SOLVED!!! '96 Yukon Cylinder 5 Misfire

Hello, I have a 1996 GMC Yukon with an intermittent misfire. I was cruising down the highway at about 70mph when it started shaking, thought I had a blowout. Then MIL started flashing. Rough idle and loss of power with code p0300. I took it to the shop and they saw on their scanner that at idle Cylinders 3 and 5 were misfiring, at 2,100 RPM no misfires, and at 2,900 RPM Cylinders 5 and 6 were misfiring. They told me the most likely cause was injectors, so I replaced those and it didn't fix the problem. Now I have a p0305 code. I replaced cap, rotor, wires and plugs less than two years ago, but I replaced the plugs yesterday just to see if that was the cause. It was not. If anyone has any answer as to why this is happening please respond, I have already sunk close to $800 into this problem and I really don't want to have to spend much more.

Moved you here from General Discussion since this is an Engine & Drivetrain issue.

Replace the spark plugs and spark wires. Under $100 if you DIY, or buy a friend a 6pack of beer and a 6pack of Boylan's Ginger Ale.
Also, when you replaced the injectors, did you keep the old ones, or did you give them to the mech?

Strange that they went straight for the injectors (most expensive part) and not the plugs/wires which are the cause a good chunk of the time. Were the new distributor cap and plugs/wires you installed 2 years ago OEM? I would definitely start there since you've identified 2 cylinders and not a "random misfire".

tooleyondeck said:

Strange that they went straight for the injectors (most expensive part) and not the plugs/wires which are the cause a good chunk of the time. Were the new distributor cap and plugs/wires you installed 2 years ago OEM? I would definitely start there since you've identified 2 cylinders and not a "random misfire".

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They said based on what they were seeing on Identifix or Mitchell or whatever they use, that the most common fix was injectors. I replaced the injectors myself and opted for the MFI upgrade. I threw out the old injectors as some of the plastic tabs broke off when I removed them. The cap and rotor were both ACDelco, but I will replace those and see if that rectifies the issue.

Marky Dissod said:

Replace the spark plugs and spark wires. Under $100 if you DIY, or buy a friend a 6pack of beer and a 6pack of Boylan's Ginger Ale.
Also, when you replaced the injectors, did you keep the old ones, or did you give them to the mech?

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Already replaced spark plugs myself and those alone were $100 for all 8. I’ll get a fresh set of wires. I threw away the old injectors because some of the plastic hold tabs broke off upon removal, and I made sure none I’ve those went into the intake. Also upgraded to MFI.

You should look at your ignition coil as well.

XBT7 said:

Already replaced spark plugs myself and those alone were $100 for all 8. I’ll get a fresh set of wires. I threw away the old injectors because some of the plastic hold tabs broke off upon removal, and I made sure none I’ve those went into the intake. Also upgraded to MFI.

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You could swap the plug/wire from #5 to one that isn’t misfiring, see if the misfire follows. A spark tester would let you compare the actual spark - could just be a bad cap.

Distributor

Pull the distributor and look for excessive wear on the gear. The teeth can get worn similar to what is seen in the pic below. My '98 2500 I had, had this issue. It cost me a lot of dough and time before I finally figured it out.