greenjeepster

My jeep is a 1999 and I have owned it since 2002. It is a low mile jeep for it's age, only 135K. So here is the issue.

Starts fine and runs fine, but about 5 minutes after it is initially started it starts to run rough, lope, pops and sputters. All this for about 3 minutes and then it goes back to running fine again.

What makes it worse? High humidity.
What makes it better? High octane fuel and pulling the vac hose off of the brake booster.

Things that have been replaced already: throttle position sensor, idle air control motor, throttle body cleaned. Lucas oil run through it, spark plugs, wires, distributer rotor and cap. It is not putting out any codes. It has been checked multiple times for vac leaks

It does have a mild rough idle all the time, but it has been like that since I owned it.

I have had a few shops look at it and have gotten everything from "it needs fuel injectors" to "it needs a new ICM. I hate to have to take it in to the dealer, I know it is something simple, just gotta figure out what?

tjwalker

For something strange like this, I would want to have this vehicle hooked up to an engine analyzer or a scantool and then watch all the live data very carefully as the symptoms occur.

Has this been done?

This absolutely does not sound like the computer.

952door4x4

It's a shot in the dark, but maybe it's emissions related? On my camaro it has an EGR (exhaust gas recirculation) valve that can get stuck open or closed but I'm not sure if Jeeps have them or not. Maybe check out and clean the MAF sensor? If it only happens at start up it sounds like something related to it warming up from a cold start. Does it ever happen when it's already warmed up, like starting it up after going to the store?

greenjeepster

Yes, it happens after almost every startup, hot or cold. The only time it really doesn't do it is if I shut it off and start it right back up, if it sits for a few minutes it does it again.

If I run premium fuel in it it does it less, but still does it. Fuel pressures are fine.

I haven't had it on an analyzer... I am in CT and getting a car looked at at a shop is spendy around here... well over 100.00 an hr for diagnosis. I wa kinda hoping somebody had a similar experience and would know what the issue is.

wildbill1

Has it thrown any codes? Having it run better with a huge vacuum leak sounds like the fuel trim is way off, but I would think it would be showing a CEL.

VTJeep

o2 sensors. i win.

greenjeepster

No codes.

freegdr

map sensor possible it regulate fuel according to pressure in the intake system

fishtaconc

You can find a scanner with data for under $200 online.

93gc40

Sounds Fuel supply related to me, Pump/Filter Injectors. Could so be a Plugged Cat. I'd start by seafoaming the intake and in an almost empty fuel tank. have you had the fuel pressure tested?

dennis461

high humidity is a clue.
When dark outside, engine running
look under hood for sparking/glowing plug wires or coil.
Look underneath at the coil

freegdr

op stated pulling hose off booster makes it run better this would add more air to the mixture never heard of fuel pump pumping to much fuel?

greenjeepster

Thanks for the input, the plugs and wires have been replaced multiple times, gaps checked etc. Haven't checked the coil pack, but will. Scanner does no good, it isn't putting out any codes.

JeepLV96

[QUOTE=greenjeepster;936691]What makes it worse? High humidity.
What makes it better? High octane fuel and pulling the vac hose off of the brake booster.

QUOTE]

Pulling the vac hose off the brake booster will cause a high idle, and make your brakes lock up.

x2 on this one. I had this similar problem on my 96. I didnt have a CEL on and i didnt throw any codes. Try disconnecting the up-stream o2 sensor and see if the problem goes away. This will throw a CEL and the jeep should run better, at least mine did. It will burn more fuel however since the ECM is relying on default parameters.

SeriousOffroad

x Howevermanytimes.
Why?
After 5 minutes from a cold start the engine should reach normal opperating temperture and switch from Open Loop mode to Closed Loop. O2 sensors are not monitered during Open loop Mode.

Check your O2 fuse in the junction box.