Diagnosing Rough Idle Hell

[DylChrFla]

Knock counts are way high mine are 0. When it see knock it pulls timing. Tps voltage at idle should be .50 to .60 you might double check that also.
Strangely it runs way better without the ALDL cable plugged in, why could that be? Possibly the wrong cable for my ECM or the service mode messing with my engine.

 

[tsr2185]

I just noticed my TPS % shows 30% with no input and 220% at WOT. Hmmmm
My idle also seems to rise whenever I plug in the ALDL cable.

that your problem? ive seen this with someone else. they had to manually adjust it from a aftermarket slotted TPS. They showed 10% in park. Yours may be screwed? or ECM?

 

[PlayingWithTBI]

Strangely it runs way better without the ALDL cable plugged in, why could that be? Possibly the wrong cable for my ECM or the service mode messing with my engine.

Some ALDL cables come with a 10K resistor jumper for older ECMs. You should be able to pull it to get your idle down and run better.

Knock Counts: 67 (seems pretty high)

When did you get the KCs, when starting the engine (no biggie)? The PCM doesn't always pull timing unles you get multiple KCs all at once. It'll continue to count them though. Do you have a loose exhaust pipe or something banging/hitting while accelerating/driving?

 

[DylChrFla]

Some ALDL cables come with a 10K resistor jumper for older ECMs. You should be able to pull it to get your idle down and run better.

When did you get the KCs, when starting the engine (no biggie)? The PCM doesn't always pull timing unles you get multiple KCs all at once. It'll continue to count them though. Do you have a loose exhaust pipe or something banging/hitting while accelerating/driving?

I always had one KC when starting. I will pull the resistor and see if that was a fluke, my exhaust is mounted rock solid So that shouldn’t be an issue. I can’t hear an audible knock or or anything out of the ordinary.

 

[DylChrFla]

Alright, after taking the resistor out and resetting the IAC yet again, my idle seems to be perfect, KC are next to nothing, engine is pulling strong as ever. The distributor seemed to be the culprit of all my idle issues. Thanks to everyone who helped out and gave me advice.

 

[1993GMCC2500SLX]

I haven't thrown anything away that I have taken off this truck, except for the idle air control valve I cleaned while my new ac delco unit came in, I just wanted to see how well I could clean it and I gave to a buddy of mine for his truck that ran fine when driving but it's idle was allover the place.. and the only good plug wires off the truck I gave to a friend who's truck kept having a miss in the engine and that's because the silicone boots were about gone and the jacket coating on the plug wires weren't much better..

 

[1993GMCC2500SLX]

OOOOooooo..."notorious"!

I like it!

Sur the newest laptop I own is a Compaq Armada which needs a replacement battery pack, only works if plugged in, anyways think of the ibm Thinkpad with the mouse pad square..lol it's a little old

 

[1993GMCC2500SLX]

Gotcha. I replaced all my lines and still same issue. Im offshore right now and I just learned about the booster check valve and vac line going to it (im till a noob). I used a butane torch to check for leaks but I didnt find any either. Didnt use starter fluid, but will try that when i get home, along with the datalogging and TBI rebuild.

You did a compression test, fuel pressure test, and put a vac guage on it? I havent put a vac guage on mine, but the other two check out fine.

Let us know. Ive searched dozens of posts about rough idles, and 90% of the time the O.P. never follows up with their solutions lol.

I usually use pb blaster,or brake kleen flammable brake parts cleaner because that's what I have on hand, already used up the tbi cleaner I had when I was cleaning the iac valve and rebuilding the fuel pressure regulator. And that got me thinking about my earlier statement if this turns out to be a 305 tbi not off a 350 then the regulator spring is probably wrong pressure as well so I'm not keeping it either,I'll just clean and rebuild another one or buy a injection pod off eBay, but I'm not sure I want that can of worms either?! Anyways a walker gm Rochester 220tbi rebuild kit is cheap insurance.. and I'll know that the fuel pressure regulator is not leaking.

 

[1993GMCC2500SLX]

No. It's the programmed idle speed that the computer is trying to achieve by adjusting the IAC.

So, for example, the computer wants 650 rpm, but the engine is actually running at 725; you can expect problems with the IAC, the IAC wire harness, a faulty ECM, or there's an air leak that's beyond the ability of the IAC to compensate for. You'd be looking at the IAC counts to see if the computer is trying to adjust the speed.

But if the computer wants 650, and the engine is actually running at 650, and the IAC counts are reasonable, it's likely that the whole speed-control system is Okee-Dokee.

I've actually experienced that before tbi vehicles I had a car that I thought would keep losing power to the electronic choke it would start up after being a throttle singing nightmare when cold, start to kick on high idle, about the time it would normally come down off high idle by touching the gas pedal, it would sputter and die, after several checks with a dvom I realized that wasn't at all what was happening, what was happening is that it had a short in the wiring going to the TPS sensor and the computer feedback e4mc quadrajet and O2 sensor kept adjusting the timing stalling the engine thinking it was running rich,not a idling cold engine, terminals inside the TPS sensor were loose so it needed replaced, 86 Buick regal sport coupe Oldsmobile 307 v8 roller cam 6 pin ignition control module..

 

[1993GMCC2500SLX]

I've actually experienced that before tbi vehicles I had a car that I thought would keep losing power to the electronic choke it would start up after being a throttle singing nightmare when cold, start to kick on high idle, about the time it would normally come down off high idle by touching the gas pedal, it would sputter and die, after several checks with a dvom I realized that wasn't at all what was happening, what was happening is that it had a short in the wiring going to the TPS sensor and the computer feedback e4mc quadrajet and O2 sensor kept adjusting the timing stalling the engine thinking it was running rich,not a idling cold engine, terminals inside the TPS sensor were loose so it needed replaced, 86 Buick regal sport coupe Oldsmobile 307 v8 roller cam 6 pin ignition control module..

I forgot to add that Buick regal was a California emissions car.