3.6 manual cutting out at low revs once warmed up

[SimonR]

Hello everyone,

First of all - apologies if you also read the Jag-Lovers forum as I've posted this there as well. So far the Crank Position Sensor and main Engine ECU are suggested as culprits, however, I'd really appreciate any other insights that you may have as this could be a right pain to track down!

This is a thread that I thought I’d probably be posting, given the four invoices in the past 5000 miles for work to try and rectify an issue with the engine cutting out.

I took the car for its first long-ish drive in my ownership this afternoon - about an hour in mixed conditions - some 70 mph some in town and country lanes, with a few shut downs and start-ups. All was well until about 45 minutes in when it was idling and crawling along in slow traffic and the engine just cut out. It immediately re-started on the turn of the key and I continued, but the same thing happened shortly afterwards, again in traffic and then when I pulled up in front of my house and was selecting reverse to get into my drive it cut out for a third time. It’s possible that it’s cutting out as the revs drop naturally to idle but I can’t be sure.

Each time the car restarts immediately with a turn of the key.

I’ve checked the service history and the first instance of this issue was in 2011 (5k miles ago), then 2015, when a new crank sensor was fitted and ‘main relays and ECU connections’ checked. In 2018, the fuel and ignition systems were checked again along with the engine control module (ECU), then in March 2021 the car was returned to the local Jaguar Specialist, where it received Injector Cleaner, and had its engine sensors tested, airflow meter cleaned and emissions adjusted again.

So it looks like this is an intermittent fault that’s been present for some years, and I guess it may account for the lack of mileage in the last decade.

I’m thinking it’s an electrical issue - perhaps some part that degrades with time and lack of use? - but it could be anything - if you guys can work it our then you’re doing better than a Jag main dealership and local Jaguar Specialist.

Perhaps the only other thing that I’ve noticed that may or may not be related is that the car has a slightly lumpy idle and seems to be hunting ever so slightly - it’s worse when cold, but once warmed up it’s not as smooth as it could be. I’m hoping that a service (including new plugs and oil) this weekend may help this problem.

The car is a late (1989) 3.6 so it has the later MAF air intake set up.

[davidr]

Has the temperature sensor been checked? If it's out of spec it may be causing the over fuelling which would make the car lumpy when hot.
It may be worth the modest investment to renew it with your fingers crossed? It's only a 5 minute job.

[poprivet]

Worth cleaning, adjusting and re-setting both the throttle body and the stepper motor. Also a careful look at the air hoses there as any pinholes or splits will affect idle.

The Haynes XJ6 (XJ40) manual covers the procedure quite well.

[greeban]

As many of the usual culprits have been replaced, as Pop suggests could be an air leak as you have a lumpy idle. Try spraying some brake cleaner around the various vacuum hoses and throttle body - if the revs rise you have an air leak.
Another possibility is the air flow meter but these rarely give problems but I changed mine for a non OE replacement as one of the lugs on mine had broken. Afterwards my engine would cut out intermittently when idling. Put the old one back and all was well - I had gone through the hassle of changing the fuel pump before I tried this as thought the new part could not be an issue - nightmare!

[SimonR]

Gents - I really appreciate those answers, thanks a lot for taking the time to reply.

I'm going to give those possibilities a thorough investigation and report back. Time is a little difficult to find at the moment with a 2-year-old and a major gardening project (to keep the 2-year old happy and safe), however, I'll get to it asap and report my findings.

Happy Jagging for now

[SimonR]

Just a little follow up on this issue as I *think* I may have solved it.

First of all thanks to all who replied - I checked the temp sender and also used WD 40 to test for vacuum leaks - I didn't find any faults.

As Pop suggested, I had a look at the XJ40 Haynes manual but before I got into doing any resets, my (newly-fitted) trip computer gave me a fault code and also I've been able to experiment a bit with the fault. My best guess is that I've got an intermittently bad throttle potentiometer, but what do you think after reading the below?

As I've been able to drive the car a bit more, I've been able to see exactly what's happening. The actual fault is that it stops registering throttle inputs temporarily. The engine revs drop to idle and then if I release the accelerator the engine will keep running, albeit the trip computer reads FUEL FAIL and gives an FF6 code. The engine only cuts out if I press the accelerator pedal and open up the throttle butterfly - so that means that the mixture is leaning out as more air rushes in but no more fuel is injected.

I checked here: http://www.jagweb.com/aj6eng/fault_codes.php and FF6 means "Airflow meter low/throttle potentiometer high - incompatible."

My interpretation of all this is that intermittently the throttle potentiometer is failing, so that the ECU thinks that the throttle is closed when it's not.

The only thing that is making me hesitate is that every time, turning the ignition off and back on again clears the problem and it immediately behaves as normal again- this isn't necessarily what I'd expect from a mechanical potentiometer failure!

I'm tempted just to buy a new part and see if the problem goes away - I'm hoping that it's not an ECU issue that resets when it's powered off though.

[PaulGover]

"Airflow meter low/throttle potentiometer high - incompatible." would be a "plausibility error" on a more modern ECU - it can't believe both inputs - from what you've described, the airflow meter says there's no air, but the throttle pot say it's wide open. So at a guess, the air flow meter is not working. In my older cars, the airflow meter is a moving flap, and squirting carb cleaner in can often ungum the bearings/remove the deposit sticking the flap to the pipe, but I'm not up on the 3.6 engine.

My guess about the restarting clearing the fault is that all the movement from the starter, plus perhaps the engine bursting into life, shakes it enough to clear the issue.

I've had failed throttle pot on my V12 - it gets roasted. AFAIR, the part was no longer available; I found a similar spec device on-line, but it had the wrong direction of rotation so I had to fiddle a mount that let me turn it over top to bottom.

(On the V12, the throttle pot does not control the fuel flow; it's used to determine if the throttle is opening fast, in which case the mixture needs enriching - rather like the damper dashpot in the old SU carburettors - or closing fast, in which case it cuts fuel to prevent popping back. The fuel flow is controlled by the air flow meter. I expect the 3.6 is similar.)

[SimonR]

Thanks Paul - it remains a bit of a conundrum in that case.

The airflow meter on the 3.6 is a hot wire type rather than mechanical and visually mine appears to be pristine. It would be a fairly simple and cheap operation to get a new air flor meter and see if that cures the fault, I suppose.

I think my next step is to follow the instructions in the Haynes to see if all the voltages and resistances are correct on my car on the air flow meter and re-check the temp sensor in the thermostat housing, however, it's all pretty academic as the fault is intermittent.

I shall keep scratching my head until I get the time to look at it properly.

Cheers for now,

Simon.

[poprivet]

A long shot but one worth checking. The mass air flow meter should have an earth connection, a ring under one of the small nuts. It is easy for this to get left off become corroded.

In fact its no bad thing to check all the other earth connections, especially the ones that go on the panel above the radiator and not forgetting the main large braided earth to the engine. All these can look ok but have gained electrical resistance over the years.

[SimonR]

Thanks for the suggestion Pop, I'm happy to report that the earth wire on the MAF is definitely in place and not corroded - I had it apart a few times while I was playing with the K&N filter and also replacing the brake flexi-pipes a little while ago.

Your mention of the earth points on the top radiator panel has me thinking though, because at some point someone has sheared off a stud from that panel, not far from where the air cleaner goes, and that whole assembly is flapping around slightly. It's on the list of things I need to fix, but it wants removing, stripping and re-painting.

I'll check those earth points and clean it all up - it could explain an intermittent fault.

[SimonR]

Well, this is a bit of a weird one! I undid, cleaned and re-seated all the earths I could find in the engine bay but have to say that none of them looked to be corroded or anything other than factory-fresh.

That was last week and I celebrated by taking the car to Duxford Imperial War Museum (about 5 miles away). The problem happened 6 times on the way there - each time I flicked the ignition off and back on and carried on with my journey. At the end of the day, I got back into the car with trepidation, expecting the worst on my way home, however, I got home without any problem at all, and have covered another 100 miles since with no reliability problems at all.

I'm not saying that the problem has gone away, but am left wondering if it is symptomatic of the car having only been driven for a few hundred miles a year in the last decade. Hopefully it's waking up a bit now and fingers crossed that more miles on the odometer will do it some good.

[SimonR]

The trouble-free motoring didn't last very long - I took the car out last week and it just kept cutting out on me so something had to be done.

The FF 06 FUEL FAIL code identifies a mismatch between the throttle potentiometer and the air flow meter, so seeing as the latter is fairly inexpensive to source second hand that's where I started. A suitable unit from a 1986 car was therefore purchased on eBay for £45 and I fitted it this weekend.

It's a bit early to be conclusive, however, after its 40 mile test drive this afternoon, it appeared to pull better, certainly idled better and didn't cut out once.

I wonder how long it will last!

As an aside, I tried to trace a suitable new air flow meter for my car and although the ones for the 4.0L engine are fairly easy to source, there was nothing for the 3.6L engine anywhere that I looked. Having my original one rebuilt is something I may look into - does anyone know of a company that does this?

[poprivet]

When the mass air flow meter is changed, it may be necessary to adjust the Idle fuel screw (the allen key screw on the body).
Half a turn of the screw will change the CO reading by 0.3 to 0.4% Clockwise to enrich and anti clockwise to weaken.

Some years ago I changed mine and it failed the next MOT on emissions, luckily the garage knew about it. This isn't adjusting the air mass meter circuitry but is signalling the ECU which makes compensations to injector pulse width.

The adjustments will only work if the engine is at normal operating temperature, all throttle settings are correct and there are no fuelling failures on the system.

[SimonR]

Cheers Pop - I'll get it to my friendly MoT tester over the road and see if it needs to be altered. I checked the fuel economy on the trip computer whilst test driving it, and it's doing roughly the same MPG as with the faulty unit, if not slightly more, but I know that's only a ball=park figure.

[SimonR]

Hello again chaps,

I've re-visiting this thread because unfortunately I've still not managed to find a solution and have hit a bit of a brick wall. Hopefully someone can advise about using an older-style throttle potentiometer, and fitting a later-style plug.

The current situation is that even with the new MAF fitted, I'm still getting intermittent cutting out with FF 06 FUEL FAIL messages. This suggests to me that there are only three other scenarios possible:

1. Throttle potentiometer is intermittently giving false readings
2. The ECU is intermittently causing the error code
3. A wiring fault.

The most likely scenario as far as I can see is that the throttle potentiometer is the culprit, and I'd already bitten the bullet and purchased what I thought was the correct replacement part, DBC2889. However, on receipt, this turned out to be for later cars and it has more wires/pins than the multiplug on my car, and is a different shape.

I phoned David Manners, whence it came, and they confirmed that this is the wrong part, and that the correct part is NLA. I also can't find the part I need anywhere in the second hand market. I should have asked then what the correct part number is, although I now know what it will look like when I see it.

From what I can see, the actual potentiometer (i.e. the red part that sits on the bottom of the throttle spindle) looks exactly like the part that is used on the earlier V12 cars, EAC9634, and has the same number of wires, but with a later multiplug on it. I am wondering if I should try modifying one of these by soldering on the later round, blue multiplug from my suspected failed one - anyone know if this would work? - I suspect it depends on the resistance range that the pot sweeps through from end to end.

Here's the multiplug on my car (the left hand wiring is from the potentiometer):