High idle when hot

[MagicJames]

Hello and sorry in advance, I know there are a bajillion threads about this topic, and I read them all, I promise. 

I have a 1990, 4.0 liter, AW4. I start it up cold, and it starts perfectly every time, idling at 850-900 rpm; you could balance a coin on the engine. The throttle is responsive, sounds good, shifts perfectly, it's powerful.

 

However, when it gets up to temp, the idle increases. between 140-190F, the idle settles at 1250-1300 

 

-new IAC - IAC gets 12V from both sides of the stepper 

-new TPS adjusted just to Cruiser54's instructions 

-new throttle body and the idle screw hasn't been touched, factory adjustment 

-throttle body is spotless inside 

-new throttle body gasket 

-new EGR gasket 

-EGR isnt stuck

-clean EGR transducer, diaphragm moves on applied vacuum, has correct vacuum routing into EGR

-new vacuum reservoir 

-zero vacuum leaks, I ran smoke through the system, and nothing 

-new intake air sensor - reads 4-5V

-new exhaust manifold gasket - no leaks from here 

-new PVC/CCV fittings 

-new EGR solenoid - reads 12V from both sides 

-new O2 sensor, the expensive one from NTK

-all new grounds, no paint, metal to metal, and the extra from the -ve battery terminal

-new valve cover gasket

 

-Renix engine monitor (Nickintimes) reads no fault codes and very good readings from O2 sensor/ TPS.. ect

 

I spent the last few months throwing parts at this thing since its a near-perfect, completely stock truck which I want as a DD. I know that one of you out there is a smarter man than I am. I'm willing to try anything at this point. I literally wake up in the middle of the night thinking about it, and can go back to sleep again. 

Please give me your suggestions or ask me what other readings you need for diagnosis I'll get them. 

 

Thanks in advance  

[89 MJ]

I’d start by double checking the intake and exhaust manifold bolts. They have a tendency to loosen up over time. 
 

Id also make sure that the IAC and TPS are functioning correctly. Just because they’re new doesn’t mean that they are good. 

[MagicJames]

Its the third TPS and the IAC works well, gets voltage, and I can see it stepping when looking down the throttle

[Ωhm]

Few questions for the REM. 
When at High Idle, is ECU in OPEN or CLOSED Loop? 
When at High Idle what are STFT and LTFT values? Are they holding steady?
 

[MagicJames]

At high idle it stays in closed loop and STFT will swing between 55-70. LTFT stays a 128. 
So the ECU is dropping the trim a bit and it's running rich. But I have no vacuum leaks, even from the manifold. 

Could it be the coolent temp sensor? the connector gets about 5V when cold. Ive tried two so far but havent read the reading when hot, it should drop right? Also this is the sensor in the block, on the drivers side, in case I'm messing with the wrong one. 

 

 

[Ωhm]

1 hour ago, MagicJames said:

Could it be the coolent temp sensor?

 

Whatever the REM shows is the value used by the ECU for coolant temp.

 

1 hour ago, MagicJames said:

So the ECU is dropping the trim a bit and it's running rich

 

Suggestions: Pressure test fuel rail for leaky injector(s). Vacuum hose between Fuel Pressure Regulator (FPR) and intake manifold, look for fuel in hose.

 

 

[Eagle_SX4]

On the REM what does your map sensor and IAC show. 

[MagicJames]

Nothing in the fuel rail vacuum line

 

when cold IAC is at 128 and will drop sequentially as it gets up to temp. When hot at high idle (1200-1300) IAC is fully extended and will read zero. 
MAP when cold is starts at 12.8 then will drop to 10.5 when hot. 

 

I think my ECU is good, its a lowish mile truck and its trying to close that IAC to bring the idle down. It has to be a vacuum leak right? Do you think I should run smoke through it again? I know a guy that has one with the UV tracer thing. If so which ports do I use? last time i used the brake booster line

[Eagle_SX4]

When I put the new engine in my truck I had a high idle when I plugged the charcoal canister straight to the manifold instead of the air box. There may be a broken vacuum hose some where that is not next to the manifold. Test each vacuum circut by pulling all the vacuum connections off the manifold one at a time and plugging them to see if your idle returns to normal.