Question about 1998 GMC Jimmy

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1998 Envoy 4.3L Misfires / P0300 code only

The engine misfirines underload. It idles smooth. The odometer shows 159K miles.

Disconnected the MAP sensor while troublshooting. This resolved the misfiring and made the vehicle drivable. Obviously this is now masking the real problem.

When reving the engine in neutral gear, the misfiring occurs in the mid-range rpms. When this happens, an incomplete combustion smell eminates from the exhaust. (must not be lacking fuel- maybe ignition timing problem?)

Checked for vacuum leaks, none found. Measured vacuum at idle- 19".

While resolving a previous problem the following components have been replaced within the past 6 months: plugs, wires, distributor, rotor, fuel filter, fuel pump, fuel pressure regulator, new fuel injectors/spider, MAP sensor, EGR valve, new vacuum hoses, new ignition module, cleaned PCV valve.

OK, I'm stumped. Any further suggestions will be greatly appreciated!

Mocksville NC

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Well for the misfiring problem you should get a code do you get as the SES light must be ON? You have a timing problem for the backfire so has it got a new cap/rotor? Then cam sensor in distributor which effects that (starts OK then problems start) but get the code - also a crank position sensor tells the PCM where to fire. Then if the fuel pressure is bleeding down possible fuel regulator leak under the upper intake. A good GM garage can find these in an hour.

Posted on May 14, 2009


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