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88 4.0 XJ running like crap

woody

woody

NAXJA Forum User
NAXJA Memorial Lifetime Member
Location
NC Sandhills
Problems started approx 2 weeks ago... feels like a miss, or a fuel problem, but very intermittent.

BTW the motor is right at 200k, and feels strong, oil pressure is the same as it's ever been (20 +-) in the 15 months I've had it.

Will idle fine, then drop way down briefly & return, sometimes the motor quits (both at idle and under load) but it always restarts.

Seems to do it more at normal temp than while cold, but again it's a random thing...does it basically anytime, enough to be pesky.

Plugs/cap/rotor are @ 8 months, air filter is fresh, fuel filter replaced in May... no obvious vacuum leaks, & ruled out 'bad gas' been through a few tanks from different stations since it started acting up. Best I can tell, MPG hasn't changed...17+- depending on my foot.

Thinking to replace plugs, then coil, then CPS (last 2 are of questionable age/condition, and can use the old parts as spares if it doesnt cure it) or am I barking up the wrong tree...

TIA for some ideas...
 
Check the CPS. Also, check the wire from the CPS to the harness. After I R&R mine it managed to sneak it's way over to the exhaust manifold. I am speculating here but it seems that the heat from the manifold cause some kind of voltage drop or increase. The wire covering never burned through but the sleeve that protects the wires sure got crispy. My XJ would stall out under all kinds of conditions. I rerouted the wires and all is well now.
 
I would look at CPS first, then O2 sensor, then coil and ignition module.
 
My vote is for the ignition module going bad, since the trouble is worse when the engine's warm. Unfortunately on these Renix's, trial and error replacement of parts is the only true way to fix it.
 
My vote is for CPS first. You can test the coil with an Ohm meter (test hot). You're SOL on the ignition module as I haven't found any one able to test them. I went through the same thing about a year ago, the CPS was my problem. Good news is I got a lot of new sensors out of the deal.
 
and my vote

Give it to your fav mechanic for a professional diagnosis.
There's no point in swapping electrical components in a costly crap shoot.
You can fix a lot of problems on your XJ, but why throw money at a mystery issue? Ask em to charge you for 30 mins of their standard hourly rate-- worth a try.
 
Re: and my vote

DeathByXJ said:
Give it to your fav mechanic for a professional diagnosis.
There's no point in swapping electrical components in a costly crap shoot.
You can fix a lot of problems on your XJ, but why throw money at a mystery issue? Ask em to charge you for 30 mins of their standard hourly rate-- worth a try.
Click to expand...

On a Renix system with no OBD scanner port, what's a professional mechanic going to do for $65/hour that Woody can't do at home with a 12-volt test light and a $10 multimeter?
 
Re: Re: and my vote

Eagle said:
On a Renix system with no OBD scanner port, what's a professional mechanic going to do for $65/hour that Woody can't do at home with a 12-volt test light and a $10 multimeter?
Click to expand...


Word.
 
Re: read the posts

DeathByXJ said:
Then do it-- I was responding to this:
"Unfortunately on these Renix's, trial and error replacement of parts is the only true way to fix it."
Click to expand...

In many cases that's exactly correct. Why pay someone $65/hour to make the same guesses you can make at home from free? I appreciate that you were trying to help, but the suggestion really doesn't seem very useful in this instance.
 
Unfortunately on these Renix's, trial and error replacement of parts is the only true way to fix it.
Click to expand...
I made this statement, and I should have qualified it by saying "intermittant" problems. As everyone knows, this is the hardest fix of all. Just when you get the multimeter out to do checks, it starts running fine! And everything checks normal. Then the next day it starts acting up again.

The TPS is easy to check, as is the coil and the CPS. I was basing my guess on the bad ignition module upon experience with other vehicles. They seem to fail when they get hot. The trouble might also be a bad stator (pick-up coil) in the distributor.....

Woody, I'd try to isolate it to either fuel or spark first. If it's a dead miss, then it's ignition related. If it simply starts idleing slower until it dies, then it's fuel related, or maybe the IAC.

Have we covered everything yet???:D
 
Thanks guys...

With four 87-89 4.0l Jeeps kicking around, I have a stash of "known good" parts to pick & swap from.

Today I bought another CPS and 02, plugs and fuel filter) which will not be wasted even if they don't help. Maybe the condition of the old plugs will point to an issue with a certain cyl.

If it ever quits raining, I'll do some swapping & see if it helped.
 
Classic EGR failure

When the EGR fails and lets too much exhaust back into the cylinders, it can choke off the engine at lower RPMs. Failure can be total or just "grouchy", meaning the valve still moves but is sticky. Easy test: Unplug the vacuum line from the control immediately beside the EGR valve, and plug the hose with a screw or golf tee. If the problem goes away, then replace the EGR valve. On my 1990 XJ (4.0 engine) I am on my third EGR. Replacement is easy, once you remove the air box (which is also easy). Use a touch of anti-seize on the bolts, to defeat the corrosion demon. EGRs are only found on the pre-HO engines. Chrsyler went to valve overlap to acheive the same effect on the HO series. 4xBob
 
Eagle said:
I would look at CPS first, then O2 sensor, then coil and ignition module.
Click to expand...

OK I swapped out the O2 and the spark plugs before a cold-start, and drove it approx 15 miles with 10 min of idling/10 min of rest at the halfway point. (IE: fully warmed up) It seemed to be doing fine, no stumble at all. It did stall once at -5mph after I restarted for the trip home, but it has done that randomly since I've owned it, and IMHO unrelated to the latest stumbling issue.

Since I am terrible for doing more than one potential repair at a time, I unplugged the old CPS before tackling the plugs, with the intent of doing a 1-2-3 assault...but hooked it back up to test what changed before swapping it. A few more trips will confirm success or failure...

Thanks for the help...and wondering what prompted Eagle to mention the O2 ahead of other potential sensor faults?

Where is the EGR valve LOL
 
Don't know what Eagle's motivation was, but I always find the O2 sensor suspect for intermittent problems for three reasons:

1. It's the one computer sensor designed to wear out/be replaced.
2. When the O2's bad, the ECU automatically goes into closed-loop fail-safe (dead rich) condition to get you home and to a mechanic.
That extra-rich condition then causes plugs to foul, Cats to plug, and other undesirable conditions.
The same things happen when the wire or connector to the O2 go bad.
3. It's a cheap and easy first step in a trial-and-error analysis, and you'll eventually need a new O2 sensor anyway.
 
Makes sense to me...

My cat is -30k, and the old 02 was 20k older than the 180k on the odo when I got the Jeep, so swapping in a fresh ($50 Bosch) one can't be a bad thing.

The old (Bosch Platinum) plugs if anything looked lean to me...whitish residue, no tan or brown. These plugs don't have any sort of post below the electrode. I replaced with $3.99 AC Delco Rapidfire that have a 'splined' post and one electrode. :dunno: what that's all about, but we'll see when I pull one after a few days.
 
Wow, Woody, there's alot of benefit to being president:) I had basically the same complaint earlier and got about half the response!

We are still hunting ours down. I suspect a wiring harness problem, but have no doubts it's ignition related. The misfires are very complete misfires.

Good Luck and keep us posted on solutions.

WillySLC
 
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