1995 XJ 4.0 - Misfire at idle

[anthonygubbin]

Originally Posted by Sobek

No play in the distributor shaft. As an observation, when warmed up and sitting at idle, oil pressure is reading about '2' (The quarter mark, I'm guessing this is kPa so about 29psi?) which seems to be within norm, however when it's stumbling at idle the oil pressure is jumping up and down. Not sure if that means anything.

I also neglected to gap the spark plugs so I think I'll do that tonight just to be certain. Haven't tested the map sensor yet, but does anyone have any other suggestions while I'm at it? Oh and also, I was driving it about half an hour ago and for the first time, I had a longer than usual crank. Our XJ's ALWAYS been a one-turn-start kind of car, never hesitates, but today I had to hold the key over for a good second or so before it would fire up. That was worrying. Still goes like the clappers though, absolutely no loss of power.

Might look at running some injector cleaner through it for good measure too.

Long crank can sometimes means a battery is getting close to its useby date. XJs need a good battery to start properly. Sometimes XJ batteries might be no good in the Jeep but will go fine for another 2-3 years in a different car.

Regards A

[Roler]

Mine has been a long crank as long as I've had it.
Have you checked any fuel pressure at idle yet? Just a thought, since you now also have a long crank, and I have to prime the fuel pump a few times before starting to significantly reduce crank times..

[Sobek]

I'm pretty certain the battery is good in this case, it's not too old and gets run regularly - no pulsing lights, no odd starting, everything seems good. Today was the only time the car has ever had a longer than usual crank, and it only did it once. I started the car a further 3 times after that over the course of the afternoon and each time was instant... No idea why it did that.

I haven't checked the fuel pressure, but I just remembered that these motors have a schrader valve on the fuel line right? I'm so used to European cars - no tranny dipstick, no schrader valve, no this or that etc etc. I've actually got a fuel pressure tester sitting in the garage that I purchased recently so I CAN test that pretty easily.

Anyone know what the specs are on fuel pressure for these motors?

[rainman]

31 pis at idle
39 at wide open throttle = 39 at idle with the vac line disconnected from the fuel pressure regulator and the vac line plugged.

[Sobek]

Great, thank you. I'll test that either tonight or tomorrow depending on when I get the time.

I have a feeling this will wind up being something simple. The car's been such a rock for so long, no doubt some tiny little thing has just thrown a spanner in the works. FYI, still no codes present, just reads 55 every time.

[junglejuice]

One thing to be aware of is that Xj's will run rough when the battery is getting down a little, it can be still strong enough to start "normally" but it can still be weak enough to run poorly.
I had a similar situation years ago with it playing up, would start fine but was running poorly, it was suggested that the battery wasn't up to it, I needed a new battery in my XR6 at that time so I bought a new battery for it and tried it in the Xj, ran perfect, put the "tired" battery in the XR6 and it was good for another year or so in it...

[Sobek]

Alright, well I won't rule that out then. Would you expect the voltage gauge to read low on the cluster, or would you expect that to still read proper volts anyway? I don't know if I have a spare battery that'll suit the XJ at home, but I'll see if I do and give it a go.

It's hard to capture it in video, but for the sake of it I recorded a little clip of the exhaust right after it did that longer than usual crank, as it was stumbling pretty badly then. You really notice how it's struggling when you hear the exhaust... A friend thought it sounded a lot like it was over-fueling, but you can really hear it struggle a few times in the first 20 seconds. The video will be live in about 30 minutes or so;

www.youtube.com/watch?v=fRKXx61rNo0