GMC Repair: Cylinder #1 Miss on a 97 Jimmy, coolant system, head gasket

Question
I went to start my truck one morning to warm up.  When I came back out I noticed the check engine light was flashing and the engine was running poorly.  On my way to work the problem seemed to clear up.  But I stiil took it to a shop to check the codes.. The computer was showing a o2 sensor fault, as well as a Cyl #1 Misfire.  The guy checked the entire electrical system and said it was fine.  He said the only thing he could think of was an intake mainifold gasket leak.  I had that gasket replaced a year ago.  So I took it to another guy and he said it was probably something wth the fuel system.  One thing i noticed is that when you first start the car it runs fine.  Its only after about a minute that the problem begins.  What do you think this might be?
Thanks in Advance

P.S the engine was rebuilt last april
Mike

Answer
What size engine and how many fuel injectors ?.



The FIRST thing I would do is to determine if it is a fuel or ignition related fault and then go from whichever it is.

If the fuel and ignition checked out OK, then i would run a compression test on the suspect cylinder BOTH when the engine is HOT and COLD and compare it to the other cylinders.

next would be a cylinder leakage test on the suspect cylinder when the engine is BOTH COLD and HOT. This would verify a possible head gasket fault.

Next would be a cooling system pressure test on the cooling system when the engine is COLD and HOT to see if there is a slight pressure drop in the coolant system.

You could swap injectors with another cylinder IF each cylinder has it's own injector to see if the miss moves to the other cylinder.

Another trick is to use a digital thermometer. Wheras you touch the probe to each of the cylinders right next to the injector and read the cylinder temperature and compare it to all of the other cylinder temperatures. IF the cylinder is running COLDER than the other cylinders, This would indicate an ignition problem at the spark plug or wire or ignition source. If the cylinder is running HOTTER than the other cylinders, This would indicate a probable fuel injector or fuel rail problem.

Make sure ALL the tests are within the manufacturers specifications and let me know what you find.



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