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1994 GMC Sierra 5.7L pings,bogs down, no power, backfire through TB only sometimes

04-15-2011, 10:51 AM
Newcomer here, and I have a question that is really bothering me..its lengthy sorry.... I have a 1994 GMC Sierra that I purchased... "well" used. Anyway, it has the issue of pinging and knocking (sounds like little gremlins in the engine with ball peen hammers) when accelerating or mainting a speed above 50mph. When it does this there is practically no power. It more or less has to be floored to maintain 60mph. If I quick slam the pedal down to the floor, release, and slam it down again it will backfire through the throttle body, recover and then run fine for about a mile or so. Am I correct that is because its running lean?? Then it will feel like it loads up, and the whole process starts again. It doesn't do it everyday, or as bad everytime. It seems to matter the most what the weather is, or temp outside. When cold and a strong headwind, it is horrible. The exhaust system is all new (cat included) and a new O2 sensor was just installed. The plugs were replaced last year, and the wires are newer. I did have an engine code a while ago and it was the EGR valve. The local shop told me to remove it, install a plate over the port, and plug the vacuum lines to see if it changes at all....just for eliminating one possibility. It is still the same. I was wondering if it would be the MAP sensor since air temp/weather has an impact on how severe it is??

I used a multimeter and the MAP sensor is getting 4.7V with engine not running and key on. When running, and I disconnect the plug nothing really happens. If I disconnect the vacuum line, the engine almost snubs out, them recovers most time...has died a couple of times.

I also read that it may be the coolant temp sensor? When running, I pull that plug same thing...engine almost snubs out then recovers. Plug it back in no change.

Both injectors are pumping fine it seems and even from what I can tell.

So am I correct in thinking its the MAP?? They are only $50...but money is tight right now and don't want to spend it on something that has nothing to do with my symptoms. Thank you in advance for any help you can offer!!

04-21-2011, 04:43 PM
Check your distributor. I'd be willing to bet that the gear on the shaft is pretty badly worn. I had this exact same thing on my 98. Different fuel systems, but same problem.

04-21-2011, 06:22 PM
I actually purchased the MAP sensor before your response...and WOW what a difference. It almost feels like I have a new engine. No more bogging down, stuttering, and I have to try HARD to get it to backfire at all. Gas milelage has improved probably about a 3rd. How can a $50 piece of plastic affect that much???

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