Engine miss fire problem above 3500 RPM

[gary350]

My 1999 tahoe runs great. If I am on the highway I always set cruise control. I cruise along at 70 mph RPMs are about 1800. Everthing if fine, gas mileage is about 18 mpg. When I come to a steep hill like a mountain cruise control down shifts to a lower gear and RPMs go up to about 3500 it only takes 1 second for check engine light to come on and the vehicle starts miss firing so bad it will not pull itself up the hill. Quick I tap the brake to shot off cruise control, it shifts to another gear then engine starts running perfect at a lower RPM, engine goes up the hill like nothing is wrong. I continue to get 18 MPG on flat roads and 150 miles later check engine light goes off. Any time the engine RPMs goes up to 3500 check engine light comes on and engine miss fires so bad it will hardly run. 150 miles later check engine light goes off. I installed new spark plugs, new distributor cap, new rotor, new wires. I still have the same problem. Engine runs perfect on flat road at low RPM but at 3500 RPM it miss fires so bad it acts like it is running on 2 cylinders not enough power to pull itself. ???????????????????? It has been doing this for over a year and I can not figure out what the problem is. Dealer worked on it 2 times cost me over $900 total it still has the same problem. The engine has a new fuel injector spider old one leaked on 2 cylinders. Engine only has 131,000. miles. When cruise control makes gas peddle go to the floor it miss fires at 350 RPM but with no cruise control if I make sure gas peddle is not pushed hard enough to reach 3500 RPMs I have no trouble driving up the mountain and engine runs perfect up and over the top, no problem below 3500 RPM. If I am driving on flat road 70 MPH engine runs perfect but if I punch gas peddle to the floor and transmission shifts and engine does 3500 it instantly miss fires so bad it loose speed and has no power until I let off the gas pedal and RPM drops below 3500. CODE SAYS, Engine miss fire on multiple cylinders.

 

[IROCDave]

I don't know what caused the issue you described but I saw it once with one of my old trucks, a 98 GMC heavy 1/2 ton.

I bought it new in 99 and daily drove it for 5 years without issue. The first time the CEL came on was the first time I towed our new travel trailer up a steep grade, I had to cane that poor truck to maintain the speed limit. That was the first time I had put a heavy load on the truck and matted the throttle for any duration of time. It freaked me out and I stopped at an auto parts store on the other side of the hill and picked up a scanner. The code said a miss fire happened. I reset the CEL and that never happened again. The truck had about 50K on it at the time. I'm assuming it was just carbon build up that got knocked loose and tripped the code.

Might try some GM top end cleaner (be careful and read the instructions!!). That stuff is amazing, when you done just look at the pile of crap on the ground under your exhaust tip. Just don't breath it.

Just my .02

 

[branduhlyn]

Thisss!

This is happening to my 99 Tahoe as well. I also replaced the fuel injector, fuel pump & filter as well as MSD wires & spark plugs. While each replacement definitely made leaps & bounds in the right direction, unfortunately the misfiring issue persists.