libengan

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!

EZEARL

I'd be getting the codes read for a start.

libengan

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.

EZEARL

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?

libengan

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...

EZEARL

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.

Fred/N0AZZ

Wonder if you may have gotten a bad batch of dirty fuel and plugged up the filter.

cruiser54

The octane isn't the issue.

Check this first:

libengan

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!

Fred/N0AZZ

Good call cruiser!!

westran2002

Any part numbers or names for this tube? I believe I have this issue also. Wondering where to get a replacement?

cruiser54

No replacement available........

XJwonders

you would have to improvise

cruiser54

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.

westran2002

Thanks for the info. I'll MacGyver it I guess!