1997 GMC misfire issues at 70+mph or uphill?

So I ve been asking questions about this damn GMC. its a 5.7l 350ci vortec motor.

I m having this persistent misfire issue. Up around 70+mph on the highway, it starts to shudder like hell. If I keep laying on the throttle without making it downshift it gets worse and just throws a P0300 random misfire code. I ve covered all the bases. Pump, checked injectors and regulator and they all function to spec, filter, no vacuum leaks. Intake gaskets are all in awesome condition. I m left to think its a spark issue. I ve changed cap and rotor a few thousand miles back, but before this the truck ran 21k+ without needing new cap and rotor, only new plugs and it never acted like this. Could it need it again?

I ve been told either distributor itself, cap and rotor, hall effect sendor or distributor gear. Sorry for the description but the more accurate the description, hopefully more accurate the answer...or so I hope. I m keeping this truck as a memorial to somebody dear to me who passed but I m tired of throwing money at it and would like decent advice on what direction to go next. Thank you in advance!

4 Answers

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  • 5 years ago
    Favorite Answer

    With no other codes, I suspect the EGR valve is opening too much. Most engines do not have excessive flow codes (because too much EGR is not an emissions issue) so excessive EGR is a silent nuisance. Disabling the EGR for test will confirm or rule it out.

    Otherwise, throttle sensitive misfire is almost always ignition weakness. Separating ignition weakness from insufficient fuel delivery is not hard - if the misfire occurs under high throttle coming off the line also it is ignition, if it only happens at high speed/high throttle where fuel demand is high it is fuel starvation. If you expect to keep the truck long enough for it to need another tuneup, I would be inclined to buy the cap, rotor and wires and give them a try. You can save the parts that are not bad for next time.

    I've had a six cylinder with a broken distributor gear, and although it was worse at high throttle it didn't do anything right except start. Idle was lumpy, acceleration at any speed was ridiculous.

    It doesn't sound like the crankshaft Hall effect sender, which is usually night and day - runs fine or cuts out completely. I would not rule out cam sensor but those are hard to pin down except by replacement. I have had several cam sensor failures in my Ford work trucks and none of them threw a code. I know what you mean about getting real tired of throwing money at it.

    One more thought... have you checked the spark plug color after a bout of misfire? If it is running rich or lean during this you will see the coating of soot or the whiteness. I had a pre-OBD turbo Volvo with a bad connector to the engine temp sensor and it ran really rich under boost but I didn't know why it was sagging until I saw the plugs.

    Good luck - I fear you will need it.

  • CB
    Lv 7
    5 years ago

    Have you pulled the spark plugs (try do it immediately after making in shudder for along time- pull over let engine cool) if it is spark issue you should see evidence by the color of the spark plugs. Sooty black plug is the cylinder(s) you will want to concentrate on OR very light white plugs (too lean). Ohm the spark plug wires on the holes with the sooty black plugs, If no plugs running foul or all running fouled replace the coil (about $25) Replace plugs if due. Do a compression check on all cylinders. All the above is free or relatively cheap and will help you baseline the engine.

  • J J
    Lv 7
    5 years ago

    I doubt that it is spark related as you would have the problem at any speed. I do believe it is fuel related, the engine is starving for fuel. I would make or buy a fuel pressure gauge hook it up and run the gauge under the wiper blade and see if the pressure is dropping off at high RPM under load.

  • 5 years ago

    Fuel pump.

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