Over fueling

Hi all,

First time on this forum and hoping to get some guidance. I have a '75 XJ12C that has an over fueling issue. Current symptoms are:
* All spark plugs get fouled after only a short period of time (with no oil smell) - have always used NGK BPR6ES and on recommendation of a fellow Jaguar owner have tried the hotter BPR5ES with no improvement around how the engine runs
* Engine feels lazy at low throttle acceleration and backfires
* When idling both banks seem to have some missing



Have tried the following:
* Fuel rail pressure - have upgraded the fuel rail to the simpler and higher flow later V12 rail and monitor the pressure with a gauge in the dash - pressure set at the lowest end of the specified range at 28.5 psi - details at the following link xj12c.org/category/engine/vapor-lock-current-project/
* Vacuum leaks - almost impossible to be certain that there are no vacuum leaks on this engine but I am pretty sure there are none. I recently removed and resealed the intake manifolds and injectors and replaced all of the vacuum hoses
* Air intake flow - the stock intake boxes have been replaced with high flow aftermarket K&N filters
* Coolant and air intake temp sensors have been checked for resistance values across the temperature range
* new spark plug wires and newer cap, rotor and coil
* Original Opus ignition system replaced with Re-Opus upgrade
* Both mechanical and vacuum advance working
* Last but not least - the MAP sensor has zero leaking and passes the electrical tests in the workshop manual

Here is my current theory. The car started to run worse after we recently moved from Calgary to Denver. I believe the high altitude has something to do with the issue and maybe the MAP sensor is not working even though the diaphragm is not leaking and it passes the resistance checks. Does it make sense to have the MAP sensor checked out further, or are there other ideas of things I should try?

Really appreciate any ideas and help with this.

Hallo, man without a name...
we like to call each other by name here, would you please tell us yours?

And a second question: Did you measure the resistance values at the sensors or at the ECU plug?
Most errors are located in the harness, therefore only measuring at the ECU plug makes sense and gives reliable results.

Regards
Norbert

Hi UNKNOWN,

welcome to my D-Jetronic website.

if your are in the US and have a US engine, you will have a MAP sensor that will not leak vacuum. It is a type 3 MAP sensor with black cap. DO NOT open and detune it as some crazy guys in the US recommend.

There are 2 recommendations from my side:#

1. Please verify that BPR6ES is the correct spark plug. R means has a 5 kOhm resistor against radio interference. My MB car should not have that and a Jaguar E-type with same engine but carburettor also not. But I am not too sure on your car. If you have resistor in spark plug connectors like myself, you will have too weak a spark
2. Verify your ignition timing

Your MAP sensor (which Bosch / Lucas no ?) should have an altitude compensation.

Sorry for the name confusion. Thought it would show up on my post as I entered it when joining the forum. Thanks for the quick responses.

Norbert....I did check the sensors themselves not at the ECU. That could definitely be the problem, but I have replaced the engine bay wiring harness. Could definitely be grounds though.

Volker...you could be on to something with the resistor spark plug. NGK lists the BPR6ES for my year and engine, but the Jaguar spec is Champion N10Y which I don't think is a resistor spark plug. I will try a BP6ES.

The number on my MAP sensor is 0280100123.

Regarding timing, I am running more advanced than spec as I found a few years ago this engine is happier there and with the NA low compression V12, I don't have any knocking or pinging.

Thanks again both of you. Will try the spark plugs and report back.

John

Hi John,

what a pity that you are located in the US. I would love to take a reference measurement of this MAP sensor. Unfortunately too far away. Hopefully no one removed its cap and detuned it. This is unfortunately a very bad habit with Mercedes-Benz 450 SL MAP sensors in the US.

After replacing wiring harness it would be a good idea to check the sensors on ECU connector. Espeically look for engine temperature sensor. If is it missing you have triple fuel.