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90 XJS V-12 Problem

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Michael J.

Jedi Warrior
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Well, Finally got running following engine rebuild, and a host of problems that never seemed to end. Finally got things sorted out, or so I thought.

Was driving the car. This car has a Keisler 5 Speed Conversion. Occasional rough running and almost cutting out followed by smooth. Roughness seemed to be at 200- - 2500 rpm. I was attributing this to a half a tank of seven month old gas. Added water absorber, injector cleaner and another half tank of gas. Still occasionaly rough with low power, but less often. Gas seemed to be it.

Suddenly almost shut down when reved to about 4,000 RPM. Limped home at idle. Now car idles smooth, although at about 1100 rpm (was that high before the problem), but when the throttle is opened even a little, runs rough and rpm surges up and down almost 2000 RPM. Take your foot off the gas and it idles smooth.

I looked for disconnected vacuum lines. I did not see anything disconnected in the underhood wiring.

I have no idea where to look next. Any ideas? I need help!

Thanks
 
Have you checked the fuel filter? If the fuel additives seemed to work, there is a possibility of some fouled injectors. Check the lines from the tank to the fuel pump, in the boot, see if the are soft, may collapse. But, I would check the filter and think about stopped injectors, the alcohol is not Jag tolerant and old fuel with solvents lets sludge loose. A couple of bad injectors will play a lot of havoc on the V-12. Keep us informed will help as much as possible.
Larry
 
Michael. I have encountered this problem and I think I know what is wrong. The V12 has a logic mode built into the ECU that shuts off the injectors if it senses a non standard condition. This can lead to up and down surging of the RPM (about once every two seconds). The culprit that caused this was the throttle potentiometer calibration. I am going to check my records on this car and confirm what I did to correct the situation. It is a very disturbing fault but is relatively simple to correct. I will get back with you probably tomorrow. Hang on till then. Jack Farrington
 
Here is what I have found out thus far. I changed the fuel filter. That did not affect the problem. The fuel that I drained from the filter did not appear to be contaminated. The filter seems to have a rattle. I will cut it open after the gas fumes are removed to see what is inside.

I pulled the exhaust and looked at the catlytic converters. I did not find any evidence of breakdown.

I had another throttle potentiometer from an 87 model. I installed it to see if it affected the problem. Because I had the whole tower assembly I did not have to change the setting at all. It is a different type in appearance.

It did not solve the problem.

I look forward to hearing back from Larry & John on how to adjust the potentiometer and/or how to trouble shoot if the spare was also defective.

Thanks for the suggestions thus far.!
 
Michael. This problem is a result of the logic built into the ECU. When it detects that the throttle is not being opened, (no gas pedal depresion) but the engine RPM speed is over a built in limit, the ECU thinks that the injection system does not need to be fed gas so it shuts the injectors off. When the RPM goes lower than the ECU limit, it allows the injectors to work This condition can be caused by the trottle potentiometer in some XJS models and by the idle speed adjustment stops, etc; in other models. You must tell me what specific ignition system your XJS is equiped with and what year XJS. Is your ignition a Marelli type or is it an earlier version of an HEI system? I am assuming that your car is an HE V12 which came on the market in 82-83 time frame. If it is a modified system that is not Jaguar standard, I am not sure I can help you over the internet. With this information I can lead you through the trouble shooting and fix but it will be difficult as compared to my hands on the trouble. Let me know, Jack Farrington
 
The car is a 1990 Marelli ignition car. Corsaro Electronics in Houston had looked at the ECU earlier this year (in March) when I could not get the car to start after the engine rebuild. They changed the Prom Chip and told me that the reason that I was having black smoke on heavy accelleration (a problem that i was trying to solve when I had a head gasket failure resulting in the engine rebuild) was that Kiesler Automotive had set the Prom for a full rich setting. The last step in the Keisler kit installation is to send them the ECU to make the change to address the lack of kickdown (?) during braking.

Cosaro told me that he used a standard XJS V-12 prom program and that the only thing that Kiesler Automotive should have changed was one setting to address that the car no longer had an automatic. He said that he made the setting change to the standard chip. It was only recently that I sorted out all the other problems and was driving the car for the first time and faced by the present challenge. I don't know if there is an ECU problem here or not. Corsaro was recommended to me by Pauls Jaguar in Fort Lauderdale.
 
I checked the Throttle Potentiometer according to the procedure to test for a six cylinder XJS since there is not a procedure to test the V-12.Testing with a volt-ohmeter between yellow and green wires indicated 5.5K resistance through whole range of throttle opening. Testing between red and green wires gave 5.5K to start and dropped smoothly and gradually to 800 ohms.

Factory manual does not say how to set the position by adjusting the potentiometer in the three slots. The six cylinder procedure calls for a test unit that illuminates an LED at the correct setting.

Any ideas on how to determine if the potentiometer is in the correct place?
 
Michael, I am having trouble sending you this response. This the third attempt. Now that I know you have a Marelli ignition, your problem is with the throttle potentiometer. You must have the correct part for your application and it may be calibrated without the Jag tester. I will research my records for this procedure (requiring a multimeter) next week. I am sure that this problem can be solved. In the interim, back off of your idle RPM as much as is practicle. Also check your linkage to each throttle butterfly valve for irregularities or free play. Best regards, Jack Farrington
 
Since I last posted, I reinstalled the original pot, checked for vacuum leaks and found that the butterfies were off. I reset them to .002. I also found possible air leaks at the red plastic line that provides vacuum to the ignition module and fuel injection ecu.

The problem appeared to be corrected. My son and I drove the car for about 30 minutes. Still seemed a little rough when letting off the throttle, but otherwise was running good.

I let it sit for about two hours. The mrs. and I went for a ride. Ten minutes into it the problem resurfaced and we limped home at idle again.

So, Is this heat related? I'm going to make sure all connector plugs are in place. Perhaps something came loose. I still have no clue what is causing this.

Any other ideas or suggestions?
 
By the way, I cut open the fuel filter that I removed with a tail pipe cutter. For those that have never seen the inside, it sesembles a 3" diameter toilet paper roll with corregated paper. The tube is a plastic piece sealed at ine end.

I did not find what ever was rattling inside. The element appeared to be clean. I don't think this is a fuel issue.
 
Here is where I am at, as of tonight. I noticed that if I disconnected the idler switch (at the rear of the left intake on Marelli Ignition Cars, the hunting occured at idle. Reconnected the switch, back to smooth idle.

On the throttle Potentiometer Pedestal is a switch that is engaged at a certain throttle position and rides on one of the "ramps" for the throttle pulley. The Jaguar Parts book RTC9900CA January 1987 on, depicts the switch as item 23 in the exploded view for Throttle Potentiometer 5.3 Litre up to eng no. 8S61794. The Repair Operation Manual Incorporating XJ-S HE, 5.3 & 6.0 Supplements AKM3455 Edition 4/3 refers to the Throttle Potentiometer as a Throttle Switch and the procedure for the throttle switch adjustment is actually for the Potentiometer. I was not able to find a procedure for adjusting what the parts catalogue called the throttle switch. It did not seem to affect the problem though.

At least two separate persons suspect the Potentiometer adjustment as the culprit for the hunting RPM.

The procedure listed for adjusting the Potentiomer decribes either an older or a newer Potentiometer. The one for the 90 model has a three wire connector with Red, Green and Yellow wires. The manual says to measure with an Ohm meter between pins on a five pin potentiometer between pins 12 and 17. The procedure (19.22.35) says to insert a .050 feeler guage between the pulley closed stop and the arm for the throttle push rod lug. It then tells you to adjust the potentiometer until the multimeter flicks to very high resistance and then lock the two screws. There are also two hex head screws on the top of the pulley that are apparently adjustable. The procedure is not clear about which two screws are being adjusted.

What I cannot find, is which two wires (Red, Yellow, Green) on my Potentiometer correspond to the pins (12 and 17) on the newer/older model described in the procedure.

Any thoughts or comments, please?
 
"Michael, I am having trouble sending you this response. This the third attempt. Now that I know you have a Marelli ignition, your problem is with the throttle potentiometer. You must have the correct part for your application and it may be calibrated without the Jag tester. I will research my records for this procedure (requiring a multimeter) next week. I am sure that this problem can be solved. In the interim, back off of your idle RPM as much as is practicle. Also check your linkage to each throttle butterfly valve for irregularities or free play. Best regards, Jack Farrington"

John I would be interested in this info on the Throttle potentiometer for I am facing replacing mine on my 92 XJS V12. Could you e-mail me the info as well as posting it?? TKs
mdallair@capecod.mass.edu or mdallaire@prisons.com
Tks
 
MDAllaire;

Michael J. here, what does your potentiometer look like? does it have a five pin connector? As I said in the string above, both the 87 and the 90 (my car is a 90) have a three pin pigtatil coming out of the pot that connects to the wiring harness. What is installed in the 92 model?

M.J. Baker
thebakers@centurytel.net
 
I need some help from other V-12 XJS owners
You can answer the question that I have just with a quick look under the hood. I may be able to help us both with the information that you provide. If you look at the rear of the throttle pulley you will be able to see whether there is a long flat connector with numbers or just three wires for the Throttle potentiometer.

If you should happen to have the 5 pin connector style, the potentiometer will have numbers visible near where the throttle cable housing attaches. The first two will be 2, 14 with two above the fourteen, 17 by itself, 12 and 47 with 12 above the 47, 20 by itself and 9 by itself.

What I would like to find out is what color wire corresponds to each of the five pins. If the model you have has the same three wire type that I have, we will need someone with a five pin pot to respond after which I can convert the procedure in the book to how to adjust the three wire style. If yours is five pin, I will type out the procedure for adjustment from my book for you.

This style pot apparently includes what is a separate switch on earlier cars into the potentiometer body. What I am trying to figure out is which color wire corresponds to each of the numbers.

Mike Baker
 
Here is where I am today on this. Over the weekend I attended a jaguar club meeting in New Orleans. I got to talking with a guy that had an extreeeemly nice 72 series three XKE.

What he told me was probably sacrilage. He advised that I should jo to Jag-Lovers.org and download the FREE online book called Help For The XJS Owner by Kirby Palm. Palm compiled varoius strings from a competing chatroom and various other sources into a 702 page handbook discussing numerous problems and how people fixed them. If you have not downloaded a copy of this book and you own an XJS you NEED this book.

The first thing that I learned was that one can't test the throttle potentiomer with a dial type volt/ohm meter. Apparently one needs a digital meter, which I now have.

The developement of potentiometer problems, regardless of the style, has to do with the wiper that completes the variable resister circuit. According to several sources in the book, the wiper has a pointed contact that eventually wears through the foil resister arc. What apparently happens is that the potentiometer wears though the foil causing intermittent contact rather than the gradually increasing resistance signal needed.

So I'm going to pull the Pot and see if that is the symptom while waiting for the new pot that I ordered to arrive.
 
Some of the comments in the manual that I printed suggested that another source of rough running was spark plug gap. I pulled the distributor cap and found that the B bank contacts had very little signs of spart jumping from the rotor to the cap, whereas the A bank had quite a bit.

I still had the original cap that was on the car and noted that a true Mirelli cap was twice as heavy as the Beck/Arnley replacement, the Marelli cap had brass contacts and the Beck/Arnley had aluminum.

I cleaned it up and put the orininal back on. Second, I pulled a plug to check the gap. Although I had set the gap at .025 as is called for on Marelli ignition cars, The shop that I had work on it had reset the gap to .035 as would be correct on Lucas ignition cars.

If you have changed a set of plugs on a V-12 before, you can imagine how thrilled I was at the prospect of having to pull and regap the plugs. That will take a few evenings to do.

The new pot arrived. I checked it with a digital VOM and got pretty much the same as the one I removed. I did notice that the wires for the original pot had peeling insulation where all three wires entered the pot.

mdallaire emailed a procedure for adusting the pot by measured voltage. I will test it out and tell you how well the procedure worked.

More to come
 
Here is the info I got for all to share:
From:Houston, Tx.Registered: Jan 2001 posted 11-18-2003 07:53 AM All you need is a few hand tools and a digital voltmeter. A cheap Radio Shack type will work fine. When you install it, leave the screws a tad loose. Plug it in and turn the key on. Ground one lead of the voltmeter and slide the other under the rubber boot of the throttle pot connector so that it makes contact with the red wire's terminal. Adjust the TPS so that the voltmeter reads as close to .370 volts as possible. Tighten the screws and make sure the reading is still .370 or LESS. Higher voltage is a bad thing but too low is as well. Try to get it between .350 and .370 volts with the throttle fully closed. Once you have that, just put everything back together and you're done. ------------------Phil PrinceASE Master TechnicianFactory trainedJaguar & Land Rover Tech

Thanks Phil...would it make sense to check the voltage reading on the present one prior to considering a new part?? Or once they go the go.------------------Mike92 XJS V-12 Coupe

Wouldn't hurt to check it, but I think yours has an intermittent problem. It is basically a rheostat and they sometimes get to a worn spot and don't send the proper signal. ------------------Phil PrinceASE Master TechnicianFactory trainedJaguar & Land Rover Tech

Just pop the two rods off and remove the 4 7/16" nuts that hold the turntable in place. You will have to disconnect the kickdown switch and full throttle enrichment switch wires. It doesn't matter which way you plug them back in, BTW. As long as the right wires are on the right switch that is. After that you can turn the turntable upside down and get to the TPS. ------------------Phil PrinceASE Master TechnicianFactory trainedJaguar & Land Rover Tech
 
This will be the last post for this string. I think that the problem has been resolved. That is not to say I don't still have a problem. I have looked at virtually every possible source of vacuum leaks and still can not get the engine idle lower than 1100 rpm. That is with the idle adjustment "screw/bolt" turned all the way in.

I am going to start a new string seeking to lower the idle.

Well, I pulled the plugs. 2A-6A and 2B-6B were set at .035". 6A was apparently broken at the shop and they replaced that one plug with an autolite plug in place of the Bosh Platinums I had installed. I regapped all to .025" as per the owner's manual for a Marelli ignition car. In spite of some intermittent hunting, after following the procedure for setting the Throttle Potentiometer (a very special thank you to mdallare), I was able to drive the car. I did encounter sporatic roughness, but nothing like the original problems. I also set the idle mixture on the ECU 3 clicks CCW when looking at the adjustment. Does anyone know if that is rich or lean?

Thank you to everyone that took the time to offer suggestions.
 
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