454 TBI stalls returning to idle

[TheApocalyptican]

I've read a bunch of topics all over the web on both 350 and 454 tbis stalling. Only saw one similar to my situation with no solution. In park, I can rev the engine and after letting off, the engine stumbles when near idle rpms, but recovers and the idles perfect. This same situation in gear usually results in the engine stalling out.

I just recently bought this 95 2500 suburban. 202k. Ill be honest that I haven't gone thru all the typical tune up stuff yet. I did just replace the iac as it he old one was frozen in place. Also had to fix the vac line for and adjust the throttle actuator, which from my reading on the internet, not many are aware of as its a big block only thing. I'm aware that I need to check everything out still, but am looking for a direction to look at my issue as it runs strong and idles fine, even when shifting from park to drive, just doesnt like to return to idle. I see in tunerpro the the iac counts freeze when revving, but then drop to 0 during the stumble before going into the 40s when it recovers. Possibly a slow responding iac?

[Tiger Joe]

We had an issue with a 454 tbi randomly stalling when we pushing the clutch in to stop. Ended up replacing the whole distributor because the bushing was shot

[speedygonzales]

A "lazy" IAC as you were wondering usually does the opposite of your problem. When you approach a light and decelerate, the iac stays open more than idle until you come to a stop and goes back in to it's idle position. When they are "lazy", it stays out longer than normal and the idle stays high much longer than normal until the IAC gets to it's "rest" position.

I think your problem is related to the IAC Counts going to "0" which would indicate it's at it's fully extended position and closing the bypass port. It could not idle at that position and you indicate it does not idle, it shuts off when in drive or stumbles when sitting in park.

So the IAC valve itself appears to be ok but rather it's being commanded to do what it is doing. I seems that you will have to determine why it is being commanded to do that.

I believe your problem lies in the TPS setting/reading or the adjustment of the throttle plates. You might want to check on the reading of the TPS through out it's movement. Then possibly the setting of the throttle plates. If the plastic cap is missing from the throttle plate adjustment screw, it's possible the plate stop has been changed and needs to be readjusted.

At this point it would be a good idea to get chevytech involved. I'm not versed on TBI motors.

[TheApocalyptican]

Forgot to mention some stuff in the 1st post. Tps has been checked for dead spots. It is currently reading 0.6v at idle. I've read that 0.5 is ideal but that 0.6 was acceptable. The idle screw has never been messed with. In fact it's one of the hardened cap ones that I'd need to drill the throttle body to punch it out from the backside. Just more info on what I know is good. Map sensor appears to be working. Coolant temp sensor is working. Haven't checked fuel pressure but there is no issue at higher rpms. After looking today, plugs cap and rotor all look old but ok as well. I will try getting the tps right on at 0.5v and see how that does. Will also check the dist play. Any other ideas?

[TheApocalyptican]

So no more issue. I set the base timing to 0 instead of 2atdc and set the base idle. Not sure which fixed it. While the idle screw was never messed with and had the hardened cap, once I got to it I was able to adjust it properly. Due to have a '7427 pcm in the truck, I couldn't jump the aldl pins. I had to unbolt the iac, then start the truck, which resulted in super high rom, but put the iac at 0. Then shut it off, screwed the iac back in, but had it unplugged. Then set the base idle. As I said, not sure which fixed it, but it returns to idle with no issue now.