1990 XJS V12 5.3L with Marelli Ignition.

I finally have the air pump system working and all of the vacuum hoses and wires returned to original condition. I also rebuilt the AAV valve and it appears to be working properly.

The engine runs very smooth and idles at the correct RPM. The only problem now seems to be a lack of power during acceleration.

I dont have any misses, surging or other issues, it just seems to not have the power that it should. It feels like its being held back.

Troubleshooting suggestions would be greatly appreciated.

I'd like to test the Air Temp sensor. Anyone know its operating range?

Thanks!

Mark

 

Brakes dragging?

Exhaust restricted?

Fuel filter clogged?

If you're talking about full-throttle acceleration....are the throttles actually opening all the way? Is the transmission holding in the lower gears as it should?

Is the full load enrichment switch working?

Others will chime in with more thought starters

Cheers
DD

 

My responses in RED.

 

you may consider a fluid and filter change in the transmission , also the kickdown switch and vacuum for the transmission modulator may need looking at ,

BB

 

I had a problem with the trans on my '88 which caused poor acceleration. It was a torque converter fault. Heavy drag feeling coming off the line but OK once up to speed. I can remember the exact details of the problem, though. It was ages ago.

Cheers
DD

 

Well, that covers all the easy stuff that I can think of.

Are there any additional details? Low power only when hot, or only at certain road speed ranges, or only in a certain gear....or anything?

Or is it low power at all times, period?

I'll try to find the air temp sensor specs but I'm not very confident it is the root of your problem.

Cheers
DD

 

The engine seems to run fine at all temperatures. The lack of acceleration is constant.

The engine did seem to have more reasonable acceleration before I restored the air pump. I know the pump consumes some horsepower but the reduced acceleration is far more that I can attribute to the pump. After all, the pump was running before it just was not pushing its air into the system.

Hummmm....maybe the air pump check valve is stuck closed? Worth checking.

I have not checked the transmission vacuum. I will do that this weekend.

I also dont think the air temp sensor is the problem but I want to check it just to eliminate it. I also want to check the water temp sensor.

I plan to do a full tune up this weekend and try to check the fuel pressure. I will let you know if I see any improvement.

Thanks for your suggestions.

Mark

 

does it stumble when you gas it or just not accelerate much?

 

Nope. No stumble. Just feels like weak acceleration.

 

How does the Marelli system advance the timing? If yours was Lucas I'd say your centrifugal advance was seized.

Just wondering

 

There is no centrifugal advance in the Marelli distributor. All timing is controlled by the ECU.

But I do suspect that timing advance might be part of the problem. I will be checking the vacuum to the ECU this weekend to make sure it is seeing a good signal.

 

Just read Kirby's explanation of the Marelli system. He says the computer that handles the timing is near the passengers feet and has a vacuum line to it

I guess that would be in addition to the one to the ecu in the trunk?

Perhaps others may confirm

 

A red vacuum line, as I recall





Right !


Cheers
DD

 

I know that I have the red vacuum line connected to the engine but I have not confirmed its connection to the ECU. I will check it this weekend.

Thanks!

Mark

 

I disconnected that red line when I was working on my Marelli car to get it running. Unlike the vacuum line going to the Lucas ECU, it seemed to have no effect. Yet, I can't help but believe it is something to do with timing, engine load sensing.
What I was wondering, could this possibly be the engine running on one bank? The Marelli car will run on six cylinders pretty well, as it fires the engine evenly on each bank. The engine would run like a inline six cylinder that wasn't misfiring, but be low on power because the six that weren't firing would be pulling the other six along with them. I feel that the a.i.r. pump installation has little to do with low power.

 

The red plastic vacuum line acts like a vacuum advance on a distributor telling the Marelli ignition module to advance. Check where it connects to the module in the R/H footwell on a L/H Drive car.

 

Maybe the TPS? or possibly fouling injectors or low fuel pressure possibly?

My opinion throw away the Auto gearbox, the Air Pump, The Mareli distributor, the Jag ECU , all other parts Lucas and by Magic it will become High Performance. HaHA!

 

Last weekend I began giving the car the "Ultimate Tuneup". I was not able to finish the work because I had to order some parts. Fortunately the parts just arrived and I will be able to install them this coming weekend.

When I finish I will have replaced;

Spark Plugs
Spark Plug Wires
Distributor Cap
Rotor
2 Ignition Coils
2 O2 Sensors
2 Fuel Regulators
Fuel Injector Power Resistor
Fuel Filter
Air Filters
Throttle position Switch
Transmission Kick-down Switch
Auxiliary Air Valve
PVC


I have verified that both the ECU and the Ignition module appear to have a good vacuum signal and that the electrical connections are clean and tight.

I have adjusted both throttle body blades and linkages.

I have measured the Throttle Position Sensor and it appears to be fine.

If all goes well I hope to have all of the work completed by late Saturday or Sunday. Wish me luck!

Mark

 

re: your points in reply to Doug, I have a couple of ideas in blue:
Originally Posted by Doug

Exhaust restricted? NO How can you tell it is not?

Fuel filter clogged? NEW This could VERY likely be your problem. I think it would be worth replacing it again. Once, years ago, I spent months and a fortune on an original 2 door Range Rover chasing the exact problem you have, only to find that it was a duff new filter (exactly the same as fitted to the V12).

Greg

 

Vacuum check?