HELP! Engine Issue, can't diagnose!

[birdie2000]

Check the fitting/pipe that goes from your PCV to the port under the throttle body too. It's easy to forget about since the intake elbow is in the way.

[Darren]

all these suggestions are great, and I would do each and every single one of them, but I would do it only after I did a full tune up on the car. Meaning get 6 & 8 out of there. I am willing to bet if they are that hard to get to then the previous owner probably replaced 6 out of 8 plugs when they did a tune up as well. That is the first thing that I would do!

[1BADAIR]

Delco you buy at the parts store are NOT the same as what a dealer has...

[nascarnate326]

Have you checked the MAF?

[93formulalt1]

Check the PCV valve, make sure it is clean and not stuck open.

Pull and plug the brake booster vacuum line to rule out a bad brake booster.

Check to make sure EVAP purge solenoid isn't stuck open.

There is a vacuum elbow that commonly cracks on LT1's, can't remember what one off the top of my head.


Just some things I'd check first. They're free

Installed new PCV valve last night, replaced 2 vacuum boots, and fixed a leaking vacuum line. Still sitting at 10-11iHg at idle. Car is advancing 40*+ of timing at 1000RPM.

Decided to check the O2 sensors again. . . Turns out the passenger side O2 is "lazy". Bought a new pair of O2's, replaced the driver's side, started working on the passenger side, I apparently don't have tools that are capable of removing that sucker. About to go back out there, pull the starter (replace with a spare I have laying around), unbolt the y-pipe, spray it with PB, and go at it again with an open-end 7/8" wrench. Connector and a couple places on the sensor side of the wires are burnt to hell, engine harness side of wiring is all fine. Just a matter of getting at the O2 sensor to replace it. . .

[85z28kid]

Do a back pressure test on the pre O2 sensor port when you get it out and let us know what it comes back with

[93formulalt1]

Delco you buy at the parts store are NOT the same as what a dealer has...

Rock Auto carries 2 versions of the AC Delco wires, one is the same part number as OEM wire set. Those are the ones I'm going to buy. They have the stock style boots.

Have you checked the MAF?

Ruled out the MAF last night, unplugged it and noticed a change, but not necessarily an improvement.


I took the car for a spin today, after replacing some vacuum connectors. Car ran GREAT until it went into closed loop, then it started bucking, spark knocking, hesitating, and afterfiring, like it was all day yesterday. Something I did yesterday made the car run better, and the right side O2 is reading a nearly constant value while the car is running. Inspected the part, the O2 sensor and sensor side of the wiring is toast. Bought replacements, should have them on soon enough, the right side O2 is a PITA to get to, and I'm stuck with basic hand tools again. When my neighbor gets home, I'm going to go see if he has a 7/8" crow's foot socket, I think I can get it from under the car with that tool. Otherwise, I'm going to have to pull the flippin' header, and I'm really not too enthused about that.

[93formulalt1]

Do a back pressure test on the pre O2 sensor port when you get it out and let us know what it comes back with

I wish I could, but I have to be back on the road before 8pm, including getting my tools, clothes, and computer packed into the car. Have things to do tonight, and can't be working on the car too late.

[93formulalt1]

Had my neighbor help me out after work, we put it up on his hoist and replaced the O2's and starter, brought it down, started it up, and he checked all of the info on the scanner's datastream. . .

I have a stretched timing chain. It will take about 10 hours and $200 worth of parts to replace the timing set. Hardest part is removing the oil pan, the last time I pulled it, it took about 7 hours to replace it. . . If I pay his shop to do the work, labor alone will cost $690. If we do it in his garage next door, he won't charge me for labor, but I'm sure I'll be buying alcohol and food.

I'm in a bind right now, I have to drive back to the West side tonight, work at 3pm tomorrow. Car is going to have to hold it together enough to get me to the west side, and it's going to be parked until I have the parts and extra funds assembled to do the work, then I'll drive it home, it's going under the knife, and all will be right with the world.

Until then, I will park her in the garage, and she can sit there and think about what she's done.

[Darren]

So the mechanics who told ya that were right, could have saved you a lot of coin and frustration.

[93formulalt1]

So the mechanics who told ya that were right, could have saved you a lot of coin and frustration.

Honestly, the work that was done needed to be done. Fuel pump was going out, starter went bad, O2 sensors were bad, etc. I will do the timing set when I can afford it, and drive the car until something else goes bad.

And the mechanics who told me that before hadn't even seen the car in person, they were diagnosing over the phone based on what I told them. I've had 2 mechanics take rides in the car, both in the last 2 days, and they agreed, so until I replace the timing set, I'm not spending another dime on the car.

Also, found out that the MAF is reading too high, so it's going to need replaced also. He diagnosed a couple other things as well (exhaust bolts coming loose, etc.), that will be corrected when I swap the long tubes in.

[Sic96]

pull the headers and replace those two plugs... pita but it rules that out at least. When I deleted my EGR my car ran like shit. Like really bad! Everyone said that the EGR had nothing to do with it. Got it tuned out of the pcm and it's screamed ever since. It was the problem despite being told it wasn't 100 times. Since your car is OBD1, I doubt it's the EGR though. This may very well be a pcm issue, have you tried swapping another stock pcm in there? I didn't read all 5 pages so some of this may have already been addressed. I'd love to help, nick, I know a thing or two about these cars, but you're just too damn far away now. This almost has to be an electrical issue. You've replaced everything I could think of.. icm, coil, plugs, wires, opti.. I'd throw an LT4 knock module in there, see if that cleans up the knock. The only thing it could really be is those two plugs or something in the pcm. I'd say check the cat but iirc you said it was replaced. Muffler good too? those being plugged up will will surely cause a stumble and loss of power, and bad idle. I'm giving you the benefit of the doubt, but is the opti for sure right? as in installed right? those things can get finicky. Is it the non-vented like the car calls for? idk, just thinking here. hopefully you get it figured out. And I wouldn't sell it either, just missing something here.

[93formulalt1]

pull the headers and replace those two plugs... pita but it rules that out at least. When I deleted my EGR my car ran like shit. Like really bad! Everyone said that the EGR had nothing to do with it. Got it tuned out of the pcm and it's screamed ever since. It was the problem despite being told it wasn't 100 times. Since your car is OBD1, I doubt it's the EGR though. This may very well be a pcm issue, have you tried swapping another stock pcm in there? I didn't read all 5 pages so some of this may have already been addressed. I'd love to help, nick, I know a thing or two about these cars, but you're just too damn far away now. This almost has to be an electrical issue. You've replaced everything I could think of.. icm, coil, plugs, wires, opti.. I'd throw an LT4 knock module in there, see if that cleans up the knock. The only thing it could really be is those two plugs or something in the pcm. I'd say check the cat but iirc you said it was replaced. Muffler good too? those being plugged up will will surely cause a stumble and loss of power, and bad idle. I'm giving you the benefit of the doubt, but is the opti for sure right? as in installed right? those things can get finicky. Is it the non-vented like the car calls for? idk, just thinking here. hopefully you get it figured out. And I wouldn't sell it either, just missing something here.

Opti is the non-vented that it's supposed to have, the vented opti is a pin drive, the non-vented is a spline drive, so I'd have had to swap cams to make a vented opti work at all. Opti was installed correctly, I already thought that was the issue, so I pulled it again and re-installed, no change. Muffler is fine, cat was actually deleted, so there's a straight pipe in it's place. Muffler is fine too. Car will be getting long tubes and SLP 2OTL after this issue is resolved. The car is definitely only doing this under load, I can rev it all the way through the RPM range without backfiring with no load, but as soon as I put the engine under any load, it starts doing what I'm describing. I have an LT4 knock module laying around somewhere, when I find it, I'll swap it in, but I'm not banking on that fixing the issue. I have not swapped in another PCM, as I don't have a spare laying around at the moment. I have a '93 (PROM/SD) ECM in my parts car, and a '97 (OBDII/MAF) PCM in my parts pile, but neither will work in my car. I'd need to obtain a '94 or '95 PCM to test that theory.

I need to get a video to show what I'm explaining the problem as, if you guys saw what we saw on the test drive tonight, I think you'd all be just as puzzled as I am. Honestly, the timing chain makes the most sense at this point, as it explains both why it's doing what it's doing, and how it could have started as a minor misfire and evolved slowly, over time, into the major issue it is now. Any other issues would have presented all at once as a sudden change in the way the engine runs, the timing chain, however, is capable of becoming looser over time, causing the symptoms to worsen.


I should also state, this problem has existed since I bought the car, and while the work I've done did help improve the way the car ran, it did not completely solve the problem. At this point, I think my neighbor's diagnosis of a stretched timing chain is correct. Still open to other options, but at this point, I'm not throwing any more money at the car until the timing chain is ruled out as the culprit.

And I've decided I'm not selling the car because of this, it's become my mission in life to emerge from the battle between my intellect and my car's mechanical issues victorious. I will not accept defeat, I will beat this.

[85z28kid]

What codes are you getting? To bad you can't put freeze frame up so we can see some data on it.

I'm not convinced timing chain.

[birdie2000]

FWIW, while I have never done this personally while an engine was still in a car, you shouldn't have to completely drop the pan to do a timing set. You can just lower the front of the pan enough to get the timing cover in & out.