Every now and then, while idling, the engine will stumble:
RPMs will drop to ~300rpms, kick up to around 900~1000rpms, and then back down to a steady 600rpms. The engine keeps running, but it has stalled a couple of tmes when this has happened.

This only seems to happen after the ECM has gone into closed loop.

Here is what I have done to troubleshoot so far:

- I sprayed some carb cleaner around the vaccuum hoses to check for any leaks, but didn't find any.

- Replaced my ADS chip with the stock one; but the problem remained.

- Replaced the AFPR with the stock one (thinking that maybe the 13lbs of fuel pressure was causing both my stumble and a rich condition), but even at 11.5lbs the car still stumbles at idle.

- Replaced:

• Knock Sensor
• IAT Sensor
• Spark Plug Wires
• Dist Cap
• Dist Rotor
• O2 Sensor
• Injectors

Next, I looked at WinALDL (Thanks Jonas!) to see if I could figure it out, but I don't notice any sensor reading changes when the stumble happens.

To fix my rich condition, I think I will be getting another PROM burned. The guy doing the PROM work told me that the IAC setting could cause the idle to do this.

WinALDL reports the IAC is reading betwween 8~11 when idling. He said that as long as it doesn't go over 40, then there is something else wrong causing the low IAC and the stumble.

Can anyone offer some insight/advice to solve this? I've done a few searches on this topic, and turned up a few posts; but no one seems to be able to resolve it .....

Thanks for any help,

Thomas.

 

Sounds like the IAC to me.

 

Could be the IAC itself, your votage regulator going bad, or a lean condition where when you put it into gear it stalls because it can't react fast enough to the change in engine load since it's already running lean. Too much base timing can also cause this problem.

 

once you get it moving does it start running good? and does it only do it sometimes....mine did the exact same thing and i replaced a crapload of sensors...turned out to be the fuel pump going bad...mine would start right up, jump up and down and stall it i didnt rev it or get it moving....but for some reason once i got moving good and even if i stopped for a redlight it didnt stall...take the car to a shop and have the fuel pressure tested...sounds like that could be it

 

Is there a way to test the IAC with a multimeter? I turned the IAC adjustment screw 1/2 turn clockwise, and the IAC reading went to 0. I then turned it one full turn counterclockwise, and the reading went up to 15. I was going to keep turning it until I got it up around 30~40, but the guy doing the chip work (Ed Wright) advised me not to. He said if the reading is that low, it's probably something else.

I thought about the alternator (or regulator) too, but the voltage meter stays pretty steady right around 13.5v; even when I turn everything on in the car. I've got a pretty good sound system that pulls a lot of juice, and the car does the stumble no matter what I have turned on. I'm thinking about upgrading the alternator to a high output one anyway, so maybe my problem will go away when I do.....?

The engine doesn't stall when I put it into gear, the stumble only happens at idle.

It does happen only sometimes, but when I get up and go, the car runs great! I have a fuel pressure guage under the hood, and I've never seen it go below what the FPR is set to. When it started to stumble once, I got out and checked the guage and it was right at 11.5lbs.

Thanks for the info guys, hopefully I'll have this resolved soon

Thomas.