Transmission or fuel problems??

CLTX11 · February 9, 2007

I cannot tell which one it is so hopefully some of you can steer me in the right direction. First of all I have 98 Z71 auto.

When I am driving at a constant rate of speed the RPM's seem to jump up and the truck feels like it is loosing power. When I am doing 65 or more when the RPMs are at about 1900 or higher I do not notice the problem.

I have tried replacing the fuel pump and also a tune up (rotor, cap, wires, plugs, PCV valve) and nothing helped.

Reason I am leaning toward maybe a fuel problem, I added some seafoam and it seemed to get worse. After I refilled with fuel it is not doing as bad as when seafoam was added.

The transmission doesnt seem to be shifting funny when taking off. But it does seem when it shift into over-drive around 40 MPH it goes really low as in feels like it bogging down. Keep in mind the problem is ocnsistant from low speeds up til about 65 or so.

What would bad injectors make the truck do? O2 sensor kicks on the engine light correct?

ChevyTech007 · February 10, 2007

Could be the TCC. When it is acting up, slightly push your brake pedal to disengage the tcc. If it stops you know it is the trans. A very small amount of water in the trans will cause this. Usually comes in through the dipstick tube.

CLTX11 · February 10, 2007

Could be the TCC. When it is acting up, slightly push your brake pedal to disengage the tcc. If it stops you know it is the trans. A very small amount of water in the trans will cause this. Usually comes in through the dipstick tube.

Thanks I will give that a shot. I am guessing it is the trans, this morning I started it up and put it in gear, it is hesitating to kick in. It is revving up but going slow. It will not kick in gear hard but the RPM's will slightly go low and speed will pick up. After that initial take off, all the other take off's are normal. Once the truck is warm and I turn it off and restart it and put it in gear, it will not hesitate, only when its cold.

Also, will I need to go ahead and get the transmission rebuilt to solve the problem?

95Sierra2500 · February 11, 2007

A few things to look at:

check your fuel pressure (both key on and running). If key on is not 65+psi, and running is not 55+psi, your fuel pump or pressure regulator could need replacing.

consider having your injector system cleaned by a dealership using their Top Engine Clean procedure. This can free up your sticking poppet nozzles that can cause misfires.

Check your cap/rotor/wires, these caps are notorious for having burn-through issues.

Catalytic converter may be failing/plugging up, but the problem could go away after warmup.

This is a problem I chased for 6 months in my old '99 GMC Jimmy. The injector cleaning helped, I did have a bad fuel pressure regulator, and the cap did have a burn-through, twice. Ultimately the problem turned out to be the catalytic converter plugging and causing excess heat in the engine bay. Replaced it and the problem went away.

I would recommend having a shop take it for a drive and read the history, I'm betting you have a bunch of misfires stored in the computer. I doubt it's a transmission problem.

CLTX11 · February 11, 2007

A few things to look at:
check your fuel pressure (both key on and running). If key on is not 65+psi, and running is not 55+psi, your fuel pump or pressure regulator could need replacing.

consider having your injector system cleaned by a dealership using their Top Engine Clean procedure. This can free up your sticking poppet nozzles that can cause misfires.

Check your cap/rotor/wires, these caps are notorious for having burn-through issues.

Catalytic converter may be failing/plugging up, but the problem could go away after warmup.

This is a problem I chased for 6 months in my old '99 GMC Jimmy. The injector cleaning helped, I did have a bad fuel pressure regulator, and the cap did have a burn-through, twice. Ultimately the problem turned out to be the catalytic converter plugging and causing excess heat in the engine bay. Replaced it and the problem went away.

I would recommend having a shop take it for a drive and read the history, I'm betting you have a bunch of misfires stored in the computer. I doubt it's a transmission problem.

Thanks a lot, that is a little better hearing I guess than having a transmission problem. I will try these out and see if it helps.

KB06YUKONXL · February 18, 2007

Change your fuel filter as well. It may be clogged.

CLTX11 · February 19, 2007

I changed my fuel filter not to long ago, it didnt seem to help but I might throw another on there since it takes all but 3 minutes.

I think I narrowed it down hopefully to either the fuel pump or fuel pressure regulator. Is there any way to tell which it is before replacing it?

If it were a bad regulator would it take a little more to start the engine and if it were the pump would it feel as if the engine is bogging down?

Also hooking it up to a fuel pressure tester, would it read the fluctuations from the pump or from the regulator as well? All in all what would be my best bet?

I wanted to try and just replace the problem instead of a hit or miss if possible.

CLTX11 · February 20, 2007

Another thing to note, its idleing at about 6600 RPM's but its seems to be really rough not quite to the dying point but it'll make my heart skip a beat sitting at a red light.

CLTX11 · February 20, 2007

I cant edit my post via phone so i wanted to correct myself. Its idleing at about 550.

CLTX11 · March 1, 2007

Well I finally took it in. They cannot seem to find a problem, everything is coming back ok. He said when he drove it he cannot feel anything wrong.

He said he would go ahead and clean out the throttle body for me and just charge me for the diagnostics. I am trying to find a way up there now to show him.