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Unread 09-13-2009, 01:23 PM   #1
proffessor
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RPM matches MPH while coasting

97 Grand Cherokee LTD / 318 1/st Jeep

Problem: The engine RPM matches the MPH when in OD or even in neutral when coasting. Tested on hill in neutral and 65mph =2,000 rpm.

I have disconnected battery and had it idle down the first time I tested it , then it quickly goes back to matching the mph. Tested #3 times; twice with battery and once by clearing with code machine (no codes)
In the test with the code machine it never idled down even on the first try.

Seems to drop to idle ramdomly below 30mph.

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Unread 09-13-2009, 01:26 PM   #2
SILASPRIT
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might try replacing your IAC (idle air control) or your speed density air sensor.
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Unread 09-14-2009, 09:09 AM   #3
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Hey Silas; thanks for the answer. Like I said tho, 1st jeep ... where are they & what do they look like?
are they cleanable? **** could you point me at an IPB?***
I am on a fixed income and cannot troubleshoot by replacing parts.
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Unread 09-14-2009, 09:16 AM   #4
ezflip
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Sounds like its just not down shifting when it should. Check your Transmission fluid levels (on level surface, engine running and to normal operating temp, in neutral.) Could be a speed sensor, or governor pressure sensor in the transmission. If the fluid looks ok (level, color, and smell) I would take it up to a tranny shop.
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Unread 09-14-2009, 04:41 PM   #5
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Quote:
Originally Posted by proffessor View Post
97 Grand Cherokee LTD / 318 1/st Jeep

Problem: The engine RPM matches the MPH when in OD or even in neutral when coasting. Tested on hill in neutral and 65mph =2,000 rpm.

I have disconnected battery and had it idle down the first time I tested it , then it quickly goes back to matching the mph. Tested #3 times; twice with battery and once by clearing with code machine (no codes)
In the test with the code machine it never idled down even on the first try.

Seems to drop to idle ramdomly below 30mph.
it's normal, the computer doesn't know it's not in gear and is matching idle input to what would be needed with a load on the engine at that speed.

also, since your torque converter is locked up in overdrive at those speeds, the engine rpms will NOT drop with letting off the pedal, they will drop with vehicle speed untill the torque converter unlocks and allows variation between the motor and the transmission.
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Unread 09-14-2009, 04:53 PM   #6
MurdaJs
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Quote:
Originally Posted by ratmonkey View Post
it's normal, the computer doesn't know it's not in gear and is matching idle input to what would be needed with a load on the engine at that speed.

also, since your torque converter is locked up in overdrive at those speeds, the engine rpms will NOT drop with letting off the pedal, they will drop with vehicle speed untill the torque converter unlocks and allows variation between the motor and the transmission.
My jeep doesnt do that when in neutral going down a hill.
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Unread 09-15-2009, 06:02 PM   #7
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Murda... you're right! thats' the beauty of a Mopar type trans ! when you let off the gas; engine rpms drop to nothing & you save fuel. My daughters' '97 drops to idle and millage on the VIC goes to 99 if you are above 50 mph. She has the 6 cylinder tho. SO, if the V-8's are not a totally different acting system I still have not gotten a specific solution.

I have been studying and found that the throttle position sensor on the V-8s' is a dual sender and signals both the engine and transmission. Has anyone out there had the same problem and found the TPS to be the cure?????
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Unread 09-25-2009, 04:40 PM   #8
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Update: I cleaned the IAC and the RPM's dropped correctly the first 9 mile trip over & down the mountain. When I started to pick up speed the trans even kicked in with a slight hold-back and just a little increase in RPM. Perfect! On the way back same old ****; RPM matched MPH. Don't know if it was the cleaning or disconnecting the battery that did it.

Does anyone know what Silas means by "speed density air sensor"? It's not on my Electronic engine control component locations chart.
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Unread 09-25-2009, 10:25 PM   #9
ratmonkey
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Map sensor. Manifold absolute pressure. We don't have speed density/mass airflow units.
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