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Old 05-25-2008, 08:17 PM   #1 (permalink)
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Hypermiling killed my PCM?

Newb here

'96 Jeep Cherokee
4.0
I6
Auto

I bought my jeep new in '96. It's always gotten 16-17 mpg, with ver little difference between city/hwy.

About a year ago, I began using some hypermiling techniques to improve mileage; slowing down to 60, or less, shutting the engine off at stoplights and drive throughs, coasting up to lights and stop signs, etc.

I decided to go a step further and began shutting the engine of when coasting. Not long afterwards, it developed two problems:

1) While cruising at highway speed, the converter would lock, unlock, lock, partially unlock, etc. There was no rhyme or reason to it. Sometimes it did it as long as it was in overdrive, sometimes it did it and quit, sometimes it didn't do it at all, sometimes it did it, quit, started back up, etc.

2) From the day I bought it, it has always started perfectly, within a revolution or two, every time. Now, it starts perfectly when cold, but, after it's been run for even a few minutes, it will not restart unless I floor it, and it wouldn't idle until after it had been running again for a few minutes.

I reset the pcm, which cleared the converter problem up completely. The starting problems continued, though.

At the suggestion of a friend, I checked the IAC valve. It looked pretty crappy, so I replaced it. The starting problems remained unchanged, but the idling problem after the restarts has cleared up.

I had my ace mechanic look at it; he can find nothing else wrong, and suggested I have the PCM reflashed, which only a dealer can do. For big $$$$$

Also, my gas mileage has been steadily dropping. After beginning to use hypermiling techniques, I'd been averaging 20 mpg. It's been steadily going down, my latest tank was 16 mpg, which was where I started.

Now, I just found out the other night that all of the gas available in my area is E10, which I believe could be affecting it, but a 20% drop?

Does anyone have any ideas?

Thanks

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Old 05-25-2008, 08:20 PM   #2 (permalink)
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Doesn't sound like anything I've ever heard hypermiling could do, but it might just be messed up with age and sadness, cars get that way sometimes.

How much does a pcm reflash cost? Sounds like a waste to me, just buy one, at least for hondas they're like 30 bucks,

Welcome to ecomodder! Hopefully the jeep people on here will have a little bit more help for you.
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Old 05-25-2008, 08:23 PM   #3 (permalink)
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How many miles on the Jeep? Have the O2 sensors ever been replaced? Maybe it is starting to die and it has not thrown a code yet. Welcome to the site. Lots of Jeep drivers here someone will chime in.
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Old 05-25-2008, 09:09 PM   #4 (permalink)
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Quote:
Originally Posted by Lazarus View Post
How many miles on the Jeep? Have the O2 sensors ever been replaced? Maybe it is starting to die and it has not thrown a code yet. Welcome to the site. Lots of Jeep drivers here someone will chime in.
80,000 miles.

I pulled the O2 sensor, it looked OK. It could still be bad, I guess, but, I find it odd that it started at exactly the same time as the converter locking/unlocking, right after I started cruising with the engine shut off.
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Old 05-25-2008, 09:10 PM   #5 (permalink)
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80,000 miles.

I pulled the O2 sensor, it looked OK. It could still be bad, I guess, but, I find it odd that it started at exactly the same time as the converter locking/unlocking, right after I started cruising with the engine shut off.
Maybe the transmission is slipping or fried. Does it say in the owners manual if it can be flat towed?
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Old 05-26-2008, 01:47 AM   #6 (permalink)
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Maybe the transmission is slipping or fried. Does it say in the owners manual if it can be flat towed?
No, it doesn't, but the transmission problem resolved itself immediately after I reset the pcm. The transmission has been working perfectly ever since.

The lingering problem is that it starts perfectly when it is cold, but, after even just a few minutes of running, I have to floor it to get it to restart.
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Old 05-26-2008, 02:23 AM   #7 (permalink)
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Could be your TPS. As for you FE going down, thats most likely your O2s on their way out
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Old 05-26-2008, 01:17 PM   #8 (permalink)
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Can you get a pin diagram of the ECU connector/ wiring harness most of these sensors can be checked with a Digital Volt Meter.(DVM)
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Old 05-26-2008, 02:36 PM   #9 (permalink)
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I have a similar problem with the converter.

If your vehicle is equipped with a Transmission Control Unit (or similar) that controls TC lockup, then it may be "confused".

On mine, if I engine off coast, the "Hill Logic Control" function tries to perform calculations of engine load, speed changes, and throttle input to decide on shifting (it doesn't have a way to tell that you're on a hill to hold the gear, other than performing calculations).

My theory: During EOC, the load value is 0 and the TPS is at stock minimum (9) and the speed is high. This brings the running average of the LOD and TPS values down significantly. It generally takes either time or deceleration for the TC to lock up. It also has fuzzy logic that tries to learn driving habits for shifting. That system is likely in a very "illogical" state.

To eliminate the problem, a complete engine-off without turning the key back to the "On" position should do it -- but you lose ABS (if equipped), turn signals -- and all that.

RH77
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Old 05-26-2008, 02:56 PM   #10 (permalink)
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Quote:
Originally Posted by RH77 View Post
I have a similar problem with the converter.

If your vehicle is equipped with a Transmission Control Unit (or similar) that controls TC lockup, then it may be "confused".

On mine, if I engine off coast, the "Hill Logic Control" function tries to perform calculations of engine load, speed changes, and throttle input to decide on shifting (it doesn't have a way to tell that you're on a hill to hold the gear, other than performing calculations).

My theory: During EOC, the load value is 0 and the TPS is at stock minimum (9) and the speed is high. This brings the running average of the LOD and TPS values down significantly. It generally takes either time or deceleration for the TC to lock up. It also has fuzzy logic that tries to learn driving habits for shifting. That system is likely in a very "illogical" state.

To eliminate the problem, a complete engine-off without turning the key back to the "On" position should do it -- but you lose ABS (if equipped), turn signals -- and all that.

RH77
Again, resetting the PCM eliminated the converter lockup issue. It's been months, and it hasn't resurfaced again. I just don't coast with the engine off anymore.

The remaining problem has to do with warm restarts requiring me to floor the throttle.

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