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post #1 of 7 Old 02-20-2013, 12:10 PM Thread Starter
88jeep2b
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low vacuum

i have a 88 jeep cherokee 4.0. it kinda bounces between 13 to 14" of vacuum on a warm motor. ive search high and low for a vacuum leak and cant seem to find any. change out the intake gasket with no change. blocked off the egr. the compression seems to be good, 140 for a low and 150 for a high. does not seem to have a blocked exhaust. what could be causing this ?

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post #2 of 7 Old 02-20-2013, 03:03 PM
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Did you use the vacuum gauge to check for restricted exhaust?

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post #3 of 7 Old 02-21-2013, 01:16 AM Thread Starter
88jeep2b
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yes . open throttle it goes to about 0, close t blade it jumps to about 20 " then it jumps back to about 17" then moves slower to 13"
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post #4 of 7 Old 02-21-2013, 05:26 AM
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Do it this way to check for clogged exhaust. I don't like seeing that it slowly goes down from 17 to 13.

CRUISER’S VACUUM TEST FOR EXHAUST RESTRICTION

Your vacuum gauge should come with an instruction booklet outlining the procedure. Hook the vacuum gauge up to a source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum. Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle. Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

If you own a Renix Jeep, you'll want to check this out.

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post #5 of 7 Old 02-21-2013, 12:59 PM Thread Starter
88jeep2b
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ok this is what i have. 600 rpm = 13", 2000 rpm = 17", closed tb jumps to 20", moves steady back to 13". cut exhaust pipe off before the cat and no change. from what ive been reading its kinda leading to rings but the compression seem to be good. a low of 140 and a high of 150. can you still have good compression with worn rings.
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post #6 of 7 Old 02-21-2013, 05:25 PM
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http://www.classictruckshop.com/club...ts/vac/uum.htm

If you own a Renix Jeep, you'll want to check this out.

An index to over 30 write-ups can be found in the link below

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My Jeep Technical Photos

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post #7 of 7 Old 02-22-2013, 12:53 AM Thread Starter
88jeep2b
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i think ill do a leak down to check the rings. really dont want to pull the timing cover if i dont have too. thanks for all the help
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