Misfire, surge, eratic idle at startup

Ok, I got a '93 s-10 Blazer with the 4.3 Vortec. I have been having a misfire I have been chasing down for some time and am running out of options.

Under a very light acceleration I get a series of misfires right before it shifts gears. It also idles erraticaly when first started which goes away quite quickly. The idle is good with an occasional stumble after this. It also surges under medium throttle when climbing hills.

I started with a compresson test and found a blown head gasket. I replaced that and the misfire went away for a few months. Checked the compression again when the miss returned and it's still good.

Thought maybe it was fuel pressure. Checked the fuel pressure, and it was low. Pinched the return line and pressure shot up. Replaced the regulator. Pressure is fine now...no difference. There was also no fuel wash in the Plenum....no leaks there.

I checked the vacuum and have a steady reading around 15 in.

My EGR kept getting clogged with carbon and found that it was acting a bit weak. (It was alot easier to move the pintle in the old one vs. the new one.) I replaced the EGR. Ran great for a day then the symptoms returned. It also clogged the new EGR a week later so I ended up putting a screened gasket in.

I ran some sea-foam through it and it exaggerated the problem which went back to how it was on the next tank of gas.

No engine light. No oil loss, No coolant loss.

I know that light throttle misses are fuel related and heavy throttle misses are ignition but heck, the distributor shaft wobbled a bit and the coil is origional so i put in a new coil, distributor, wires, plugs. I did get a hotter spark but still a miss. Played with the timing a bit, still runs best at 0 degrees.

Here's where the uncertanty comes in with the surging. In the middle of the winter something in the transmission froze up. Reverse was like neutral. Ran some sea-foam through it, it unstuck reverse, changed the fluid and found it to be a little bit burnt. Also a random hard shift went away. Now this is the first automatic i have ever owned however, I keep thinking torque converter, although I have never experienced a bad one. I'm basing this on somewhat burnt fluid (mind you it may have been factory fluid. who knows) and surging.

The bottom line is I'm not quite sure if I am dealing with two seperate issues or just one. Something is telling me that a cylinder(s) is running lean, because my spark plugs have minimal deposits if any on them. I guess i just wanted to see if I was missing anything before i put in a new IAC valve, nut kit, and spider assembly.

-Thanks,
Mark

"I checked the vacuum and have a steady reading around 15 in."

Vacuum is low, which can indicate late ignition timing, should be up around 18" to 20". Did you disconnect the EST wire when checking/adjusting timing?

A fuel pressure and leakdown test will tell if the nut kit is ok, don't replace it "just because". Key on, engine off, fuel pump running: fuel pressure must be 55psi to 61psi and must remain above 50psi for 3 to 5 minutes after the fuel pump shuts off.

I know you said no check engine lights, but you don't mention codes. Did you check for any?

I had somewhat of the same problem with my 2000 Jimmy. You are describing almost exactly what mine did. I described mine as more of a sputtering problem.

Anyway, My problem turned out to be a bad distributor cap and Rotor. It took months before mine threw its first code which was P0300 Random Misfire. If you haven't replaced your cap and rotor for awhile or if you replaced it recently with anything but an AC Delco products, this could be your culprit.

I was thinking after your reply that when I checked the vacuum it was normal. I checked it again and it was at about 18 in. Sorry for the mis-information.

Anyways the EST wire was disconnected when timing was set but that thought did jar something loose in my head. The idle when the EST disconnected would be erratic unless the engine was fully warmed up. As in I would have to drive it 20+ miles then immediatly check the timing for it not to idle erratically. The timing would stay correct throughout the rpm rises/drops suggesting to me that the erratic idle isn't ignition related.

I did the leakdown test today. It stayed above 50psi well past 5 minutes.

Something else I found though. My IAC valve does have some side play on the pintle. I'm not sure if this would be a real problem though as the pintle would be guided into the cone hole on the manifold anyways. Unless it never quite seats. Any thoughts on this?

-Thanks
Mark

Unless the IAC is leaking vacuum where it bolts to the throttle body, I wouldn't worry about it. What is the fuel pressure with the key on engine off, pump running? Is the fuel filter restricted?

Thanks for the ideas Rick.

I did replace the cap and rotor when I replaced the distributor. In doing that I really didn't see any improvement at the time so I feel I can rule out an ignition cause. I do have a healthy blue spark and the distributor is advancing as it should.

I did check for soft codes in the PCM and had none. The thing with the '93 is that the OBD-I only reports about 20 different codes. (I think there's more possible codes in the ABS controller. Ha) It also seems that you have to basically unplug something and drive for a week before the computer even realizes a part is missing. But enough complaining for now.

-Thanks
Mark

Fuel Filter was changed in the fall. Less than probably 6k on it. Fuel Pressure was 55 with the motor off.

I may pick up an O-Ring for the IAC just in case tommorow but it still seemed alright.

I did replace the cap and rotor when I replaced the distributor. In doing that I really didn't see any improvement at the time so I feel I can rule out an ignition cause. I do have a healthy blue spark and the distributor is advancing as it should.

Mark, did you replace the old distributor with a new AC-Delco Distributor?

Seriously, I am not one of those Purists who tell you to buy OEM stuff just because it is OEM stuff.

I replaced the entire distributor on my 2001 S10 4WD with an AIRTEX aftermarket Distributor and 6 months later the cap shorted out internally, throwing a whole bunch of codes and disabled the truck.

In addition to that, here is what happened to my 2000 Jimmy SLE, See this thread:

http://www.automotiveforums.com/vbulletin/showthread.php?t=975186

PLEASE, PLEASE, PLEASE, DO NOT THINK JUST BECAUSE IT IS NEW THAT IT IS GOOD. IN BOTH CASES A NEW AC-DELCO CAP SOLVED MY PROBLEMS, and in both cases the caps were approximately 6 months "NEW" when they took a dump. Please read my thread and just see if this sounds familiar.

Yeah Rick, I went down that road trying to fix things in my ford the crappy aftermarket electronics wern't worth it. I went through three cam sensors in that thing, 2 AC controllers ($400 a pop) and I was broke and bought a rebuilt alternator that kind-of works. (It slowly drops to 12V as the engine hits about 6 Grand and the voltage randomly drops from 14V to 12V and is faily annoying at night.) Anyways I ended up getting a new one at the dealer cause Advance didn't offer anything but a rebuilt one. So the ignition is quality. I would assume the cap is too as it came from the dealer.

I live out in the sticks. It's a pain to get parts but the dealer is nearby.

-Mark

Alright Old Master, getting closer. Figures that the misfire turned out to be a faulty 99 cent O-ring on the IAC. Thank you very much.

I am still having a surging problem though although is actually is greatly reduced. Can you get a surge from a bad IAC? I ran it in neutral today and the rps would wander with a feather light touch of the gas. Without changing pedal pressure eventually it would jump to 2000 rpm and stay there steady. The motor just seemed to have a hard time running at a consistant speed below 2000.
A cold or warm engine seemed to have no effect on it.

-Thanks again
Mark

A vacuum leak can cause a surging idle, so can low low fuel pressure. A fuel leak inside the plenum can also cause surging, but it's likely to set a rich mixture DTC also. Post #2 gives you fuel pressure and leak down specs, might want to check it.