96 Blazer Idle problem

[blazin08]

Hey everybody. I've been reading every post I can find and haven't found the problem with my idle problem. I've replaced the wires, plugs, cap, rotor, map sensor (that I broke while trying to remove the plenum to check for washout - found none) and still can't figure it out. When I first start it up, it runs smooth. Once it seems to warm up, then it starts "stumbling" when I put it in gear. When I sit in gear at a light it stumbles really bad as if it is a misfire or about to die. I've sprayed ether all over the motor but haven't found a vaccuum leak. I did a compression check when I changed out the plugs and all cylinders registered the same, 175. I've cleaned out the EGR even though it looked fine and the pintle moves up and down easily. Like I said, it seems to miss (stumble) worse when in gear. In fact, when I come to stop now, I have to put it in neutral and it seems to idle fine with a slight miss that seems to go away until I put it back in gear. If I start the truck and just let it sit there, it idles fine. It almost seems to miss more when I'm in reverse.

My brother says "Generally your fuel system will not progressively misfire under load, it will misfire all the time. When it misfires under loaded conditions it is the ignition system." and I tend to agree. That's why I changed out all the standard tune up items. Like I said, it runs smooth as glass when it's not stumbling, so it makes me think that everything fuel wise is ok such as the injectors, filter, pump, regulator. I replaced the fuel filter at the same I replaced the fuel pump about two years ago. Fuel pressure seems to be steady so I figured the fuel regulator is doing it's job. Granted the truck has 225,000 miles on it, but it drives great when it's not stumbling. Yesterday, while I was driving down the road with a trailer, it started really stumbling bad as if it was going to die, but then it cleared up. It's possible that it may be slightly missing while I'm driving and I just don't feel it though.

There's no ticking or misfires that I can see or hear. I've raised the hood at night but haven't found any misfires. I bought a scanner because the for the past few months the CEL has been on (because a bad O2 sensor), but the truck has run fine all that time - until lately. The scanner only shows the O2 senser being bad. But it has never affected the idle like this. When I erase the codes to turn off the light, for a couple of days, the scanner doesn't show anything and says everything passes. I read that if the TPS is loose or bad, it can make the truck idle rough, but the scanner doesn't show anything there or any other sensor for that matter. So, if all the sensors are operationg correctly (except for the O2), then do you guys have any suggestions? When I pulled the cap off, the rotor assembly is nice and snug and again it runs smooth as glass at times, so I'm stumped.

Any ideas?

Thanks.

[old_master]

The 4.3L engines have a known problem with intake gaskets leaking. They can leak coolant, and vacuum, both internally and externally. (They can also leak oil externally.) Propane enrichment done properly will confirm an internal vacuum leak. Keep in mind that if/when there is a misfire, the ECM will identify which cylinder it is, set a DTC, and illuminate the SES.

Oxygen sensor response time slows with mileage. After 100,000 miles, the sensor has slowed enough to noticeably affect engine performance. If the SES light is coming on and indicating an O2 sensor problem, and the O2 sensors have over 100,000 miles on them, replace them they're due.

If there is an intermittent problem with ANY sensor, the SES light will come on and a DTC will be set in memory. If there is no DTC for the TPS, the sensor and its circuits are fine.

"My brother says "Generally your fuel system will not progressively misfire under load, it will misfire all the time. When it misfires under loaded conditions it is the ignition system." Usually true, but not always, especially with fuel injection. A simple 5 minute check of fuel pressure and leakdown rate will confirm or eliminate any problems with fuel delivery to the injectors. Key on, engine off, fuel pump running, pressure must be 60psi to 66psi. When the fuel pump shuts off, fuel pressure must remain above 55psi for 3 to 5 minutes. Hope this helps.