any smoke out the tailpipe? Do have the correct TFI module (not a cheap replacement, and NOT black in color)? Do you have the correct IAC valve? RonD mentioned this when you asked about the fuel pressure loss. In fact, there is an issue with newer IAC's on older vehicles that a lot of people appear to not be aware of. You mentioned that your Ranger ran better with the '89 ECM, and that made sense if you are using an IAC that's compatible with newer Fords. Did you change your vacuum line routing prior to the surging issue? This won't be an issue if you use an '89 ECM, but it could cause problems with the stock '87 EGR system if it was routed wrong.

I could be wrong, but I'm thinking (primarily) about the IAC. Read what RonD replied to you back in April '22, and also the attached Ford IAC article. I've attached a few other things that may be helpful. It doesn't show it on the diagram, but on the vacuum reservoir the front port goes to the intake, and the rear goes to the EVR solenoid. Oh, and the '89 ECM you have is for RWD (JA or JB), not 4WD (VA or VB). Some people think it makes no difference, but IIRC it has to do with the VSS (speed sensor). And '88 was the last year for EGR on these Rangers.

 

[Cees Klumper]

If I were in your situation I would try a few more different (CA compliant) computers until I found one that works both the overdrive and solves the idle/surge issue, because from what you describe it's really the determining factor. And they can still be found easily for under $100 in good operating condition from parts places. I just picked up one spare one myself, after my old one went bad overnight - I always want to have at least one good spare on the shelf. Whatever doesn't work for your particular truck you can just resell for what you paid for it, so no big deal, considering how much time and expense and effort you've already put in, and you're almost there!
My '90 Bronco II that I've had for 3.5 years now has had ocassional idle / stalling problems, and each time it had to do with vacuum leaks or poor quality, semi-collapsed vacuum hoses. I've had to replace defective/bad brake booster check valve, the hose that goes to it from the intake manifold, and the hose from the throttle body to the pcv valve. Other than those occasional issues, idle has always been, and is today, rock steady, both at cold start and when warm.
Incidentally, my truck failed smog last Saturday because of EVAP leak, and I found it was a dried out, split fuel tank vent valve seal, and a dried out leaking 2 inch hose section at the fuel filler tube. Cost me several hours dropping the tank and all that, but I think I got it fixed, tank / system is now holding air pressure with the vent line to the vapor canister up-front blocked off. $10 in new seals/hose. Fingers crossed she'll pass when I go for re-inspection this week, she passed easily on all the other emissions tests.
Best of luck finally fixing your issue, that'd drive me nuts as well.

 

[RustyDusty]

Well EGR is not used at idle or until engine warms up
And lack of EGR system with EGR computer would be CEL(check engine light) and pinging/knocking while accelerating

1989 Automatic computer will try to activate the 3/4 shift solenoid which 1986/7(maybe 88) wouldn't have, but wouldn't cause stalling or idle issues

I would rule out or confirm a computer issue
On the throttle linkage is the Anti-diesel screw, looks like an idle screw, but everyone knows you can't have an idle screw on a fuel injected engine, lol, right

Any way, after engine is warmed up and idling unplug the IAC Valve
RPM should drop to 500 or so or engine may even stall, either is good, no vacuum leaks
If engine stalled turn Anti-diesel screw clockwise 1 full turn
Plug IAC Valve back in and restart
Unplug IAC valve again
Adjust anti-diesel screw until idle is about 750-800, needs to be high for automatics "in gear" test that
Drive it this way to see if you have surging or stalling with NO IAC Valve connected so no computer control of idle

Cold starts will be an issue, so you may want to plug IAC back in for that

Computer with IAC Valve should be +/- 4 rpm, its that fast to respond to higher or lower idle, which is why the surging or stalling is way out of character for the setup
Could be the TFI is sending incorrect RPM to computer, computer is "not seeing" the drop until its too late
Or its intermittent, it sees the drop which cause the surge, but drop was false, so just the surge
All computers, of any kind, have this one unchangeable rule, "Garbage IN = garbage OUT"
Not using the IAC Valve means computer has no control of RPM

TPS is there to give the computer a "heads up" to add more fuel quickly, or there would be a stumble when accelerating, it does what the Accelerator Pump did in a carburetor, not really part of idle control


PCV system, pull off the hose on PCV Valve and plug it, see if surging and stalling are gone
PCV Valve is a "controlled" vacuum leak, valve can stick open or closed randomly, just plug the hose and see if symptoms go away


Shift to Neutral before stopping, to see if the TCC(torque converter lockup) is the issue, unlikely but possible, would cause surging and stalling both

My truck doesn’t have a CEL light actually. It has the label for it in the lens of lights but there’s no wiring for it and no hole for the bulb for it.For some reason this year just didn’t have it. I haven’t noticed any pinging with this 89 computer.

I’ve fiddled with the anti-diesel screw quite a bit, I’ve done the idle reset procedure many times where you unplug the IAC, rev the engine for 2 minutes etc.. I will try it again exactly as you explained just to see if anything different happens.

I’ve had several TFI’s in the truck since they was my very first theory when the symptoms only showed up while warm. None of them have affected the issue. Currently it has the new grey
TFi module that came with the remanufactured distributor I put in recently. I replaced the distributor because my new computer gave me a PIP sensor code and from what I read on the forum most likely means that sensor is on its way out and it’s best to replace the whole distributor. So timing is set and good, distributor and TFI are good. I’ve jiggled the TFI wires trying to see if there’s some kind of short but I couldn’t get any reaction from the engine when doing that.

I’ll try plugging the pcv hose as well and report back. It’s had the hose, valve and grommet replaced but it’s easy enough to check again

what should I be looking for when shifting to neutral before stopping?

 

[RustyDusty]

Well EGR is not used at idle or until engine warms up
And lack of EGR system with EGR computer would be CEL(check engine light) and pinging/knocking while accelerating

1989 Automatic computer will try to activate the 3/4 shift solenoid which 1986/7(maybe 88) wouldn't have, but wouldn't cause stalling or idle issues

I would rule out or confirm a computer issue
On the throttle linkage is the Anti-diesel screw, looks like an idle screw, but everyone knows you can't have an idle screw on a fuel injected engine, lol, right

Any way, after engine is warmed up and idling unplug the IAC Valve
RPM should drop to 500 or so or engine may even stall, either is good, no vacuum leaks
If engine stalled turn Anti-diesel screw clockwise 1 full turn
Plug IAC Valve back in and restart
Unplug IAC valve again
Adjust anti-diesel screw until idle is about 750-800, needs to be high for automatics "in gear" test that
Drive it this way to see if you have surging or stalling with NO IAC Valve connected so no computer control of idle

Cold starts will be an issue, so you may want to plug IAC back in for that

Computer with IAC Valve should be +/- 4 rpm, its that fast to respond to higher or lower idle, which is why the surging or stalling is way out of character for the setup
Could be the TFI is sending incorrect RPM to computer, computer is "not seeing" the drop until its too late
Or its intermittent, it sees the drop which cause the surge, but drop was false, so just the surge
All computers, of any kind, have this one unchangeable rule, "Garbage IN = garbage OUT"
Not using the IAC Valve means computer has no control of RPM

TPS is there to give the computer a "heads up" to add more fuel quickly, or there would be a stumble when accelerating, it does what the Accelerator Pump did in a carburetor, not really part of idle control


PCV system, pull off the hose on PCV Valve and plug it, see if surging and stalling are gone
PCV Valve is a "controlled" vacuum leak, valve can stick open or closed randomly, just plug the hose and see if symptoms go away


Shift to Neutral before stopping, to see if the TCC(torque converter lockup) is the issue, unlikely but possible, would cause surging and stalling both

Ok so I have the computer for my truck in (not the ‘89, I unplugged the IAC and idle dropped a bit but I had to turn out the diesel screw a little to get it to idle closer to 750-800 but its surging as usual and the idle goes from 500-800 over and over again even with IAC unplugged.

I plugged the PCV hose and it had no effect on the surging, it continued as usual.

 

[RonD]

Three things can cause idling engine to surge, more air or more fuel or spark advance change

IAC unplugged, warm engine

Timing light on engine, should be 20-22deg BTDC on warm engine, see if it changes prior to surge
If so disconnect SPOUT and retest, if timing is stable at 10-12deg BTDC then computer's timing advance is causing it

Test fuel pressure while its surging, 30-35psi is expected
Also remove vacuum hose from FPR and plug the hose, retest fuel pressure, 41-43psi is expected


A4LD, pre-1989, will have a vacuum hose to intake, longshot, but check that hose for fluid, at high vacuum, idle, it could be sucking in ATF

 

[RustyDusty]

Sorry to see that you're still having to deal with this. Did you ever figure out what was causing your fuel pressure to drop so rapidly? I've read that bad injector(s), a bad check valve (built into the high pressure fuel pump), and maybe a bad fuel pressure regulator would cause the drop. And the bad FPR would cause a vacuum leak and a rich fuel condition. None of which is going to work in your favor with the surging problem.

Any trouble codes? Have you checked the timing when the engine is warm and surging, to see if it is stable (just a thought). Is the SPOUT connector in? Is the alternator voltage output in spec? I once had one that was putting out 18+ volts, and it was causing problems with sensors, etc. Another source of vacuum leaks can be the vacuum modulator on your A4LD. Any smoke out the tailpipe? Do have the correct TFI module (not a cheap replacement, and NOT black in color)? Do you have the correct IAC valve? RonD mentioned this when you asked about the fuel pressure loss. In fact, there is an issue with newer IAC's on older vehicles that a lot of people appear to not be aware of. You mentioned that your Ranger ran better with the '89 ECM, and that made sense if you are using an IAC that's compatible with newer Fords. Did you change your vacuum line routing prior to the surging issue? This won't be an issue if you use an '89 ECM, but it could cause problems with the stock '87 EGR system if it was routed wrong.

I could be wrong, but I'm thinking (primarily) about the IAC. Read what RonD replied to you back in April '22, and also the attached Ford IAC article. I've attached a few other things that may be helpful. It doesn't show it on the diagram, but on the vacuum reservoir the front port goes to the intake, and the rear goes to the EVR solenoid. Oh, and the '89 ECM you have is for RWD (JA or JB), not 4WD (VA or VB). Some people think it makes no difference, but IIRC it has to do with the VSS (speed sensor). And '88 was the last year for EGR on these Rangers.

I think the fuel issue you mentioned was a post I made about my engine stalling while I had the fuel pressure gauge hooked up and I noticed the fuel dropped to 0 in about 5 seconds. RonD replied "If your engine is stalling with key on then when RPMs drop low the computer WILL shut off the relay, but injectors stay on, so pressure will drop as engine stalls out." Every time I do a fuel pressure test the numbers seem great so I've kind of moved away from investigating that system. When I first got the truck I was pretty sure it had a fuel issue and I posted on here about it but I ended up taking it to a mechanic and they figured out it was the ignition switch contacts going bad. If the truck hit a bump in a certain way it would just turn the truck off and sometimes it wouldn't start up. So far fuel delivery hasn't actually been a problem for this truck as far as I can tell.

No trouble codes. When I put the new computer in I finally got one which I got really excited about. It was for the PIP sensor and thats what lead me to replace the distributor. Unfortunately my surging problems didnt go away but I havent seen the PIP sensor trouble code again. I did check/set the timing with the engine warm when I installed the distributor. The spout connector was out when I set the timing of course and the timing appeared steady and consistent when I was setting it. The SPOUT connector has been in ever since.

You know I really thought I had it in the bag at one point when I started looking at the alternator, I was thinking maybe the diodes were going bad and it was putting AC current through the truck but as I did more testing I couldn't actually find any issues but I will test it again today and report back.

I tested the modulator and replaced the hose to it with silicone tubing when I redid all the vacuum lines. I actually opened up the transmission valve body a few months ago to try to fix a really frustrating hard shifting issue. I cleaned out the entire valve body and got everything moving smoothly and reassembled with a new modulator and when I took it for a test drive it wasn't shifting at all. I was stressed to say the least. First thing I decided to do was put the old modulator back on and the truck shifted better than it ever had before. No more erratic hard shifts. I could investigate the modulator again though since it is the original that ended up back on the truck if you think that could be an issue.

No smoke from the tail pipe

I believe I have the correct TFI module, but now I cant remember if it was Motorcraft or not. I've put 3 or 4 different motorcraft TFI modules in, one was new, the others are all from junkyard trucks. The one in there now is a new one that came with my remanufactured distributor. All of them have been grey, not black

What do you mean by correct IAC Valve? I'm not sure where the one came from thats on the truck now but it's motorcraft. Originally when I got the truck it had a sparkly clean new off-brand one but as I started investigating the issues I learned these trucks need a motorcraft or Hitachi IAC so I've got several from the junkyard, threw the one from my Econoline on and even tried a brand new motorcraft one and none had a positive affect long term. A few times when putting a new one on the truck ran great and I thought I figured it out but I think it was just because I disconnected the battery and forced the computer to reset and once the computer finally got its leggings again the idle gets nasty again. I've since returned the new motorcraft IAC. I havent tried the Hitachi one but the fact that I've tried several IAC's without change and the surging happens even with the IAC disconnected made me stop really thinking about that as a possibility.

The vacuum routing is similar although I didn't send the line to the little valve on top of the air cleaner that opens the hot air intake valve because the hose to the exhaust manifold was shredded. I live in Southern California and I didnt really feel that system was useful enough to repair. Without vacuum going to that switch the valve stays shut and all my air comes from the cold intake behind the grill. The front port on my vacuum reservoir goes to the intake and the rear port goes to my EVR solenoid. And actually now that you bring it up, one thing that is on my list is replacing the EVR solenoid because after I shut off the truck I can hear it wine like its letting vacuum out of the little filter/cap on top. Im not sure if its supposed to do that, I guess since theres a filter under the cap its not necessarily supposed to be airtight but Ive wondered if its letting it out faster than its supposed to. But I've messed around with plugging the vacuum to that valve which stops the wine it makes when I shut the truck off but it doesn't fix the idle issue. curious if anyone has anything to say about this.

Thanks for the research you put into all of this with the diagrams and looking up the computer! Interesting about the speed sensor. That was something I had thought about as well and while it may not be a thorough test I did unplug the wiring harness to the transfer case and the speed sensor with the truck running warm (idle surging) just out of curiosity and it had no affect on the idle.

So are you saying 88 was the last year with EGR on these rangers so the 89 computer definitely is not an EGR computer?

 

[Cees Klumper]

Have you tried putting in a new oxygen sensor? I did not see that on the list of improvements. The sensor is only used when the engine is warm (when 'the loop' closes or opens, always forget which is which). A bad oxygen sensor could have a serious effect on warm running conditions?

 

[RustyDusty]

Three things can cause idling engine to surge, more air or more fuel or spark advance change

IAC unplugged, warm engine

Timing light on engine, should be 20-22deg BTDC on warm engine, see if it changes prior to surge
If so disconnect SPOUT and retest, if timing is stable at 10-12deg BTDC then computer's timing advance is causing it

Test fuel pressure while its surging, 30-35psi is expected
Also remove vacuum hose from FPR and plug the hose, retest fuel pressure, 41-43psi is expected


A4LD, pre-1989, will have a vacuum hose to intake, longshot, but check that hose for fluid, at high vacuum, idle, it could be sucking in ATF

IAC unplugged, warm engine, surging idle, SPOUT connector plugged in the timing is consistent at around 25deg. with SPOUT unplugged its consistent at 10deg. Timing seems consistent and unaffected by the surging idle with or without SPOUT connected.

Fuel pressure is around 32psi, I see it slightly fluctuate with the rhythm of the surging idle between 32-33 psi. When I unplug the vacuum to the fuel pressure regulator its completely steady at 41 PSI, doesn't fluctuate with the surging idle.

 

[RustyDusty]

Have you tried putting in a new oxygen sensor? I did not see that on the list of improvements. The sensor is only used when the engine is warm (when 'the loop' closes or opens, always forget which is which). A bad oxygen sensor could have a serious effect on warm running conditions?

Nope I havent put a new O2 sensor on. I threw matching one that I had from my Econoline on there just to see if it changed anything and it didn't. I remember testing both and they tested good but this was when I first started trying to tackle this issue a year or so ago. I could buy a new sensor, I'm kind of just tired of throwing parts at it.

 

[RonD]

Well then, there is no actual surging you just think there is...................

Out of ideas
Plug Power brake booster hose
Put volt meter on battery to see if alternator voltage is "surging"