SOLVED!!! '96 Yukon Cylinder 5 Misfire

[XBT7]

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.

 

[Fless]

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

 

[Marky Dissod]

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?

 

[tooleyondeck]

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".

 

[XBT7]

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".

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.

 

[XBT7]

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?

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.

 

[YukonGTmaster]

You should look at your ignition coil as well.

 

[east302]

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 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.

 

[strutaeng]

Distributor

 

[Hoesgottaeat2]

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.

 

[tooleyondeck]

I'm gonna beat Joe to it and say this qualifies as SAGE ADVICE lol

 

[Eman85]

Everyone wants to buy parts, I'm old school and cheap. Do some diagnosing first, can't believe a shop just says injectors going by common failure without proof. You had the plugs out, good time for a compression test. Before wire replacement unless I saw something obvious it costs nothing to do a poor man's scope test. A spray bottle of water, get high tech and put a little dish soap in so it clings. Have someone power brake the truck a little and spray a mist at each wire start at the plug. A good spark tester that is adjustable will test the coil to see just how good the spark is. If I read correctly it looks like all of the misfires are on the drivers side bank also, did they check fuel pressure and the regulator?

 

[exp500]

A cheap android scanner on live data will help a great deal. I agree- throwing parts at it is not the answer. Common failures in Ignition are in this relative order for me-
Distributor cap/rotor Delco/Denso/United DCR820x only. Plug wires. Spark plugs(NGK very good and inexpensive). Coil. Ignition moduleDelco/Delphi. Distributor(united a good choice).
Watch fuel trims and misfires on live data while Driving steady state at various rpms. A fuel pressure gage with long hose under wiper blade and a spark tester at coil, coil wire, and individual terminals on cap rounds out most testing at under$100.
Good luck.

 

[XBT7]

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.

View attachment 434193

Got it. I will check that tomorrow after work and will report back here. Hopefully once I'm done this won't be the 350 of Theseus.

 

[XBT7]

Everyone wants to buy parts, I'm old school and cheap. Do some diagnosing first, can't believe a shop just says injectors going by common failure without proof. You had the plugs out, good time for a compression test. Before wire replacement unless I saw something obvious it costs nothing to do a poor man's scope test. A spray bottle of water, get high tech and put a little dish soap in so it clings. Have someone power brake the truck a little and spray a mist at each wire start at the plug. A good spark tester that is adjustable will test the coil to see just how good the spark is. If I read correctly it looks like all of the misfires are on the drivers side bank also, did they check fuel pressure and the regulator?

When we had the plugs out at the shop we did a compression test and it was great between cylinder 1, 3, and 5. I bought a spark tester and tested the plugs on cylinders 3 and 5 and both were flashing erratically. This weekend I will borrow a timing gun from my uncle to see if there is an issue with timing.

 

[exp500]

When we had the plugs out at the shop we did a compression test and it was great between cylinder 1, 3, and 5. I bought a spark tester and tested the plugs on cylinders 3 and 5 and both were flashing erratically. This weekend I will borrow a timing gun from my uncle to see if there is an issue with timing.

These engines REQUIRE a scanner to set timing. Spec is 0deg +-2 at !000-1200rpm. Name of timing something like cam/crank correlation depending on which scanner. If you loosen distributor you must reset timing or code sets.
This might help.- https://www.gmt400.com/threads/88-95-service-manuals.43575/

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[Eman85]

Timing "gun" or timing light isn't used on a Vortec engine, need a scan tool that can tell you the readding. A flashing spark tester isn't testing the power of the coil, just that an electrical impulse occurs. What compression numbers are "great"?, how does great compare to the other 5 cylinders?

 

[XBT7]

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.

SOLVED!!! Thank you to user Hoesgottaeat2 for providing me with the exact information I needed to fix my problem. Thank you to everyone else who helped me with this problem, you are very much appreciated! It was in fact my distributor gear that had worn down and caused my timing/rough idle issue. It just so happened to be cheaper and faster to replace the whole distributor than it was to just buy the gear haha.

 

[Hoesgottaeat2]

SOLVED!!! Thank you to user Hoesgottaeat2 for providing me with the exact information I needed to fix my problem. Thank you to everyone else who helped me with this problem, you are very much appreciated! It was in fact my distributor gear that had worn down and caused my timing/rough idle issue. It just so happened to be cheaper and faster to replace the whole distributor than it was to just buy the gear haha.

View attachment 435068View attachment 435069

Thanks for circling back and letting us know.
Happy to hear that was the fix!