texmedic49

I need some help...

2004 stock Grand Cherokee, 170K-ish miles on it. I've been having rough idle and hard start problems for a little while, and it has progressed to dying outright in traffic. I got to sit for a while tonight because I couldn't get out and push due to the rain.

Good times.

The same thing happened Monday, I hooked up and pulled several codes- Cam position sensor, and 2 random evap codes. Replaced the Cam sensor Tuesday, fired right up without issue. Drove it to work yesterday and it died like someone flipped a switch. The weird thing is, it seems to be associated with how warm the engine is, or how long it has been running.

When it died Monday, I was about 7-8 minutes down the road. Same thing yesterday, and again tonight.

So it goes like this...cold start, idle, drive for 7 or so minutes, dies. Crank for about 5-10, then start rough. After that it seemed to be fine and had no other issues.

Not sure where to look at this point. No codes, no obvious signs something might be wrong.

I'm kinda thinking about going down the following fault tree:

1. Test the fuel pressure at the rail with a gauge. I've never seen a fuel pump falter before complete failure, but I guess it is a possibility.

2. Test the air temp sensor and IAC, TPS and see if any of those are bunk. I would figure at least one of them would throw a code if it was mucked up.

3. Test (?) and/or replace the coil.

4. Refresh the main ground. I'm leaning towards this, as it seems to be related to heat, and bad grounds love that.

5. Last but certainly not least, consider the ECM as the culprit. Not sure how to prove it though.

6. Neutral Start Switch- One other seemingly incidental finding is...I had trouble cranking it while in neutral and moving. It ended up grinding several times, and didn't at other times. Not sure if this is related or not.

Does anyone else have any reasonable suggestions? I don't want to throw parts at it, but am unsure of which direction to move in.

Thanks to any and all who reply!

dave1123

Failing crank sensor. They can be intermittent when they are close to dying completely. They rely on a magnetic field to sense windows in the flexplate to signal the PCM to fire the spark plugs. They won't set a code until they die completely.

My advise is to replace the CPS with a Mopar part because of aftermarket quality issues. The Mopar one uses a neodymium magnet where aftermarket ones don't. That's why they are more expensive. Check the magnet with a screwdriver when changing it and see the difference.

This is totally my opinion, but I went thru the same thing with mine.

texmedic49

Several questions...


1. Exactly how similar were your symptoms?

2. Why would the problem not continue within the same "use" of the engine? In other words, once it came up to the temperature that it fails at (opens) why would it no do so again after cooling off again and having the offending part of the sensor make contact? I don't doubt you, I would like to learn from this.

I'm going to go with whatever sensor I can get tonight if this is the way to proceed, and replace later. Got to get to work tomorrow.

texmedic49

I posted this elsewhere...

I picked number one to go first because it will produce hard data. At this point things are a bit subjective and inconsistent. I like hard data.

That said, I went back out to the vehicle and started it up without issue, just to see if I could reproduce the "service time" finding. I could not. Sat at idle for a good 30 minutes with no rough spots or bobbles. Not sure what to make of that. I even tried giving it some throttle and easing off, just to see if I could get it to die on the downswing of the RPMs. It didn't. I also tried roughly the same thing shifting back and forth into gear. Still nothing. I am hesitant to continue tonight because of the rain and the fact that it is dark. I'm going to pick this back up in the morning.

An incidental finding that is maddening...there is no test port on this damn fuel rail, at least not one that I could see. On my XJ it is in an easily accessible place. Unless I'm missing something, I don't think this rail has one. There is one spot that I can't quite see though under the mount for the air intake, and I will check that out in the morning when the sun comes up.

If anyone else has input, I'm all ears.

dave1123

My jeep would stall for no reason at any time and sometimes would catch itself and burst back to life if it haden't already spun to a stop. It would often start right back up, but sometimes you had to wait a few minutes before it would start. When it stalled, it was like you turned off the ignition. It generally happened at low speed or when slowing down to stop. No stumbling or bucking, just died. It showed no code from the key or scanner.

This is a 4.0 straight 6, BTW. It had 175K miles on it when the problem started. It hasn't happened since then and it has 230K on it now.

texmedic49

I just went and got a new one. Ended up not going to work.

Wish me luck.

texmedic49

Got it done. I'd love to track down the individual who designed that and have a...chat.

Thanks guys.

dave1123

LMAO! Welcome to Jeepin'

Jeep=Just Empty Every Pocket!

texmedic49

This ain't my first rabbit hole to go down with a Jeep. Thanks for your help!

dave1123

Glad to help! I just hope the CPS was your only problem. It's very hard to diagnose using a keyboard! You supplied enough information that it was pretty easy. Thanks for that. Funny, I still don't know which engine you've got.