GM 3400 / 3100 3.1 3.4 Engine Misfire Cause & Causes Miss Missing P0303
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If your car is missing out, first try the following. Check for spark and check the spark plug, wire and coil. A test you can do if it randomly misses is in the dark, spray a mist of water around the plug wires. If you see blue arcing, you've found the problem. But be careful not to zap yourself, IT HURTS!
If the spark is good the listen at the injector. Is it firing and clicking? If not, you've found the problem.
If both those seem good, then check for compression. Do this with a compression tester available at any Autozone with the loan a tool program for @$40.
If all 3 of those check out then it's a possible vacuum leak. May be time to do an intake manifold gasket repair as they are known to go out. But, if you do have to do an intake manifold gasket fix, go ahead and replace the head gaskets too. You'll be glad you did. Not fun taking it all back apart again. Check out my youtube channel for these videos both on doing the lower intake manifold gasket and the head gasket.
Got questions about this engine? Leave a comment in the section below and I'll try to help!