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Poor Idle while in Park, decent while in Drive

RustyDusty

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It's once again time to smog my 1987 ford ranger with the terrible 2.9. Its been idling rough and I can't seem to figure it out. When in park (especially when warmed up) the idle surges then drops real low and then surges then back real low rhythmically and sometimes it will stall. It will sometimes die when I put it in drive or reverse but once its idling in Drive or reverse it smooths out and is fine but if I put it back in park its back to the rhythmic up and down over and over. Does this point in any certain direction? is there a sensor or something the computer pays more attention to for idle while in park that is bypassed when its in drive?

All my searching on here led me to believe its a vacuum leak. I've gone through every hose I could find and replaced it with high temp silicone tubing. The throttle position sensor is new, the IAC is new, the ECT sensor is new, the MAP sensor is new, the air charge temperature sensor is new, new spark plugs and wires, new cap and rotor, new air filter, throttle body is clean, what am I missing?

I'm about to give up and take it to a shop for them to figure out but thought I'd ask you guys first.

Thanks
 


RonD

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Clean or replace PCV Valve, its a controlled vacuum leak
It has a weighted valve inside that high vacuum(idle) should pull up to lessen the vacuum "leak" at idle but then open PCV valve more when vacuum is lower(driving)
But should do the same rhythm in Drive(at idle), so long shot, but not a no shot, lol

Does it only do it after warm up?
On cold start, idle should be higher(choke mode, open loop), does it also do it then?

The rhythm can be because the computer is working and so is the IAC Valve, as RPMs drop computer opens IAC Valve more to increase RPMs but then they go too high and it closes IAC Valve more, then too low and opens valve more, ect.............

When you shift an automatic out of Park/Neutral the computer gets that signal and it will bump up "target idle" 50-75RPM, which can make it more stable, also the load on the engine can stabilize a fluctuating idle

3rd party IAC Valve can cause this
Motorcraft or Hitachi brand IAC Valves are true solenoid valves
3rd party are solenoid/stepper valves, and to generic, made to work with other car makers systems
If you have the original Motorcraft IAC Valve put it back on and see if its better

Spark timing, a glitch in TFI wiring can cause computer to get momentary incorrect RPM signal, also a long shot
Just have a look at the wires,maybe unplug TFI connector and plug it back in to clean the contacts

Dirty fuel injectors
Add a can of Seafoam or similar injector cleaner to gas tank
Dirty injectors DRIP fuel at idle, instead of spraying it, causing poor air/fuel mix
 
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RustyDusty

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Clean or replace PCV Valve, its a controlled vacuum leak
It has a weighted valve inside that high vacuum(idle) should pull up to lessen the vacuum "leak" at idle but then open PCV valve more when vacuum is lower(driving)
But should do the same rhythm in Drive(at idle), so long shot, but not a no shot, lol

Does it only do it after warm up?
On cold start, idle should be higher(choke mode, open loop), does it also do it then?

The rhythm is because the computer is working and so is the IAC Valve, as RPMs drop computer opens IAC Valve more to increase RPMs but then they go too high and it closes IAC Valve more, then too low and opens valve more, ect.............

3rd party IAC Valve can cause this
Motorcraft or Hitachi brand IAC Valves are true solenoid valves
3rd party are solenoid/stepper valves to generic, made to work with other car makers systems
If you have the original Motorcraft IAC Valve put it back on and see if its better

Spark timing, a glitch in TFI wiring can cause computer to get momentary incorrect RPM signal, also a long shot
Just have a look at the wires,maybe unplug TFI connector and plug it back in to clean the contacts
Thanks for the quick response. I do have the old motorcraft IAC and unfortunately the problem persists when I run it with that one as well. It's possible that one has gone bad and the new one was never good as its not motorcraft brand. I might just have to drop the cash on a new Motorcraft one to troubleshoot.

Good idea with the TFI. When I first got this truck two years ago it had a issue where it would die and not turn back on then by some miracle it would decide to turn back on eventually. I thought for sure it was the TFI after doing research and replaced it. Thought I fixed the problem for a few weeks then it returned. Eventually I figured out it was actually the ignition switch going bad. Since then I've kinda stopped thinking about the TFI but its worth looking back into.
 

RustyDusty

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Clean or replace PCV Valve, its a controlled vacuum leak
It has a weighted valve inside that high vacuum(idle) should pull up to lessen the vacuum "leak" at idle but then open PCV valve more when vacuum is lower(driving)
But should do the same rhythm in Drive(at idle), so long shot, but not a no shot, lol

Does it only do it after warm up?
On cold start, idle should be higher(choke mode, open loop), does it also do it then?

The rhythm is because the computer is working and so is the IAC Valve, as RPMs drop computer opens IAC Valve more to increase RPMs but then they go too high and it closes IAC Valve more, then too low and opens valve more, ect.............

3rd party IAC Valve can cause this
Motorcraft or Hitachi brand IAC Valves are true solenoid valves
3rd party are solenoid/stepper valves, and to generic, made to work with other car makers systems
If you have the original Motorcraft IAC Valve put it back on and see if its better

Spark timing, a glitch in TFI wiring can cause computer to get momentary incorrect RPM signal, also a long shot
Just have a look at the wires,maybe unplug TFI connector and plug it back in to clean the contacts

Forgot to respond to a few things. The PCV valve is new, so is the tubing leading to it as well as the PCV grommet.

It does sometimes have this issue on a cold start. Other times it starts up just fine in the morning, sounds good, then after its warmed up I put it in drive and it dies, I start it up again, put it back in drive and we're off, good to go to wherever I'm going, then when I put it in park I usually shut the engine off because I'm at my destination and I don't actually notice the issue until I get back in the truck and start it up. Then is when I notice it most because I like to let it run for a second before I put it in gear to drive off and that's when it will show these symptoms most since its already been warmed up that day.

Back to the TFI talk, is the PIP sensor also to be suspected? I'm not too familiar with that one but I've seen it mentioned when doing research on the workings of the TFI.

Also in unrelated (I think?) news one of my new motorcraft spark plugs exploded on the freeway today. The ceramic insulator shot right past the metal crimp of the #2 cylinder spark plug while going 60mph. I pulled over and saw what happened and luckily still had the old plugs in the truck from doing the tune up the other day. Threw an old one back in for now until I pick up a new one. Thats a first for me. I'm hoping it was just a simple manufacturing error on the spark plug.
 

RustyDusty

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And for what its worth I just unplugged the map sensor wiring harness and it stopped doing sporadic idle. It sounded way more consistent but not necessarily healthy to my ears. I assume this is just because without the map sensor signal its just forcing the truck into some kind of limp mode or bypassing the computer's control of the idle?

I unplugged the IAC with the truck running and the rhythmic hunting for idle stopped as well and the truck didn't die. It sounded more consistent but, again, not very healthy.
 

RustyDusty

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Also maybe worth noting: Sometimes the sporadic idle issue goes away momentarily and to get it to come back I just give it a little gas (while in park) and the issue comes back, it starts surging and hunting for idle. Im not sure if that helps identify anything. I think I'm going to just buy a Motorcraft IAC and see where that gets me.
Does anyone know if a 1987 ford ranger with the 2.9 should have one of those ford coffee can vacuum reservoirs anywhere because I've looked around and cant seem to find one anywhere.

Thanks
 

RonD

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In 1987 it would only have a vacuum reservoir if it has an EGR system

You could check the throttle plate and throttle linkage, maybe its loose, so plate is not closing the same way each time
When you are coasting to a stop engine vacuum goes very high maybe its holding the plate open a bit, and tapping the gas pedal changes it slightly
 

RustyDusty

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In 1987 it would only have a vacuum reservoir if it has an EGR system

You could check the throttle plate and throttle linkage, maybe its loose, so plate is not closing the same way each time
When you are coasting to a stop engine vacuum goes very high maybe its holding the plate open a bit, and tapping the gas pedal changes it slightly

Hmm my truck does have an EGR system, do you happen to know where the vacuum reservoir is supposed to be?
 

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Here's a short video showing the engine idling more steady with the IAC disconnected then the up and down issue once it is plugged back in. Make sure to turn the sound on in the video:

 

RustyDusty

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@RonD I think you nailed it with the suggestion that off-brand IAC's cause these kind of issues. I took the crusty old IAC off my 1989 ford ecocline which appears to be the original and threw it on my ranger to see what would happen and the sporadic idle issue vanished. I'm going to keep it on there for a day or two to make sure but it looks like I'll be purchasing a new motorcraft IAC for the ranger! Problem potentially solved!
 

RustyDusty

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@RonD Turns out it wasnt the IAC after all. The issue returned shortly after so I put a brand new Motorcraft IAC on, still same issue. I decided to take it to a shop near me and they had a few guys trying to diagnose it. They tried most of the same tests I have already done, checked the timing, looked for vacuum leaks, checked the fuel pressure, Tried some new parts, MAP sensor, IAC etc.. and they were completely stumped. They thought it was strange the computer didn't throw any codes even with the MAP sensor or the IAC unplugged. They said it might be a computer or wiring issue and suggested I do some tests on the computer.

So far I just checked the voltage on a few of the pins on the EEC with the key on, engine off. The numbers seemed good except the reference voltage pin. I only got 3.24v on that pin when it should be about 5v. This seems like it could create a strange issue right? What might cause this? What further testing should I do? I did open the computer when I first noticed this issue and checked the capacitors and none are leaking, it looks brand new in there.

Some of the EEC pin voltages I got:

Pin 7 (engine coolant temperature sensor) 2.7v
Pin 21 (idle speed control) 12.43v
Pin 25 (air charge temp sensor) 5.05v
Pin 26 (5v reference voltage) 3.24v
Pin 47 (TPS) 0.88v
 

RonD

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Pin 26 should be 5v, : http://www.auto-diagnostics.info/ford_eec_iv

Sensors that use the 5v are TPS, IAT(ACT), ECT and MAP(and EGR Sensor if you have one)

Test pin 26 again, but unplug one sensor at a time until voltage comes up to 5volt, could be a sensor is effecting voltage to all the rest
 

RustyDusty

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Pin 26 should be 5v, : http://www.auto-diagnostics.info/ford_eec_iv

Sensors that use the 5v are TPS, IAT(ACT), ECT and MAP(and EGR Sensor if you have one)

Test pin 26 again, but unplug one sensor at a time until voltage comes up to 5volt, could be a sensor is effecting voltage to all the rest
Excellent, I will do that today, thanks!
 

RustyDusty

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I went to test the reference voltage on the EEC today while unplugging different sensors but before I started I decided to clean up the ground wire on the EEC harness that screws into the body under the glovebox area to make sure it’s got a good ground since it’s just screwed into a painted surface. Then when i connected my multimeter leads to the reference pin and the ground I just cleaned up o got a reading of 5.05v so I guess that issue disappeared? I started the truck and it still has the same idle issue. I’m wondering if this is going to end up being a bad eec issue but I highly doubt it, the EEC looks immaculate inside. I checked the part number it does seem to be a match for a 1987 ranger/bronco with the a4ld and 2.9 engine. Can’t really think of anything else right now other than a bad EEC. I know people on here say the 2.9 just has a shitty idle when it’s running properly but mine is swinging way too wide and stalling to the point where the smog people don’t really want to deal with it.
 

RustyDusty

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Had to take the ranger out for a 45 minute drive today to pick something up. On the way back shortly after getting on the highway I had a new issue that might be related. I had a split second loss of power. Felt like I had instantly took my foot off the pedal then back on in a fraction of a second. There wasn't anything gradual about it and there were no residual sputtering loss of power. Just a fast blip. It happened two more times later in the drive. When I got home I looked up this symptom specifically and found a thread on here with someone noticing the same thing with their truck. A few people suggested that the alternator could be going bad (bad diodes?) and sending AC current into the system which will confuse the computer and what not. That got me interested because It seems like my EEC is confused even though the voltage on each pin is accurate with key on engine off. Maybe once the alternator is brought into the picture it starts throwing off the EEC? I'm still not getting any codes unfortunately but figured I should rule it out. Can I simply disconnect my alternator from the equation and start up the truck to see if the symptoms persist? It was also suggested in that thread that the TFI was to blame. I did test it with my multimeter and it passed the test but who knows. It is still in its original location attached to the Distributor. Does anyone else have any ideas based on this new symptom? I've tested fuel pressure already and it was great but I guess I can check again.. ¯\_(ツ)_/¯
 

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