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Old 03-30-2011, 11:13 AM   #1
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bad thermostat cause rough idle?

can a bad thermostat cause rough idle?

i have changed the spark plugs/wires, rotor, distributer cap, air filter, cat/muffler/tailpipe, put a new exhaust manifold gasket on, bolted the exhaust manifold securely to engine (last bolt was broken) and it has no cracks in it, use lucas fuel cleaner, use marvel mystery oil in my oil changes, changed trans fluid, change transfer case fluid and have done the OBDI procedure and it gave me code 17 (bad thermostat).

once the weather gets warmer i wil replace the fuel filter and see if that does it.

the engine runs great, doesn't over heat, and does warm up to normal temperature. there is NO check engine light either.

i get a rough idle when i am at a dead stop whether i am in gear or neutral. Im talking about my seats vibrating, my mirrors shaking, my shifter shaking and anything in the the jeep shaking (bottle of water, etc.) its like my jeep is getting a massage from one of those seat massage/vibrating things.

it is a 1995 fuel injected 2.5L 4-cyl, 115,000 original miles on it. everything is stock, no engine mods, no suspension/tire mods. NO A/C also. NO fluid leaks of any kind.


motor mounts look ok too.


This only happens when i am in idle at a dead stop.

any ideas anyone?

i have been searching the web and can't come up with anything. Im just getting tired of this...

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Old 03-30-2011, 12:41 PM   #2
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A bad thermostat can cause your engine to overheat when stuck closed or make it take a really long time (if ever) to get up to operating temp when stuck open. I don't think it can cause a rough idle by itself.

What is your idle engine RPM? My 94 with the same engine idles a little under 1k. Does the idle RPM bounce around? If you are stopped and in neutral, does revving the engine smooth it out?

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Old 03-30-2011, 12:55 PM   #3
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Originally Posted by MetsFan978 View Post
A bad thermostat can cause your engine to overheat when stuck closed or make it take a really long time (if ever) to get up to operating temp when stuck open. I don't think it can cause a rough idle by itself.

What is your idle engine RPM? My 94 with the same engine idles a little under 1k. Does the idle RPM bounce around? If you are stopped and in neutral, does revving the engine smooth it out?
right under 1k just likes yours. nope doesnt bounce around, its smooth. yup it smooths it out, i dont get vibration when i am driving.
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Old 03-30-2011, 01:07 PM   #4
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There is an air temp sensor on the intake that affects the amount of fuel to the injectors....or you might have a vacuum leak somewhere, check all the vacuum lines, especially of they are the OEM plastic ones.
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Old 03-30-2011, 01:09 PM   #5
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Originally Posted by Garyk
There is an air temp sensor on the intake that affects the amount of fuel to the injectors....or you might have a vacuum leak somewhere, check all the vacuum lines, especially of they are the OEM plastic ones.
Ok I will check that out next. What's the best way to check vac lines?
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Old 03-30-2011, 01:37 PM   #6
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not sure if this has anything to do with it but i was just crawling around under my jeep and the 2 rubber donut washers and torque arm bolt isnt bolted to anything, it is loose and can be wobbled around.
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Old 03-30-2011, 04:11 PM   #7
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Ok I will check that out next. What's the best way to check vac lines?
Best way is visual and feel, also check the rubber connections(vacuum) at the intake too, they deteriorate.

Where is this torque arm?
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Old 03-30-2011, 05:21 PM   #8
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Dang, I had ideas if your idle RPM seemed to be off. If you are idling at the right RPM and it's steady, I would guess your sensors are reading fine and you probably don't have a vacuum leak, but I wouldn't rule it out.

An easy way to check for a leak is to use a vacuum gauge. They cost like $20 or so if you don't already have one. Pull off a vacuum line that you don't need while the Jeep is sitting there, like the break booster line, and connect it to the gauge to see what your idle vacuum is. It should be around 18 and hold steady.
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Old 03-30-2011, 07:39 PM   #9
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Best way is visual and feel, also check the rubber connections(vacuum) at the intake too, they deteriorate.

Where is this torque arm?

ok ill will see what i find.

best way to explain the torque arm/rubber bushings and bolt is for you to crawl under your jeep, find the 2 nuts/bolts that hold the trans bushing to the trans and look for a 3rd nut right next to those on the driver side, then peak your head up over the skid plate. let me know if yours is tight...
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Old 03-30-2011, 07:42 PM   #10
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Dang, I had ideas if your idle RPM seemed to be off. If you are idling at the right RPM and it's steady, I would guess your sensors are reading fine and you probably don't have a vacuum leak, but I wouldn't rule it out.

An easy way to check for a leak is to use a vacuum gauge. They cost like $20 or so if you don't already have one. Pull off a vacuum line that you don't need while the Jeep is sitting there, like the break booster line, and connect it to the gauge to see what your idle vacuum is. It should be around 18 and hold steady.


ok here is a picture of my RPM and temperature at IDLE after driving for like 20 minutes...maybe they arent normal...






i will look into the vac line check also.
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Old 03-30-2011, 08:27 PM   #11
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ok ill will see what i find.

best way to explain the torque arm/rubber bushings and bolt is for you to crawl under your jeep, find the 2 nuts/bolts that hold the trans bushing to the trans and look for a 3rd nut right next to those on the driver side, then peak your head up over the skid plate. let me know if yours is tight...
EDIT: i was browsing through my factory service manual looking for a better description of the torque arm and i found the picture below. now i also discovered that i dont have that bracket piece on the head pipe before the cat...does anybody have that on there? maybe since i dont have it on there it is making the exhaust shake more than it is suppose to be...unless im going crazy here lol


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Old 03-30-2011, 08:58 PM   #12
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i also found a DIY from another website on the trans mount to show you the torque arm/bushings...

http://www.jeepforum.com/forum/f12/h...ement-1075968/
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Old 03-30-2011, 09:14 PM   #13
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RD, I'm certainly no expert, but your temp looks quite low, as well as your RPMs. (at least compared to my '95 2.5L) At running temp, I am just over the 200 degree mark on the temp, and just below the 1000 rpm line at idle. Maybe someone else will chime in with more insight.
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Old 03-30-2011, 09:21 PM   #14
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Hey I have an 89 YJ with a newly rebuilt engine ,All parts new down to the pistons and my temperature gauge read the same as yours before and after the rebuild I don't know if that helps but thought it might
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Old 03-30-2011, 09:22 PM   #15
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Mine is also a 2.5 the 4cylinder
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Old 03-30-2011, 09:25 PM   #16
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RD, I'm certainly no expert, but your temp looks quite low, as well as your RPMs. (at least compared to my '95 2.5L) At running temp, I am just over the 200 degree mark on the temp, and just below the 1000 rpm line at idle. Maybe someone else will chime in with more insight.
yeah my temp goes just over the 200 degree as well when it is in driving mode but when it idles it drops down to where it is in the pic...

any opinions are welcome!
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Old 03-30-2011, 09:37 PM   #17
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Hey I have an 89 YJ with a newly rebuilt engine ,All parts new down to the pistons and my temperature gauge read the same as yours before and after the rebuild I don't know if that helps but thought it might
ok cool yeah that helps thanks

what does your RPMs read?
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Old 03-31-2011, 02:12 PM   #18
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If that "torque arm" is not connected it will let the trans rock, could be part of your problem.
I don't have the bracket forward of the cat and mine does not shake.
My temp goes to about 200 and stays there, it should not drop, you might have a bad thermostat.
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Old 04-02-2011, 08:49 PM   #19
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Originally Posted by MetsFan978
Dang, I had ideas if your idle RPM seemed to be off. If you are idling at the right RPM and it's steady, I would guess your sensors are reading fine and you probably don't have a vacuum leak, but I wouldn't rule it out.

An easy way to check for a leak is to use a vacuum gauge. They cost like $20 or so if you don't already have one. Pull off a vacuum line that you don't need while the Jeep is sitting there, like the break booster line, and connect it to the gauge to see what your idle vacuum is. It should be around 18 and hold steady.
So what are your ideas if my rpms are alittle off?
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Old 04-02-2011, 10:18 PM   #20
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That's where mine idles, the temp it too low though. Either your temp gauge sensor is bad or thermostat is ....
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Old 04-03-2011, 05:07 PM   #21
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I think your idle is fine. The temp shouldn't drop when you idle though. Once it's warm the temp should pretty much stay around 210 degrees whether you are moving or not. You may have been on the right track asking about the thermostat.

The idle speed for our jeeps is controlled by the PCM. Idle RPM should be about 800 and you'd need your PCM to be in closed loop mode. For that to happen, the coolant temp sensor (different than the one for the dash gauge), MAP, oxygen, and MAT sensors all need to be working right and your engine has to be at operating temp (~210 degrees)
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Old 04-03-2011, 07:12 PM   #22
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I think your idle is fine. The temp shouldn't drop when you idle though. Once it's warm the temp should pretty much stay around 210 degrees whether you are moving or not. You may have been on the right track asking about the thermostat.

The idle speed for our jeeps is controlled by the PCM. Idle RPM should be about 800 and you'd need your PCM to be in closed loop mode. For that to happen, the coolant temp sensor (different than the one for the dash gauge), MAP, oxygen, and MAT sensors all need to be working right and your engine has to be at operating temp (~210 degrees)
x-2 If the engine temp drops that much the coolant temp sensor will be sending the wrong signal to the PCM
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Old 04-04-2011, 08:30 AM   #23
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Is replacing the thermastat a fairly easy job? What thermostat is the best replacement?
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Old 04-06-2011, 03:47 PM   #24
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Just a few bolts and gasket. I run a 195...make sure the one you put in has a bleed hole or you'll end up with air in it...pellet side is down..
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Old 04-06-2011, 04:33 PM   #25
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Just a few bolts and gasket. I run a 195...make sure the one you put in has a bleed hole or you'll end up with air in it...pellet side is down..
is this one cool?

Original Replacement Parts 83501426 - 195 Degree Thermostat for 72-95 Jeep® CJ Series, Wrangler YJ, 84-01 Cherokee XJ with 2.5L Engine & 87-01 XJ with 4.0L Engine - Quadratec
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Old 05-04-2011, 08:52 AM   #26
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Bump
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Old 05-07-2011, 02:07 PM   #27
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Yeah, that looks OK, why not just go to local parts store, by the time you pay shipping it would be a lot easier, don't forget a gasket, make sure surfaces are clean..
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Old 05-08-2011, 12:35 PM   #28
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ok i changed out the thermstat this morning.... wasnt hard at all, took less than a hour. didnt disconnect any hoses, just lifted the housing up and rested on valve cover. i didnt drained the radiator, i placed 2 buckets under the jeep to catch the coolant when it comes out after i disconnect the housing. popped the old thermo out and stuck the new one in with a new gasket of course. put the housing back on. ran the engine until it started to get hot then shut it off and listened to it sucking the coolant in. waited 15 minutes started it up again, kept adding coolant alittle at a time until it was full again. now my thermostat guage is reading correctly.

thanks everyone!

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