Intermittent rough idle

Tiny
KNARF1518
  • MEMBER
  • 1996 JEEP LAREDO
  • 4.0L
  • 6 CYL
  • RWD
  • AUTOMATIC
  • 180,000 MILES
1996 Jeep Laredo. 4.0 Straight six. Rear-wheel drive.

A few years ago my Jeep began running rough and the check engine light came on. A few days later the rough idle smoothed out and the check engine light went off. This became a reoccurring pattern, but with increased frequency and duration's of rough idle/ high gas consumption.

I bought a basic reader, but it only tells me multiple random misfires. Sometimes it is on cylinder 2 only or cylinder 1 and 5 only and other times (Now) it is on all 6 cylinders.

I tried fixing it by changing out plugs and wires, checking the gap on the plugs, cleaning the carburetor, and finally taking it in to my mechanic. He suggested changing out the fuel injectors, which didn’t help. No clogs in the rail either.

Recently my mechanic suggested it might be a bent exhaust valve and that it wasn’t worth fixing due to the potential high cost of needing to replace more than just the valve once he got into it. He said new found compression from fixing the problem might cause further problems.

Luckily it passed its smog test this year. Of course 3 days later it started running rough again, which triggered the check engine light.

The rough idle seems to be provoked by my lead foot. If I drive nice the problem occurs less often and for shorter periods of time. This last incident though has lasted over two weeks. The check engine light is still on but the idle has smoothed out a bit.

My questions are:

1. How could a bent exhaust valve cause the engine to run rough sometimes and normal others?

2. What is causing the rough idle?

3. How do I fix it?

I love the vehicle, but the frustration is getting to me. I’ll have to sell it soon if I can’t find and fix the problem!
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Monday, March 14th, 2016 AT 12:00 AM

4 Replies

Tiny
MHPAUTOS
  • EXPERT
Check that the distributor shaft bushes are not warn, get the dwell checked if it is fluctuating beyond allowable limits this may well be the problem, I go down this path as I have seen quite a few distributors on these fail.
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Monday, March 14th, 2016 AT 3:00 AM
Tiny
KNARF1518
  • MEMBER
Thanks MHPAUTOS,

I replaced the distributor five years ago, so I didn't think to check it but I will this weekend. I'll post what I find.

Thank you.
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Monday, March 14th, 2016 AT 12:27 PM
Tiny
KNARF1518
  • MEMBER
I took off the cap and inspected the rotor, cap points, and the distributor shaft.

* The distributor shaft did have some play in it.

* The rotor looked okay.
* Cap points were brown on the face only so I cleaned them.
* All the plugs looked good, no oil or excessive carbon build up.
* Wires checked out.
* Oil and filter change too since I had some time.

Since performing the inspection the jeeps performance has declined. (Rougher timing)
I'm assuming this means the bushings on the distributor are bad and it needs to be replaced.
This is the longest period of time the vehicle has ran this rough without smoothing out.
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Thursday, March 31st, 2016 AT 10:51 AM
Tiny
MHPAUTOS
  • EXPERT
It will depend on the dwell reading and how stable it is, this can be checked quickly by a mechanic and make sure that the points are not arching due to a faulty capacitor you really do need to check these things electronically to determine how bad they are.
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Thursday, March 31st, 2016 AT 1:38 PM

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