4.3l Engine Idle troubleshooting

YJJPWrangler

YJJPWrangler

Well-Known Member
Joined
Jul 19, 2005
Location
Charlotte
1992(ish) 4.3L TBI in the Willys.

Symptoms: High Idle. It typically idles around 1500rpm. The warmer it gets outside, the higher it goes. If I blip the throttle, the rpm comes down to around 900-1000 for a few seconds and then shoots right back up. I've been chasing this thing for a while now. I found that my TPS sensor was reading higher than normal voltage. Normal voltage at closed throttle(idle) is supposed to be between .45 and .55 volts, I was around .85. I was able to manually adjust the TPS down to .48. I also replaced the IAC and did the ECM reset where you jumper the A and B pin. Still no change.

Right now it will idle around 1600-1700 rpm, pedal response is flat. It won't rev past 3k and I'm getting some backfiring. It will also just randomly die, continues to drop idle until it kills itself.

Ideas?
 
I struggled with high idle on that motor with the same conditions. I chased it also and then learned to live with it. When I built my current 4.3, it idles high at times but I'm used to it now I guess. I thought it was something with my ECM or sensors, or...? I replaced everything for one reason or another throughout the process.
BUT it would rev to the moon. Backfiring and dieing isn't good. (That was never an issue with me). I'm wondering what the condition of the distributor is?
The S10 groups on FB is a pretty good resource, I've been using them for help with Sweenz which also has a problematic 4.3
 
@R Q , I think it's the IAC reset. Before I went into the TPS/IAC, it would rev fine, no backfire etc. Once I changed those out, that's when the problems started. I'm going to pull the IAC back out, clean it and the TBI thoroughly and redo the reset. I've also thought about smoke testing it to see if there are any major vacuum leaks.
 
Pulled the IAC out again, cleaned everything well. Did the reset procedure. Pulled a chunk of crud out of one of the TBI's orifice. Confirmed that the IAC passage into the TBI is clear. Still no change. Motor stumbles hard. When it does idle, its around 1500rpm but it quickly dies. Now won't rev past 2500.

It smells very rich. Injector spray is in a cone but it is not a mist.


Prior to this work, the jeep ran fine, although it idled high. The ONLY thing I did was TPS reset and put on a new starter. I measured the TPS voltage again yesterday and it is at .50 nominal voltage at rest. Specs are between .45 and .55. I didn't touch distributor/timing.

What I think is happening: IAC is fully closed and is not actuating. Still need to physically verify the plunger moves in/out. I think the IAC plus the TPS is causing it to run in a rich position.

Thoughts?
 
Did some more troubleshooting on it this afternoon. Realized I'm an idiot...

MAP sensor was plugged into one of the front vacuum ports. The front ports are metered, with the exception of the PCV valve. Picked up another MAP sensor because why not, built a new vacuum line and connected the MAP to the rear of the TBI where its supposed to go.
Initial start was good. Ran decent for 30 secs and then died. Did a full IAC reset. Again, ran great for 30 secs and then died. I think it all points to a big vacuum leak somewhere. Checked the PCV valve, rattles around and doesn't seal at all so that's a likely culprit. Will pick one up tomorrow and test again.

If it continues, I'll start spraying carb clean around the TBI and the intake and see where that takes me.
 
IACs don't really plunge well in my experience and they're easy to screw up when you mess with them. I put new injectors in Sweenz even tho it had a cone, it wasnt atomizing the fuel very well. I also rebuilt the throttle body and regulator and it made a big difference. I got new injectors off Amazon but there is also places that you can send them for a rebuild that costs less.
 
@R Q, all good points. Ive got new injectors and a new regulator in this unit. I think the only reason it was running prior to this work was because the TPS was set so high, it could overcome unmetered air. Now with the TPS set to nominal range, IAC reset the ECM is trying to bring the idle down to where it's factory set but dies due to the unmetered air from the worn out PCV valve. I could blow air both ways through the valve while I had it out.

When it would run for 30 secs or so, there was little to no pedal response and I couldn't keep it running by feathering the pedal but it wasn't running rich. Engine wasn't to temp but it was somewhat warm so I think the ECM was trying to get it out of closed loop cycle but couldn't due to the airflow.

You mentioned this motor used to rev to the moon, I never could get it up above 3500 prior. Hopefully the PCV valve takes care of this. If not, signs point to throttle body gasket or intake gasket. All the other vacuum lines are new or plugged(EGR). It could be the coolant temp sensor but that's a relatively new one(3 yrs). It wouldn't explain it running super rich.
 
I have not heard that jumping A and B resets the ECM. But jumping A and B on the diagnostic port will let you read codes via the SES light. (CEL light). Have you done that and get any codes?
I have a complete troubleshooting guide that came with my wiring harness from Howell Fuel Injection that is very helpful. I can make you a copy if you would like. Also do you have manual or power brakes? There's a check valve at the booster that, if bad, it will shut the motor down.
This is the manual #M258 - TBI Service Manual
 
@R Q, I should have added some punctuation. You are correct, jumping the A and B does not reset the ECM. What I was trying to convey was that I did the "IAC reset" comma "ECM is trying to idle down". I never could get the CEL light to work properly, I think it's tucked up under the dash somewhere. I have a wilwood booster for braking. I agree that if the booster is bad it will shut it down for sure.

I sent my wiring harness out to Jims Performance in Maryland to get cut down. I told him exactly what I was running and he sent everything back.

Picking up some more parts in a bit.
 
intake leak? Have you done a smoke test?
 
If the CEL light isnt working or is grounded out it can create issues. I had to remedy the one on Sweenz. Its the brown wire coming out of the plug that goes into the ECM, I can give you the wire number later, I'm prepping for work now.
 
CEL light is hooked up and functions. Had a code 15 which is coolant temp sensor. Replaced it and now no codes. PCV valve didn’t fix anything either.

Jeep runs for 30 secs and then dies. It won’t crank afterwards for 15 minutes and acts like it’s flooded. I’m going to check base timing because we set the timing when it was running rich.

My understanding is: For the first 30 seconds, the ECM is running on base timing. Once the ECM “stabilizes” it then sends signal to advance/retard timing via the distributor. Since my initial timing is so advanced(map sensor in the wrong port, it thinks it’s in a heavy load condition which commands more fuel). I’m going to re verify timing now.

Getting there…

Edit to add: while it was running, I sprayed carb cleaner all around the intake and TBI. No change.
 
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