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-   -   question for master mechanics about idle? (http://www.wranglerforum.com/f210/question-for-master-mechanics-about-idle-42753.html)

autopc25 01-14-2010 09:54 AM

question for master mechanics about idle?
 
I have a 97 2.5L with 130K miles, auto trans and A/C.

The car does not throw any fault codes when scanned with a Snap On scanner?

I had a broken exhaust bolt that caused a intake vacuum leak. A Jeep mechanic replaced the broken bolt, intake, exhaust gaskets. There are no more vacuum leaks. He also did a full tune-up and replaced everything, wires, plugs, rotor, cap, air filter, changed engine oil and filter, all trans fluid and filter, all gear and diff oils. He cleaned the throttle body and replaced the idle air control motor (the idle is a lot smoother now but the rough idle and hesitation is still there. The idle also dropped from about 850 rpm to about 750 rpm after the idle control motor change). He also replaced the thermostat and radiator cap, flush and filled the radiator ( the car is between 200-210 degrees at full operating temp). I also ran some Chevron fuel injector cleaner in the gas tank.

The car always starts on one crank on cold days (0-25 degrees) and on warm days. The car will idle smooth at 1000-1100 rpm for 1-2 minutes then the rpm slowly drop to about 850 rpm and idles smooth. The idle gets rough after 1-2 minutes of idling smooth at 850 rpm ( the idle is rough like it is missing and the rpm jump up and down between 600-1100 rpm). The idle will return to normal and the car will drive fine if I rev the gas for a few seconds. The car will drive fine and normal all day until I turn off the engine. If I turn off the engine and re-start the engine after 5-10 minutes the rough idle will return (after a minute of two of idling smooth) until I rev the gas for a few seconds again ( then the car runs fine until I turn off the engine). The car will not have these symptoms if I turn off the engine and re-start the car right away or after a few minutes (the problem returns if the car is off for 5-10 minutes). The car will buck and hesitate if I drive the car right away (1 minute after starting with no warm up). The car will buck, hesitate and sound like there is slight backfiring when trying to accelerate until I push the gas pedal up and down a few times (the car runs fine after I push the gas pedal up and down a few times).

The bucking, hesitation is a lot better after the tune up and intake leak fix but is still there?

Other people seem to have the same problem? What fixed your problem, fuel pump? I was told it also could be a bad fuel injector that is leaking or clogged? Others have suggested it might be a bad throttle position sensor, MAP sensor, O2 sensor, coolant temp sensor, A/C request signal? The car does not throw any codes and I do not want to wait until it does. I don't want to get stranded on the side of the road but I also don't want to replace parts by guessing?


Anyone?
Thanks

MouthfulOfGrass 01-14-2010 02:50 PM

I had a problem that sounds a lot like yours.

I had the injectors cleaned/serviced. And an O2 sensor replaced.

Problem went away.

With the year model, and the amount of mileage, if you haven't had an injector service that you know of, it could be time.

koz555 01-14-2010 03:42 PM

good question, i have a 98 sahara that is having the same issues, and i dont want to replace parts that i dont need to either, im not throwing any codes..anymore input

rrich 01-14-2010 04:58 PM

By your symptoms - Sounds like it still has a vacuum leak. Since the broken bolt was bad enough to make it run bad, even though it's been replaced the leak is still there. Dirt gets in the hole, when you tighten the new bolt down it just squeezes the dirt, the leak is still there. Sometimes you can get lucky, but it sounds like you didn't.

But first - don't panic - make sure that's it before condemning it.

Get it warmed up to where it's idling bad. Use a BURNABLE carb cleaner - most don't burn like gas. Gummout works. To test a carb cleaner if it's going to work for this test, dribble a few drops down the throat. If the engine speeds up, use it. If it slows down it won't work. Neither will water.

Spray around that bolt - see if the idle changes. If it does, even a little, it's leaking.

Or better yet - use propane. Remove the tip of a propane torch, slip a hose over the tube. Blow propane around the bolt (unlit of course.) The propane gas will get underneath where the liquid carb cleaner can't. If the idle changes, it's leaking.

The entire intake and exhaust manifolds have to be removed, cleaned, and the gasket replaced. Sorry, there is no simpler way.

If it does not make a change - use the propane again, spray around the throttle body base and vacuum ports, again, looking for an RPM change.

Propane burns just like gasoline, but it's far safer than any liquid - it doesn't make standing puddles. Plus it can get under things where a liquid will run off. And, often a liquid will wash dirt in the leak, sealing it temporarily for a few moments, making it really hard to find.

Gonda 01-14-2010 08:28 PM

I am wondering if you have a sticky lifter that is not allowing a valve to open. Sounds like it leaks down after 10 minutes and then takes a while to get pumped back up. Reving the engine increases the oil pressure and pumps it up faster.

autopc25 01-14-2010 09:15 PM

Quote:

Originally Posted by MouthfulOfGrass (Post 523667)
I had a problem that sounds a lot like yours.

I had the injectors cleaned/serviced. And an O2 sensor replaced.

Problem went away.

With the year model, and the amount of mileage, if you haven't had an injector service that you know of, it could be time.

I have a feeling you might be right? Did your mpg go up after the fix?

Did you service the injectors first or replace the 02 sensor first? Did you replace all the 02 sensors of just the one upstream in the exhaust manifold?

The Snap On scanner did not show any fault codes and the 02 sensors seem to be working, the mechanic said the sensors were bouncing back and forth and making adjustments and corrections. He also said it still could be the front 02 sensor even though they looked fine on the scan. He based that on experience and history from working on cars. A fault code would be a lot easier!

I am taking it to the mechanic again tomorrow for a fuel pressure check and he is going to check the throttle position sensor with a oscilloscope. He is also going to test the MAP sensor and injectors also. He doubts this but he is also going to triple check for a vacuum leak. One way or another I will know tomorrow.

Thanks

autopc25 01-14-2010 09:18 PM

Quote:

Originally Posted by rrich (Post 523721)
By your symptoms - Sounds like it still has a vacuum leak. Since the broken bolt was bad enough to make it run bad, even though it's been replaced the leak is still there. Dirt gets in the hole, when you tighten the new bolt down it just squeezes the dirt, the leak is still there. Sometimes you can get lucky, but it sounds like you didn't.

But first - don't panic - make sure that's it before condemning it.

Get it warmed up to where it's idling bad. Use a BURNABLE carb cleaner - most don't burn like gas. Gummout works. To test a carb cleaner if it's going to work for this test, dribble a few drops down the throat. If the engine speeds up, use it. If it slows down it won't work. Neither will water.

Spray around that bolt - see if the idle changes. If it does, even a little, it's leaking.

Or better yet - use propane. Remove the tip of a propane torch, slip a hose over the tube. Blow propane around the bolt (unlit of course.) The propane gas will get underneath where the liquid carb cleaner can't. If the idle changes, it's leaking.

The entire intake and exhaust manifolds have to be removed, cleaned, and the gasket replaced. Sorry, there is no simpler way.

If it does not make a change - use the propane again, spray around the throttle body base and vacuum ports, again, looking for an RPM change.

Propane burns just like gasoline, but it's far safer than any liquid - it doesn't make standing puddles. Plus it can get under things where a liquid will run off. And, often a liquid will wash dirt in the leak, sealing it temporarily for a few moments, making it really hard to find.


How much work is it to remove and replace the intake and exhaust manifolds again? How many hours?

rrich 01-14-2010 09:49 PM

Again? I thought you said they just replaced the bolt.

If they R&R'd it, it should still be under their warranty.

Or look at your bill.

MouthfulOfGrass 01-15-2010 11:45 AM

Quote:

Originally Posted by autopc25 (Post 523840)
I have a feeling you might be right? Did your mpg go up after the fix?

Did you service the injectors first or replace the 02 sensor first? Did you replace all the 02 sensors of just the one upstream in the exhaust manifold?

The Snap On scanner did not show any fault codes and the 02 sensors seem to be working, the mechanic said the sensors were bouncing back and forth and making adjustments and corrections. He also said it still could be the front 02 sensor even though they looked fine on the scan. He based that on experience and history from working on cars. A fault code would be a lot easier!

I am taking it to the mechanic again tomorrow for a fuel pressure check and he is going to check the throttle position sensor with a oscilloscope. He is also going to test the MAP sensor and injectors also. He doubts this but he is also going to triple check for a vacuum leak. One way or another I will know tomorrow.

Thanks

My MPG has never fluctuated much, no matter what service/repair has been done - not enough for me to get excited about anyway. The only thing that has changed my MPG was to switch from 33's, to stock tires, for long driving trips with no off-roading scheduled during the trip (a difference of 2-4 mpg).

I do not remember which O2 sensor was replaced. It was when I first got the Jeep. I also had taken it in to the dealer for the recall on the exhaust tube-cat. converter replacement (everything in between the exhaust manifold & the muffler). It was 3 or 4 years ago.

I just remember that I had that same type of idle issue - sounds the same anyways. And the fix was injector cleaning & I'm pretty sure O2 sensor also.

autopc25 01-18-2010 10:00 AM

Quote:

Originally Posted by MouthfulOfGrass (Post 523667)
I had a problem that sounds a lot like yours.

I had the injectors cleaned/serviced. And an O2 sensor replaced.

Problem went away.

With the year model, and the amount of mileage, if you haven't had an injector service that you know of, it could be time.

Bingo!

My mechanic checked all the usual suspects like throttle position sensor, MAP sensor, vacuum, fuel pressure, etc, everything checked out OK. The last 2 possibilities were fuel injectors and upstream O2 sensor. He thought the O2 sensor would be the problem because of warm up time? He replaced the upstream O2 sensor and it fixed the problem right away. It's funny the bad 02 sensor did not throw any codes and a scan with his Snap On scanner showed the front and back O2 sensors to be working?

I think the broken exhaust bolts contribute to a intake or vacuum leak which causes the car to run super rich. The O2 sensor gets fouled up from running too rich. Probably a common problem if the manifold bolts and studs are prone to snap off over time?

My MPG went up after replacing the bad O2 sensor.

I hope this helps anyone else with this problem.

Thanks!

mrcarcrazy 01-18-2010 10:24 AM

O2 sensors have a limited lifespan anyway...If they haven't been replaced in 100k miles it should be done as a "maintenance item".

mruff5 01-19-2010 05:52 PM

Quote:

Originally Posted by autopc25 (Post 525705)
Bingo!

My mechanic checked all the usual suspects like throttle position sensor, MAP sensor, vacuum, fuel pressure, etc, everything checked out OK. The last 2 possibilities were fuel injectors and upstream O2 sensor. He thought the O2 sensor would be the problem because of warm up time? He replaced the upstream O2 sensor and it fixed the problem right away. It's funny the bad 02 sensor did not throw any codes and a scan with his Snap On scanner showed the front and back O2 sensors to be working?

I think the broken exhaust bolts contribute to a intake or vacuum leak which causes the car to run super rich. The O2 sensor gets fouled up from running too rich. Probably a common problem if the manifold bolts and studs are prone to snap off over time?

My MPG went up after replacing the bad O2 sensor.

I hope this helps anyone else with this problem.

Thanks!

I have been having the same problem as you were, cold it bucks on take off, idles rough until warmed up. Which o2 sensor is the upstream sensor. Also would your Jeeps RPM's fluctuate when you disengaged the clutch?.....


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