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frdgra
New User
Aug 8, 2011, 4:48 PM
Post #1 of 2
(1805 views)
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99 yukon missfire
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i have a 99 gmc yukon 5.7L R engine, with a missfire on cyl 5 only at idle. it goes away at 1200-1500 rpm.
compression is about 150 psi, i have spark to the plug, (checked it with a spark checker).
i replaced plugs about a month ago, but i switched #5 and #1 plugs just to rule that out.
i replaced the spider injector assembly,(that wasn't cheap).
manifold vacuum is 19 inches at idle and i checked for leaks but can't find any.
i did a top engine clean on it tonight and that didn't make any difference.
truck has 178000 miles. any more ideas would be great.
i thought maybe a weak valve spring but i don't have any engine noise or any popping through the
throttle body
thank you
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Sidom
Veteran
/ Moderator
Aug 8, 2011, 8:20 PM
Post #2 of 2
(1776 views)
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Re: 99 yukon missfire
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Well sounds like you've already done some work to it....
Some times it can be hard to tell the difference between a misfire caused by a vacuum leak or mechanical problem....
If you have the means to safely run some propane into the intake system this would help narrow the search. If propane smooths it out & it runs good, then you have a vacuum leak that needs to be traced down.....
If not try to run the compression test while it's hot & acting up. Also a running compression test might help. If you're 150 on all cyls, you are going to want to see 75 lbs on the bad cyl & a good one for a baseline. If you have a valve sticking it should show up with intermittent lower comp, 10 psi or more.. (This is where a pressure transducer would be worth it's weight in gold,,,,hmmmm actually they are pretty close to being worth their weight in gold....lol)
Just for the heck of it, if you have the old cap, put it back on & swap the wire as well......I would thing a crossfire issue would go away at higher rpms but you never know....Those caps are a bad design & I've had bad ones right out of the box. I pretty much will only use A/C parts on those any more.....
(This post was edited by Sidom on Aug 8, 2011, 8:22 PM)
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