Truck stall when come to a stop, 87 FI 454

[Eric M.]

I can give that a try, but considering I have swapped the IAC (it was part of the entire TBI unit I swapped) and swapped the ECM, I'm doubting it is either. But, it could tell me if there is a problem with the wires to the IAC.

Thanks,

Eric M.

 

[dyeager535]

It sounds to me like IAC is working....I'm out of touch on the tuning side now, but it sounds like your IAC is going exactly what its supposed to be doing...preventing the engine from stalling.

Plugs, wires and timing are all good, correct? When you manually manipulate the EGR (I understand with idle being as poor as it is, you'll need to keep the RPM's up) does the engine noticeably run worse?

It still sounds like a vacuum leak, albeit bad. How common are intake leaks on the 454's Scott?

 

[Eric M.]

Plugs / wires are good. Timing is right on (the balancer was replaced about 15K miles ago due to spinning and incorrect timing mark location).

I opened the EGR as you mentioned and the idle gets worse.

I don't think there is a vac leak on the top of the intake manifold (i've checked by spraying with choke cleaner and listening for RPM change), but someone did mention it could be leaking from the underside.

One other note, on occasions I can get it to idle for a few minutes while in neutral, the IAC count will be high (80 or above) and I get a Rich flag. Once I start driving, the Rich flag goes away (or will bounce from Rich to Lean like it should). From what I understand, this would mean things are too rich at idle.

Thanks,

Eric M.

 

[dyeager535]

What are your BLM/INT numbers? Only other thing I can think of is that it's forcing too much fuel in, and IAC is trying to compensate by adding air, but too much fuel should show with the BLM/INT numbers.

I've got the GM manual, I should get off my butt tonight and simply look at what they have for "poor idle".

 

[Eric M.]

Dorian,

Sorry to take so long to post up, I wanted to make sure I had all my trouble shooting done.

A friend called saying he had the same problem in his 91 and it turned out to be the distributor. I swapped the distributor from my other truck, got it running, set the timing and no change, it still died at idle. I put a brick on the accelerator to keep the RPM at about 2000 while it warmed up while I went through the service manual for more clues as to what was going on. After about 15 minutes, I pulled the brick off the peddle and it started to die, I played with the gas pedal a few times as the idle dropped to near stalling, then, all of a sudden, it smoothed out.

I’ve been driving the truck for 3 days and not a single stall. When the truck is hot and you turn it off for a short time (like at a gas station), when you turn it back on, the idle will be very low (maybe 450) for about two seconds as it gradually works its way up to 750, that’s about the only issue, if you could even call that an issue on a truck with 174K miles.

I put the scan tool on and pulled a few readings at idle.
Idle Air Motor 70
Oxygen sensor 900mv
Rich / Lean flag Rich (always, never bounces to lean)
RPM 750

It’s rich, and with the idle air motor at 70, it doesn’t have much room for adjustment. But for now I guess it’s fine … it’s running fine.

Just for kicks, I got the same reading off the other truck.
In Park and in Drive (on this truck the reading were different from P to D).
Park / Drive
Idle Air Motor 8 / 10
Oxygen sensor 4mv / 300 - 700 mv
Rich Lean flag Lean / Rich & Lean
RPM 1000 / 750

Almost the opposite of the first truck, lean and with the Idle Air Motor at 7 and 10, there isn’t much room for adjustment.

From what I got from the Service Manual, the IAC uses 3 sensors, the temp., TPS and distributor.
If the TPS is fully closed the IAC will function, if it is open at all, it won’t function.
If the engine temp is low (cold engine), the IAC will adjust for less air at idle.
The IAC will adjust for more or less air to maintain the proper RPM based on RPM signal from the distributor.

I did check the RPM reading on my scan tool while trouble shooting and it looked correct. I’m guessing at lower RPMs it was incorrect and I was never able to get the engine to remain at lower RPMS long enough to see if the reading as erratic.

So, there you have it. It was the distributor.
Thanks for all the help. I hope I don’t have to reopen this thread!

Eric M.