4.6 erratic high hidle

[66stingray]

I posted a while back regarding cel light and codes i got with the mounty p0171 and P1152. They eventually went away and found out the probable cause, i had the infamous broken rubber elbow connecting the pcv valve and the throttle body,

Anyway, My erratic high idle ( meaning it revs high usually at start up to 1,500 or higher before settling down after a couple of minutes and if I blip the throttle it usually hangs high before kicking down to normal ), didn't start happening until I did a fuel pressure test. After I hooked a gauge to the fuel rail, my idle has gone bonkers. Read up a bit on idle conditions, found the vacuum leak with the broken elbow and I replaced the assembly. While I was at it I replaced the pcv valve. Still the same. Replaced the IAC valve with a standard brand and thought that initially cured it, but after a day of driving it returned to the same condition. Cant find any other obvious vacuum leaks. Thinking the only thing left would be the TPS sensor, I replaced that, still no change.

Idle is high at start up, blip the throttle and it hangs for a while before kicking back down. Driving the car, coming to a stop, the tranny kicks back to first and idles high, making the car surge and then after coming to a stop, kicks back down to normal idle. I read numerous posts regarding the issue and the cause usually is a vacuum leak, but can't find any other than what I found before. I have no current cel codes. I am at a loss. My remaining theory is that the IAC valve took a dump after a day and was thinking of getting a Motorcraft one.

Btw, my fuel pressure readings were within specs when energizing the fuel pump, no loss of pressure after shutting the motor off, but was 3 psi higher than spec in running condition, 68 vs 65 psi. Don't think this is a major issue.

 

[Joe in NY]

This happened when I cleaned the TB on our 07 suburban. It has fly by wire. I disconnected the TB and cleaned it really. Well it turns out GM ECU completely loses all learning of the idle and TB position when you disconnect it... I have no idea why... But found out when I started speeding while coasting to my first stop and had to really lay on the brakes. The relearn procedure was:

Disconnect battery for at least 10 min with key on.
Hook back up with key off.
Start car and let run for 5 min with out adjusting anything! Just start and walk away.
Then shut it off for 15 min.
Go back out and restart and let run for at least 5 min.

That is the GM method.. I did it and it helped tremendously. Although, years after that, it still tries to idle a bit high when slowing down.. Not always though.. Just sometimes. Strange.

But, give that a try and see. It is free and just takes a little time and gas.

 

[66stingray]

Thanks,

Actually my car has cable controls, but thought of that too. Tried resetting idle strategy after hooking battery back up, but no improvement.

 

[Joe in NY]

Yes, but you still have a PCM controlled IAC module. It is a variable solenoid that controls the amount of air into the motor when the TB is closed. Still has to learn the car's sweet spot.

 

[lincolnshibuya]

usually when the IAC is at fault you get erratic idle rpm and sometimes stalling when coasting or turning, no CEL either. with a vacuum leak, you will definitely get an erratic high idle and a CEL.

 

[66stingray]

Ok, good to know, reinforces my theory about the iac valve, i also read that the motorcraft one doesn't have a spring type plunger as opposed to the Standard brand one that I got, maybe the design difference isn't compatible with my application. Another poster wrote that after noticing the same differences in design and getting an oem Motorcraft one the problem resolved itself. Just didn't want to spend more money if I didn't have to.