Topic: Help!!!!! '96 XJ Misfiring (Read 6344 times)

General Confusion · « **on:** November 08, 2015, 07:30:26 PM »

A friend of mine has bought my old XJ from the guy I sold it to for his son. First time out in it ( which was Cheapfest) it started misfiring when the engine got up to normal running temperature. Since then it has steadily got worse.

He has now changed: HT Leads, Distributor Cap, Rotor Arm, Coil Pack, CPS and Spark Plugs. He has also removed and cleaned the fuel pump and filter.

None of this has made the slightest bit of difference

and the next step is the upstream O2 sensor.

Has anybody got any other suggestions please?

bigjeepzj · « **Reply #1 on:** November 08, 2015, 08:06:03 PM »

if it has been stood for a wile it could have a sticky valve mite need an oil flush

General Confusion · « **Reply #2 on:** November 08, 2015, 08:15:04 PM »

Been in regular use since I sold it 4 years ago.

bigjeepzj · « **Reply #3 on:** November 08, 2015, 09:11:23 PM »

checked the PCM connectors ??

JamesH · « **Reply #4 on:** November 09, 2015, 06:45:23 AM »

TPS, fuel pressure test, MAP sensor. Just to help rule things out at least.

General Confusion · « **Reply #5 on:** November 09, 2015, 05:47:02 PM »

Quote from: bigjeepzj on November 08, 2015, 09:11:23 PM

checked the PCM connectors ??

Could you point me towards those please?

General Confusion · « **Reply #6 on:** November 09, 2015, 05:48:13 PM »

Unplugged the upstream O2 sensor today. Ran like a bag of spanners!!!!

JamesH · « **Reply #7 on:** November 09, 2015, 07:18:00 PM »

PCM Connectors are a known ZG/J problem but is less often an issue on XJs. Not sure where the PCM is on a 96 but the facelift ones are front right in the engine bay, behind the passenger dude headlight

Jonny Jeep · « **Reply #8 on:** November 09, 2015, 10:15:45 PM »

If it runs bad with the upstream O2 sensor disconnected I'd be looking at map and coolant temp sensors I think. The computer would be relying on those two more if it has no input from the O2 sensor.

General Confusion · « **Reply #9 on:** November 09, 2015, 10:19:16 PM »

Is the coolant temp sensor the one that switches the aux fan?

Jonny Jeep · « **Reply #10 on:** November 10, 2015, 09:37:57 AM »

The aux fan relay is controlled by the computer using the input from the temp sensor on the thermostat housing. Pre facelift models have a second sensor on the rear of the block but this is for the gauge only. The temp sensor resistance can be measured against the chart below. The same chart is for the intake air temp sensor on the intake manifold too.

General Confusion · « **Reply #11 on:** November 10, 2015, 10:59:25 AM »

Sorry to appear thick (I probably am!!!) but is that the two wire sensor on the stat housing? If we unplug that one, the auxiliary fan cuts in.

JamesH · « **Reply #12 on:** November 10, 2015, 01:59:54 PM »

That's the one and only coolant sensor

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Mike Pavelin · « **Reply #13 on:** November 10, 2015, 07:28:35 PM »

Pre '96 have a second one at the rear of the head, but that only does the temp gauge. The one on the stat housing is the one the Pcm uses.

JamesH · « **Reply #14 on:** November 10, 2015, 07:48:55 PM »

Listen to Mike, pre facelift are more different than I thought.

General Confusion · « **Reply #15 on:** November 10, 2015, 08:02:34 PM »

I know there are two, just wasn't sure if I was talking about the right one at the front.

LairdScooby · « **Reply #16 on:** November 25, 2015, 09:28:59 AM »

I'm going to throw in an unusual suggestion but it does work, assuming the fault is what i think it may be.

Add between 1/2 and 1 litre of Dexron ATF to the engine oil. Drive it around and hear the engine get quieter, feel it get more responsive as the hydraulic lifters become cleaner and drive past the filling station as you won't need so much fuel!

Ok, Ok, i make it sound like a "snake oil" but the purpose of adding it is to clean the hydraulic lifters and it is a job it does very well. When i first got my XJ 4.0 a few months back, it had a pronounced knocking from at least one lifter and a misfire on idle. A new set of leads, rotor arm and dizzy cap helped somewhat (it already had new plugs) but adding the Dexron has seen the biggest improvement. Not only is it smoother and quieter but the economy is up (last fill up returned 24mpg on LPG!!! Admittedly i'd recently fitted a new LPG control system (but that only ever "self-learnt" because the software/lead package i bought doesn't work - don't buy anything LPG from wtv.uk - they sell rubbish and their customer service is a joke, a very poor one!) but the economy on petrol seems to be up as well although i don't check the petrol as it runs on LPG within a short while of starting it.

Adding Dexron to the engine oil is a tried and tested method for me, i use it regularly on both my Rover 827s to prevent the exhaust "tappet rattle" that these Honda engines are famed for and it helps the economy on them as well - i regularly get over 30mpg from both and did at one point see 43mpg from the saloon.

If this Jeep in question has been sat around a while then the hydraulic tappets could well be sticking and causing the misfire. Also worth checking the breathers and the vacuum hoses from things like the MAP sensor to the inlet manifold, fuel pressure regulator etc just to eliminate the obvious.

General Confusion · « **Reply #17 on:** November 29, 2015, 11:15:19 AM »

Sadly the ATF trick appears to have made no difference.

LairdScooby · « **Reply #18 on:** November 29, 2015, 11:43:00 AM »

It took a while to make some difference on mine, give it some time. What i found with mine was that at first it seemed to make no difference and then matters slowly began to improve.

General Confusion · « **Reply #19 on:** November 29, 2015, 12:50:07 PM »

We'll give it a whirl. Trouble is he's scared to drive it too far.

LairdScooby · « **Reply #20 on:** November 29, 2015, 01:15:17 PM »

I've just re-read your original post and am wondering if perhaps i missed something. Have you done a compression test? Also is it a regular misfire or is it random,all over the place?

Next question is does it run smoothly when cold and then once hot starts misfiring again?

General Confusion · « **Reply #21 on:** November 29, 2015, 01:20:54 PM »

Done a compression test. Random misfire. Only happens when engine reaches normal operating temperature.

LairdScooby · « **Reply #22 on:** November 29, 2015, 02:06:19 PM »

Hmmm, this is a weird one but it's happened to me on at least 3 different vehicles and given the proximity of the heater coolant pipes to the ignition system could be a likely candidate for being the culprit.

Once the cooling system is up to temperature, the pressure also increases. This can cause very small coolant leaks that are difficult to find as the spray that comes out is so fine it's near enough invisible but it's still enough to cause a misfire when it sprays the HT leads, coil and plugs. Being so fine on a hot engine, the evidence evaporates before it can be found!

The 3 different places i can remember i found leaks are :

1) the metal stub where a coolant hose joins - the end of the hose was slightly perished, cutting it off and refitting it with a new Jubilee clip fixed it.

2) Water pump - this was actually leaking onto the belt so was sprayed harmlessly round the periphery of the belt until the engine was hot enough to bring the cooling fan in at which point the spray was blown all over the HT leads! Luckily the pump was ok and it was one of the rubber hoses on it causing the leak, just needed the clip tightening.

3) The other place i found a leak was the hardest to find. Two reasons for this, one was it only effected the front bank (it was on a tranverse V6) most of the time until the cooling fan kicked in and then it played havoc with all 6 cylinders by spraying over the distributor which was mounted on the nearside end of the front head. The other reason was it was a pin-hole in a coolant pipe (top hose in fact, where the A/C coolant temperature sensor mounts) and only visible once up to temperature with the engine running or by bending the hose which was tricky because of the other stuff in the way.

If all other possible answers have failed, this has to be worth a look. Perhaps getting a polythene sheet and taping it round the dizzy cap/leads and coil/king lead to protect it all from water spray might give a clue you're on the right track. Doesn't have to be a permanent thing so won't matter if it looks rough but as long as it protects the HT leads, coil and dizzy cap from possible water spray it should yield a result if it is a tiny coolant leak spraying the engine bay.

General Confusion · « **Reply #23 on:** January 23, 2016, 07:47:59 PM »

Problem finally solved. Faulty fuel pressure regulator in the fuel tank module.

The Recovenator · « **Reply #24 on:** January 23, 2016, 07:53:57 PM »

There you go

Author Topic: Help!!!!! '96 XJ Misfiring (Read 6344 times)

Mike Pavelin

LairdScooby

LairdScooby

LairdScooby

LairdScooby

The Recovenator