'91 4.0l Pouring out grey smoke...... - JeepForum.com
 
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post #1 of 5 Old 03-18-2017, 01:26 PM Thread Starter
BGDG280
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1991 XJ Cherokee 
 
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Location: Thornville, Ohio
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Question '91 4.0l Pouring out grey smoke......

after engine runs for say a min maybe two, then it begins smoking, not black, like a dead rich condition, not blue like excessive oil burning, not white like coolant contamination.

Its grey, looks sooty if I had to try to describe it. Its only under heavier throttle input.

The engine does not miss, hesitate nor seem down very much on power. I did a full tune up about 400 miles ago, this is My toy, doesnt see much street time. The plugs looked pretty sooty too at that time. I replaced the fuel pump and filter like 1000 miles ago. Oh, it does backfire upon deceleration if I leave it in gear and just coast in gear.

I have replaced the O2 sensor with no change, I was advised to check the EGR, but I cant find one. My Haynes manual ends at 1990 on their listing for an EGR.

I'm a very accomplished wrench, but My weakness is drivability. Any help or ideas I can look into would be greatly appreciated!!


91 XJ, Yellow, Dirty ALOT, 34 x 10.50 x 15 Super Swampers. Looking for Trails and Off Road fun!
95 XJ 2 Door 5 spd, 2" lift, 31 x 11.50 x 15 Super Swampers....this thing is a bada** for being nearly stock!
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post #2 of 5 Old 03-18-2017, 01:36 PM
ou2mame
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Have you looked at the new plugs? Could be a bad injector causing a rich condition.

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post #3 of 5 Old 03-18-2017, 03:54 PM Thread Starter
BGDG280
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Quote:
Originally Posted by ou2mame View Post
Have you looked at the new plugs? Could be a bad injector causing a rich condition.

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No, I have not, but when I changed the plugs, shortly after this condition showed up, they were all pretty equally sooty and grey.

I have considered the possibility of an injector, but there is no smoke at idle and the Jeep starts right up, no hard starts, warm or cold. I would think a stuck injector would have both of those issues.

I will check the plugs, but I'm fairly sure they will look the same as before.

Thank you for answering.

91 XJ, Yellow, Dirty ALOT, 34 x 10.50 x 15 Super Swampers. Looking for Trails and Off Road fun!
95 XJ 2 Door 5 spd, 2" lift, 31 x 11.50 x 15 Super Swampers....this thing is a bada** for being nearly stock!
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post #4 of 5 Old 03-21-2017, 08:29 AM
cruiser54
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Make absolutely sure your MAP sensor hose to intake manifold is perfect as well as it's electrical connection.

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post #5 of 5 Old 03-21-2017, 09:01 AM
tjwalker
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Backfiring is often caused by a fuel air imbalance. This is how I would proceed if it were mine.

1. Verify MAP sensor connections, test that sensor and be sure that the vacuum line is not rotted, which is not uncommon.

2. I know you replaced the fuel pump, but fuel testing is absolutely in order here in the process of elimination. You should have 31 psi at idle. Now disconnect the vacuum line leading to the fuel pressure regulator (be sure there is no raw fuel in that vacuum line) and the pressure should jump up to approximately 39 psi.

3. There is no EGR on your 91.

4. Consider replacing all of the grommets/elbows/lines for your CCV system, coming and going from your valve cover. These rubber parts clog, get brittle and rot over time. It's a very important system to let your engine breathe!

5. A simple compression test on all cylinders is never a bad "snapshot in time" on how things are doing internally. The compression spec. for the 4.0 engine is 120-150 with no more than a 30 psi variation between cylinders.

Good luck and keep us updated!

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
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