Today's goal will be to pressure test the fuel system and swap out the fuel filter. That arrived today. Given time the exhaust system pressure before the cat will be checked too.

Have not done a compression test yet. So I will give that a try and see what happens.

Kind Regards

 

Video update

Various maintenance was done to fix other problems today but this is a short video clip outlining the problem.


Might have to replace the throttle body and the throttle positioning sensor.

It is literally the only thing I have not replaced so far.

 

Update with Video

So today I compleated a more in depth service involving mostly other things on the car.

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I decided to record a short video outlining the problem below. As the throttle is applied it shakes, and as you can see I punched the throttle a few times and it does not respond. Might have to replace the throttle body including the Throttle positioning sensor. Have literally replaced everything else.

Note at the end where it dies right off, and it takes a bit to get it going again.

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Full volume required

 

I changed the fuel pressure regulator on my 3.6 XJ40 to fix this problem, after a while, it started again so I removed the fuel rail and injectors to clean them and found a rusty fuel rail.
I have now replaced the tank, fuel pump, fuel filter and cleaned the injectors, perfect!!

The idle adjustment stepping motor is next.

 

Will certainly give this a check. If it persists after the throttle potentiometer is replaced I will be looking at the injectors and the rail.

The tank in this one is good; however, you never know until it is too late.

Cheers

 

Hi Terry,

I have merged your new thread from today with your old one so all the related posts are together. Discussing the same malfunctions in multiple threads leads to frustrating confusion when others can't review all the previous replies and all the services you've already performed. If you would like me to retitle this thread, just let me know.

I'm sorry we're not coming up with more ideas and I hate to see you throwing so many new parts at the problem.

I'll be curious to hear the results of the compression test.

A failing fuel pump might account for your symptoms: perhaps it can provide sufficient flow to run the engine at idle but can't supply enough for the engine to run at higher speeds.

Another new crazy idea: Have you checked the condition of the engine ground strap and thoroughly cleaned both ends of the cable and the points on the body and starter/bell housing where it connects? If there is corrosion on the braided cable inside the ring terminal crimps, perhaps when the engine tilts in response to throttle inputs the ground connection is interrupted intermittently. This is the ground path for the spark plugs and all engine-mounted sensors, so it's more important than we tend to remember. Just a crazy idea but maybe worth checking.

Cheers,

Don

 

No worries.

Compression test is good.

Awaiting the TPS sensor in the mail, and will see how that goes.

This problem has stumped more than one experienced mechanic, so will see how it all turns out.

Soon as this is over I can focus back to detailing and servicing the drivetrain.
It has been quite the headache.

 

So today after waiting about 3 weeks for the throttle position sensor in the mail it has now been fitted. This took the best part of 3 hours removing the throttle body to undo the two 7m bolts that were holding it in. After marking it up; it was re-assembled and the car was put back together.

Turns out.

It made absolutely no difference,...


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Everything on the engine has been replaced now so I am very stumped as of what to do now?

 

Bummer, Terry.

I can't recall - have you checked the Vehicle Condition Monitor for any stored Diagnostic Fault Codes?

Are you certain there are absolutely no air leaks in the intake between the MAF sensor and the intake manifold gasket?

I know you observed the fuel flow, but it might be worth actually measuring the flow rate to confirm the fuel pump is supplying sufficient flow for higher rpm.

Don

 

This week I will be trying to fit a fuel pressure gauge to formally check it out. Not sure what condition the pump is in or if there is any crud in the tank.

Would have thought it would have acted up intermittently or just completely given up and died by now.

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The car went from fully functioning to not being able to push past 20 overnight due to the poorly running engine. Then in less than 12hrs, it could not push past 10kph or 1000rpm.

Seems to have stumped 1 master technician, the local dealership mechanics, and me [automotive studies teacher].

Whatever the solution is, it is going to be either very simple or something completely overlooked.

 

Quick story about a 1992 XJ40 4.0 I owned years ago. The crank damper failed so I sent it to be rebuilt. I got it back and bolted the crank sensor 'timing ring' to the damper and installed it.
The engine idled fine but would misfire when the engine RPM was increased.

I installed a timing light on each plug wire in turn and revved the engine to determine if it was ignition and what cylinder. The timing light would go 'dark' on cylinders #2 and #5 when the misfire occurred. I knew it had something to do with the damper because everything was fine until I got the rebuilt damper returned and installed.

I rotated the engine slowly and measured the air gap between the CKPS and the timing ring. The gap changed from almost touching to a large gap. I put a small washer as a spacer between the CKPS and the mount bracket to get the distance greater so it did not touch.

The engine ran normally after that!!!!!

I don't know if the damper got warped during rebuilding????
I just know it worked for years with the small spacer.

bob

 

Hi Bob

I will check that out tomorrow, will inspect with a feeler gauge.

This one is a complete mystery as to why it runs so badly.

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In reference to earlier posts.

No VCM fault codes are shown, and the air intake has been thoroughly checked.

I also clamped off the vacuum hoses one at a time to try to see if any are causing problems. None made any difference to the running.

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Time permitting I will try put the CPS sensor on an oscilloscope too.

Just a hunch that it might be sending incorrect information to the EFI system.

Other than those theories, It is still quite difficult to pinpoint what could be causing such an error.

 

If you think it might be ignition, then putting a timing light on each plug wire can detect if and when the spark is or is NOT present. That is how I determined that the misfire was at the same spot on the timing ring, #2 and #5 are the same section of the ring.

bob

 

Ok so fuel pressure checks out, had to make up some NPT fittings from my local farming irrigation store but it is above what would be deemed sufficient in the book. Looks like someone replaced the fuel pump before I bought it.

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Today I took the CPS sensor and its bracket out for a good clean and a good inspection.
Nothing to note.

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Afterwards, I conducted a timing test. Recommended by Motorcarman, and it had been on my list as a suspicion for some time too. Backfiring being an indication of this. So I did a formal and accurate test today.

We have now pinpointed something to fix !

- At idle the car sits perfectly on the timing marks. helped by some chalk I applied.

- At a point shortly after revs are increased the timing jumps about. It seems to advance briefly to the rhs. But also advance about 15deg to the LHS when it is running rough. However, it jumps about in a wild flurry when this occurs. I can only assume the car has no idea what is going on after 1000 rpm.

Note; to fix this I am assuming I will have to replace the harmonic balancer, and or the CPS. It might also help while I am at it to get a modified or revised cps bracket.

I am up for advice.

 

I was not typing about aiming the timing light at the timing mark, just seeing if the timing light is flashing steady with EACH spark plug wire.

I checked #1 and the flashing was STEADY, #2 went dark at times and the flashing was erratic. #3 was fine as were all the plug wires except #5 that acted the same as #2 and they are the same place on the timing ring, just 180 degrees apart with the camshaft.

I don't know if this has been mentioned but you might also check the distributor to ensure that the rotor is pointing to the correct place on the cap and not trying to feed 2 terminals at once when the timing is automatically advanced by the ECM.

bob

 

Terry,

Sounds like you're making progress thanks to Bob's great suggestions.

Check with XJRengineer to confirm, but I believe his timing-advance CPS bracket will only fit the '93 and later XJ40s because they have the same wider crank pulley and CPS bracket as the X300s (for which he designed his bracket). The '92 and earlier cars have a narrower crank pulley and Andy's bracket won't properly center the CPS over the reluctor ring.

But to be sure, contact Andy's wife Helen at helenstodart@gmail.com to ask if the bracket will work on your car.

Cheers,

Don

 

Hi

So this week the car is booked into a friends garage, and we are going to investigate quite a few things.

- Accurate fuel pressure. My gauge is really dodgy.
- The accurate vacuum in the manifold. I have not got the gauges for this.
- Anything that springs into their mind too.

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Where possible I am going to put an oscilloscope on the spark leads.
Also going to follow up at home and put one for the CPS and see if they match up.
Just need to make sure that mine works, as it is very old and really dodgy.

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CPS revised bracket - There is a smaller one. But they are made in much smaller batches.
I am on a waiting list.

This work will be compleated should the car run properly again.

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Note; this week I did advance and retard the distributor and it made little to no difference to the running condition of the engine. It did die when turned fully back and shorted the two distributor posts. But as expected the distributor position made little difference. This was predicted as, the car was running fine before this happened.

Thank you, for your continued support.

 

So Fuel pressure runns at 40PSI - Sufficiant.
Vacuum pressure is steady in the green sector. - Sufficient too.
Oscilloscope and injector timing is good. - Sufficient.

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As last time I pinpointed that the electronic engine timing is the problem.

Idle it is fine.
Any applied throttle and it runs at 30deg after tdc.

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Solution 1 is to find a new EFI computer to put in the boot.

Solution 2 is to check all the earth points on the vehicle.
Might also invest in a new earth strap too.

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If anyone knows a good place to buy an EFI computer for this model please let me know.

Likewise, I will be checking the earth points this week.

 

I know you've swapped the CPS and checked the connections but is it possible the replacement CPS or the connection has a fault? After all, you say it ran ok (but was hard to start) before you changed it.

Have you tried unplugging the ECU and cleaning the connection and replugging it?

Just a few suggestions from afar

This issue must be terribly frustrating, not to mention expensive (but think of all those nice spares you'll have in stock when it's all over)

Larry

 

Funny thing was, today while I was to work someone thought of the same thing. They actually soldered the wires together while I was not looking!

They did this after they hooked it up to an oscilloscope. It works well and true.

Made no difference whatsoever, but is going to be a real pain if I have to change one over in the distant future.

Connection wise it is certainly ok.

The ECU was replaced recently, turns out that was not the problem either.

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Focusing on getting a second-hand EFI computer at the moment.

The system is firing 30 degrees after top dead centre only when the throttle is applied.

Also going to be checking the earth points tomorrow in the boot around the EFI computer.

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Frustrating but as you say, lots of spare parts for the near future.

Possibly a Jaguar that is reliable too.