Howdy all!

I recently bought a wonderful '84 XJ6 here in Southern California. It's a 55K mile original, garaged and covered for the last 18 years -- without moving. Of course, *those people* will tell you that's the exact kind of car NOT to buy, but I figured with a $700 purchase price and the help of Rock Auto's "closeout parts" list, it's a worthwhile project.

I started with the usual -- draining of all fluids, old gasoline, added new fuel hoses and filter, plus added rebuilt injectors to boot. A shot of ether, a turn of the key, and it runs. Surprisingly good, in fact. Just as smooth as one would expect.

But it only runs good cold. For about 5 minutes. Once it reaches temperature, it begins to stumble and ultimately quit. Then, it won't re-start. It partially catches, stumbles, but won't stay lit.

Any ideas? Is there a sensor which switches the engine operation once it senses operating temperature?

I truly appreciate your ideas -- without a forum like this and generous support of knowledgeable folks, a car like this would have surely ended up junked.

Thanks!

Brian

 

Brian,
Beautiful car. Looks like a great find. Two heat sensitive components to check would be the ignition coil and/or the ignition module inside the ignition amplifier.
Best of luck with the car.

Larry

 

In this case I first suspect the CTS-- Coolant Temperature Sensor. It's only job is to provide extra fueling when the engine is cold, gradually tapering off as the engine warms up. If it is faulty.....not unusual....it can fail to send the 'warming up' signal to the computer. The result is over-fueling, very much like a stuck choke on an old carbureted car. About $20 and 10 minutes to replace....so not painful as guesses go. Or, if you have an ohm meter, we can give you the specs so you can check the readings.

The prior remarks about the coil and ignition module are spot-on: they typically give trouble when hot. The only reason I'd drop them down a bit on the 'could be' list is that, most commonly, it takes 30-45-60 minutes of driving for them to get hot enough to give trouble.

Cheers
DD

 

Doug is correct (as always): the first suspect here is the coolant temperature sensor (which is easily replaced) - and it is also the first suspect if the reverse symptoms are experienced: that is, the car tending to stall before reaching the normal temperature range; in that case the fault is that the CTS is telling the ECU that the engine is already atproper operating temperature and therefore is not providing the enrichment needed during thge warming phase.In the case cited by the OP, the engine is being overfueled because the sensor is telling the ECU that the engine is NOT up to operating temperature.

 

Gentlemen,

As always, THANK YOU for such great ideas! I started with the easiest; The coil. I pulled it away from it's mount on the engine, wrapped it in a flexible ice pack, paper towel, and foil, thus keeping it nice and frosty while I brought the engine up to temp.

It started great and ran the usual five minutes before the usual and telltale sputter began; from previous trials, I knew it was about to stall. Since I was right there observing the coil, and the air filter assembly was already off, for the hell of it I placed a finger inside the Mass Air Flow Meter and pushed the flap a little bit. Sure enough, the sputtering stopped! By playing with the flap manually, I could keep the engine running as smoothly as I liked, even once hot. (Allowing it to close to where it wanted to would kill the engine immediately.) This makes some sense now as to why it runs great ice cold when it wants that rich mixture.

Thus, it looks like the Mass Air Flow Meter isn't moving it's flap for me as required to keep a warm running engine lean enough.

1.) Any recommendations on cleaning / testing the unit?
2.) Could there be another switch / sender / etc that is failing to properly signal the unit to operate properly?

I appreciate the "schooling." I just searched and downloaded quite a bit of material on the Jettronic Fuel Injection system as EFI has always beyond my purview.

Cheers! Brian

 

Also, regarding the CTS or I believe it's also known as the "Thermotime" Switch?

It looks to me (correct me if I'm wrong) that this could certainly still be the culprit, and by manually manipulating the Mass Air Flow flap, perhaps I'm just allowing enough air to compensate for the overfueling. If I understand the schematic correctly, this unit is "closed circuit" cold, and "open circuit" warm.

Thus, on a warm (or warming) engine, If I just unplug the switch it ought to revert to a warm (lean) fueling cycle?

I believe I'll go try that next and report back.

 

Thermotime switch test results:

Passed. I opened the circuit by pulling one of the clips off the switch as the engine approached warm -- no difference. Same thing. Only way to keep it running is to manually manipulate the flap of the Mass Air Flow Meter.

Hmmmm.......

 

 

Just to clarify, the flap in the AFM isn't mechanically controlled. That is, the AFM doesn't move the flap, per se. The flap moves in accordance with the the amount of air being drawn into the engine; suction moves the flap. The movement is converted into an electric signal used by the ECU to control fueling.

If we assume for the moment the the AFM is not faulty then, by manually opening the flap you are telling the ECU that more air is being drawn into the engine than what is actually being drawn in.....and the ECU would then add more fueling to go along with the additional air it 'thinks' is being consumed. Since the engine apparently likes this, it might suggest that you are actually running too lean.

I'm not saying there isn't a problem with the AFM but, personally, I'd eliminate some other possibilities first.

The only test, for DIYers, for the AFM is substitution. Back in the day dealers may have had some special test equipment. I dunno.

Cheers
DD

 

The Thermotime switch is entirely different. It operates the Cold Start Injector. The cold start circuit is operational only when the key is turned to 'start'. A problem here might account for a starting problem but not a running problem. The Thermotime switch is at the very front of the coolant rail. The CST we are referring to is the third device from the front.

The throttle blade is what let's air into the engine, not the AFM flap....although a stuck AFM flap could certainly prevent air from entering !

Cheers
DD

 

Right, and IMO the thing to do in this case is jump the plug after 5 minutes of running and see if the engine behavior changes.

Removing the plug outright adds so much mixture that the engine will outright die, immediately

Cheers
DD

 

NEXT STEPS: Sender diagnosis, update

(Thanks for sticking with me!)

I believe I found the proper sensor location, it's approximately center in the rail. It has the molded plug (white) that fits over the sensor.

I started the car as normal and it ran great as usual. As it got warm and started to hesitate and vacillate, I pulled the white plug. The engine continued to run. A little rougher than perhaps I'd like, but it ran nonetheless and stayed running (which was a first.) I used the ohm meter to determine that at operating temperature, the switch remained in the "closed" position. I also took a bent paper clip, as suggested, and jumped the innards of the white plug cap whereupon the car promptly quit.

I then unjumped the plug and it re-started promptly and continued to run.

As a final experiment, I plugged the plug back onto the sensor which is now at operating temp and the engine quit again.

Since they are so inexpensive (especially the URO Parts brand), I figure it's a good matter of course just to replace the sensor.

But, just to confirm what I'm reading here, does this diagnostic make sense so far?

Also, for clarity, what is the larger diameter sensor right above the thermostat at the very front of the rail?

Thanks again!

Brian

 

Sensors on the water rail, front to back

- Thermotime switch. Operates the cold start injector
- Temp sender for dashboard gauge
- Coolant Temp Sensor for the fuel injection
- Temp switch for the air switching valve (not all cars have this)

Cheers
DD

 

The coolant temp sensor fir the fuel injection look like this




Cheers
DD

 

I'm not sure if it's ever supposed to actually go fully 'closed', but it develops less resistance as the hotter it gets

Here's a chart

Testing the Coolant Temp Sensor



By jumping the terminals in the connector (zero resistance) you're telling the computer that the engine is fully warmed up so it (the computer) knows that cold enrichment is not needed.

The engine shouldn't die. If the engine cold or cold-ish when you do this test, it might not like it....but it shouldn't die.

With the plug un-jumped (infinite resistance) the engine should die immediately



Have you carefully examined the wiring at the plug? And the little terminals inside?

Nope.

Mulling time required. But in the meantime do carefully check those wires

Cheers
DD

 

Doug,

Thanks so much. I really appreciate your contributions to the thread and to my problem.

1.) I simply ordered a new CTS switch. The URO brand from Rock Auto. Very inexpensive and I've been happy with the URO brand of everything I've ever bought from them.
2.) My car DOES have the fourth switch, at the back, on the rail. The one called the " Temp switch for the air switching valve (not all cars have this)" Turns out that's the one that I mis-identified as the CTS switch and was playing with it (to no consequence to how it ran when warm) If you'd be willing, could you provide me a couple sentences on what this switch does?

Thanks for the link to the CTS switch testing. I'll test mine as a matter of course.

Regards, Brian

 

It operates the diverter valve ("air switching valve") at the back of the air injection pump and the electric purge valve for the fuel vapor canister. From memory it is normally closed and opens when coolant temp reaches about 150-160şF.

It also sends a 12v "+" supply to the "air switching module" mounted on the RH inner fender. The exact function of this module remains somewhat of a mystery to me and I've never seen any operational description in Jaguar literature. It appears to control, or at least have an influence on, open loop/closed loop fueling.

Cheers
DD

 

Hi Doug,

Thanks for the advice -- sure enough, that far back sensor DOES control the air valve. I notice it triggers the air pump which begins to blow about three minutes after running and the engine begins to warm. Works great!

UPDATE:
1.) I installed the new CTS switch.
2.) No change. The vehicle runs for exactly 6 minutes before it begins to stumble and spit.
3.) The car will re-start AND remain running if I unplug the CTS, but it runs rough and clearly rich with the exhaust smell.

Next test: I started the already warm car with the CTS unplugged, and then while manually manipulating the flap in the Mass Air Flow (MAF) Sensor, I quickly plugged it back in. By pushing in on the flap slightly the car would stay idling as smooth as can be. I further then manually opened the throttle and butterfly valve while moving the MAF sensor by hand. I could get the car running as smooth as can be with lots of throttle, very little, or even idle by moving the flap in the MAF just so.

Questions: 1.) Why does the MAF "care" if the car is hot or cold? (the car idles as smoothly as can be for the first six minutes from cold) 2.) Is this the final piece of the puzzle, you guess? Change out the MAF? The MAF does, at least superficially, seem to operate just fine. The flap only has spring resistance, it doesn't "catch" and when I engage the throttle flap, the MAF flap pulls in nicely too. That flap just never seems to be in the right position to have the car run correctly in any other situation than when the car is cold.

It's a Lucas 73172D.

Thanks, as always!

 

The AFM has an air temp sensor inside....but I'm not sure if it has any bearing on this situation.

Anyhow.....

I'm flummoxed by what's happening here. I'd go ahead a try a different AFM and cross my fingers. They don't fail that often so it shouldn't be hard to fond a good used one on Ebay for a reasonable price. Or, even better, give a shout to David at Everyday XJ

Great guy, fair prices

Cheers
DD

 

Dave, thanks!

Also, thanks for all of the "schooling." Between your advice, Haynes, and the factory repair manual, I've learned more than I ever expected to know about a Bosch / Lucas fuel injection system. In retrospect, it is fairly simple and straightforward once all the "bits" are understood.

As this is a solid mystery -- which, someone else will no doubt have at some point in the future -- I'll post the resolution, when I find it, and report back on a swapped AFM. The Craigslist Gods usually bless me and this morning is no different. The exact AFM I need, sitting on a shelf but tested as good some years ago, is an hour away from here in a place I must travel to for business shortly. All for $40. It's worth a swap out and see how she behaves then.

Will report back. Until then, thanks!