jg22

99 Cherokee 4.0 w/ 130k

The Jeep has had a somewhat rough idle, but only at the initial start up. After the initial start up there would be a slight hesitation when taking off for the first time, but would go away after giving it a little gas and after driving it a short distance it would idle fine at stop signs and lights. Full power when accelerating as well, only would hesitate during the first take off after starting the Jeep. It had a CEL that would randomly go away and come back. The code was P0306, cylinder 6 misfire. Used seafoam in the TB and it went away and the idle cleared up slightly.

Today while driving it started to run rough and then died. It would start back up immediately and I could drive a little bit before it did it again. Made it back home and now it has a tough time starting at all. It will crank, but won't start. I am thinking maybe CPS? Dirty injectors? IAC? I have some BG44k that I ordered before that will be here tomorrow. After this though I am unsure if that will help.

Any leads would be great.

Mr Lewis

My 97 did the exact thing. I changed the Throttle position sensor and it never died again. Well.....for that reason lol.

jg22

Didnt think of that. Thanks ill look into that as well. Did you also get a misfire CEL?

jg22

Just tested for codes again, no more misfire code. Now I am only getting the P1391

jg22

This is the CPS correct?

clawdog60

Fuel pressure/fuel filter maybe

Turbocube

I wrestled with this exact problem. Don't waste your time and money replacing the TPS, CPS, or changing the fuel filter just yet. First replace your spark plugs, and ideally, install them with a proper torque wrench (spec is 25-30 lb-ft).

Not saying those other parts aren't the problem, but when my Jeep had these EXACT same symptoms I had replaced all of them (!) and until I got new spark plugs the problem just wouldn't quit.

Think about it. A code for a misfire? That's lack of spark. If you can check fuel pressure, do that.

Finally, while spark plugs solved my problem, your problem might be the plugs or it might be the spark plug wires, or the ignition coil, and so on. But of course its best to start with the cheaper parts.

Let us know how it works out!

Kalali

YES

CCKen

P1391-INTERMITTENT LOSS OF CMP OR CKP

Possible Causes:

DAMAGED FLEX PLATE (CRANKSHAFT)
DAMAGED PULSE RING (CAMSHAFT)
INTERMITTENT CKP SIGNAL LOSS WHEN WIRING IS WIGGLED
WIRING HARNESS INTERMITTENT
INTERMITTENT CMP SIGNAL LOSS WHEN WIRING IS WIGGLED
SENSOR CONNECTOR/WIRING
SENSOR CONNECTOR/WIRING
CAMSHAFT POSITION SENSOR
CKP SENSOR CONNECTOR/WIRING
CRANKSHAFT POSITION SENSOR

Note that the Camshaft Position Sensor (CMP) is on the list, as well as the CKP (CPS). The CMP is somewhat misunderstood. Besides governing fuel injector pulse width, it tells the PCM which cylinder is on its compression (firing) stroke and the PCM ignition coil driver triggers the ignition coil for that cylinder. There have been many accounts of no start, rough running, (P1391) complaints that were cured by replacing the CMP.

Disconnect and examine the CMP and CPS connector pins and pin cavities for corrosion.

Clear P1391, run engine, and wiggle the CPS and CMP wiring to see if 1391 is triggered again.

Disconnect the battery negative post connector - a Quick, but inconclusive, test of the CPS is to disconnect the CPS, and using a digital Ohmmeter set at 200 Ohms, measure between any two pins on the CPS connector. You should see infinity (open). Any resistance indiates that the CPS is bad.

jg22

Plugs and wires looks good, but I may go ahead and replace them to be sure. Bosch plugs and wires currently on the Jeep.

Connector pins and cavities both look very clean. Also tested the CPS with a meter and there is no resistance, but that doesn't mean 100% it is still good correct?

Now the Jeep will just crank and crank but no start unless I try to give it more gas, but even when it does start it runs extremely rough and dies.

CCKen

Sometimes when the throttle is opened and it starts it's because the engine is flooded and the wider throttle plate will lean out the mixture enough to get ignition. When you checked your plugs did thay look fouled? BTW, you should be using Champion RC12CC plugs or NGK ZFR5N plugs, not Bosch plugs.

Have you checked your fuel pressure at the fuel rail?

jg22

The plugs did not look fouled at all, looked fairly new and have been there from the PO. I was about to head to NAPA to pick up these plugs and wires:

http://www.napaonline.com/Catalog/Ca...438_0275525989

http://www.napaonline.com/Catalog/Ca...444_0065776122


I have not checked my fuel pressure as I do not have a gauge. IIRC they aren't too expensive though and may pick one up while I am at NAPA if needed.


I have also been doing some more reading and came across reindexing the distributor. Could this possibly be the issue as well or am I most likely on the wrong track?

CCKen

Those Champion 438 plugs are what you need. And the Beldon wires are about the best. The only problem I ran accross with the Beldon wires is the boots seem to be too long and they are difficult to get the wire to 'snap' on to the plugs. Had to cut about and 1/4" off the end of the boots to get them to freely snap into place.

You can (refundable deposit) rent a fuel pressure test set from most auo parts stores, but I don't know about NAPA stores.

Your distributor may be just fine as far as indexing goes. But you may want to remove the cap an see how much side-to-side play there is in the shaft. A little rotational play is okay, side-to-side is NFG. While you're at it, replace the cap and rotor (NAPA - Echlin). New plugs and new wires without replacing the cap and rotor is like putting perfume on a pig.

While you're in the distributor, that black disc under the rotor is the CMP. Check it out. Make sure the CMP wires that go through the side of the distributor are not chafed.

Shortleged1

crankshaft positioning sensor. Throttle body sensor and there is one more sensor up there. I changed all mine around 150k and haven't had that issue in some time.

jg22

will do, thanks

I was just going to bite the bullet and replace sensors, but after testing them with the meter I am hesitant