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Unread 03-17-2011, 10:49 PM   #1
Tonks
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1997 ZJ 
 
Join Date: Nov 2010
Location: Newburgh, IN
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5.2 keeps pulling when it should be slowing down

I have used the search feature to the best of my ability with no luck.

97 GC 5.2 Auto 298K miles

When driving down hwy at any speed and I let off the throttle the engine RPM increase slightly and the jeep continues to make power and pulls the jeep down the road with little to no deceleration experienced. When this happens I can put the trans in netrual and the engine will rev up to 2500 rpm and will stay at that RPM until the jeep slows to around 20 MPH and then the idle will drop back to normal idle.

I am also getting P301, P303, and P300 codes that all indicate a misfire condition that I cannot rectify even after new Plugs, cap rotor, wires, Camshaft position sensor, Crank Shaft position sensor.

I sure hope someone is willing and able to assist me with possible causes for this type of drive ability symptom.

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Unread 03-17-2011, 10:56 PM   #2
bigbossd1030
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Seems like your throttle cable may be sticking, Maybe a new one may be in order.
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Unread 03-17-2011, 11:04 PM   #3
Tonks
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I was wishing it was that easy. OBD2 scan tool shows that the TPS sensor drops back to idle setting when I let off the gas. Also there is something with the speed of around 20 MPH it is like the computer commands something to allow it to drop back to idle. It really does not matter how i get it back down to 20 as I can coast, Jam on the breaks or stop normally and it will drop back to idle ever time.

The jeep was bought to be fun and a puzzle and I have had a great time with it so far but this problem has me stumped.
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Unread 03-17-2011, 11:09 PM   #4
bigbossd1030
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Faulty sensor? Im not sure on this one, Im stumped to.
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Unread 03-18-2011, 03:30 PM   #5
Tonks
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It is an interesting problem for sure....
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Unread 03-18-2011, 04:18 PM   #6
jnicewan
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Have you eliminated the possibilty of a sticking IAC? Maybe clean the throttle body and IAC. Does it idle fine, fast idle when cold, increase idle with A/C etc..? Check for possible vacuum leaks?
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Unread 03-18-2011, 04:47 PM   #7
Dyn0mitemat
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Vacuum problem it sounds like to me
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Unread 03-19-2011, 08:16 AM   #8
Tonks
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Thank you very much for the help because this problem has me by the tail as far a knowing enough about the interrelation of the different systems that could possibly cause this problem

Cleaned original IAC and no change New Idle air control valve and cleaning of the control valve port inspecting it with a mirror to ensure clean with no change.

Stable idle and changes with AC on off.

Changed from E3 spark plugs back to Autolite copper core. Spark plugs indicated a lean condition

New MAP sensor last night with a short test drive and the symptom has been reduced but not eliminated.

Would anyone know if it is normal for the RPM to increase in an automatic transmission when placed into netural at hwy speeds 50-70 mph? It is not something I am sure any of us normaly do and I might be chasing my tail.

Thanks
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Unread 03-19-2011, 09:14 AM   #9
bigbossd1030
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Quote:
Originally Posted by Tonks View Post
Thank you very much for the help because this problem has me by the tail as far a knowing enough about the interrelation of the different systems that could possibly cause this problem

Cleaned original IAC and no change New Idle air control valve and cleaning of the control valve port inspecting it with a mirror to ensure clean with no change.

Stable idle and changes with AC on off.

Changed from E3 spark plugs back to Autolite copper core. Spark plugs indicated a lean condition

New MAP sensor last night with a short test drive and the symptom has been reduced but not eliminated.

Would anyone know if it is normal for the RPM to increase in an automatic transmission when placed into netural at hwy speeds 50-70 mph? It is not something I am sure any of us normaly do and I might be chasing my tail.

Thanks
No it shouldn't, you are basically cutting the Tranny away from the engine when you do that, so the rpms, should settle down to idle. The only way they will go up is if your foot is on the gas pedal a little bit.
Your rpms will go up a little bit when slowing down as the tranny shifts itself to lower gears, but they shouldn't stay at a set rpm, they should go down, then up a little, down and up a little and down, etc.

What are your rpms at idle?

You said you are getting missfires.... try doing a compression test.
What does your exhaust smell like and look like? is it white, clear (well not completely, but not that noticable) or dark?

Its a long shot, ut it could be a missfire and when you let off the gas, those cylinders decide to go, making you move more rather than slowing down.

If your exhaust is running dark, then its oil rich if its white then its gas rich. If its white then I can see this happening, but if its dark (really dark) then not that likely.

Timming?
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Unread 03-19-2011, 11:33 AM   #10
jnicewan
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What about vaccum leaks? Do you have a scanner to look at "live" data. Maybe check the fuel trim and some other stuff
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Unread 03-19-2011, 06:57 PM   #11
Tonks
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Bigbossd idle is around 730 in the driveway according to the tach and scan tool. I will have to look at the exhaust and take a sniff. that is something I have not thought of.. The plugs were chalk white and burnt up pretty good for only 2000 miles so I am thinking it is lean.

This is a 1997 and I do not have EGR and I dont think that I can adjust the timing. I had been told that moving the distributer only changes when the injectors fire.

jnicewan I have a scan tool that will read live data. Do you have any suggestions as to what I need to look at specifically or would you have any good numbers? I have the Actron CP9145 and I have seen the fuel trims but I am not sure what I am looking at or what is right.

Thanks again for the help.
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Unread 03-22-2011, 02:43 PM   #12
Tonks
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update:
New Throttle Position sensor.
and cleared the computer maps by removing the positive batt lead and grounding, turning on key then headlights and then turning off everything. Seemed to be better unitll code P0301 and then it started reving up again.

Any idea what the idle RPM should be?
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Unread 03-22-2011, 03:05 PM   #13
csouers
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When warm, about 700 rpm. That's where my 5.9 sits. My red 5.2 idles at 600 when warm. It's a little on the low side, but never had any problems.

It'll take a little bit of time for all the settings to even back out. I would let it warm up, go for a drive, and then see what the idle is.

The whole headlights, key, thing doesn't really apply to us. There should be no residual energy in the system after the battery has been disconnected for a while. I've only heard of that trick doing anything on a TJ/LJ and that actually resets a lot of emissions related things that we don't have.
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Unread 03-22-2011, 03:11 PM   #14
98mopar
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Can you feel the misfire? When you accelerate hard or anything? Or when you are parked does your exhaust sound a little more lobey? Your PCM is reading a misfire code, so I would say your high revving is do to that. Have you checked your injectors?

EDIT:
Have you used any fuel treatment stuff? Sometimes that stuff will cause problems.

Last edited by 98mopar; 03-22-2011 at 03:13 PM.. Reason: add
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Unread 03-22-2011, 09:07 PM   #15
ZombieJeep93
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sorry, another question.
do you experience any spark knock or pinging when you apply light to moderate throttle when cruiseing or climbing a grade?
also, are you going through any oil?
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