Shudder When Turning Xj Off

[jeepsrock]

Recently something has started to happen as i turn off the jeep. After i arrive at whatever my destination is and i turn off the ignition (turn and remove key) the engine doesnt just die like it normally used to but rather it shudders and vibes a few times after i take the key out (for a brief sec).

It never did this before was wondering what could be responsible for this?

Another thing that might be related is at cold starts when i barely touch the accelerator the engine bogs down if i press it really hard the engine revs like normal but the rpm come down low enough to cause the engine to vibe a bit then it goes back up. After it warms up this thing goes away.

pete


94XJ 4.0 Aw4 /231

[jeepsrock]

Any ideas ?

pete

[splitz]

Try a higher octane gas for a while. That will probably stop your shudder.

[RichP]

Clean the throttle body and decarbon the engine, then look at your plugs.

[jeepinandy]

Check all of your vacume lines, make sure all are connected and no leaks.

[old_man]

A shudder on shut down means you are still getting the cylinders firing. This is usually caused by glowing hot carbon deposits in the combustion chamber. Run a can of BG44K in the gas and all should be fine.

Carbon buildup is normally caused by running too rich. After a tank of gas with the BG44K, Do a plug reading. Most likely your O2 sensor is off. I bet your mileage has been going down, right?

[splitz]

If you don't want to spend the money on BG44k use SeaFoam.

[TheWarWagon]

If you don't want to spend the money on BG44k use SeaFoam.

Doesn't SeaFoam cause the engine to run like hell though?

[splitz]

I've never had a problem with it. Ran it in my 94 Yota Pickup and my 95 NSX. I have heard a few horror stories, but that is from people who didn't follow the directions!

Note: I did the pickup first and drove it around a few days before trying it on the NSX.

[jeepsrock]

Actually i just did a APN manifold install this past weekend and before that about 2-3 months ago i had replaced many sensors (TPS, IAC, MAP, Bosch O2) . Before i did the sensor installs i ran bg44k and 2 treatments of seafoam thru the brake booster line.

Last month i replaced the plugs, and wires (mopar per wires and champion plugs).

Should i get some seafoam and run it thru the gas tank this time ?

Will connecting a vac gauge help ?

pete

[ChuckD]

Actually i just did a APN manifold install this past weekend and before that about 2-3 months ago i had replaced many sensors (TPS, IAC, MAP, Bosch O2) . Before i did the sensor installs i ran bg44k and 2 treatments of seafoam thru the brake booster line.

Last month i replaced the plugs, and wires (mopar per wires and champion plugs).

Should i get some seafoam and run it thru the gas tank this time ?

Will connecting a vac gauge help ?

pete


Get the Seafoam Deep Creep and spray it down each cylinder and let it sit awhile. Also if your up to it, remove your injectors and do the same.

[HeavyMetal]

is this all the same story with a shudder on start up as well? ive been experiencing that myself as of late

[jeepsrock]

Well actually the injectors are less than 5 months old, and when i removed the intake to install the exhasut manifold i cleaned the intake manifold which was pretty clean already.

And yes my mileage has been pretty horrible since this problem has started.

I really think this may be a vac pressure or fuel presure problem. Tho i should take care of the carbon in the cyclinders.

any other ideas ?

pete

[old_man]

vacuum or fuel pressure can not cause run-on when you shut off the key. Somehow you are getting ignition of the left over fuel in the manifolds. Since the computer removes the spark when the key is turned off, the only thing that can cause run-on is a glowing piece of carbon.

I overhaulled my first engine over 40 years ago and have been working with engines in all sizes and varieties ever since.

Now, a vacuum leak could cause the MAP to read incorrectly and cause the engine to run too rich, causing the carbon buildup, but running a can of BG44K will remedy the run on problem. Once that is done, do a vacuum check and test the O2 sensor. As for Seafoam, I have little experience but I have well over 20 years experience with BG44K and it is a miracle worker.

[jeepsrock]

Ok here is an update.

I went to the dealer and picked up a can of the MOPAR Combustion Chamber Cleaner, since i have already tried the other things suggested.
I followed the directions and basically sprayed the whole can in the intake with the engine idling and let it stand for a few hours. It smoked a bit upon start up and then i i took it on the freeway.

I connected a vac guage on the intake manifold and the vac isnt steady like i think it should be , it moves from 16-17, that was before the cleaner- i will test it again when i get a chance.

When i shut down the jeep, it still does it studder thing for a sec but it might have improved a small bit since the treatment.

Something that may have contributed to this is my jeep apparently hasnt been warming up bc of a bad thermostat for the past few weeks.I have fixed that and replaced the thermostat with a stant 195F unit.

Any ideas or comments ?
Pete

[RichP]

You could have a very heavy carbon build up in there, thats the only thing that will cause run on or dieseling. The carbon gets red hot and ignites any leftover fuel.

[old_man]

So answer yes or no....did you run a can of BG44K?

[jeepsrock]

Well to fully answer that question i can say that i did run a can of BG44k stuff about 2 months ago before my smog check which i amazingly passed. Since i ran previously seafoam and the BG44k stuff i decided to use the mopar stuff this time around, it smelled the worst and most toxic..lol

pete