Posted on an earlier thread that I was getting poor mileage (11 mpg max even on long highway stretches).

Slowly troubleshooting down. Next step I think is to look at my vac system. I did check to ensure the ECU is getting good vacuum (and also the idle turns to sh*& when the vacuum line is disconnected from it). Idle is pretty decent and stable at 750 rpm or so. I do get what I think is light knocking once the engine is fully warm and I give it moderate throttle. Basically sounds like someone is shaking a can of ball bearings for a few seconds somewhere deep under the hood. Heavy throttle does not seem to knock so much, and only starts to occur once the temp needle touches the bottom of the N. Currently filling the tank up with premium, which I thought was overkill...

Vac advance gives good movement when a vacuum is put on the hose. Next step I think is to look at the vac regulator and the dump valve. I looked to see if I could find a replacement regulator should mine be bad... and can't find a parts source. All the web based stores I could check list it as out of stock...

Of course this is all hypothetical, as I haven't looked at it yet (and can't until I get the car back from the body shop in a week or so), but nonetheless a little troubling as this seems to be a high failure item. Does anyone know where to source a vac regulator (EAC5157 or compatible part)?

Cheers,

- Will

 

Dear Will,
Did you find a regulator? How did you check the old one? I'm in the same situation as you and would like to know, if the regulator and the damp valve are still working as they should.
Best, Frank

 

I did not find a replacement regulator. I found the plumbing was jammed up but the regulator worked once the lines were connected correctly.

I tested it by hooking up a combination vacuum pump/gauge to the regulator outlet port with the car running. The tool I use is this, although any vacuum gauge will of course work:

Brake Bleeder and Vacuum Pump Kit

If memory is correct, for my market and model year the outlet of the regulator should read about 11" of vacuum with the car running at idle when plumbed correctly. The vacuum supply to the regulator was disconnected, leading to no advance at idle. Initially I replumbed the system running with the regulator bypassed, basically giving full advance at idle, but it seemed to have a bit of a hunting idle. As such, since the regulator was still functional, I plumbed it back nearly stock, only bypassing the cold start circuit and emissions be damned. Once fixed MPG rose to about 13-15 mpg total.

Here are a few resources for you:

Jaguar V12 H.E. A-C Emission Components

Sean's Jaguar Tech Pages - XJS V-12 Vacuum Routeing diagrams

VACUUM IGNITION ADVANCE / AJ6 Engineering

The system is overly complex and varies quite a bit by year and market, so be sure you identify the right setup before you start troubleshooting. Otherwise you won't know what it's supposed to look like when connected up proper!

Let me know if you have more questions.

Cheers,

- Will

 

Hi Will,

Thank you so much for all your explications. I just found out, that my current vacuum regulator is marked EAC4012. I will take out the whole cycle and check the regulator and the dump valve. Your way of testing the regulator is clever and I will try the same. If ever it shouldn't work, the EAC4012 is still available from Jaguar (I actually don't know the difference between 5157 and 4012).

Naturally, I also want to leave out the cold start parts, wich leaves me only with the regulator and the damp valve. I will get rid of the 3 & 2-way selonoid valve, the delay valve and the supp. air valve. Is that correct?

You're driving now at 13-15mpg. Shouldn't it rather be like 20-22mpg? That's what Kirby is saying. I'm running at 16mpg on highway, but I really think it should come to 20mpg when crusing at 100kmh. (I calculate the consumption by myslef, I don't have it onboard).

Cheers,
Frank

 

Vacuum advance system Jaguar XJS HE 1984 Europe

 

Yes!

At a steady 100 kmh/62mph getting 20 mpg should be no problem. I managed as high as 23 mpg with my XJS V12 cruisng steadily at 65 mph.

Note that *any* city driving or heavy throttle will knock that figure down very quickly. You have to drive as though there was an egg under the throttle pedal. Not exactly easy or fun. It takes discipline to drive a "king of the road" automobile in that fashion. A Jaguar, especially a *V12* Jaguar, is not intended for putt-putting along, choking on the fumes of ordinary cars.

A Jaguar belongs at the head of the pack at all times. :-)

Cheers
DD

 

DD, couldn't have said it better myself. :-) Be well, Cheers, FPT

 

Frank -

Recommend you read over the description of your system on the AJ6 engineering page. Roger Baywater was one of the engineers. North American cold start system is to improve catalyst performance and is emissions driven (lasts only 45 seconds or so). Sounds like the European cold start is to deal with higher compression, and is built to combat detonation on a cold engine and is meant to last over a 15 minute period. As such you may want to leave intact if the components are operating properly...

I was previously seeing 9-12 mpg. Now seeing 12-15 mpg. My driving is almost exclusively in the city and running errangs; I live downtown and it's about a 10 mile drive to and from work. Haven't done any prolonged freeway only runs in quite some time.

Cheers,

- Will

 

Hi Will,

I'm driving since years with the coolant temp. sensor unpluged and as far as I know that's quite alright in terms of egine safety. Since I want to check the vacuum regulator and the damp valve and redo the vac-lines, I thought I'm going to leave out the solenoid valves - and actually, I'm not even sure if they work properly, at least for the supplementary air valve, I pretty sure it's dead.

I think I know why the 15min timer was bolted in in the first place, but I can't recall any literature proclaiming to leave the timer in and to make sure it's working. I just reread the Bywater's text - didn't see any warning.

Cheers,
Frank