Engine Bog on Cold Start

[tylerforbes]

Guys,
I have been working this for the last couple days and it's really starting to bug me. Let me give you a few details then the problem. 99xj with 58k. New plugs, wires, cap and rotor less than 4k ago in nov.

The problem:
About three weeks ago I notice that starting was going from chug chug vroom to chug chug chug chug vroom. A difference of less than one second to about 1.5 to 2 seconds. Once the engine starts it revs to ~1200 rpm then drops to ~700 rpm and stays there no problems. On warm days and if the jeep has been started recently the jeep runs fine, no issues, no nothing.

However if it is cold out and I think I have narrowed it down to cold rainy days the engine starts, still 1.5 to 2 secs then idles fine. But if you touch the pedal within the first 30 to 60 seconds the engine bogs and almost stalls. If its in drive you really have to give it pedal to get it to move. After about ten seconds of bogging it runs fine. No problems.

If you shut it off and turn it right back on no problems. It's been really hard to test based on the few cold rainy mornings. If its warm out or the engine has run in the last 6 or so hours there is no problem. Im kinda stumped.

Here is what Ive done to try to solve it. At first before the bogging I thought I had a bad battery. A couple good nights on a charger and that puppy was supplying some serious amps and in good shape. So after determining it wasnt the battery the bogging started to happen. Thought it might be a dirty throttle body. Cleaned the butterfly and the iac chamber with about 100 qtips and replaced the iac after it felt pretty sticky and gunked up.

Still boggin on cold starts. So I started thinking if one of the temp sensors wasnt reading the correct temp at start up it might bog. I tested the coolant temp sensor and manifold temp sensor against published values and they were both good from below 32 degrees to about 180. Next I thought o2 sensors but was informed that they dont start reading until they are warm.

I picked up a obd2 scanner on the way home from work. No codes what so ever. Basicly I got nothing now. I have a few ideas but woudl like to hear what you guys think before swaying opinions. I could go throwing parts at this thing but would rather try to solve the problem.

Thanks,
Tyler Forbes

[mprayii]

mmmm, uneducated guess would be fuel filter/fuel line, or vacumes? honestly I really dont know...

I do know that I was having issues with what I thought was a bad battery/alternator due to simular symtoms. I bought a new battery, and I was good for a few days, then my symtoms came back. To make a long story short. I replaced my battery terminal's and a good chunk of wire between the battery and the fuse box as the wire there had corroded very badly inside its red plastic shielding. My jeep wasnt getting enough juice and that has fixed my problems.

Probably not your fix, but its a cheap place to start if all else fails.

[chrometj]

clean your Idle air control sensor, its on the throttle body!

[twistedsymphony]

tyler, the same thing is happening to me. for about 30 secs when im driving...if i jab the gas pedal it will spudder and almost die, then its fine once its run for about 30 seconds/is warm.

[Jay]

im havin similar symptoms, when u figure it out let me know, im not sure if this is still my coil issue or a new one

[tylerforbes]

Chrome,
My first step was to clean the whole throttle body and install a new iac.

Tyler

[Jay]

My dad told me today his dump truck has similar symptoms, turns out there was some moisture in his distributer, so he recommended i try taking a hair dryer and drying it out. i'll let yas know if it works. mine seams to only do this in damp cold weather.

[jps4jeep]

Didn't read anything, clean your throttle body, then if still does it, check you idle air control motor. More than not, clean your throttle body

[mprayii]

jps4jeep said:

Didn't read anything, clean your throttle body, then if still does it, check you idle air control motor. More than not, clean your throttle body

should have read it, he did all that already.

[Jay]

my problems wasnt fixed, bought new distributor, wires and cap. hopin maybe do plugs tomorrow, i smell unburnt gas cominn from the exhaust, im hopin its a plug not firin

[tylerforbes]

I have a theory about what is going on. Basicly it only happens after extremely long durations and mostly when its cold. But i think I was over focusing on the cold, that may just be making the problem worse. I think that I am loosing pressure out of my fuel rail. I now just need to get a fuel pressure guage. Put it on while it's running, then check it while its off. Then wait til the next morning before I go to work and check it. If the check valve in the pump is loosing pressure over a duration of time, which may be helped by the cold the rail will loose pressure. Low pressure will allow the rail to start on a couple extra chugs but then bog if you try to get on it right away before the pump has brought things back up.

I realized this yesterday playing with an obd2 scanner I rented. I figured I would hook the scanner up and see if it threw any codes while it cold started. Put the key in on position hooked up scanner, started right away. I was like wow, until I realized that during the 10-15 seconds between when I put the key to on and started the car the fuel pump was on most likely bring the rail pressure back up. Anybody have a fuel pressure guage I could borrow?

Tyler

[tylerforbes]

Guys,
New news. Its good to find a problem but not always what you want to hear. Just checked fuel rail pressure. Idle was ~49 psi. Shut if off and it stayed at ~47 psi. Four hours later 8 psi. Next morning nada, yep the guage read zero.

Started it up the pressure jumped back up to ~49, I got on it quick and watched the pressure drop to ~20 while I was on it good. I didnt bog on my but I was reving in park not drive or reverse. After the first drop I could not get the pressure to drop when reving the engine. Looks like pressure is bleading back into the tank. There is no visible leakage up front nor can I smell anything.

The funny thing is I have also been trying to diagnose a clunk in the back when I hit bumps. I tried the hatch, pulled everything out of the trunk, exhaust, shocks e brake lines. Still does it. Im thinking I have something loose in my pump assembly. O joy. This will be an adventure.

Anyone wanna confirm my ideas or thinks other wise.

Thanks,
Tyler