4.3 Stumble off of idle

[dacstuff]

This thing is driving me nuts. Got a 96 Caprice with the 4.3 V8. It was running fine then started to stumble off of idle. Got real bad. Ran compter scan, no codes, checked it with the computer, nothing showing. Took the TPS off, checked it and put it back on. Seemed to solve the problem. Started it again, so figured that a new TPS would solve the problem. NO. There is a very slight stumble off of idle on light acceleration. If you cram your foot to the floor it takes off great, but if you drive normal you get this stumble. If you feed the gas slowly at that point it actually starts to die a little then catch. I just had it out again and I came around a corner with a little power then let up and turned in a drive and gave it a little more gas and it died. Shut right down. Now I think, not sure, but will check this, that I felt the trans shift just before I applied the gas. Is it possible that we have shift point problem with the trans that's causing this to load up? Any suggestinons on this? The car runs good other than that. This has about 76,000 miles on it. Had one guy suggest it was a problem with the ignition. Suggested a tune up. Don't mind doing this but, if you have ever done a tune up on one of these you know that it's not a fun job and can get real expensive, just on the hunch that might be it. It really does not act like a ingition problem, other than possilbe low idle, or timing, but again it runs fine after you get it up and going. Acts like a fuel problem to me. Thinking a sensor or other computer generated problem. If anyone has an idea I would sure like to know about it. Thanks guys

[misfit2004]

Got me.check your EGR valve for vaccum leaks,hoses for leaks,and your intake/header gaskets for the same.Sounds like you might be running lean?

[CD Smalley]

Stumble off idle in L99 and LT1 cars is usually caused by a buildup of carbon on the back side of the throttle body. Take it off, clean it with carb cleaner and you'll be amazed at the difference...

[misfit2004]

Could buildup of carbon cause stumble during idle?Like,perhaps a small irregularity in idle quality noticible only if youre paying attention?Also,could this be an issue that would affect fuel system response at all?
Ive went thru all the service/troubleshoot proceedures on my 305;EGR condition,replacing the old vacuum hoses,checking gaskets,etc.Even with the efficency in combustion Ive built on my motor,I still find these issues.It would be really cool to just have to pop off the TB unit and clean it to fix these problems...................

[dacstuff]

Thanks for the response. Couple of other facts. We had this diganosed as a bad EGR valve. Did the EGR Sol. and cleaned the TB unit, checked all hoses. Condition was so, so then we replaced the TPS and it got bad. Stalled three times on test drive. All same coniditon. Start with light gas feed from stop, travel about 500 feet, let up on gas feed to make a turn. Apply light gas feed and engine cut out.

[dacstuff]

Couple of things I forgot. Response well with agressive driving. If you take off fast from a start it will responde well. Only does this on light driving conditions.

[CD Smalley]

You did the EGR Solenoid. But did you replace the EGR valve itself???

[burnM]

Have you checked the IAC (idle air control) valve? Carbon build-up on it will make your car lope during idle, and in extreme cases can cause stalling. Clean or replace it, making sure to adjust it properly before re-installation.

.... another shot in the dark..... change the fuel filter and see if there is an improvement. It could be clogged just enough to cause problems at a lower fuel pressure.

Short of everything else everyone has said, I think it's tune-up time!

[dacstuff]

Thanks guys. Here's what we did so far. Had already replaced the fuel filter. No did not replace the EGR valve, but cleaned the TB and all around it. Checked to see what TPS was used and it was an aftermarket unit. Got rid of that and went to OEM unit. Much better but still not quite right. Used the scan tool, scope and computer. No codes, everything looks good. Checked for leaks on the shot that we are creating a lean condition, which is what this feels like, before the MAP and the computer doesn't know it. Everything looks good. May try a MAP just to make sure that's not it. Next is to take it up to my son, who builds high performace cars. He knows the LT1's real good and said that the 4.3 uses the same type of computer controls. He will put it on a computer and dirve the it. Does not do it when you rev it up in park. Does not do it in drive with foot on break when you load it up. Only happens on the road. What is driving me nuts is why does it get better when we replace the TPS? These are zero calibration units. The aftermarket one rendered the car almost undrivable. The OEM corrected most of the problem but not all. What are we missing?

[CD Smalley]

As I understand it, there is some adjustment in the TPS units. The ones I have seen on my cars all have a slightly oval mounting holes in them. This allows a very small amount of adjustment when installing them. But there is a small amount there.

Since you haven't replaced the EGR yet, I would sure do it. And get a GM replacement as well. Don't mess with aftermarket EGR valves. I've had nothing but bad luck with them.

[dacstuff]

Thanks. You are right. A lot of the early TPS units did have adjustments on them. My 85 Eldo for example. The new units do not. They are self Calibrating. You can only put them in one way in one position. They will then calibrate themselves when you turn the key on. Ain't computers great! Our first thought was that it was a EGR but just not acting like an EGR now. And again you are right about the aftermarket items. The aftermarket TPS made the car almost undrivable. It make this condition a lot worst. The OEM TPS put it back to about where it was piror to changing it. The thing that has me baffeled is that it acts like a lean mix, which should indicate a air leak after the MAP, but we can not find one. Thought of replacing the MAP just to make sure it's not out of range. Not sure that's the problem. Would help if this would show up on the computer as a code. Gonna go with my gut and continue to look for some kind of air leak after the MAP. Any other good words?

[CD Smalley]

Acts lean huh? How about an exhaust leak? These cars are known for breaking exhaust manifold bolts.....

[Larry1957]

[quote=burnM]Have you checked the IAC (idle air control) valve? Carbon build-up on it will make your car lope during idle, and in extreme cases can cause stalling. Clean or replace it, making sure to adjust it properly before re-installation.

BurnM
You say to "...adjust the IAC valve before replacing it". I have one that is giving me fits, can you tell me how to adjust it ?

Thanks

[burnM]

Measure the distance between the tip of the pintle and the gasket mounting surface. It has to be less than 1-1/8" so as to not damage the valve. Hold it like you would a roll of quarters with the pointed end (pintle) facing out (toward your thumb). Rock it gently back and forth while pushing it in with your thumb.

I don't know which engine you have, so...

- If it's the screw in type, be careful when you install it! I found out the hard way about overtightening it, and snapped it in half. I ended up rebuilding another old TB unit, because I couldn't get the threads out of the casting.
I think the proper torque is 13 ft-lbs. Don't forget a light coat of RTV on the threads.

- If it's the type with mounting screws, adjust in the same way, with the same measurement,install a new o-ring, and it's gravy!

Something else to watch out for... On a model 220 TBI unit, the pintles came in two different sizes. 10mm and 12mm in diameter. Make sure you have the proper one for your TB or the pintle tip may not seat properly.

Good luck! Keep us updated.

[Larry1957]

Thanks BurnM
I guess I should've given you more info. It's a 92 4.3 in a GMC Jimmy.
The IAC is a screw in type. This thing is running very (rich?) (to much fuel). Idle is very rough. Stalls and surges very bad.
To date my trouble shooting efforts have included the following - replaced: plugs, wires, cap, rotor, O2 sensor.
Cleaned and tested EGR
Tested MAP
Tested temp sensor
Set timing (disconnect / re-connect advance line under dash)
Visually verified movement of pintle of IAC in tbi
Verified voltages at IAC connector awa stepper pulses w/ O'scope.
Checked for vacuum leaks using carb/injector cleaner.
MIL is functional but no codes are set.
Fuel pressure = ~18psi
Swapped ign module w/ another not new one
Swapped coil w/ another not new one

Any help / advice anyone could give me here would be very much appreciated.