Hi,
I have a 1990 xj6, had it since early 90's.

Suddenly one day it started running rough, missing and stalling, can't go anywhere with it. It does have very short periods of running smooth, but returns to rough and soon shuts off. It had been many years so I changed the pump, filter and regulator, I did find quite a bit of lose crap in the filter and regulator. I have fuel rail gauge and its presently holding a steady 40lbs of fuel rail pressure at start up and while running. I clearly have spark or it would not run at all. I checked the throttle sensor and its delivering correct voltage from idle to full throttle. The EGR valve is good and operates correctly. All the injectors ohm out to spec and so does the crank sensor.... but can it still fail anyway? Also can the ignition module fail and yet still allowing engine to run rough in this way? I have a video of how its running click on link, any suggestions what could be causing this are welcome.
Thanks,
rj.

https://docs.google.com/file/d/0Bxyo...?usp=drive_web

 

Thanks for the video, it will be of good use on the forum.

You've owned it since the 90's as well? Pretty impressive.

No cps, no start. Eliminate that
No ignition module, no start. Eliminate that.

These issues are tricky. My 90 vdp when dead on me today traveling at 80mph on the highway. I had to get flat-bedded home. while I was waiting I kept trying to start it and it would try but not run. When it did run it idled fine, as soon as I gave it gas it stalled. When it got home, of course it runs as before, no issues at all. Brain damage has be done though, and I don't trust it.

I will be doing the same things you will need to do, because it has the same symptoms as yours.

First thing I suspect is bad gas, or water in the gas. This is a vicious issue, and road I have been down before. So instead of doing everything as I've done before, I'm starting there first. Draining the tank and putting in fresh gas.

Your Mass Air Flow Sensor could be the culprit. They are tricky because if they work fine so does your car, but if not it causes havoc and we tend to look everywhere else not figuring that could be it. It's easy to take off and clean.

Start there, keep us posted. as I will.

 

Yea I bought the car around 1994, its always been good to me, has 111k miles on it, not too high for its age.

In fact I drained the tank and put five gallons of bran new fuel in it when I did the pump, filter etc., I basically drained the whole fuel system, lines and all air blown clean The test port on the fuel pressure gauge allows me to take a sample and its all good.

As of the Mass Air Flow sensor all the voltages are spot on, I removed it and cleaned it anyway, it was not that bad shape inside.

Could it be the old injectors, I do have a new set but not quite ready to try them yet because It once in a while for very short periods runs nice. I would think bad injectors would be bad all the time.
Just went under dash and checked all ECM cables and plugs, they all look good no corrosion.
Its still a mystery why its running bad.

 

Hi rj:
I'm no kind of mechanic, but here are my thoughts anyway. Given how conscientious you are, I'm assuming there are no codes on the VCM? So many of the fuel related problems would likely throw a CEL... Does your car normally start that hard, or is that part of the problem you are experiencing? To me it sounds like it is not getting the fuel it needs - again, odd considering the obviously good pressure. How clean is your throttle body?

Could vacuum cause this?

Also, what about some of the other general tune-up things like plugs, wires, cap, or rotor?

Just thoughts, I guess...

Cheers,
Scott

 

It usually always has started immediately, cold or hot, in fact started so quickly its almost scary.
I am not getting any codes on the VCM, turn key on and push the button the mileage goes blank for a few seconds and comes back with just mileage.

I cannot find any vacuum leaks or problems with feeds anywhere.

I did replace the plugs, they were fowled pretty good, but it did not make a difference. The timing at distributor is where it should be on TDC #1 and no play in rotor. Cap and rotor is almost new, wires are somewhat older.

It was running fine and all of a sudden started running bad in a matter of flipping a switch, first impression it wasn't getting enough fuel, but regardless of new fuel system parts it has not recovered. But yet it does act like its still got a fuel starvation problem, so the only thing left in fuel system is replace injectors. Since I have a new set I'm going to put them in tomorrow, it either fixes it or eliminates one more thing its not.

 

Update my engine problem.

I changed the injectors and nothing changed, didn't really expect it to but now everything regarding fuel delivery is new, the rail pressure is good, diode blinking showing injectors triggering, so I can move on. I pulled one of the new spark plugs and its already fouled black.

Still No codes from VCM.

Looking again at the Mass Air Sensor, out of four voltage test readings, the only one I can't do is on pin 3. With engine off and ignition on it has the correct 0.42 volts (then start motor to complete test, but now it won't even run long enough to do test) As you increase rpm the 0.42 volts should increase up to a max of 2.0 volts. Its an important test and still an unknown, gives me pause. Anyway, ...fouled Plugs are telling me the injection and spark have lost harmony. But is it something screwing up the ECM,.. or is the ECM screwed up?

The Engine Management System Diagnostic manual says the crank trigger can produce a "misfire" condition. I have only known these sensors to be a work/no work component, and mine ohm's ok, but I will probably replace it anyway, its old. Sensor misfire troubleshooting says to check its gap and grounding, so I may as well do that replacing it and also remove any doubt with it. See what happens.

In the beginning I simply pulled it in the garage to do a bunch of under hood A/C work. When I was finished it was running fine, good A/C, then after about 5 min of parked and running.. poof! Started running badly, kept stalling and getting worse, it would not even start today.

But I'm just happy it did it in the garage.

 

Is it only when idling? If so, maybe a leaking EGR valve? I had a couple weeks of rough idling and stumbling after installing a brand new Jaguar OEM EGR valve. Turned out the pintle was leaking EGR gasses into the intake, causing the rough running.

With the engine idling, you can manually open the EGR valve by using thickly gloved fingers to press up on the diaphragm on the bottom of the valve. The engine should run noticeably rougher.

It's a simple test that only takes a minute or two to rule out the whole EGR system.

-Nick

 

When it was willing to start it barely idled, but that's even degraded now and its not even willing to start anymore.

I already took off the EGR Valve and inspected it, its still in good shape and working correctly. Cleaned it and put it back on, its good. But thanks for the suggestion!

 

Hi Phydeaux1
Has your car got the pump in the tank; If so the short lead between the pump and tank fitting could be faulty I had this problem on a customers 1990 4 litre about a year ago it took some finding but replacing that short lead fixed it. You could also check the tank is venting properly.I have noticed that after arun in some XJ40s when you remove the fuel cap there is quite a lot of pressure hissing out I don't no if this is normal. There could also be some thing floating around in the tank and blocking off the main fuel outlet . listening to your video it sound like a fuel problem. Hope this is of some help
Cheers Ray

 

It doesn't sound like a fuel issue so much. Your fuel pressure is good and steady and you have replaced your injectors. When your say the plugs are fouled, I'm assuming you mean the the electrode and insulators are covered in soot and not covered with wet oil? This would indicate the fuel mixture is too rich.

It sounds like you might not be getting a strong enough spark. Have you checked the coil primary and secondary resistance? Also re-check the ground connection for the coil.

Do you have the correct type of spark plugs installed and are they gapped properly? Have you checked the plug wires and the king lead?

If all the ignition side checks out, you might want to check the MAFS connector. These often give problems (more so than a dirty sensor). Clean them with contact cleaner and make sure they are seated properly. I will assume that your air filter is not clogged.

If all that fails, you might want to check engine compression

Cheers,

Allan

 

Hi Ray,
It has the external pump, and yes the tank is venting ok, I ran it for a while with no cap on the tank and still no difference. I'm keeping a steady 40lb rail pressure while cranking it, and if lucky it starts the pressure holds fine.

The new spark plugs I put in the other day fouled to carbon black in just a few runs (when willing to start.) The plugs are showing if anything a very rich condition. Things that could cause fuel system to go rich (EGR, Oxy Sensor, throttle position sensor, Air Mass Sensor) are checking ok.
So maybe coming from bad timing, the Injector pulse timing could be wrong or even ignition spark timing, but impossible for me to check because both controlled by ECM. The fundamental checks I can do are all coming back good. I have good distributor rotor position for TDC #1 firing, strong spark at the plug, and when cranking the motor my diode tester is telling me the injectors are triggering.
It would be nice if the ECM would give a code(s), some kind of hint, but its not telling me anything. For now I continue to read the Engine Management System Diagnostic Guide for clues and probably next try a crank sensor, its related to timing. Then again the ECM itself may be bad, considering its not giving me any codes at all for this bad run condition is suspicious.

But any suggestions are welcome!
Thanks! rj.

 

Ok, Mine was basically a regrinding of the rotor and the insides of the distributor cap. I also changed the lead coming from the coil. Shes running strong again.

These coils need to pump out a lot of juice, and be sealed bringing it to the cap.

The CPS is interesting. It may ohm out, but that little nub at the end can wear with time, which causes the distance issue. They may be your issue. You can't really move it closer because the frame holding it can't be adjusted.

 

Rj,

Yes, as I said above your problem is the mixture is too rich. If you clean your plugs and put them back in will the car now start? If not, then you may have two different problems.

Also you might have flooded the engine, try starting the car with the accelerator floored.

The ecm might be on the fritz, even though the connectors look ok. I would take them off and give them a clean. Obviously disconnect the battery first

If the car starts but still exhibits the rough running and fouling, I still would suspect the ignition system.

Cheers,

Allan

 

I cleaned ECM connectors several times and plugs real good but no better, its a no start. Every time I check spark at the plug its strong, and the ignition parts are recent, I am not finding a problem in the secondary part of ignition. But a good suggestion because its also characteristic of ignition problem.
It was running fine and something went bad suddenly, but whatever it was allowed the engine to keep running with short periods of smoothness interrupted with bogging, missing and stalling and back to smooth. (That's why I replaced everything on the fuel system, seemed the logical fault and it was old anyway, but replacing did not make any difference) When it shut down it would start after some cranking and run again rough for short periods. That was last week, now it won't even start, but yet all my tests are coming back saying I have air, spark and fuel. All my testing of vital sensors across the engine are testing clear for correct voltages and ohm's.
Sooo..just to be sure, I will have to dig up a compression gauge and confirm compression, regardless that the engine sounds normal with starter spinning it. I'm not that familiar with what happens to a 4.0 if maybe loses a chain tensioner? Is this a known weakness on this engine?

But if compression good and back to it being maybe a timing problem with the ignition or injectors, and I still get no codes out of ECM, then probably see what a new ECM does. It controls the ignition and injector timing voltages, and see what happens.

But I appreciate all and any input!!!!!!

Thanks, rj.

 

Take out the CPS. Clean it and see if it makes a difference.

 

Ok will do, I've only looked at it and its pretty nasty, lot of road grime and oil on it. Let you know if helps. Thanks!

 

A bad connection or a problem with/at the ignition module maybe?

 

I pulled out the Crank sensor and gave it a good clean, put it back in and nothing changed.

So I went back to one of the first tests I did which was check for spark at the end of the plug wire to #1 cyl. Its been showing spark here when tested couple of times before, or before it stopped running all together. Now today I cannot get any spark at the end of #1 plug wire. But when I pull the coil wire off cap and check directly off the coil I have massive spark. The dist rotor is turning correctly. So although the cap, rotor and wires are not really that old, I ordered a new everything except coil. Going to clean up the plugs and I will know more when parts arrive.

Thanks!
rj

 

I found a Ignition Amplifier Check list I printed out a long time ago, not sure if its been kicked around here before. It may be helpful to someone.


Positive (+ white/pink wire) side of ignition coil should be hot at 12V (its connected to battery via relay). Same wire goes to ignition amp B+ pin. Remember that battery voltage will drop when engine cranks.

Negative (-- white/black wire) side of coil goes to C-pin of amp. This should also be at battery voltage when not cranking. This is the line the amp normal pulses to ground and then opens to fire a spark (where the points would be on a old car). If you have battery voltage when NOT CRANKING measure voltage between battery+ and this line - should be small not cranking and then rise. If it doesn't rise start suspecting amp module and carry on tests.

The B terminal ( -- white/green wire) on amp should be 5V above ground. (Mr ECU drives this one)

The E terminal (white/red wire) on amp goes to ECU (--) should rise when cranking. If this does NOT rise, disconnect amp module and repeat test. If it rises the amp is bust (take out and replace the GM module assuming you don't find a more obvious case) If it doesn't rise you have to check back to ECU.

 

All though you replaced the rotor and dis cap may be check that the carbon tip in the centre of the cap is making good contact to rotor. they can some time fall out or stick up. May be also check the ground or earth wires at the base by ignition amplifier and the one connected to the airflow metre.