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Good compression low(ish) vacuum?


cp2295

cp2295

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My compression is between 130 and 140 in all cylinders, so that's good. However the vacuum is at 15-16" hg at idle.. I know it should be between 18-22 (unless these engines have low vacuum?).. The needle would fluctuate as the idle did. My truck idles kinda bad.. Just bounces around about 50 rpms is all though, so not that bad.. Can anyone tell me why it's got low vacuum?
 


cp2295

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Is the timing retarded? I hooked a timing light to it a while back and the #1 cylinder was firing at 10 degrees btdc when I revved the engine. However at idle it was all over the place. (From what I remember, I'll double check tomorrow)
 
adsm08

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Exhaust restrictions can cause low and erratic vacuum. Loosed up the muffler and retest, if no change loosen the y-pipe and try again.
 

Earl43P

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That 3.0 has a 90 degree elbow at the rear of the head that is prone to cracking. It is for the PCV and will cause quite a vacuum leak. I'd suggest you look for that.
 
cp2295

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adsm08 said:
Exhaust restrictions can cause low and erratic vacuum. Loosed up the muffler and retest, if no change loosen the y-pipe and try again.
Click to expand...
Just loosen them don't take the muffler off or anything? It seems like if my exhaust was blocked id have trouble getting up to speed, I know with a clogged cat you can't go past like 30, and i have no trouble bringing the rpms up..
 
cp2295

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Earl43P said:
That 3.0 has a 90 degree elbow at the rear of the head that is prone to cracking. It is for the PCV and will cause quite a vacuum leak. I'd suggest you look for that.
Click to expand...
Oh forgot to mention I also got a leak test and no smoke came from anywhere except Iac valve.. He said it was normal, but I replaced it anyways.
 
cp2295

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Really needing some help I'm stumped.. I took out one of the 02 sensors and the vacuum looked about the same.. Hovering around 15-16 and the needle bouncing.. I have no idea what it could be! Should I undo the other sensor or would that be pointless? I'm also max 300 ft above sea level so that shouldn't have an effect
 
Last edited:
cp2295

cp2295

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My credo
If you ain't first you're last
i posted a video on youtube showing my vacuum at idle, and around 2000 rpms, and when snapping the throttle. hopefully this is a little more helpful than my explanations.

http://www.youtube.com/watch?v=OETcowMUD6E
 
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