95' Tahoe TBI 350 trouble

americanmuscleman · February 6, 2003

Hi guys, my truck is running ruff, and I thought I might ask you guys if you have any idea whats going on.

My problem originally started when I would start my truck on a cold mornings. It would (and still does) idle abnormally rough until it runs for 20 seconds or so.

My truck seems to missfire at anything more than about 50% load above 2000rpm. It idles and runs great under 2000rpm. This leads me to think its got to be ignition related, or my fuel pump is wearing out. My fuel filter only has about 10k on it. I have 20k miles on the set of plugs in my truck, and new wires with only about 5k on them. My 02 sensor only has about 20k on it. When I rev the engine with the air cleaner assembly off, the injector spray pattern looks prefect, but this is also just revving it, not putting it under load.

I'm not certain what the problem is, but before I just start replacing parts to find out what it is, I thought I would post and see if any of you have had similar problems before. Thanks!

By the way, it isn't throwing an SES light.

February 7, 2003

I had a similar problem with mine a while back. It turned out to be a corroded distributor cap. Swapped the cap and rotor, no more trouble. Of course I also changed plugs and pcv valve at the same time. Check your timing, too.

americanmuscleman · February 7, 2003

Thanks for the ideas Friz. I'm going to put a new PCV valve in tomarrow and see if that helps. I'll probably replace the distributor cap after that, and then check the fuel pump and injectors.

Thanks for the response!

americanmuscleman · February 8, 2003

Well, I changed my PCV valve, cap and rotor, and I've still got the problem. I'm going to change the plugs next.

February 8, 2003

Did you check the timing? If it's too far advanced, you'll get detonation and sputtering/bogging down.

americanmuscleman · February 8, 2003

Friz, no I didn't check the timing. How would the timing just suddenly become advanced or retarded though? For the last 20k miles its been spot-on. I thought to change the timing you'd either have to pull the distributor and turn it, or change the orientation of the timing chain on the cam and crank sprockets. If there is a way to adjust the timing any other way, I'm not aware of it. Can you just manually advance or retard the timing by turning the distributor? I didn't think you could do that on these motors.

Thanks for your help!

February 8, 2003

Yes, you can change it just like any of the old motors with distributors. The computer advances or retards it as needed, but the base timing has to be on the money.

There is a brown wire with a connector on the passenger side under the glove box. Disconnect it and put a timing light on #1 plug wire. The timing should be at "0". Mine is 3 degrees advanced and runs fine. Some of the guys here told me to try it and it seems to work okay.

americanmuscleman · February 8, 2003

Friz, I'll be sure to check that out. Thanks! I didn't realize these distributors were adjustable.

timk49 · February 8, 2003

That sounds like my old 95 Silverado when the catalytic converter failed. I hope that isnt your problem but those were my symptoms. A mechanic tried checking fuel pressure and electrical. He then installed a new O2 sensor. No good. Then he dropped the exhaust and the problem went away. I installed a cat back system to fix it.

Good luck.

americanmuscleman · February 8, 2003

timk, thanks for the idea. I actually installed a new Catco catalytic converter about 3 weeks ago. I had a rattle in my stock one, so I went ahead and replaced it. I didn't see any change in the way the engine ran. Thanks for the idea though!

It will probably be a week or so before I get a chance to check my timing. I'm out of town, and don't have a timing light with me. I have a pretty good tool box, so swapping the cap and rotor and PCV valve was a cinch, but I don't keep a timing light with me ;)

Thanks for your help guys, and I'll find out next weekend how the timing looks.

Steve

timk49 · February 9, 2003

When the Cat breaks up I believe the pieces can get caught up in the stock muffler blocking the exhaust flow. Did you change the muffler at the time you installed the new cat.

If I'm wrong on this someone let me know. I was told this by a mech that I trust and I have no other way of verifying it.

Does anybody know if there a way to accurately test your exhaust system to see if there is a blockage?

americanmuscleman · February 9, 2003

I don't think my muffler is blocked because I have a cutout before my muffler, and the truck runs exactly the same with it open and closed. My factory cat hadn't broken down either. The honeycomb mesh still looked good inside it, it just had a rattle. My muffler is a 40 series flowmaster which is basically a straight-through muffler with one "V" shaped piece of steel the exhaust moves around. I can tell you for sure my muffler isn't clogged. Thanks for the idea though!

Friz, whats the significance of dissconnecting the brown connecter under the glove box?

February 9, 2003

The brown wire is the computer hook up. It takes the comp advance out of the distributor. Basically, it's the same thing as disconnecting the vacuum advance on older stuff.

americanmuscleman · February 9, 2003

Thanks! Did you have to make a chalk mark on your damper for the timing light to read? Just glancing in there, it doesn't look like the slot on my damper is obvious enough for a timing light to read it. The problem is getting progressively worse, so I'm going to go see if I can get a timing light tonight to check it out.

Thanks a lot for your help!

Shaners · February 9, 2003

If you could somehow do a pressure teset on the fuel system, you would be able to tell if it was a fuel pump or fuel filter or perhaps a partially clogged fuel line.

It's also possible that a spark plug wire that you replaced is going bad.

Does your truck have a coil??? Maybe you should look at that too...