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24-10-2019
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Full Flexer
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Join Date: Aug 2007
Location: Melbourne, VIC
Age: 35
Posts: 1,053 What Jeep do I drive?: XJ
Likes: 10
Liked 117 Times in 56 Posts
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Missing/Rough on both ULP and LPG
Hey folks,
So I'm chasing a rough running/miss condition on my 1996 XJ dual fuel.
It is both under load and at idle, AND it is present on both LPG and ULP.
I checked spark plugs and ran a compression test yesterday with the following results:
So that's 68, 70, 70, 70, 75, 75 PSI (at 5 cranks each)
I did forget WOT during these tests, but given that I'm more interested in relative compression it looks okay.
Plugs looked alright, though I did manage to tear the #1 lead on removal so there's a chance it was already damaged - I've ordered a new set already.
I've also tried swapping out coil (which is fairly new) for the original, with no change. Distributor Cap and rotor look fine.
Perhaps the new leads will fix it up, but was just throwing this out there for any other ideas.
Cheers in advance,
Alex
__________________
1996 XJ Sport
4" Lift, 31x10.5R15 Federal Couragia MT, LPG/ULP
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24-10-2019
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Rock Munki
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Join Date: Mar 2008
Location: Kellyville NSW
Posts: 2,381 What Jeep do I drive?: XJ
Likes: 178
Liked 351 Times in 223 Posts
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umm that is low compression. like im suprised its running low,...140 150psi is unremarkable.
suggest u use a different compression tester. on the up side at east they are all similar.
possibly review yr technique/ method.
plenty on u tube
good luck with it.
__________________
Obd1 scanner.
'95 sport,4.6 stroker, 4.5" lift, 32"s, 4:10's, 8.25&SYE, taurus fan
Last edited by rainman; 24-10-2019 at 03:57 PM.
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24-10-2019
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AJOR Bronze
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Join Date: Apr 2013
Location: Blackall Outback Queensland
Posts: 1,736 What Jeep do I drive?: XJ
Likes: 1,286
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as rainman said .... one of 2 things your compression tester is stuffed or your engine is you should be getting at least 120 psi
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_.(o)_)__(o)_)--o-)_)
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96 XJ Limited LPG Conversion......
99 XJ Sport ... Finally on the Road
RC Mini Me
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24-10-2019
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Full Flexer
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Join Date: Mar 2016
Posts: 1,133 What Jeep do I drive?: XJ
Likes: 109
Liked 640 Times in 436 Posts
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Quote:
Originally Posted by Hamsteh
So that's 68, 70, 70, 70, 75, 75 PSI (at 5 cranks each)
I did forget WOT during these tests, but given that I'm more interested in relative compression it looks okay.
Alex
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not ok at all, something is very amiss there..should be much higher, even with no throttle and a "push in" tester.
confirm your tester is not defective, and redo the test with WOT
__________________
'96 4.0 Sport 370k+
'96 Limited 263k (unreg spare)
'96 Sport 335k (got smashed, farmtruck)
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24-10-2019
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Full Flexer
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Join Date: Aug 2007
Location: Melbourne, VIC
Age: 35
Posts: 1,053 What Jeep do I drive?: XJ
Likes: 10
Liked 117 Times in 56 Posts
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Hey so I came across some conflicting information on how to run the test. Some said MAX pressure over ~10 cranks, some said just run 4/5 cranks consistently for relative pressure comparison. I know the first cylinder hit >120PSI after more than 5 cranks for instance.
Is max pressure per cylinder the preferred method? I was just looking for consistent results.
EDIT: I did notice that it climbed a lot slower than what I've seen in some of the YT tests.
__________________
1996 XJ Sport
4" Lift, 31x10.5R15 Federal Couragia MT, LPG/ULP
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24-10-2019
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Rock Munki
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Join Date: Mar 2008
Location: Kellyville NSW
Posts: 2,381 What Jeep do I drive?: XJ
Likes: 178
Liked 351 Times in 223 Posts
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i go till it stops climbing, i try to do it with a warm engine, all the spark plugs out, and WOT. go 1-6 if the last cylinder is down i redo it after charging the battery 6-1.
i realy struggled with the push in type tester and had to make a screw in one out of a old spark plug.
I also use a push-button switch on 2 leads that plug into pins 30 & 87 of the starter relay reciptical in the PDC. this allows you to be local to the pressure tester, and does not activate the injectors while cranking.
__________________
Obd1 scanner.
'95 sport,4.6 stroker, 4.5" lift, 32"s, 4:10's, 8.25&SYE, taurus fan
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24-10-2019
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Full Flexer
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Join Date: Aug 2007
Location: Melbourne, VIC
Age: 35
Posts: 1,053 What Jeep do I drive?: XJ
Likes: 10
Liked 117 Times in 56 Posts
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Quote:
Originally Posted by rainman
i go till it stops climbing, i try to do it with a warm engine, all the spark plugs out, and WOT. go 1-6 if the last cylinder is down i redo it after charging the battery 6-1.
i realy struggled with the push in type tester and had to make a screw in one out of a old spark plug.
I also use a push-button switch on 2 leads that plug into pins 30 & 87 of the starter relay reciptical in the PDC. this allows you to be local to the pressure tester, and does not activate the injectors while cranking.
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Awesome thanks Rainman, I'll give that a shot tomorrow hopefully (though cold as I'm not going to rebuild it to strip it down again haha).
I have the screw-in type of tester, but its hard to get a decent amount of thread as there isn't room to tighten it down by anything other than the hose.
__________________
1996 XJ Sport
4" Lift, 31x10.5R15 Federal Couragia MT, LPG/ULP
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