No, there’s several sensors on the water rail, the small one with the single wire is for the gauge and the similar one with the square plug is for the ECU. There’s a trick to bridge (with a resistor?) or disconnect the sensor so you can tell whether it works.
Concur.
If the correlation is cold running problem then the temp. sender for the ECU is usually the culprit. On the Bosch system disconnecting it will cause the ECU to revert to baseline mapping and the car should be driveable when cold. And to confrim what David says, the sender for the gauge has nothing to do with the sender for the ECU.
It’s a NTC thermistor, and it should read something in the area of:
8-12,000 ohms at 14 deg. F
2-3,000 ohms at 68 deg. F
700-1,000 ohms at 122 deg. F
200 ohms at 176 deg. F
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While the CTS should certainly be checked, Jack; was there any exhaust smoke during the acceleration?
A specific fault in the AFM causes overfuelling during warm-up acceleration/load; see ‘cap-mod’ in Archives, and is easily fixed.
Except for exhaust smoking the symptoms of too little and too much fuel is easily confused, and observing the exhaust is one diagnostic. However, since the symptoms initially appeared, as your first post indicates, during normal running, engine warmed up and lasted briefly - situation is not that clear…
You may have a bad CTS connection, causing intermittent fat/lean running (exhaust will tell). To test; disconnect CTS connector and measure resistance between the two connector wires - it should read some 2 Kohm. Then wriggle the connector/wires - the resistance should not vary - if it does; it is a possible source of your problem. Which otherwise may require some deeper search…
A failed CTS should of course be replaced - but it is not an intermittent fault source. It either runs the engine too fat (hot) or too lean (cold) causing constant symptoms…
Frank
xj6 85 Sov Europe (UK/NZ)
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The failure pattern of the NTC sensor is that of too little resistance- so the symptoms are worse at cold start/cold running as the computer cannot compensate. As the engine warms up, it moves closer to the false data of the failed sensor. It’s not intermittent, it is constant in the cold start/cold running phase.
Again, the easiest way to check is to do a cold start with the sensor unplugged. If driveability markedly improves, chuck a $7 new sensor at it.
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Exactly, Paetersen…
The reported intermittent character of the symptoms doesn’t make sense if the CTS itself has failed. First reported while driving with the engine hot.
The xk doesn’t like lean fueling, but thrive on fat - but there is a limit…
Frank
xj6 85 Sov Europe (UK/NZ)
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Except for the location of the master cylinder, the picture reminds me of the way my engine bay looked circa 2001. Perhaps, not as clean, Sigh…
Carl
Very good suggestion
FWIW, the entire fuel injection system is grounded/earthed at the rear of the water rail; about 6 black wires. Worth checking
Not a bad idea. Normally you check pressure first but with an intermittent problem like this you’d have to check/test while the problem was actually occurring. This isn’t always possible. As guesses go, not unreasonable or too expensive.
Just use a paperclip. The ECU with then provide ‘fully warmed up’ fueling
I can virtually guarantee that the result here will be the engine stopping…instantly. The typical (and plausible) explanation is that the resultant over-fueling is so massive that the engine cannot run. I’ve also heard, but could never positively confirm, the the ECU will simply not pulse the injectors without a signal from the CTS.
Cheers
DD
Might be better to say “third from the front” rather than “rearmost”. Depending on market and year there is often a fourth thermal sensor or switch on the water rail. You can see the un-drilled provision for one in your picture.
Cheers
DD
Yup, but he said he has a UK car so I know they’re the same - mine also stopped immediately when the plug was disconnected. I now remember. I can’t find any information on whether it increases or stops fueling - the only thing I could find was another L Jetronic that assumes 80° when the plug is disconnected.
Carl, thank you, it certainly looks better on the outside than internally, I don’t regret selling it just yet and the new owner knows. It ran very well though who knows how much longer!
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A hot engine would certainly stop with the CTS disconnection, Doug; the xk likes fat - but there is a limit…
The ECU just measures either the current flow, or voltage drop, through the CTS - an analogue function. The CTS resistance table goes to 9,2 ohm (in my manuals) at -10C - but the engines are certainly meant to start at lower temps than that. And that the CTS’s resistance continues to increase progressively (asymptotic?) as temps drop further? Easily tested with the CTS
…placed in a freezer.
The ECU uses the CTS reading to widen the injector pulse for increased fueling, but for all I know there might be a deliberate, built-in, limit to pulse width. The amount of fuel required is limited to the amount of air entering the cylinders - which is limited by cylinder volume. Which means that, as injection happens once every engine revolution; adequate fuel is delivered within a limited pulse width.
Further pulse widening is surely counterproductive - the temperature compensation is to take care of inefficient combustion during start and warm-up. Again, the CTS table shows a minimum resistance of 190 ohms at +100C - but the resistance drops further as temp increases; and the engine will run OK, hot, with the resistance of 0 ohm…using the paper clip. At least until it reaches the melt-down point
Ie, I then assume that there is a limit at both ends; infinite resistance, plug disconnected, will still run the engine, injectors operating - but only if it is very, very cold…?
Frank
xj6 85 Sov Europe (UK/NZ)
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Robert!
Have you or anybody you know have any experiences with any auction co.
to find out the time it takes to auction a vehicle from start to finish?
Thanks,
WalterWould anyone be so kind as to highlight the exact location of the Coolant Temp Sensor on the above engine image please? Apologies, I am still very new to these cars and engines!
I am also unsure of whether the test should be to bridge the connection or to unplug to see if my possible lean-running issue is resolved?
Frank, I have not noticed any exhaust smoke coming from behind the car so I am hopeful it is the CTS. I shall keep everyone posted!
Meantime I have also bought a new fuel filter - I don’t think it’s the culprit but the previous owner hadn’t changed it in a while.
Best regards
Jack
Jack, follow the coolant hoses. Big and small enter in the front, a medium sized out the back, it’s a black casting with 4 sensors going in and it’s the rearmost one aka third from front.
If you folded the picture horizontally the sensor would be on the crease, one third of the way in from the left if that helps.
You jump the connector with a paper clip with a hot engine.
Hi David
Have I circled the correct sensor?
I am not sure why jumping the connector with a hot engine would help me - my issue is only occurring when the engine is cold (we suspect the engine is running lean due to ECU thinking car is warmer than it is, hence the issue vanishing when the car warms up because the false reading better aligns with true temp)
So, will bridging the connections make the car think the engine is warmer or colder than it is?
Thanks
14 degrees centigrade here- resistance reading looks to be accurate on the CTS!
On to the next idea?
Separately I pulled the main lead from the ignition coil to the distributor and noticed it had quite a bit of oil/sludge on it so I’ve cleaned that up a bit. I might take some close-ups of the engine bay for experienced jag-heads to comment if they see anything obviously out of line - some of the wiring looks like it could do with a haul over
Thanks
Correct sensor. I think @Doug_Dwyer can help you best regarding the sensor. Maybe it’s the wiring or the temperature signal is not the problem. Good luck!
1 Like
Ok quick update:
Replaced fuel filter today, old one was full of crum so that’s good to have done.
I fired her up and drove to the supermarket, and the issue was still present, runs absolutely fine until you give it quite a lot of gas (1/2 to 3/4 pedal depression) when it just can’t translate that into revs. Interestingly you can coax the revs up gently to past 2500rpm but it’s when you just open the taps straight away she will just limp along maintaining speed. With foot depressed and engine hiccuping a bit at 2000rpm I did notice some greyish smoke in the rear view but not loads - to be expected I suppose given taps are open but revs aren’t rising - fuel has to go somewhere.
So, on to the interesting bit- I decided to pull over with her cold, switch off the engine and unplug the coolant temp sensor. She refused to start, just cranking but no fire. So along with the accurate OHMs resistance reading i am inclined to rule that one out.
So I went about my shop, came back to the car and drove a little until she warmed up a bit.
And lo and behold, again, she turned into a big cat absolutely rearing to go, one of the most positive sounding engines I’ve had the pleasure of experiencing, just wanting to go and go! Not a sign of any problem.
So I’m beyond convinced something is causing the issue to go away once there’s some heat in there. Any other ideas apart from CTS?
I took a look at the AFM and noticed some kind of sealant around it and you could prise a little - does that sound bad?
Sorry for the confusion, but now I think about it I have only experienced this with a cold engine