98 pulling troubles

I am hoping someone can give me a new idea what my problem may be. On my 98, 5.7L I am having a bad vibration while the engine is pulling when it shifts out of OD and then back into it. It will happen when there is only resistence, such as pulling a trailer or a bed full of something. It is worst when going up hills. The transmission will do a normal shift from OD to do the pull. The engine will naturally jump in RPMs. When it shifts back into OD and the RPMs drop, the truck will begin to vibrate severly. I can back out of the gas for a minute and it will generally goes away. The SES light will come on and it will read either a cylinder 5 or cylinder 8 misfire and once read a mass air flow error. I have replace the plugs, wires, and cap. Had the injectors cleaned. Did not help. The transmission was already in bad shape, so I had it rebuilt. The problem improved a little but still occurs. The torque converter is new also. Otherwise the truck runs great and does not have any problems. The engine sounds good and smooth at idle and high RPM's. Does anyone have any other ideas??

Thanks in advance for any help!

I read a thread this winter with the same simptoms & it ended up being a bad torque converter.I know you have codes that say motor but maybe this could cause the codes???

You should keep it out of OD while pulling a trailer anyway, or at least shift into manual drive when getting close to a hill.

I know on my 93 Suburban, there is a sticker on the door that says "Do not tow with OD on". I don't know if 98 has the same but if you are towing in OD the transmission is probably getting very hot and you will most likely shorten its life.

Yea... I don't know of ANY vehicle that is is ok to tow with OD on. Maybe some of the tonner trucks, but even on those, I'd use D just to be safe.

NEVER TOW IN OVERDRIVE!!!!!!


That is the worst thing to do to a tranny!!!!! Even if it's just on flat ground, DO NOT put it in OD!

my truck does the same thing. I know the tranny needs to be rebuilt but I can't afford to have it done right now. The only advise I can offer you is whenever the vibration starts to occur (which is the tranny slipping) give the engine a little punch to make to tranny shift down or throw the torque convertor into high pressure. This isn't a fix its just the way I've dealt with the prob. until I can afford to fix it.

Wouldn't it be MUCH better to downshift manually?
Stabbing the throttle is just going to put a much greater amount of stress on the tranny.

I agree with you there. Downshifting manually would be better than punching it. I just give mine enough extra throttle to make it change gears. What im trying to stress is (if you must drive it) not to let the tranny continously grind down the road and destroy the torque converter further. Down shift manually or give it a little more gas. In my opinion I wouldn't even drive it in OD since it seems to slip in this gear the most (at least w/my truck). Hope this helps in the short run. In the long run (just like the others have said) you need to get your torque converter rebuilt/replaced.

The transmission was already in bad shape, so I had it rebuilt. The problem improved a little but still occurs. The torque converter is new also.

I'd get your truck back to the guy that rebuilt it asap.

I also own a 1998 Chev with a 5.7. Had a problem very similiar to what you are having. I could drive down the highway going any speed I wanted and no problems. When pulling a loaded trailer up a faily steep hill is when the problem started. Truck would downshift and every thing would be fine for a few seconds. Would slowly start to lose rpms and truck would vibrate. I could make it come out of this condition if I put tranny in neutral for a few seconds then put back into drive. If I let it go I would get codes for random misfires and air flow sensor. If I let it go long enough engine would actually die. What the problem actually was is carboned up exhaust valve stems. Under a load the exhaust valves would hang open due to the carbon. I cured this problem by running GM's upper engine cleaner thru the system. I used two cans. Ran first can thru and then let truck sit owvernight. Thfe next morning I ran the second can thru. Went out and retested by pulling loaded traler up steep hills, problem was gone. a few months prior to this problem I had replace intake gaskets due to anti-freeze leak. I replaced distributor back exactly as it came out. Come to find out I still 12 degrees off. I believe this caused poor combustion leading to carbon build up on exhaust valves. Hope this helps.

Working with a local mechanic, we have narrowed it down to two things. Either it is the valves sticking due to carbon build up like was mentioned in the previous post or the fuel injectors are going bad. We called a chevy dealership and they said this is a common problem on these trucks. They suggested trying to clean the carbon out of the engine first as this is the problem for most people. They recommended running automatic transmission fluid in the gas. The transmission fluid has alot of detergent in it. I have done this and the problem has improved. And I have ran some Chevron Techron fuel injector cleaner in it. I am going to run a couple more tanks thru and then see what happens. I am almost to my last resort of the costly replacement of the fuel injectors. I will keep you guys posted on the outcome and what finally fixes the problem. I appriciate all your suggestions!