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2004 gmc envoy_sle_xl
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Discussion Starter #1
Hey Everyone, I'm new here and have an issue I'd like to get some better insight on.

I have a 2004 Envoy, I6 with a continuing misfire P0300 issue. 150,000+ miles. Several weeks ago, the engine started to misfire (blinking check engine light, running terribly). I put in AC Delco plugs about 3,000 miles ago. I cleaned the throttle body as part of an unrelated issue about 1500 miles ago. Fuel filter has less than 5,000 miles on it. Less than 4,000 miles on oil change (oil level fine). Good Air filter.

Step 1: I disconnected the wire coming out near the fire wall, cleaned it, and reconnected. Problem fixed for about 2-3 days (75-100 miles).
Step 2: Cleaned Camshaft position sensor and actuator. No result.
Step 3: Diagnosed and isolated Coil 1, replaced. Problem fixed (very proud moment).Problem returned about a week later.
Step 4: Began isolating coils again and found that the problem resolved itself after disconnecting/reconnecting individual coil wires. This lasted about 5 days (125-150 miles)
Step 5: Just diagnosed and isolated coil 5 as the problem. I swapped coils 4 and 5, found the issue is the coil again, not the wires/plugs.

Note: Coil 4 doesn't have that tricky problem where water is getting into the coil. It's as dry as all the other coils.

I've been buying higher octane gas and buying from different, high traffic gas stations (don't think it's bad gas). I am wondering, is it best to go ahead and replace all five old coils, expecting they'll all go out around the same time?
Should I just replace number 5?
Could the intake manifold gasket be the culprit and it's killing the coils one by one? Based on my understanding, this is common, but I would expect the gasket at 1-2 to go out, isolating the problem to those two coils.
Should I try a vacuum pressure check?

I don't hear any knock, vacuum hoses appear to be in good shape, no cracks in the system I can see. Please let me know if there is anything else I should look into before junking my beloved envoy. :confused:

Thanks in advance!

2003 gmc envoy_slt
628 Posts
A P0300 is a random misfire, not isolated to one particular cylinder. With a random misfire you MAY also have one, or two cylinders are are often included with the P0300

If you have one bad coil, ONLY that cylinder is affected. You may in fact have a bad coil, but you also have other causes of a random misfire.

Unplug the MAF and drive it after clearing codes. You will get a code because the MAF is unplugged, but you should not get any misfire codes, IF the MAF is the cause. If the misfire codes are gone, the MAF is the cause of the misfire. A dirty MAF will show less air is going into the engine than actually is, so the PCM reduces the fuel accordingly, the mixture is now lean and a misfire is the result. Also possible to get lean codes with a bad MAF.

If the misfire codes do not return, then try cleaning the MAF with MAF cleaner, if that fails you will have to replace it.
Note that on start up with the MAF disconnected the PCM will be confused and the engine will likely stall and need a restart, running rough for a few seconds until it realizes that it cannot use the MAF data.

If disconnecting the MAF does not improve anything, check fuel trims to see if it is running lean. It can run lean enough to cause a misfire and yet not show a lean code.

Long Term fuel trims should be under 10 for sure. While driving at less than 3/4 throttle, short term fuel trims must be under 10.

At WOT (clear road ahead) the Oxygen sensors !, Banks 1&2 MUST be at .9 volts and stay up, or else you are not getting enough fuel into the engine.

This graph shows the oscillating voltage during easy driving, then, it jumps up when the throttle is wide open.
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