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Unread 07-18-2009, 08:44 AM   #1
Muddeprived
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short rough idle after start up

Help me out guys so I know where to start lookin.

When I start up the jeep for the first time in the morning it would start up just fine but rumble and shake back and forth for about 6-8 seconds then it idles out fine. The rpms are normal during this time. It's just a rough rocking-type shake. It only happens right after you start it up, almost like there's no oil and it's trying to run dry and something like a motor mount broke and the motor shaking loose. hard to explain but has anyone had this issue? Over the past couple of months I've noticed the idle at a stop sign or stop light has a very slight rumble every couple seconds too. It might be related. It only does this during the very first start after sitting for 1/2 day or so.


Thanks

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Unread 07-18-2009, 09:28 AM   #2
CJ7-Tim
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Quote:
Originally Posted by Muddeprived View Post
...(1) When I start up the jeep for the first time in the morning it would start up just fine but rumble and shake back and forth for about 6-8 seconds then it idles out fine. ..... (2) I've noticed the idle at a stop sign or stop light has a very slight rumble every couple seconds too.
1) Your fuel pressure check valve is probably starting to fail. It is a common issue, not a problem, just an annoyance. SEARCH, it has been covered.

2) Clean the Idle Air Controller (IAC).
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Unread 07-18-2009, 04:37 PM   #3
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Quote:
Originally Posted by Muddeprived View Post
Help me out guys so I know where to start lookin.

When I start up the jeep for the first time in the morning it would start up just fine but rumble and shake back and forth for about 6-8 seconds then it idles out fine. The rpms are normal during this time. It's just a rough rocking-type shake. It only happens right after you start it up, almost like there's no oil and it's trying to run dry and something like a motor mount broke and the motor shaking loose. hard to explain but has anyone had this issue? Over the past couple of months I've noticed the idle at a stop sign or stop light has a very slight rumble every couple seconds too. It might be related. It only does this during the very first start after sitting for 1/2 day or so.


Thanks
Mine does this too or at least used to...

I just got through running a tank of gas with a can of SeaFoam in it (don't hate) It idles a little better now...shaking isn't intolerable at least.

I thought that it was dirty injectors so that's why I decided to do the SF. FWIW, it used to be real bad...I cleaned TB and IAC first which helped some, but didn't completely cure it then i went with SF.
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Unread 07-18-2009, 11:02 PM   #4
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Ok I'm gonna try seafoam. I remember using seafoam last year when I first bought it. I didn't have a cloudy afternoon coming from my tailpipe but it ran much better afterwards. I'll try that again.
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Unread 07-19-2009, 06:50 AM   #5
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Originally Posted by CJ7-Tim View Post
1) Your fuel pressure check valve is probably starting to fail. It is a common issue, not a problem, just an annoyance. SEARCH, it has been covered.

2) Clean the Idle Air Controller (IAC).
Here's my IAC:





What's a good way to clean that tip? Do you think it's bad enough to cause my idle issue?
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Unread 07-19-2009, 06:59 AM   #6
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Spray it liberally with throttlebody cleaner, then gently wipe the carbon off. Be sure to clean the hole it goes into well also.
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Unread 07-19-2009, 08:41 AM   #7
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Quote:
Originally Posted by CJ7-Tim View Post
1) Your fuel pressure check valve is probably starting to fail. It is a common issue, not a problem, just an annoyance. SEARCH, it has been covered.

2) Clean the Idle Air Controller (IAC).
I saw you mentioned how to check the Fuel pressure check valve in another thread that I found via searching. You mentioned to release the valve when the engine is warm and then do it again in the morning. When I do it in the morning, do i drive it around and warm it up first or just squirt it while cold?
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Unread 07-19-2009, 09:50 AM   #8
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Originally Posted by Muddeprived View Post
release the valve when the engine is warm and then do it again in the morning.
What you get, is a comparison of the fuel rail pressure when the engine has recently run (approx 50 psi) to the fuel rail pressure after having sat overnight. Perform the second test with a cold engine that has not run for 8-10 hours. The results of the test are subjective and will vary based on the condition of your check valve. Most likely you will find little, to no fuel pressure in the morning, confirming that the check valve is a partial or potential source of your rough running.

My check valve failed several years ago. The fuel pump will continue to run normally for many more years and miles. I am too cheap to buy a new fuel pump and too lazy to drop the skid plate and tow hitch to perform a non-essential repair.
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Unread 07-20-2009, 05:30 AM   #9
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I cleaned the IAC and no change.
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Unread 07-20-2009, 09:52 AM   #10
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You may have a leaky fuel injector(s). If an injector starts to leak while sitting, there is excess raw fuel in the cylinder(s). When the engine starts, it has to burn out that excess fuel which results in a rough idle/stumble for a few seconds.

A failing injector could also create the idle problem.

What is the year/engine/mileage on original injectors? Always helpful to have this information and will result in more accurate replies.
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Unread 08-12-2009, 10:56 AM   #11
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Quote:
Originally Posted by Muddeprived View Post
Here's my IAC:





What's a good way to clean that tip? Do you think it's bad enough to cause my idle issue?
Hey! This is a family forum!

By the way, I also have a hardstart problem. Fuel is good. Spark is good. Starts with starter fluid. I think it may be my IAC also.

-Bruce
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Unread 08-12-2009, 11:09 AM   #12
_litz
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One symptom of a bad IAC is you start the engine, and it immediately dies (e.g., not a no-spark condition).

You can idle it manually with the gas pedal, though .... but soon as you release the pedal, it dies.

Jeep's drivable like this, but it's a PITA to have to shift to neutral at every red light, and idle the engine ...

- _litz (who's gotten home before doing just exactly that ...)
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Unread 08-12-2009, 12:29 PM   #13
brucebo
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1989 Renix XJ.

Most of the time it starts fine. My problem is an intermittent/occasional hard start, and even less occasional sudden death konk (no sputter, just the "lights" suddenly go out) when the throttle is closed (ie, when slow coasting or idling). It's never died with an open throttle.

During the hard start condition.

1. Runs fine after start.

2. The engine cranks vigorously. No ignition.

3. I measured both the fuel pressure and performed a 30 minute leak down test. Both tested out perfectly.

4. Sparks fine. Plugs, wires, distributor all new.

5. Starts with starter fluid. Then runs fine. (Need to further check to see if throttle is open or not when it starts here (am I pushing the gas pedal or not).

-Bruce

Last edited by brucebo; 08-12-2009 at 02:09 PM..
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Unread 08-12-2009, 04:42 PM   #14
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rough idle after start up is an air pocket in the fuel line, typically caused by a failing check valve in the fuel pump body assembly in the tank, or a leaking fuel injector that lets pressure bleed off.

two things to check:

replace old fuel filter if possible, that is essential on 96' and older jeeps, and easily done. an aftermarket vw check valve can be purchased and installed in the fuel line near the filter if it is truly a problem.

For leaking injectors....disconnect injector rail quick connects, unplug main spark coil wire, and crank motor over. (this will prevent fuel/spark from each cylinder, and cranking will pull in leaky fuel that's sitting on top of intake valve during the crank. you don't want spark or more injecting fuel during this test steps)

next, pull all plugs and see which one is wet with fuel. this is your leaking injector. ford 5.0 mustang injectors are a direct, inexpensive and proven drop in replacement. Sometimes a bottle or two of techron with a full tank of gas/driven until empty will clean leaky injectors and sticky check valves.

on 99 and later models, rough idle after hot soak/start is the result of #3 fuel injector overheating and vaporizing fuel, and can be fixed with a $10 dealer part which is a heat wrap around #3 injector. earlier models with hot start issue like this can also be tried but is a item with the 99 manifold and heat rising from motor and migrating to #3 injector.
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