Hello forum friends, I’m having trouble with my '86 Jag and I’m lost on what to troubleshoot next. My XJ starts up well when it’s cold but then it doesn’t idle well. I have to keep the idle around 800 RPMs to keep it running. Once I start down the road it will often stall and then need to crank for 30 seconds or so to start up again. When the engine is hot it seems to run very well. However, once I shut it off, it’s very, very hard to start again. I tried the following:
Replaced all the vacuum lines.
New fuel pump
I believe I verified the fuel pressure regulator is working (maintains around 30 psi)
Hi Adam, I have just spent a bit of time making sure my Coolant Temperature Sensor was working ok. I had similar starting problems from cold and in the end it turned out that the sensor wasn’t plugged in. (Thanks to the previous owner) Once plugged in the Beasty started easily. What happens is if the CTS is not working the ECU thinks that the car is already warmed up and doesn’t give the motor enough fuel (think no choke as on an older car). You can check it with a ohm meter. When the motor is cold measure the resistance across the two pins on the CTS it should show very high ohms. Warm the car up and check again, the resitance should be dropping, I currently measure 319 ohms with my motor at 75 degrees Celcius. Here is a data list that was supplied to me from Aristedes as a guide:
Hi Adam
Is it a V12 engine? Do you have black smoke at the exhausts? Maybe then running too fat caused by a broken temparature Sensor (on the B head in front which gives the watertemp signal to the ECU). Try to take the Bosch plug out of the sensor and shortcut the two contacts on it with a wire. If it works well, its the sensor.
Best regards, Gregor
Everything else working as it should, the amount of air regulates engine rpms. No pedal input should be required when starting and idling; enough air is delivered through the throttle gap, the idle screw - and, when cold, the AAV provides extra air to compensate for cold engine ‘drag’.
With incorrect settings the idle will be low and the engine may quit when pedal is released. While driving the opened throttle will provide the necessary air - and the idle settings are overridden/bypassed.
Verify idle settings by removing air duct, clean throttle body and verify/set throttle gap to 0,002". Check AAV function; remove top air hose on the AAV and check slide position. When cold the slide should be about half open - and closed with the engine hot.
With the engine hot; try adjusting idle screw to the prescribed idle. If the engine does not respond to the idle screw; you may have an air leak. Which may cause bad idling and difficult starting when hot - when cold; the extra fuel provided by the CSI may partially compensate. Check air duct connections between the AFM and the throttle body - and verify that there are no tears/rips…
Crudely; air leaks, up to a point, have minimal effects at speed, the idle setting have none - but both will be evident at starting and idling…
There may be deeper causes for a bad start idle and running…
As an aside; the xk is strictly a ‘feet off’ starter when correctly set up - the engine will baulk if pedal is used. Check that the throttle cable does not bind - and has the spec slack…
Hi Con, I believe my CTS only has one lead. Does that sound right? I linked to what I believe the CTS looks like on my vehicle. When I unplug it the temp gauge quits working. Can you confirm this is the sensor you’re referring to?
No. There is some information somewhere as to which of the three (?) it is; the one you show is for the gauge only!
Try a search - coolant rail sensors ore something.
Hi Adam, The one you are looking for is the second to last one on the coolant manifold. It is next to the one that has 2 little tubes sticking up with no connections. I’ll post a pic later if you can’t find it.
Con
I was able to do some testing today. If I tested the correct sensor (second from the left on the coolant rail) and my ohm meter is functioning correctly then the sensor is probably bad as I didn’t see any variation in resistance as the coolant temperature changed. I did notice that the engine temp never exceeded 140 degrees which seemed low to me. I’m guessing in addition to the coolant temperature sensor (CTS) being bad I might have a bad thermostat. I read somewhere that the factory thermostat doesn’t run the engine hot enough and that I should look into a thermostat that would bump the temp by a few degrees. Any opinions on that?
Also, does anyone know if the '86 xj6 has a crankshaft sensor? I was thinking that could also be contributing to the rough idle.
Lastly, if someone could help point out what the different sensors are on the coolant rail and their function I would greatly appreciate it. Moving from left to right I know that the number two sensor is the CTS and third sensor is for the temp gauge. Does anyone know what 1 and 4 are used for?
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The CTS has two prongs, Adam, but you seem to be measuring the correct one. What resistance did you actually read…?
Your 140F is far too low, but the dash gauge is not that reliable - an infrared back-up reading would be useful. If the temp is that low, the thermostat is defective - but the standard thermostat is just right…
The frontmost sensor is the thermotime - which operates the CTS.
Running cold will not compromise with engine operation, it should run perfectly at any temp. But a defective CTS will give wrong temp information about temp to the ECU - and the mixture would be wrong. But the engine will run perfectly during
the time the resistance in the CTS fits…
Since my Jag has a hard time starting when the engine is hot, is there a chance the thermo-time switch isn’t working, and it’s sending extra fuel to an already warm engine, flooding the engine?
Lastly, I can’t seem to find a part number for the crankshaft sensor. Is there an easy way to identify the correct part # without removing it?
Adam,
When your 1986 XJ6 was new it came with two documents that in my opinion are absolutely necessary for owners to have and read, the green “SERIES III HANDBOOK” and the white “XJ6 SERIES III MAINTENANCE HANDBOOK”.
Both documents contain a wealth of important information about how the cars operate, many useful diagrams of equipment locations, and specifics of how many of the systems function. There is a very detailed write-up about the Electronic Fuel Injection system on pages 50-56 of the “XJ6 SERIES III MAINTENANCE HANDBOOK” with a lot of helpful information about the sensors. If you have these two documents I recommend that read them from cover to cover as they should address not only many of your current questions and issues but also others that you are likely to wonder about in the future.
If you don’t have these documents then you can find them on eBay by searching for “Jaguar XJ6 owners manual”. I just looked and there are several of them listed in the $20-30 range. Jaguar made some changes to the Series III XJ6 over the production years of 1979-1987 but the manuals from 1984 to 1987 would be the most accurate for your 1986 XJ6. A 1986-1987 manual will be the best.
If you plan to work on your car yourself I highly recommend that in addition to the two owner’s manuals you get copies of the Series III Service Manual, the Series III Parts Catalogue, and the S57 Electrical Guide. I own, drive, and do most of the servicing and repairs on my three Series III saloons (two XJ6s and one XJ12) and I refer to these documents regularly. Using these documents has not only saved me time and money but they also removed a lot of the mystery about these cars for me, but not all the mystery.
You mean 800 rpms, Adam - right…? - [yes, 800 not 8000.]
Verify idle settings by removing air duct, clean throttle body and verify/set throttle gap to 0,002". Check AAV function; remove top air hose on the AAV and check slide position. When cold the slide should be about half open - and closed with the engine hot. -
[I will check the AAV tonight and throttle gap. This is a good tip]
With the engine hot; try adjusting idle screw to the prescribed idle. If the engine does not respond to the idle screw; you may have an air leak. -
[The engine does respond to the idle adjustment. Is there a tool for getting to the idle screw without removing the AFM]
Which may cause bad idling and difficult starting when hot - when cold; the extra fuel provided by the CSI may partially compensate. Check air duct connections between the AFM and the throttle body - and verify that there are no tears/rips…
[All duct connections seem to be good.]
As an aside; the xk is strictly a ‘feet off’ starter when correctly set up - the engine will baulk if pedal is used. Check that the throttle cable does not bind - and has the spec slack… - [I do hear the AFM “flapping” when the engine is trying to start. Is that normal?]
I got the car used off off craigslist and already fell in love with it (despite the issues). However, it didn’t come with any manuals. I’ll order them off of ebay. Thanks for the feedback.
Yes - and you can also search the forum‘s archives until your mauals arrive!
I believe the ‚flap noise‘ is normal, if it’s not backfire. There is a distributor below the air filter, but it is not points but with electronic pickup (there is an hall sensor in the distributor, goes to the ignition amplifier box on the intake which runs the coil. The coil lead goes back to the distributor which distributes the spark… you get the idea). Loosening the clamp bolt for larger adjustments and try the small screw (bottom of dist. to block) for small adjustments.
By ear you can aim for highest idle, then run up a hill in the highest gear and retard timing until it doesn’t ping/knock which you’ll hear. Make sure the engine is hot for final adjustment, of course.
No, I don’t know which way to turn - try tge search function.
On the jaguarforums.com you might find the workshop manuals as pdf under the XJ FAQ.
Adam,
Getting the correct documents and using them is the path that I took and I am very glad that I did. I have learned over the past 18 years of XJ6 ownership that there are a lot of things that are well within the capabilities of a Do-It-Yourself hobby mechanic like myself to do with occassional assistance from Jag-Lovers for specific help or questions. Some of the maintenance tasks have many complicated steps that are not intuitively obvious and some if not followed correctly (like adjusting valve clearances and working on the timing gear) can result in costly damage. Many of the maintenance and service procedures are multi-step not suitable for quick emails. When I became well armed with the appropriate documents I found that emails to Jag-Lovers helped to clarify steps or resolve issues.
**
Some have manufactured their own tool for the difficult to access idle adjust screw, Adam - gluing a cut off 5,5 mm hex to a socket. Which usually give enough leverage to turn the screw by hand. Or invented some other intricate solutions…
While checking the AAV as described is easy, and important; a full check-up of the idle settings must include cleaning the throttle body, and verifying throttle gap. Problems in this area may cause issues…
A more intricate problem with the AAV; it has the hose on top, coming from the air distribution block - and one underneath going to the throttle body. At times the latter hose may slip off - causing an unseen vacuum leak. It’s difficult to access; you may see it from under the car - or unbolt the AAV for access…
To avoid interference from the CSI; clamp the fuel hose going to it - it is a failsafe test…
The AFM is moved by airflow as the engine is cranking - flap position is a vital input to the ECU. Telling how much air is going into the engine, and how much fuel to add to get the mixture right. With constant air flow the flap is steady, but may ‘clunk’ when cranking stops…
Just for the record; remove the air filter and move the flap through the full range. There should be even resistance throughout flap motion and no binding while opening or closing the flap…
At some stage, since the car is recently required - have you checked engine compression? Just to verify unambiguously that the engine itself is not the problem…
