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Rough Idle after Gas Change

Old 09-26-2015, 01:53 PM
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Default Rough Idle after Gas Change

So a few weeks ago I foolishly filled up my 88 Cherokee i6 MPI automatic with premium gas wondering if it would improve my gas mileage. I guess this was a bad idea.

Almost immediately the Jeep's idle became rough, and my gas mileage almost cut in half with a lot of exhaust. Occasionally, the engine also "pulses" when in idle, and if I'm in gear at a complete stop, and take my foot off the brake, the vehicle will surge forward and stop at each pulse. The pulses happen about 3-4 seconds apart. But again, this pulsing only happens about 30-40% of the time that I'm at a stop.

I hoped it would stabilize after a few tanks of regular 87 gas, but it hasn't at all - in fact I think it's getting gradually worse.

Last week I filled the tank with a bottle of seafoam in an attempt to clean the fuel system and injectors, but it didn't help at all.

SO, I've been reading around and seeing all kinds of possibilities including clogged injectors, dirty throttle body, malfunctioning ECM, faulty spark plugs, failing fuel pump, clogged lines, bad oxygen sensor, etc. But the start of my symptoms at the same time as using premium gas seems more than coincidental.

Can anyone tell me where I need to start with these symptoms? Again, rough idle, occasional engine pulsing, high fuel consumption, and increased exhaust.

Thanks!
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Old 09-26-2015, 02:02 PM
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I'd be getting the codes read for a start.
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Old 09-26-2015, 02:29 PM
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Renix Cherokees don't have anywhere a trouble code reader could attach to. And they don't store any kind of trouble codes. Also they have no check engine light.
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Old 09-26-2015, 02:35 PM
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I thought on the'90 and earlier models(except for the V6's)you needed a Chrysler DRB-ll or equivalent tool to access the diagnostic system?
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Old 09-26-2015, 02:52 PM
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Hmm, you appear to be right about the DRB-II. Never seen that before. Anyway, I don't think there is a Jeep mechanic in my area that would actually have one, and I don't think it's wise to drop a grand for one either...
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Old 09-26-2015, 02:56 PM
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I wonder if your local parts stores might be able to? I know Advance Auto here will do a complete diagnostic and check codes for free.
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Old 09-26-2015, 04:27 PM
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Wonder if you may have gotten a bad batch of dirty fuel and plugged up the filter.
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Old 09-26-2015, 10:00 PM
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The octane isn't the issue.

Check this first:
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Old 11-05-2015, 08:40 AM
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cruiser, I didn't see your post, but you were absolutely right.

I came back here to post that I found that the brittle vacuum tube connection on the MAP hose had cracked. So the MAP was getting no vacuum, and and telling the ECU to dump the fuel.

Since I replaced the tube, it's running relatively smooth again, and gas mileage has improved.

Now on to the next half dozen issues...

Thanks again!
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Old 11-05-2015, 08:44 AM
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Good call cruiser!!
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Old 11-05-2015, 12:36 PM
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Originally Posted by cruiser54 View Post
The octane isn't the issue.

Check this first:
Any part numbers or names for this tube? I believe I have this issue also. Wondering where to get a replacement?
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Old 11-05-2015, 07:23 PM
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Originally Posted by westran2002 View Post
Any part numbers or names for this tube? I believe I have this issue also. Wondering where to get a replacement?
No replacement available........
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Old 11-05-2015, 08:17 PM
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Originally Posted by westran2002 View Post
Any part numbers or names for this tube? I believe I have this issue also. Wondering where to get a replacement?
you would have to improvise
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Old 11-05-2015, 08:20 PM
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Originally Posted by XJwonders View Post
you would have to improvise
Yup.

you can thread the existing hole in the throttle body ro 1/8" NPT and put a fitting in there. That plastic tubing can be bent using a hair drier.
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Old 11-06-2015, 09:16 AM
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Thanks for the info. I'll MacGyver it I guess!
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