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'88 Chief Rough Idle or No Start or Perfect Start

Hawaiian Style

NAXJA Forum User
Well, the Chief is acting up. 1988 4.0 163,000 miles. Engine rebuilt at 149k. I don't know what is going on with it. Initially I thought the problem was no spark. Checked ICM and coil and they seem OK. Replaced spark plug wires, new cap and rotor, no vacuum leaks that I can find.

It's really amazing how this works. Three possiblities when I fire her up. Perfect start with no idle problems, an awfull start where engine sputters, backfires, and refuses to get over 1000 RPM's, or my favorite, the combo start where it sputters, complains and then suddenly after a random amount of time it smooths out. I am pretty sure it's the timing because when I removed the air intake hose on the intake manifold I can see that the engine is backfiring into the intake manifold.... NOT GOOD.

Here's my checklist:

Check timing with timing gun.
Check CPS

Anything else I should be looking at? I really hope the timing chain isn't slipping with only 12k on the rebuild. I really don't want to have to throw in the towel on my Chief. It's old and ugly but I love him (A Chief just can't be female) as much as the other two Jeeps in the garage.

Crank sensor

It seems like others with the same problem on this forum found relief by changing the crank position sensor. I'm no the expert on 4.0s but that might be a good (and cheap) place to start. Just a note that without taking the ignition module out and having it checked on a test machine you can't get a good diagnosis on it.


Thanks... replaced CPS (Note to self: DON'T DROP BOLT INTO BELLHOUSING AGAIN). Still ran poorly, in fact worse. Unplugged every single sensor I could find, replaced spark plug wires, switched around spark plugs (and inspected), swore and cursed, and suddenly it ran fine. No idea what fixed it... probably a combination of all those things. Still has a slight knock and idles low. Probably needs to more attention but it is drivable again. Thanks for the suggestion.

Hawaiian Style

If you are getting a preignition knock then you I would look into replacing the knock sensor. On the non-HO engines their is a knock sensor, so if you get a ping when going up a hill go get a new one.

I will warn you it is a dealer only item, it cost me around $75 two years ago but made a big difference in how the XJ ran. The knock sensor is located on the driver's side of the block about an inch above the oil pan. It looks like an oxygen sensor with the white shaft. The knock sensor uses the same connector as does your fuel injectors. The new one is a different style but included in the package are instructions on how to install it.

An 88 XJ 4.0 should never ping due to poor quality gasoline, if it does then the knock sensor is bad.


Sounds kind of like the valves... it's really hard to say what the knocking is. It doesn't get worse when the engine is under load (as far as I can hear). The knock sensor is an excellent place to start... I read that hitting the block near the sensor with something should result in a change in the idle if it's working. Is that a good test for it? Also, the MAP and MAT sensors are also involved in the timing right? Should I be looking at them too?

You guys are the best!
I agree with the knock sensor too. But what your describing sounds like the MAP sensor. Check that you have no cracks in your vacuum supply line and just replace the sensor its only $35.00 I have an 87 Chief and was having very similar problems, turned out to be the MAP. While I was at it I just replaced all of the sensors. Over 230,000 on the odo and runs like new again.
Over the past 3 years I replaced all the engine sensors on my 88 XJ and each time I changed one it ran better. I did notice changes with a new MAP and manifold air temp (MAT) sensor, but then the knock sensor also made a change.

I think the new sensors responded faster and the computer adjusted accordingly. I also purchased a rebuild computer and that also made the XJ run better. If you plan on keeping the XJ investment in new sensors it not a bad idea.
I see that no one has offered-up the Distributor's sync sensor as a place to look -- I'll detail a few tips below -- The Sync sensor is used in combination with the CPS to determine just which piston is at TDC (the CPS can only show crank position and #1 and #6 are at TDC at the same time, one on compression and the other at exhaust)... If the data from the DistSync is ambigous or missing, the ECU will try to fire the #1 cylinder, every other time the TDC mark flys past the CPS - if the engine doesn't start, then it'll skip one of the TDC instances and try to fire #1 180* off from where it started before... This could be part of why you're getting good starts & bad starts.

The Dist Sync signal does have a tendancy to drop it's ground reference and become unpredictable -- Leme type some thoughts:

So the sync connector out of the dist is a round 3-position connector --

Pin A = Black - Ground - (chassis gound, both in the dist and for the harness - you can unplug the connector to check both grounds seperately - it's OK to add a chassis ground to this line)

Pin B = Blue - Output - (to pin C16 of the ECU - should show about 5 volts static between Pin-B and Pin-C - connected to harness, key on, engine off)

Pin C = Gray - Supply (should be about 7VDC from pin C5 of the ECU)

While you're looking at the static voltage between Pin-B and Pin-C, if you crank the engine, you should be able to see the voltage fluctuate as the sensor rotates (qty=1 wiggle per rev, so it's not that impressive)

[edit] I seem to remember the gray wire on Pin-C having a stripe as well... -- was just looking at a dist yesterday, but can't remember much more [/edit]
Something new

Hey great tip Satan. I'll have to file that one away for future reference!