97 GMC Jimmy, Need help please!!!

[Seath15]

I have a 97 GMC Jimmy, 2 wheel drive, 4.3L Vortec.

The problem is very erratic, it doesnt do it every time the vehicle shifts, and sometimes i can drive a few days without it happening, while other times it seems to happen every take off for a week. This has been going on for about 4 months now.

The actual Problem: When the vehicle is about to make a shift into the next gear (especially under a slight load, like taking off on a hill etc), The engine misfires like crazy and the entire vehicle feels like its trying to stall out and then picking back up. The Tach in the dash jumps between 200-400 rpms either way of where it is before the miss. (IE: If the tach is reading 2000 rpms before the misfires start, it will jump anywhere between 1600-2000 rpms repeatedly).

Also, when driving on the interstate going 65mph+, the vehicle will miss like crazy the whole time im driving. If i let completely off the gas it stops, and if i get hard onto the gas (3000 rpm +), the miss goes away, its only when im coasting with my foot barely on the gas.

I am a transmission technician, and have a Solus Pro Scanner from Snap-on. I have put the scanner on the vehicle and brake-torqued it, you can actually see a random cylinder misfire on ALL cylinders with no apparent rhyme or reason.

Things i have tried so far:

3 different Distributor Caps and rotors
3 Sets of Plugs AND wires
Alternator (new, not reman)
Running 2 cans of sea-foam through fuel system to clean it out
Fuel Filter
Pulling plenum and cleaning spider assembly

Putting a can of Liquid glass into the coolant system in case of a cracked head gasket (to seal it) (keep in mind the vehicle has NO smoke of any kind whatsoever)

Tested the catalytic converter
Changed the Coil Pack
Replaced all the vacuum lines i could
Pulled and cleaned and replaced every ground on the entire truck i could find New Battery



As you can see ive done everything i can think of and still no fix.
Its wierd though, when i change the plugs/wires/distributor cap and rotor, it seems to go away for a few days, then it comes back.


The only code the scanner has ever been able to pull up is a P0300, Random cylinder misfire code. No specific cylinder and no codes in months.



The only other thing i can think of is to buy the whole fuel injector assembly from auto zone for 300 bucks, but id hate for that not to fix it! I have thought about it maybe being TPS or Mass air flow, but at this point i doubt it.


Any help or advice would be GREATLY appreciated. Thanks!

 

[Molson02536]

I have a 97 GMC Jimmy, 2 wheel drive, 4.3L Vortec.

The problem is very erratic, it doesnt do it every time the vehicle shifts, and sometimes i can drive a few days without it happening, while other times it seems to happen every take off for a week. This has been going on for about 4 months now.

The actual Problem: When the vehicle is about to make a shift into the next gear (especially under a slight load, like taking off on a hill etc), The engine misfires like crazy and the entire vehicle feels like its trying to stall out and then picking back up. The Tach in the dash jumps between 200-400 rpms either way of where it is before the miss. (IE: If the tach is reading 2000 rpms before the misfires start, it will jump anywhere between 1600-2000 rpms repeatedly).

Also, when driving on the interstate going 65mph+, the vehicle will miss like crazy the whole time im driving. If i let completely off the gas it stops, and if i get hard onto the gas (3000 rpm +), the miss goes away, its only when im coasting with my foot barely on the gas.

I am a transmission technician, and have a Solus Pro Scanner from Snap-on. I have put the scanner on the vehicle and brake-torqued it, you can actually see a random cylinder misfire on ALL cylinders with no apparent rhyme or reason.

Things i have tried so far:

3 different Distributor Caps and rotors
3 Sets of Plugs AND wires
Alternator (new, not reman)
Running 2 cans of sea-foam through fuel system to clean it out
Fuel Filter
Pulling plenum and cleaning spider assembly

Putting a can of Liquid glass into the coolant system in case of a cracked head gasket (to seal it) (keep in mind the vehicle has NO smoke of any kind whatsoever)

Tested the catalytic converter
Changed the Coil Pack
Replaced all the vacuum lines i could
Pulled and cleaned and replaced every ground on the entire truck i could find New Battery



As you can see ive done everything i can think of and still no fix.
Its wierd though, when i change the plugs/wires/distributor cap and rotor, it seems to go away for a few days, then it comes back.


The only code the scanner has ever been able to pull up is a P0300, Random cylinder misfire code. No specific cylinder and no codes in months.



The only other thing i can think of is to buy the whole fuel injector assembly from auto zone for 300 bucks, but id hate for that not to fix it! I have thought about it maybe being TPS or Mass air flow, but at this point i doubt it.


Any help or advice would be GREATLY appreciated. Thanks!

If you have no DTC codes of a rich or lean condition, then there is a good chance that your fuel injection system is good. The only other thing i can think of would be a timing issue from a worn distributor gear or stretched timing chain.

Did you put a distributor cap with brass contacts on?? It helps with misfires on the 4.3 Vortec. You may want to check for slop in the distributor since there is quite a bit of plastic and the bushings could be worn also.

The other thing also that comes to mind is there is a breather screen at the bottom of the distributor plate that get's dirty and plugged up causing poor ventilation. Condensation would occur to promote corrosion to your Cam sensor. Try blowing compress air to clean out the base of the distributor plate and post back to see if that helps. Your Cam sensor my just be dirty enough to cause your timing to be off when your in the engine's mid range.

The distibutor housing holds the cam sensor, The PCM calculates the timing by measuring the crank signal and the cam signal. The turning is set by the distibutor housing in correlation between the 2 sensors. Good luck and keep us posted, and if you do have a rich or lean condition. Then you may have a dirt MAF and mentioned by you or a bad fuel pressure regulator.

 

[Seath15]

Thanks

Thank you for your fast reply! I will check these things and report back asap. Im leaning more towards a distributor related problem now too, unless its the fuel pressure regulator causing the injectors to get too much or too little gas, but i doubt it....