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camojeep88
November 27th, 2017, 23:03
i have an 88 xj with 4.0. I've replaced pretty much everything and still no luck. new cap, rotor,distributor, cps, tps, cat, plugs, wires, coil, icm, injectors, fpr, cleaned throttle body, iac, grounds, I added a ground from battery to body. fuel pressure is 31 at idle and 39 with no vacuum, did vacuum test and at idle is at 16. I'm lost as to what is wrong. it is misfiring on cylinder 6 and when cold I get no throttle response and very poor power. it's hard to get it to 55 and usually takes me holding the pedal to the floor. sometimes it idles great but most the time it just runs rough. some please help

cruiser54
November 28th, 2017, 06:07
16 is low vacuum for sea level. Perhaps your EGR valve is sticking open.

90xj06
November 28th, 2017, 06:42
double check cylinder compression too. have you owned this before it was doing this? or did you just get it?

Flyfisher
November 28th, 2017, 12:52
Not positive of desired fuel pressure (don't have manual with me), but I believe pressure should be mid-upper 40's.

old_man
November 28th, 2017, 17:46
Verify distributor indexing, spark plug wiring, vacuum leaks, clogged air filter.

camojeep88
November 28th, 2017, 20:41
I've owned it for a while now but compression is my next rest just gotta find the time to do it. as far as vacuum goes I thought it was low too but I sprayed carb and choke cleaner all over and it doesn't phase it at all

cruiser54
November 28th, 2017, 20:43
I've owned it for a while now but compression is my next rest just gotta find the time to do it. as far as vacuum goes I thought it was low too but I sprayed carb and choke cleaner all over and it doesn't phase it at all

Low vacuum can be caused by many more things than a vacuum leak.............

camojeep88
November 28th, 2017, 20:48
and from what I remember the fuel pressure on the 80s is 31 at idle and 39 with no vacuum check distributor and it seems ok when it's at tdc the rotor is pointing at the number 1

camojeep88
November 30th, 2017, 22:42
turns out cylinders 1,3,4 have 100psi of compression and 2,5,6 habe closer to 75. looks like I'm up a creek without a paddle

lawsoncl
November 30th, 2017, 23:55
Low vacuum can be caused by many more things than a vacuum leak.............


Timing chain jump a tooth?

cruiser54
December 1st, 2017, 05:45
Timing chain jump a tooth?


I'm thinking bad compression rings.

camojeep88
December 1st, 2017, 10:39
a friend told me his buddy had a low compression issue and it was a matter of head bolts coming loose just enough for it to cause issues so I figure it's worth having a look but for the most part I'm thinking it's done. no time like the present to start a striker install 🤗

Ecomike
December 1st, 2017, 14:08
i have an 88 xj with 4.0. I've replaced pretty much everything and still no luck. new cap, rotor,distributor, cps, tps, cat, plugs, wires, coil, icm, injectors, fpr, cleaned throttle body, iac, grounds, I added a ground from battery to body. fuel pressure is 31 at idle and 39 with no vacuum, did vacuum test and at idle is at 16. I'm lost as to what is wrong. it is misfiring on cylinder 6 and when cold I get no throttle response and very poor power. it's hard to get it to 55 and usually takes me holding the pedal to the floor. sometimes it idles great but most the time it just runs rough. some please help

When I had those problems over the years, it was either a loose helical gear on the distributor shaft, a Bad TPS (unusual as it tested good using FSM tests), A bent throttle body actuator rod, A dying ICM under the HV coil, leaking FPR, leaking into the vacuum line, and one you should still check is the Vac line from the throttle body to the MAP sensor, Pull it out and examine every micro inch of it for bad spots!!! Those are most of them LOL. I had one that ran like crap at idle and great at 2000-3000 rpm that had 45 psi in Cyls 3 and 4. The supper thin head gasket between the cylinders was burned through by carbon deposits there that got red hot running lean.

I would recheck the compression, and make sure you had the good o'ring seals on the tester hose threads, and use an air compressor to blow sand out of the spark plug area before removing the plugs. I use brake solvent to dissolve any oil, then blow it out (wear eye protection) so you know the o'ring sealing are is clean!!!! Then re-run the compression tests, also add some oil top side of the pistons first, about a tea spoon.

If the low compression is still there, pull the head to see if the valves, and or head gasket are bad, as that is an easy fix. 16" of vacuum is not the end of the world at sea level. 17" is better.

Ecomike
December 1st, 2017, 14:09
Is the Distributor properly indexed????

Also make sure you have the spark plug wires on the correct cylinders, same for the injectors. Way too easy to swap them to the wrong ones, do not ask me how I know, LOL.

I seriously doubt it is head bolts being loose, burned out head gasket from running too lean with past carbon fouling is very possible.

But it is way too easy to get bad test data by not getting a good seal on the SP threads!!!! Retest and verify!!!

old_man
December 3rd, 2017, 11:11
Is the Distributor properly indexed????

Also make sure you have the spark plug wires on the correct cylinders, same for the injectors. Way too easy to swap them to the wrong ones, do not ask me how I know, LOL.

I seriously doubt it is head bolts being loose, burned out head gasket from running too lean with past carbon fouling is very possible.

But it is way too easy to get bad test data by not getting a good seal on the SP threads!!!! Retest and verify!!!

X2 on everything.