renix cluster fudge... problems all over the place

so yeah, my 1990 , 170k renix xj is having some trouble... back to back style...
6 months ago, this motor was running sweet, i run 4.56 on 31's, drive like a dick, this 4.0 got used hard, ran GREAT.

3 months ago, had a not-so-typical crankshaft position sensor problem... turned out to be ecu-side.. discovered a toasted ECU --actually, cal did, thanks cal--.. replaced it. this is when i first started hearing the knock.... its a loud knock, not the 'yer 4.0 is low on oil' knock.

had some trouble w/ leaking injectors, so i mothballed the xj for 2 weeks, bought some ebay flowmatched xj injectors, it smoked really bad when i started it right after the install, checked the plugs to find them completely blackened. replaced them w/ champions, same model as before. this is when i first experienced the stumbling on acceleration...


heres the problems, itemized
-knocks at idle, real bad.... if i rev it up very slightly in N, i can quiet it down some
-stumbles coming off a stop... gf

heres the ones getting fixed this week for sure...
-i do have 2 exhaust leaks, 1 at the cat, and one at the tailpipe merge, both have black soot indicating the leaking spot
-i do have vacuum leaks, and a leaking grommet at the valve cover... anyone have a source for replacement grommets?

starting to think ive fouled my cat... anyone concur? maybe the 02 as well?

the last 2 weeks ive been not splurging on goodies, due to the fact that i think my motor's going...

any comments?

i beleive the blackened plugs is from me driving it home--short drive--, and moving it for street sweeping... with only 5 of 6 fuel injectors working... is this possible? or was it running terribly before as well? plugs were uniform black.

The "knock" you are hearing is most likely piston slap, very common on 90 4.0's.

I would adjust the TPS to try to remedy the hesitation/stumble. Also check the EGR, it should be closed except on hard acceleration.

would the egr sticking open cause this fouling? it makes perfect sense now that you mention it...

i beleive the tps is in adjustment, it shifts fine and everything, and it hasnt been touched in months, but i will throw a voltmeter on it, thanks

i do have a bad feeling about this knock though, it started when i replaced the ecu... Night and day appearance. does piston slap start all of a sudden?

is there anyway to positively identify piston slap, without tear down? i've redlined it, and it feels fine at the high end, it doesnt feel like a ticking bomb, but it sounds like one...

No way to positively identify engine sounds. Are you sure that your spark plug and injector wires are sequenced properly? Check egr function that it's closed on idle. What about doing a compression test on the cylinders to check sealing of rings and valves for compression?

i do have a compression tester, but hadnt though of using it... i will get back w/ results... got a lot of work to do this weekend. thanks

ran some tests today, compression was 160 on 1-4 --from the front--, i had 170 on 5 and 6. -- the rear two--

when i pulled the spark plugs for the compression test, i noticed they had re-fouled....--spark plugs were 1 week old, maybe 100 miles on em-- i gently cleaned them off w/ a wire wheel, and also replaced the fouled o2 sensor, w/ a take off-spare i had in the garage...

took care of the exhaust leaks, replaced the cat gasket. looking thru the cat, it wasnt plugged and it didnt have chunks floating around inside.

fixed some vacuum leaks, but apparently i still have some, as i only have 12-14psi of vacuum at idle... shouldnt this be 16 or so?

also ran some seafoam thru the intake, got a decent ammount of smoke.



any ideas on this fouling?
im gonna start checking into the egr system.

If the EGR is stuck open, you will loose vacuum and you will foul plugs. You are pushing exhaust right into the intake at idle. It should only open on hard acceleration.

How is the temp? If running cold could be fouling rich. CTS, O2, vacuum leaks.

If you have heavy carbon buildup in the cylinders you could get a bad knock. Try the Mopar combustion chamber cleaner, others have had luck with it.

Really sounds like more than one issue, maybe one causing the other.

tempreture is fine, it runs like normal as far as temp.

ordered some goodies from napa, new egr valve and gasket, and the vacuum harness i needed...

If you were still running the original vacuum harness then it was way past time to swap it out, the "O" rings inside the large connector at the intake were probably shot. The new EGR is money well spent.

I eliminated the factory vacuum harness on my 90

It sounds like CPS troubles to me.

I've got a couple of spare EGR valves I know are good, if you need to try one XCM.

i allready ordered a new egr , but thanks anyways. what do you mean by cps troubles? think maybe i re-fried another ecu i havent had any no-starts in 2-3 weeks, and for a while it was actually quite peppy, it was starting up PRONTO.. does a bad cps do something else im unaware of? also, can some varify that 12psi is low for vacuum?

Well, I'm thinking less the CPS and more the CPS plug. Corrosion in the CPS plug can cause stumbling idle, hesitation, all kinds of problems.. unplug it, clean both sides out as good as you can with a steel pick or such, and then jiggle it somewhat aggressivly as you plug it back in; and see if it helps.

Also, check the vacume lines by your MAP sensor.