I've been suspecting my XJ6 runs too rich, simply because of lousy mileage, 6.5 km/l (15 mpg) on mostly freeway and country roads to work. So I bought a CO% tester. At idle it shows 2.8%, I've read online 1-3% at idle, so that's ok'ish. At cruising speed the web says to get 1% on normal engine, 1-3% on performance engine. My stock engine shows 3.6%, which confirms it's too rich. How do I adjust the non-idle mixture on this car? I can replace fuel pressure regulator and its sensor? But can I adjust any other way?

It runs fine, but mileage has to be better...

 

There is no facility provided for such an adjustment. If you're too rich, there's a fault to be corrected....but it's likely subtle and therefore difficult to zero-in on. Or, quite plausibly, a stack-up of faults due simply to age/mileage and result loss of optimal operation of various components.

Here are some thought starters:

XJ6 Series - EFI Rich Mixture

Outright faults with any of the things mentioned could cause a gross mixture problem....but slight degradation of any of them is certainly possible and could account for what you're experiencing.

I'll mention that the Series III XJ6 was never ultra fuel efficient. I used one as a daily driver for many years. In city driving 12 mpg was typical. On the highway I could manage 20 mpg if I was very judicious but 17-18 was typical, so perhaps you're not that far off the mark. And, 70 mph was a magic number where economy dropped-off quite a bit.

Interestingly, the V12 Series III, with an additional 100 horsepower, can match the fuel economy of the 4.2 very easily, and even better it on the open road in favorable conditions. But, I digress.

Cheers
DD

 

Thanks Doug
I read the link as well. A few specifics:

1. The sender to the water temp gauge, is that also the sender that the fuel injection uses or is there a separate one? If yes, where can I find it? Gauge shows 90C, most left part of green area
2. The mentioned extra fuel pressure to avoid vapor lock, would my Swiss sold 85 have it, you know? I doubt that's the issue though, I tried to remove vacuum from the sensor that runs vacuum to the fuel pressure regulator, that led to 4-5-6-7 CO%, indicating that the system works. I'm tempted to try to plug the small vent hole in that sensor, that should lower the CO%. Makes sense?

If all fails, I'll buy new water temp sensor, fuel pressure sensor and fuel pressure regulator, and see if it fixes it.

My road to work is mostly 50 mph, some 65 mph, very few stops over 30 miles, so I think I should be at 20 mpg. Forgot to mention, Champion 12 heat range plugs are a little black, not seriously, but too black to be right.

Thanks!

 

Sorry, but you seem knowledgable, so I'll ask this too. :-). The microswitch at throttle stop, mine is hrashed, what is it supposed to do? I've tried shorting the wires, nothing happens I think.

 

The temp sensor for the fuel injection is the #3 position from the front.




Your car probably does have the fuel temp switch. it is brown, attached to the fuel rail, two vacuum ports. At xxx-temp it opens the vacuum line to atmosphere, eliminating vacuum to the regulator, allowing more fuel pressure.

From what you've described, if I understand correctly, you do have the switch and it is not the problem

Quote:

. Forgot to mention, Champion 12 heat range plugs are a little black, not seriously, but too black to be right.

They should be light tan or grey, of course, so I agree....you do have a bit of a rich running problem

Cheers
DD

 

This may be very important.

Jaguar used different switches for different purposes depending on market....so I'm not 100% certain. However, I think that micro switch provides extra fueling at xxx-throttle opening. I'll try to verify that later, or perhaps others will chime in. But, I think you may have discovered your problem

Cheers
DD

 

Just ordered another micro switch, and will test tomorrow if shorting the wires affect CO%

 

As I recall, the micro switch was merely there to detect closed throttle and cut the fuel supply to the engine until the revs had dropped to a defined value, but higher than tickover, so I don't think it will really solve your problem, although obviously it is a fuel saving device. What could cause overfueling is an incorrect input from the coolant temperature transmitter, or a malfunctioning fuel rail pressure control. This is a blow-off device that allows the pressure difference between the injector and the manifold to be kept constant.

Have a read here: -

Fuel injection and the Jaguar XJ6 4.2 Series 3 / AJ6 Engineering

I see Roger Bywater mentions in the fault finding paragraphs that engines in European cars that are slightly rich everywhere could have a faulty full load switch or disconnected vacuum line.

 

Thanks Doug and Fraser

I read the link to AJ Engineering's excellent article. And tested some stuff:

-Again I tried shorting the two wires to the now missing micro switch. At idle: no effect on anything I can see. At cruising speed: engine rpm starts to zig zag up and down rather violently

-I found the temp sender. Taking the wires off, engine dies. I had hoped I could maybe short it to see if CO% was affected. No. Probably best to buy a new one. My German parts guy shows both 215600 and 513520, which one I wonder. On epay there are plenty that look alike...

-In AJ Engineering article a vacuum switch at WOT that could be faulty is discussed, I can't find such a switch."An engine that just seems a bit rich everywhere may have a faulty full load switch or the vacuum pipe to the switch may be detached (Euro versions)."

The two adjustment points, the larger in the manifold and the smaller in the throttle body. I assume the last mentioned only affects idle, and the first mentioned affects the full range?

 

Hi,
Have you checked if the capacitor mod has been done? Here is a link to a how to https://www.jaguarforums.com/forum/a...cold-start.pdf. I had all sorts of idling problems and Rich running as well. The cap mod had been done but the capacitor needed replacing. Helped a bit along with other stuff already mentioned. Worth a look at least.
Con

 

The vacuum switch is EAC4371 (same as the V12 cars) and looks like this




But I don't know where it is located on the engine exactly

Cheers
DD

 

If you disconnect the wiring plug and jumper the terminals inside the plug, you'll simulate a 'fully warm' reading to the ECU. Worth trying to see if anything changes.

I'm not familiar with either of these. Please explain !

Cheers
DD

 

There's an Allen head bolt in the manifold, Green arrow, not visible on picture, I think it's 5/16" (you need a bent tool to turn it). There's also an Allen head bolt in deep in the hole, Red arrow, smaller size. I've read both are plugged, mine were not, maybe US only. The Green is supposed to adjust idle speed I see now, you're not supposed to adjust on throttle plate, that has to be 0.002" gap. The Red is supposed to change idle mixture.
Am I answering my own question? ;-)

 

Anybody know where to look? I haven't seen it...

 

Right, OK, idle speed adjustment and idle mixture adjustment.

Neither influences the overall fuel mixture


Cheers
DD

 

Perhaps under the intake manifold? Just a guess.

On some models a vacuum regulator for the distributor is located there, so why not an enrichment switch?

Cheers
DD

 

Thanks for input. That plus reading the service manual and googling, I've come to this:
Micro Switch, being this is a on-US non-Emission XJ, is an over-running prevention switch. It's closed when throttle is released and then it cuts fuel until it's below 1400 rpm. That's why it violently went up and down when I shorted the wires at 2000 rpm, it cut fuel, dropped rpm, below 1400 it got fuel again etc

My two things to check now are:
-Find vacuum full throttle switch, see if shorting etc changes anything. It's supposed to be on throttle cable bracket
-Try shorting the wires to water temp sensor

Need to wait fro some free time... We'll get there in the end...

 

Today I tested. Couldn't find any vacuum linegoing to anything with electrical wires on...

But progress, kinda, on water temp sensor. I took the wire off and shorted it, and CO% fell to 1.5%. Then I put it back on, and it stayed there! ???. Maybe a bad connection? In sender or wire? I'll buy new sender for sure. But not sure if I need 215600 or 513520.

 

My DOHC is gone. But, I continue to wonder as to it's eccentricities!!!

So, I have a simple generic idea. Remove the air cleaner element. Read the O's...
dirty one can enrich. Don't ask how I know that...

Carl....

 

Air filter is new, thanks