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Gary, GMC Technician
Category: GM
Satisfied Customers: 17179
Experience:  15 years gm master tech/15 years ASE master tech
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GM: I have 1995 Yukon 4x4 with a 350 cui engine. Throttle

Customer Question

I have 1995 Yukon 4x4 with a 350 cui engine. Throttle body fuel supply. Had the engine & transmission rebuilt at 290 K miles (8 months ago). Then my normal mechanic who did the work got gravely ill. Over time i developed a miss that continued to progress. At idle I can unplug the plug wire on #3 cylinder and it has no impact on engine performance.
Have had it tuned up by a "professional" (new plugs wires and compression check was OK) same miss.
Took things into my own hands and checked compression (good). Then the settings of the valves on that cylinder. Intake valve setting was a bit loose and adjusted. Re checked compression (good). Rechecked timing which I put at 2-3 degrees Above TDC. Mechanic had it at 5. Runs better but still has a little miss. I can still unplug that #3 wire at idle and it does not affect engine. Your thoughts?
Submitted: 2 years ago.
Category: GM
Expert:  Gary replied 2 years ago.
all other 7 cylinders effect the idle when unplugged ?compression was good from you are saying is there visual spark from the # ***** plug wire to the plug ?
Expert:  Gary replied 2 years ago.
just to be sure this has throttle body injection with 2 big injectors on top of the throttle body ?
Customer: replied 2 years ago.

Correct. Very puzzling situation!

Expert:  Gary replied 2 years ago.
at this point with just # ***** cylinder doing this and the compression is good and there are NO vacuum leaks the issue has to be an internal issue inside the distributor firing # ***** cylinder at the wrong timepick up a reman distributor from Napa or Pep Boys and swap it outthere really is not much more it can be as if it was a fuel injector issue it would effect that whole bank and the compression is good
Customer: replied 2 years ago.

OK Thanks. Was hoping to avoid major work. The backup mechanic who tuned it up did not impress me a bit!!! He wanted to start taking the heads off of the motor to look around.

I'll look down that avenue and see what is required to make that swap.

Expert:  Gary replied 2 years ago.
since the compression is good i am assuming the valves are opening and closing the same distance visually as the others on that bank when cranking it overthe only outside weird thing to cause this would be a sticking valve guide sticking a # ***** valve or the cam worn but you would usually pick up the sticking guide during the compression testyou could do a running compression test with the tire core removed from the compression gauge hose and see with the engine running what it reads then try # ***** and see if it reads the sameif it does at idle you could connect a vacuum gauge to manifold vacuum and see at idle when its skipping if the vacuum gauge needle bounces at idle if it does then usually that a valve issuebut at this point its a valve train issue or distributor issue
Customer: replied 2 years ago.

I have been tied up for the last week but replaced the distributor as you suggested. I was able to easily set the timing but still have essentially the same issue with the #3 cylinder not seeming to be doing anything at low rpm's. the plugs were placed on the entire engine bu a mechanic and I replaced the one on #3 cylinder a while back. I'm going to swap plugs in #1 and 3 cylinders just to see if anything changes.

Expert:  Gary replied 2 years ago.
ok let me know