'99 Yukon 95k miles, 5.7, 1500 4x4
Getting P0300 & P0307 codes. Reading it with Car Code via laptop. Misfires
showing up in cylinders 7 and 2 mostly.
New NAPA premium cap, rotor, wires and AC plugs installed, no change from
OEM old stuff.
Wires old & new ohm'ed out well in spec. .
Misfire only happens when revs are up, are erratic and not steady like a
With open old school spark tester you can see spark drop out and come back
erratically when revved up. (only checked #7 that way)
I'm thinking coil is breaking down at higher revs. Idle is dead smooth, no
misfire until it's over 1k rpm.
Looking for opinions as I don't want to swap out expensive electrics that I
I would hang a scope off the primary of the coil or the Crankcase Postion Sensor.
That will at least tell you if its a coil or the sensor.....
I never had a coil go bad except for a autometer coil, which they replaced for
Wish I had a scope, or access to one.
I had a coil go bad in my old '89 S Blazer. It would miss fire under load,
idled and light throttle it ran fine. That's what's got me going coil first.
I guess I'm gonna have to roll the dice. Don't have a stocked parts dept.
steps away to play swap'em.....
Have you checked the compression on all cylinders?
Are they within 10 to 20% from the highest, to the lowest?
Even if the compression is good, there is a well known problem of the valves
sticking in the guides a bit, or of weak valve springs.
For that, you would have to do a running compression test. If you don't know
what that is, I recommend you take it to a reputable technician.
I hope this helps.
Didn't do the compression test as I could see the spark drop in and out on
the old school open spark tester.
Definitely is a spark issue.
Just trying to see what the common failure is, coil, crank sensor, module,
etc. . Then again I usually get tested with the odd ball problem but hey I'm
a smarter person in the end.
I've had the misfire code on our '99 Suburban for years. I've been able to
isolate it to the following. It only misfires under heavy load while going
up hills at highway speeds. It is always cylinder 4 that misfires. When it
is misfiring, ease off the gas for a few seconds, let it settle and she'll
continue on without the obvious hesitation. As long as I get on it a few
times a month, it never has a problem. Let it be babied for a while and you
can bet on it happening on the next highway hill climb and mainly if you
leave it cruise along in OD.
Occasionally, when it gets pushed, some black smoke will belch forth from
the tailpipe. Best I can figure is either a weak cylinder or the valves on
that cylinder get crudded up and cause a misfire. It has been completely
tuned up and I even replaced the injector on that cylinder for grins when
the intake gaskets were redone. It's acted this way for years and is up over
105K now so I've quit worrying about it, get on 'er a bit and drive
Well the new coil stopped the misfire at a 2k -3k revved up stopped. It had
signs of arcing out of the side of it.
Still have a light throttle, cruise random misfire , mostly in #7 & #2 so in
went a crank sensor.
No change, road testing on a 1/2 mile up hill at 40-50 mph zero miss, hit
the top and light throttle cruise misfire comes right in. Stomp it, pulls to
shift points without a miss.
No codes coming up other than P0300 related ones. Any other suggestions out
there before I test the local dealer techs? Wish I had a Tech 2......
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