I have a 1986 jag xj6, 4.2L. Theirs a problem with idle. You start the car everything ok. But 2 or 3 minutes later it starts to go down. If you run the car for about 25 mts, idle goes down until it dies. I´ve check almost everything and change everything, until I bought a multimeter. When I checked the voltage coming from ecu to tps, there are only 2 wires on the terminal. One sends me 12 volts, the other with ignition on, sends me something strange, 0.100, 0300, 0.500 and then repeats the cycle. Same thing with coolant sensor, 12 volts in one the other is a ground that fluctuates. Fuel injectores have some 12 volts other 11.4 etc. Really strange things that i havent seen in other cars with EFI system. Can someone please help me, i need the voltage send from the ecu to every sensor, coolant, throttle sensor, AFM, fuel injectors. Please help me cause Im going crazy and i love my car.

 

Before going any further, have you cleaned the throttle body and set the throttle gap to .002"? This is most important.

The oxygen sensor is garden variety and should show varying voltage just like any 80s vintage car...nothing exotic about it on the Jag

The coolant temp sensor is usually checked in terms of resistance:
http://www.jag-lovers.org/xjlovers/xjfaq/testcts.htm

You can bypass the CTS by jumping the terminals in the connector...and make sure the wires and terminals are in good shape. Bypassing it makes the ECU think the engine is fully warmed up.

There is no throttle position sensor. What are you checking? There's a throttle switch working off the throttle spindle. 12v on the yellow/white wire with key on and 12v on the yellow/red with the switch closed. I can't remember know if it's supposed to be open or closed at idle.

The injectors should have 12v on both sides with key on, engine off. This should drop to somthing like 8v on the + side with the engine running.

Is there any sign that the engine is running too rich?

Is the ductwork between the AFM and the throttle body in good shape?

Are the grounds at the rear of the water rail in good shape?


Cheers
DD

 

thanks doug. Yes I know, cts is checked in terms of resistance. The deal in here is that the cable that comes from the computer to the coolant sensor sends me 12V plus a strange thing. Its almost ground but it fluctuates between 0.100V. to 0.500V up an down up an down. I usually thought all cars send 5V to CTS, and a ground.

Yes Im sorry, the throttle switch has 2 strange things.
a) switch has 3 visible conectors, cable that comes from the computer only has 2 wires. Is 1 missing?
b) one of the cables sends me 12 volts, other one sends me same thing like the cts, fluctuates between those numbers.
Could this be a ecu trouble?
Thanks doug Im going crazy, an no one can help me with this.

 

I'm stumped.

The 5v supply/reference signal thing is very common, yes, but I've never heard of it on the Jags of this vintage....perhaps because the F.I. system is a 70s design? I dunno. Maybe it use 5v and I'm just not aware of it :-).






The throttle switch was used and wired differently depending on market. Two wires is correct for yours as as far as the wiring diagram (and my memory) goes. Examing the diagrams this morning indicates it's a full load switch. You might try testing it at WOT and see what radings you get, if you haven't already.

The wire with 12v is output from the ECU as far as I can tell. The pin-out from the ECU confusing in that, unlike later/modern pin-out diagrams, only a couple of wires are noted input to the ECU or output from the ECU (and I'm not even sure if the notations are accurate)

I can't explain the results you are getting. ECU problems are rare on these cars so not many people have delved into the finer points...and the Jaguar technical data from that era is poor. They do love full battery voltage and solid grounds, though....so check all relevent pins, connections, and grounds and keep a battery charger going while you run any engine-off tests.

You obviously have some background in this sort of stuff so it's with a bit of reluctance that I stress checking the throttle body as suggested earlier, if you haven't already. There is some history of these engine having a difficult time sustaining themselves at idle and a clean and properly set t/body is an important starting point. I've had a couple where simply increasing the idle speed to about 900 rpm in "P" solved (or should I say "masked"?) the problem.

Sorry I can't offer anything concrete. If I think of anything else I'll shout back.

Cheers
DD

 

Hello and welcome to jaguarforums.com. I know that you are anxious to get started and we are anxious to meet you and talk about your car. We have a baseline / criteria that must be met before full access can be granted. Please enjoy the forum and be patient once the baseline has been satisfied full access will be granted. Please continue to post and accept my welcome to the forum.

Oh one other thing you need to put your car details in your signature. I am speaking of the Year, Make & Model. Without it you may not get the input you are looking for.

Please take a moment and read the information provided to you explaining the rules. Be patient and enjoy the site.

 

oooo, by the way yes, the fuel air mixture is way too rich, you turn on the key and 45 seconds later you cant be on the garage it smells pretty strong to almost gas unburned.
Other thing, i changed spark plugs, cables, clean the distributor, change fuel injectors, etc etc. Just to let you know and thank you very much to your advice, Ive been checking everything you tell, not a professional mechanic but still i got some kind of knowledge.
Thanks.

 

Before looking for any other weird faults I'd get the over-rich mixture squared away. Here are some thought starters:

http://www.jag-lovers.org/xjlovers/xjfaq/efirich.html

Cheers
DD

 

You're welcome. I hope you get to the bottom of it :-)

Cheers
DD

 

Attached is something I archived many years ago about XJ6 overfueling. I have used this procedure many, many times to correct what seem to be weird fueling issues; especially the exhaust that burns your eyes. I compiled it from an old bulletin board system a long time ago. People who have been around a while will probably recognize the name Brian Pel.
Adding the capacitor seems to stabilize the air mass meter signal and makes for a much better running engine, even if it's not a Canadian car!!

 

The "cap mod"...an old friend :-)

I agree that it's a good mod, even for non-Canadian cars and/or cars not exhibiting the actual double fueling problem. Many have reported better overall running after installing the capacitor.

Cheers
DD

 

Amen!
That relatively simple repair makes the average Series III owner think you are a wizard. It makes that much an improvement to the way a 4.2L runs.

Cheers,

 

Thats interesting. I took out the computer and send it to be checked. Guys, told me it was ok, everything perfect. Do you think, that maybe my trouble is due to faulty conections? Other thing is the starter relay. It has like 5 cables attached to it that come from different places. Checking everywhere, I found 2 terminals doesnt have a cable attached to it. So I checked the cables and found a black cable, not connected to the started relay, and was hired over tons of tape. So I checked ondemand software and it doesn´t tell me where it goes. Any idea? Any diagram?
second thing, I checked everything doug told me, just waiting for ecu to come. I cleaned all terminals in back of the car an front, many of them have rust. Other thing, theres 3 cables on the back, near ecu, that had 3 fuses inside a little white box with cone form. Open them, and fuses where ok, but just to old. Any recomendation?
Thank you very much

 

Where did you send it? Just curious....






Sure. About 80% of the infamous Jaguar/Lucas electrical problems are bad connections!







Some starter relays have unused terminals. No biggie. Some are probably tandem terminals....separate terminals that are actually joined together....and one lug just isn't used.

The black (or black/green) *might* be the wire from the neutral safety switch. It goes to terminal "W1" of the starter relay....which now must have a ground wire attached or the relay wouldn't work.




On general principles, cleaning every connector, fuse, and ground you can find is a great way to ensure future reliability on thes old Jags. Repeat every 10-15 years.







Wow, that's not ringing any bells at all. Got a pic?

Cheers
DD

 

Hi doug.
Where did I get the ECU? ok, first of all doug I´m not a US citizen, maybe you can find out that for the way i use to write. Unfortunately, my friend, I live in Costa Rica a central american country and we still live in the jungle. We have no good mechanics. The difference between US and CR is bsically in parts and man work. Man work in CR is very very cheap and useless, but parts are way too expensive.
Second, since I buy this car, i started reading tons of books, Internet info, plus I buy my self a scanner and fuel injector cleaning machine. HAHAHAHA many people even though Im not a mechanic ask for my help and pay me for the service. It has been really cool. I also have my tester and everything but i dont have any mechanic degrees of any source, just books i use to buy in the internet and some things I read in the web. Until the moment i think from all i have studied, analize, and ask people for, your comments and the ones from other members have been the best ideas I found. Books give you an idea, but only experience can show the way.
Ive did what you told me and I turn it on. Ive been having problems because i have it stabilized, it doesnt turn off, but idle is working at 1500 rpm, I have checked the ignition timing with the timing light and its ok. Ive played with almost all the screws in there but havent put it in the point. At least thank to all of you Im getting closer.
Thank you
Again any ideas?

 

So how are you again its me. Thank you for all your help. Now I have a new problem. I have change, fuel injectors, air flow meter, fuel pressure regulator, extra air valve (idle valve i think so), coolant sensor, all hoses, change all gaskets from throttle, etc etc and vacuum lines. The car starts at 1700 rpm if I set it in 17 degrees BTDC as manual says. With vacuum disconnected it goes higher. Check the ECU, clean all grounds etc etc.

Im desperate, and have spend almost $1000 on parts and its not working well. What else can I do, replace cold injector?
When I opened the throttle, I so inside the manifold some fuel thats being storage in there. Plus the mixture is to heavy, 10 minutes and your eyes burn. So it has extra fuel going to the manifold. Could it be a cold injector leaking when car starts? Dont have a clue, thinking of posibilities.
Please someone help me