90 XJS V-12 High Fuel Consumption & Low Power

[Michael J.]

Ran the car over the weekend. Problem still the same. Noticed that the left exhaust felt cool when you stick your hand in the stream. Right side was hot!

No white or gray smoke at the time.

Thoughts anyone?

 

[John S Farrington]

Michael, Did you actually switch the harmonic damper? If you didnt, dont overlook the possibility of the entire assembly rotating due to the key or inner hub having a problem that permitted rotation. That could allow the timing to get out of synch with the crankshaft. I have had a situation where the entire damper rotated on an XK (XJ6) crankshaft. The fastening bolt loosened up and allowed the key to get loose which allowed the damper to spin. I am going to examine my problem V12 for a similar situation. AJ6 engineering in the United Kingdom has confirmed that enough rotation of the damper could allow improper Marelli ignition timing. I have narrowed my problem down to slow ignition timing. Keep me informed on your progress and I will try to help. Jack

 

[John S Farrington]

Michael, Have you rectified or made any progress on your XJS? I have explored my harmonic damper for any irregularities and found it firmly attached. You can tell that it is in the correct clock position from underneath the car. Get underneath the front of the engine and crank the engine until you have the Jag supplied TDC marker on the rim of the damper. It is marked with an arrow (V) and a 1A for 1 A bank. From this position you will see one of the three magnetic tips of the sensor wheel a few degrees ahead of the 1A mark. This orientation is verification that the damper has not turned on the crank. What is your status on finding your V12 gremlin?

 

[Michael J.]

John;

I have not checked it yet, but it should be fairly easy to confirm with the method that you describe, so I will check this in the next day or so and get back to you.

Mike

 

[Michael J.]

John;

I checked the timing mark position using the procedure that you described. I have attached a photo of the mark, the sensor and the "finger" position.

I am rulling out the harmonic balancer based on this, unless you notice that the finger is not in the correct position.

I'm going to have to trace each wire for the Fuel ECU one by one checking for open circuits and cross shorts. This has to be something stupid like a broken wire, or something similar, that was damaged when the engine was pulled and reinstalled. This will take a couple of weeks to sort out. I will post what I find.

I'm still open to suggestions.

 

[Dave Russell]

Pardon the intrusion. It appears that the "finger" is about 10 degrees behind the pointer. Assuming that the damper is on the front of the engine & the engine rotates CW as viewed from the front. Maybe Jags are different? I'll go away now.
D

 

[Michael J.]

Re: 90 XJS V-12 High Fuel Consumption & Low Power

Dave;

The harmonic balancer is on the front of the engine, but I'm going to have to check on the CCW VS. CW rotation. I used to know, but am drawing a blank now.

 

[Michael J.]

Not sure how "amp" got added into the title?

 

[Michael J.]

I pulled the harmonic balancer over the weekend. This is easier if you remove the front pulley first. Getting the four bolts for the front pulley out was a problem. There was not much room to work.

Sitting the harmonic balancer side by side with the used one from a 92 that I had purchased and comparing the distance from the three fingers to the timing scribe mark, both were identical.

The key in the tapered cone was intact. I removed the tapered cone and also checked the key for the crankshaft and found that to be intact.

I went ahead and installed the balancer from the '92 because the rubber between the inner and outer section was in much better shape that on the original.

I'm thinking about trying to order a complete front engine harness through Jaguar. I do not know if it is available or not. I would like to be able to rule out wiring as the cause.

I test drove the car after reinstalling the balancer and pulley, just in the hope that the problem would have been solved in some way. The same low power and high fuel consumption are there. Very frustrating.

 

[Michael J.]

Everyone: Here is what I think is the answer to the problem.

On the '89 on Marelli ignition cars, the ignition amplifiers consist of two DAC 4607 units mounted on aluminum plates that act as heat sinks. there are four wires to each amplifier. The cable passes from the top of the engine near the left side of the compressor to the radiator top support and then splits into a "Y" feeding each amplifier separately. I don't know if I would have chosen to mount them on top of the radiator though. Also mounted to the top support are two clamps or brackets that support the air conditioning receiver drier. The clamps attach to two threaded studs on each side that stick up vertically. The clamp/bracket is securred with nuts and washers to those studs.

The cable "Y" passes on each side of the l/h rear stud. Each amplifier is secured with two screws that connect to nut plates.

When I was connecting everything back after reinstalling the engine, the cable for the r/h amplifier apparently passed right over the top of the stud. There is very little clearance. When the hood (bonnet) was closed it crushed the wire on top of the stud hard enough to cut through the glass fiber cable sheath and through the plastic insulation on one of the wires shorting out the r/h amplifier.

It could have just as easily been the l/h amplifier cable.

The car would not start, so I openned the hood and started checking wires. I did not find anything, but apparently moved the wire harness from on top of the stud. Without the short, the car started, but the damage was done. I don't think the r/h amplifier has been working correctly since then.

In examining the front engine harness looking for anything that I might have overlooked, I discovered the cable outer sheath cut and the insulation cut on one of the wires.

I swapped out the amplifier with a spare unit and got an immediate improvement in power and fuel economy. The spare amplifier unit I had was suspect so I ordered a replacement.

The Original DAC 4507 has orange plastic. That has been superceded by DAC 11520 that has white plastic. I do not think the two can be intermixed. They also cost $285 each!!!!

John Farrington;

Check the 92 you are working on to see if either cable is cut. If not, consider swapping amplifiers out to see if the problem resolves! I had an intermittent problem before in which the amplifier would short internally when hot and the ignition would kill. If the car sat for a while, the amplifier would cool and the short went away and the car would start. I never could determine which was the problem at that time and had to replace them both. Now this problem.

I don't have full power yet, I'm going to have to check the r/h side cats to see if they are damaged as a result of all of this.

In my case, the wire that shorted still allowed the ignition to work once the cable was moved. The 92 may have shorted a different wire to the amplifier and "fried" it.

 

[Michael J.]

Received and installed replacement amplifier, but got only a modest improvement.

Disappointing. I thought that I had solved it.

Back to possible damage in the wiring harness, unless anyone has any ideas?

 

[Lynn Kirkpatrick]

So I'm curious to find out if you got it going. Was it the pinched wires?

 

[Michael J.]

I pulled the rear catalytic converters so that I could look at the back of the front cats over the long weekend. The cats do not show any signs of melting down, even after all that they have been through. I was pretty sure that they were ok because the purple hoses for the air injection to the exhaust ports fail if there is too much back pressure.

It was an immense disappointment that the pinched wires were not the cause. I would have liked to point to a specific cause. I still think that I must have damaged the wiring during the pulling of the engine to change the clutch and just have not found the right wire yet and don't remember which were disconnected at the last minute while jacking the engine.

I have not been able to get a front engine wiring harness yet, I guess that I will have to ask Jaguar if a front harness is available. I need a DAC-5895. I am also going to replace the Marelli ignition Harness DAC 4876 if I can find one.

I'm a little frustrated and out of ideas.

 

[Michael J.]

Paul of Paul's Jaguar recommended sending Rodger Attwater of AJ6 Engineering (who was involved in the V-12 design) a copy of this thread and requesting suggestions for testing equipment and procedures that might get to the bottom of this. I got an email address from the Aj6 website, but have not heard back yet. I hope Mr. Attwater choses to respond.

In the mean time, I am still trying to locate a new or used DAC 5895 Harness - engine 5.3l from V148782 and a DAC 4876 Harness ignition from 89 MY. I asked Jag-bits, but am also widening my search.

 

[Michael J.]

Jagbits has a source that can rebuild wiring harnesses. I'm in the process of pulling the engine harness to send in for rebuild.

Some surprizes in removing the harness. It passes through the inner fenderwell on the r/h side at the rear near the Teves Brake pump and accummulator. You have to remove the inner fender well at the rear. 4 screws. Then the harness passes into the passenger footwell. I have not found the connector yet.

Before I send it in, I plan to check each wire resistance and check it again when the harness comes back. I am hoping to find the "smoking gun" somewhere in this harness.

 

[John S Farrington]

Michael, I have solved the 92 XJS problem (I used my mechanics infrastructure to get very good feedback). Although I had tested the ignition to B bank for spark, I did not conduct a quality test (strength of spark). In my naivette I went on to more complicated failures of the ignition and fuel injection. The culprit turned out to be a weak B bank coil. It was a great learning experience and I must pass it on to you. I wish I could look at your problem with hands on!! A fresh hands on view point is absolutely essential when you begin revisiting the same suspected fault. Hope this helps, Jack

 

[Michael J.]

The wiring harness is one piece from the engine all the way back to the F.I. ECU! I'm still working on removal. I'm having trouble where the harness feeds over the top of the RR wheelwell. I'm working on removal of the r/h rear passenger window motor to get clearance.

Treat your wiring with care. You do not want to have to pull the harness as I'm having to do.

 

[Michael J.]

It was even less fun reinstalling the fuel system harness than it was to remove. The harness was sent for rebuild and testing through Jagbits. I had it back in about two weeks. This is what was found:

1. Poor connections for the coolant sensor on the left rail.

2. Poor connections for the air temp sensor on the left air cleaner were found.

3. A poor ground termination at ECU Shield pin 18 was corrected.

All new connectors were installed except the main connector at the ECU.

Although about 20% better, the problems persisted.

Next was to pull the Marelli Ignition Harness for inspection and repair. It was also sent to Jagbits and routed to a vendor to test and repair. I had it back in about 2 1/2 weeks.

The coil connectors had cracks and some cracks in the wiring insulation at the connectors. These connectors were replaced. This harness corrected 75% of the remaining problems. There was obviously intermittent shorting at these connectors.

The false indication of running rich was actually intermittent ignition for the left bank, with similar problems less often on the right bank. I still have some adjustments to make to improve cold starting.

As improved as this harness made things, the power was not there for the high end. 70 in fifth gear required a downshift to maintain that speed when mild hills were encountered. 70 was the top end.

Looking at all the posts for this problem brought me back to the fuel pressures observed at different speeds and gears and the comments that the pressures seemed low. The car had a Pektron fuel pump with a black plastic case installed. I ordered a replacement from Advance Auto Parts. The replacement pump is rated for a maximum 120 PSI stated on the side. It has a metal sheath and came with a rubber sleeve to fit over the outside. This pump weighs twice as much as the Pektron, which was also not the original bosch pump.

The combination of repairs to the Marelli Harness and the new pump have solved the problem.

The remaining problems are most likely fine adjustments needing to be made.

The sole new problem is that the Tach no longer works.

Anyone familiar with which connector supplies the Tach on a Marelli equipped car?

The Marelli harness has a 1" diameter, 3" long connector with two wires, one white & slate which I think is for the Tach that sits just behind the throttle turntable. I could not find any wires needing to be connected. I did not have this problem until the wiring was disconnected to remove the harness for repair.

 

[Don_Neff]

The Marelli system supplies the Tach signal from the plug on the A-Bank coil.

 

[Michael J.]

I unplugged and reconnected the coil connectors and the tach worked. I apparently did not have it inserted correctly the first time. Tach now works.