Unstable Idle, especially when cold

[mpyusko]

'96 Vortec 7400, 50,000 miles (low mileage, 5 year old create engine)

I'm having an issue where the truck has been hard to start (especially when wet, but dry too) and a screwy idle. When it is idling, either at rest or driving, it will run smooth for several seconds and then dip low (200 - 250 RPM) almost stall, then rev to 1900 and back to 675 (idle). Occasionally.... very occasionally.... it will actually stall. When I do a stall test (punch the throttle to the floor for an instant) it coughs, dips and returns to idle. On the street, I have to ease into it because the starting with too heavy a foot, makes it dip, recover and launch abruptly. Gearshifts are... interesting because of the way the Engine and Transmission computers work together. (Transmission says "I need to shift". Engine says, "OK, I'll back off on the power briefly", transmission says, "all done, let's play", engine says, "awesome....POWEEEEER!!!") When the transmission calls for a shift, the engine stutters, and then the shift is complete, the erratic performance throws you around. Normally the transmission shift so smooth you can't feel it, cruising towing or smoking the Prius next to me. (Lets be honest... it's a 7,000 truck... as powerful as it is, it's not a racecar.) These engines are surprisingly powerful and without the ability to manage them properly, they can really throw you around.

The last tune-up was 2 years ago. Cap, Rotor, Plugs and Wires. No codes. -24 degrees timing advance at idle, Fuel pump running smooth. I was thinking EGR, but it's electric and I can't tell. MAF gets good, stable signal. Everything "appears" to be running normal. (Has smog pump)

Thought's

 

[SAATR]

You need a scan tool with live data. Long and short term fuel trims, MAP and MAF readings at idle, IAC desired position vs actual, throttle position, misfire counts, etc. Lots of things that won't set a code that can still cause a problem.

Edit: If you already have one, your readings would help with recommendations.

 

[mpyusko]

You need a scan tool with live data. Long and short term fuel trims, MAP and MAF readings at idle, IAC desired position vs actual, throttle position, misfire counts, etc. Lots of things that won't set a code that can still cause a problem.

Edit: If you already have one, your readings would help with recommendations.

All I have is Torque as far as a scan tool goes. It doesn't show any misfires per-se, not saying it's not. It definitely feels like it's missing. I have a new cap/rotor/plugs/wireset I was going to put on it, but 20,000 miles and 2 years seems a bit soon to be replacing them.

The Injectors are original, so they have about 250,000 on them.

 

[HotWheelsBurban]

That's what my truck did when the throttle position sensor was going out. Last summer the Passlock was starting to get really goofy, so when starting the truck, no guarantee it would stay running. The throttle response had been a little bit off last couple times we drove it, didn't think much of it. It is an old truck after all....
So that morning, it stayed running, but the idle was absolutely crazy, going up and down. Called my retired fleet mechanic cousin, barely had the symptoms described to him and his diagnosis was the TPS. Apparently quite common on vortecs. You said your truck is a 96 so it's a vortec I presume. My Burb is a 99,5.7 vortec. The sensor was easily replaced, and the old one was very wobbly instead of nice and tight like the new one. I also replaced the idle air control valve; it was also the original one and was going out as well. Hope this helps

 

[mpyusko]

I drove it around yesterday and the fuel trim seemed pretty stable with no erratic behavior. However it was over 90F outside and it usually only shows up cold/damp/wet. I ordered a TPS and IAC. TPS seems to read smoothly throughout it's range, and the IAC seems to react ok when the idle suddenly dips. But I agree, they could both be worn out.

I noticed my passenger side knock sensor's lead is broken off. Could that mean anything?

 

[Pinger]

I noticed my passenger side knock sensor's lead is broken off. Could that mean anything?

I don't know so very much about these engines but on more recent ones, that would send the ECU into limp/safe/get-home mode where power would be limited to preserve the engine from possible damage.

Is it possible an earlier engine would only begin to shut down at a TPS threshold when the ECU cannot 'see' the knock sensor? Reconnecting it won't take you long....

 

[stutaeng]

I drove it around yesterday and the fuel trim seemed pretty stable with no erratic behavior. However it was over 90F outside and it usually only shows up cold/damp/wet. I ordered a TPS and IAC. TPS seems to read smoothly throughout it's range, and the IAC seems to react ok when the idle suddenly dips. But I agree, they could both be worn out.

Why did you order new IAC and TPS if they are reading normally?

It could be many things. You can check your EGR with a voltmeter. Very easy. I once was testing an EGR valve and dropped into my 4.3 (same valve as 5.7) and when I tried to accelerate on a light the engine stalled.

Can you check with vacuum guage? You should have something like 16-22 in. Hg. reading.

Did you check fuel trims at different RPMs? Are they within +/-15%?