The search continues for my mysterious surging idle/stalling issue

[RustyDusty]

With a vacuum gage hooked up with the engine warm, idling and surging it goes from about 15 to 16 and fluctuates with the slow rhythm of the idle surge. If I remember correctly these engines should get around 18-20 but correct me if I'm wrong. I'm at 3500 feet above sea level and I believe, my vacuum reading should go down about 1 in Hg every 1000 above sea level the engines running at so 15 should be acceptable for a truck at 3500 feet. Does that all sound correct?

 

[RustyDusty]

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"

I've taken off each hose from the intake and plugged it just to see if I could get the surging to stop and none of them stop the surging. With a voltmeter on the battery the voltage varies between 14.15v 0 14.16 volts in accordance with the surging idle. I assume thats just because when the idle drops so doe the speed of the belt turning the alternator. I'm not detecting any AC current on the battery terminals.

 

[RustyDusty]

You have a good point about how subjective a term like surging can be. When the engines warm it always fluctuates about 300 RPMS. Sometimes even more which is why it will die on me on a warm start, sometimes 3 times in a row. and if I shift into gear when the RPM's are on the downswing in the surge it will often die too. When I brought the truck to a mechanic he was baffled by the surging and said the RPMs shouldn't swing like this. So while we may have different ideas on what surging is I think whatever might call these symptoms, its safe to say its not how a healthy truck should be running. The thing that baffles me most about all of this is the fact that the wrong computer for this truck runs it better than the correct ones. It makes me think its a really stupid simple fix. I guess I should go pull some more computers from the junkyard and find one that works and is a match for my truck. Or just keep living with this problem. Maybe the mechanic that told me I won't ever fix this issue was correct ¯\_(ツ)_/¯

 

[RustyDusty]

I still love this truck, does everything I ever ask it to do, it has no issues when its actually on the road. Its just embarrassing sometimes when I try to start it up in public or with friends and it dies 3 or 4 times in a row and I assume everyone just feels bad for a poor guy who can't afford a working truck haha

 

[Josh B]

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

I was thinking heavy on the transmission, then the next comment kinda went there too, even tho my first thought was working outward from the ECU, now I guess it's both

 

[RustyDusty]

I was thinking heavy on the transmission, then the next comment kinda went there too, even tho my first thought was working outward from the ECU, now I guess it's both

What do you think the transmission could be doing to cause this idle issue? Its kind of the most obvious difference about a computer from an '87 ranger and an '89 ranger but like someone else said on here it shouldn't cause a surging idle.

 

[RonD]

Yes, vacuum on any gasoline engine is relative to outside air pressure, not specific to engine size
18-21" is expected, at sea level with 14.7psi outside air pressure

Older higher mile engines will be on the lower end, as compression is also a factor

Yes, as outside air pressure goes down, higher elevation above sea level, vacuum level also goes down
2000-3000 ft. 16-20"
3000-4000 ft. 15-19"

So yes, if you were on the lower end because of high miles then 15" would not be unreasonable

This is why turbo-charged engines are popular in places like Denver, lol, need it just to have any power at all

Vacuum in the intake is "power", it means more air flow into a cylinder so more fuel can be added, as vacuum goes down so does power level, engines get very sluggish at higher altitudes
Turbo or super chargers add more power by forcing in more air so more fuel can be added and more energy released

 

[RustyDusty]

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

So to return back to this. If my timing is steady, my fuel pressure is steady and the surging is still happening with the IAC disconnected are you saying theres no way for the computer to be causing this issue? There's obviously a chance theres a vacuum leak somewhere I cant find unless the vacuum gauge readings I shared earlier negate that. I've sprayed water, propane and wd40 everywhere I could think of and couldn't find any leaks, the mechanic I took it to did the same and came up empty handed. When I opened up the valve body I checked the vacuum modulator while I was down there and there wasn't any ATF in the line. The modulator held vacuum. I checked vacuum from the hose connecting to the modulate up to the hose connecting to the intake after I replaced them with new silicone tubing and it held vacuum. It really feels like the surging idle happens suddenly, as if the computer is switching modes and starting to receive data from a different sensor. It doesnt seem to me like a gradual thing like you might expect from some part or gasket getting warm enough and creating a leak that only happens on a warm engine.

 

[RustyDusty]

Here's a video of the engine "surging." Let me know if this is what most would consider surging or not. The RPMs are going from about 600-900. Make sure to turn the audio on, I think its off by default.

https://imgur.com/YRm0TOX

 

[RonD]

Only other thing I can think of is MAP sensor, unplug it, 3 wires

 

[RustyDusty]

Only other thing I can think of is MAP sensor, unplug it, 3 wires

Yeah I’ve replaced it a few times, unplugging it definitely changes the idle but it doesn’t really make it better

 

[RonD]

Yes, at your altitude(3,500ft) it would idle poorly as "no MAP" would default to 1,000ft elevation or so, but does it surge

When you first turn the key on the Computer gets Base Air pressure and uses that and engine vacuum in the air fuel calculations

People who drove from sea level up into the mountains would notice rough engine running, until they stopped for gas, then engine ran better after that, the same on the way back down
Because MAP systems only reset base air pressure with key off and then on again

MAF systems do that on the fly, so outside of the loss of power with lower air pressure engine runs fine at most elevations

 

[Josh B]

What do you think the transmission could be doing to cause this idle issue? Its kind of the most obvious difference about a computer from an '87 ranger and an '89 ranger but like someone else said on here it shouldn't cause a surging idle.

I have no clue, I was just hoping to help you find one
This is one thing that gave that line of thought "The thing that baffles me most about all of this is the fact that the wrong computer for this truck runs it better than the correct ones. It makes me think its a really stupid simple fix. "
There's just so many ways this could work, and I also wonder if the surge comes from an air leak or choke point which starves and then gets a dose through (gas/air/whatever)

 

[RonD]

Computer can't control idle directly with IAC valve unplugged but it still controls injectors OPEN time
Which is why I asked about MAP sensor, MAP and IAT sensors are used to determine air weight/mass so part of calculations for open time for injectors

If you add more fuel you will get a surge, i.e. searching for vacuum leak with ether or other flammable fuel, and finding the leak, lol, you get a surge
Engine can also surge from lack of fuel, Lean surge, dirty injector dripping fuel now and then vs spraying, so poor air/fuel mix(lean) at idle, at higher RPM and faster air flow the drip doesn't matter

And computer also does Spark advance, but you tested that

 

[rusty ol ranger]

88+ 2.9s did NOT have a EGR.

The 89 ECM basically bypasses the EGR and the knock sensor, as the knock sensor went away with the 88+ models as well.

I would pull the EGR valve and see if its stuck open, or if the passage behind the EGR is plugged with carbon. My 87 was plugged rock solid.

Also it could be an issue with the EGR vacuum solenoid (the little can on the passenger side with little red and green vacuum hoses running to it, as well as a plug)

There is also an EGR pulse sensor, that tells the ECM what the EGR is doing. Its a little black box held on by two clips right in the vicinity of the EGR valve. There is a little hose running to the bottom side of it from the EGR valve, and an electric plug

Also, do not overlook the knock sensor. Its job is to adjust timing if it senses spark knock. However, if you have alot of valvetrain noise (ticking) it will sense this as spark knock and retard timing.

The sensor itself could also be failing.

Since the 89 ECM clears everything up i would bet on either...

1- EGR valve/system issue
2- Knock sensor issue/valvetrain noise
3- Failed ECM.

Listed in order of which is most likely to least likely.