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Runs rough,multi misfiring, 98 gmc 5.7


MichaelEd72

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98 gmc 1500Z71 5.7 vortex

Started out with misfire #7 when heavy on the throttle. Did a time up plugs (ac delco)and wires. Still when heavy on the throttle usually at 80+mph check engine light comes on motor runs like it's misfiring. Code now has become multi misfire.

 

So now I do cap, rotor,wires,change to platinum plugs gap .06, fuel pump,filter, battery, coil ignition mod. Truck low end performance improves, but still runs bad( like its misfiring) at around 80 and requires all of acceleration to keep it there.

 

After frustration over a couple of months I ran it very hard as it started acting up and now runs so bad it can barley idle.

 

Codes now are multi misfire, MAF,MAP sensors, throttle position sensor.!!!

 

Those have been check and replaced and also spider injector replaced, with still motor running like its on half the cylinders!!!

 

Any ideas?? 207,000 miles

Taped out mentally and financially..

 

 

Sent from my rotary phone.

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if exhaust is restrictive it will show up in any gear at any speed under hard acceleration(plugged fuel filter behaves the same way as well). From a dead stop if you floor it, it will hardly pull away. Rarely feels like rough running, it acts more like traction control kicking in. You can almost always get it to show up in park or neutral as well by flooring the gas pedal and watching how long it takes to clear 4k rpm. And having someone check the volume of exhaust will also show very low flow under WOT in park or neutral.

 

Don't see any results from a compression test, you have checked compression haven't you? Should be near the front of the line when diagnosing an old engine.

 

With the truck being as old as it is, how confident are you on it being the original engine?

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Makes one hell of an intake honk with a carburetor too ... :lol: It will be louder than the exhaust.

 

A simple vacuum test will let you know too. If it drops at high RPM, light throttle ... bingo.

 

Depending on the laws in MS, take the cats to the scrap yard and cash them in. :cheers:

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Thanks for your suggestions. Issue has been found(no compression #7&5 cylinder.

Possible crake in head. Just slapped in a new crate motor from GM, and it's running like a BoSS!!

 

 

Sent from my rotary phone.

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