P1398 help

So I bought my 97 xj knowing it had this code. I did a tune up, and reset the pcm. When you first start it up when cold it has a rough idle fire a bit, and when you first drive it it stumbles/hesitates and missfires, then it clears up and runs fine. Noticed it has an aftermarket crank position sensor. I'm going to replace that with a mopar one, maybe do the cam sensor too. Next step is to have the pcm checked for the tsb.

i had a very similar issue, mine was a 99, would stumble once every time i started the jeep, usually when i would start to drive it, after that it would drive and run perfectly, after i shut it off and restarted it it would do it 1 time again. my problem ended up being the first O2 sensor, it was damaged physically. did not throw a code until finally it crapped out completely rendering the jeep almost undrivable, that time it threw the code and was i able to fix it, after that my 1 stumble per start cycle was gone for good.

Interesting. I’m out of town and my wife told me the check engine light came on. ‘97 XJ. My son checked the codes and it was P1398. He erased the code. When he started it, it sounded normal and the light didn’t come back on, but it hasn’t been driven yet.

I expected the pcm reset to clear up this issue, maybe I haven't driven it enough, or maybe the fault is still there causing the issue which would make me lean more towards a sensor problem in my situation.

1996-97 have a TSB for a P1398. Does yours have the updated PCM software ?

Is the rough start and stumbles caused by a faulty check valve ?

Tim_MN said:

1996-97 have a TSB for a P1398. Does yours have the updated PCM software ?

Is the rough start and stumbles caused by a faulty check valve ?

Click to expand...

yes I know of the TSB, how do I check the software level? I have a Modis scanner. Where is the check valve located and how do I test it? When its cold the first crank is a little long, it has a slight rough idle, and hesitates/bucks/and misfires for about 30 seconds to one min after putting it in gear, it will surge, then run and idle perfectly fine after that.

The PCM part number gets updated when the software gets update, So what's your PCM part number?

Empty_Pockets said:

Where is the check valve located and how do I test it? When its cold the first crank is a little long, it has a slight rough idle, and hesitates/bucks/and misfires for about 30 seconds to one min after putting it in gear, it will surge, then run and idle perfectly fine after that.

Click to expand...

The 1997-2001 fuel pump check valve is supposed to hold pressure in the fuel line while the engine is off. The check valve is located on the fuel pump and located inside the gas tank. A failed check valve allows the fuel to drain back to the tank. This results in one or more of the following symptoms:

SYMPTOMS

• Longer than normal cranking times
• Never starts on first try, almost always starts on the second try
• Rough idle for a few moments of idling
• Rough running for first ½ block of driving
• Little or no fuel pressure at the fuel rail test valve similar to this -

1- Key in on position; fuel pump primes for 2 seconds then stops. 0 PSI
2- Cranking over the engine. 2-3 PSI
3- Engine starts and idles for 20 seconds. < 5 PSI
4- Engine idles and fuel pressure creeps up to 45-50 PSI.


POSSIBLE CAUSES

• Failed check valve on the fuel pump in the gas tank.
• Leaky fuel injector(s).

Testing of the fuel injectors should show if any are faulty and are allowing fuel to drain into the cylinder. Either problem can allow heat soak to vaporize the remaining fuel in the fuel rail, and you may be dealing with a bit of vapor lock as well, especially in warmer weather.

1. Connect the gauge at the fuel rail and start the engine. The gage should read 49 psi plus or minus 5 psi.

2. Turn the engine off and immediately clamp the fuel line just ahead of the fuel tank. Watch the pressure gage and see how long it takes to loose pressure.

If the pressure remains at 49 psi for an extended period of time then the problem is likely in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably one or more leaky injectors.

I was poking around the xj tonight while putting a fab shroud and e-fan in that were missing. The connector for the upstream o2 sensor is a huge ball of electrical tape so I'm thinking of starting there. I'll check the fuel pressure since it's easy to do.

So I just wanted to update this. Been raining here or really hot that I haven't wanted to work on it. Got a break in the rain today and dove into the tape mess that was the o2 sensor connector. The connector was broken on the sensor side. Ordered a new ntk sensor off rock auto, but in the meantime I've left this one unplugged and it's been running great. P1328 has not returned since I did the hard reset on the pcm