D-Jetronic pressure regulator- can just one cope?

Ive seen a few write-ups of projects to use a later style fuel rail in order to tidy up the engine

It would seem neatest to use just one regulator- but is it up to the job?

Are there other designs that can be adjusted to the D-Jet pressure?

1 Like

G’day Russell,

My car, a 1995 XJ12, with the 6 litre engine has a fuel rail with only one fuel pressure regulator which regulates to 3 bar. I cant see why it wouldn’t work on other engines.


Thanks Jeff

The early d-jet system is totally different- uses adjustable bosch regulators at a lower pressure than more modern systems

i’m interested in the performance of those particular regulators

Certainly big 4.5L mercs only used one. I’m not sure if jag used two because of the flow specs, or due to their rail design

My 1988 V12 XJS has 3 bar inlet and 2.5 bar outlet. Many have questioned why the inlet, and many have removed it with no difference. I think Jaguar removed it either by the facelift Xjs or 6 liter.

I left mine becsuse it was easier than replumbing to fuel rail.

my question was very specific to the properties of the D-Jetronic regulator

it is totally different to yours

Ahhhh, sorry. Is that pre HE?

I changed the fuel rails on my 77 Pre HE to a single HE fuel rail some 10 years ago and have not had any issues, using just one regulator.
I have adjusted the rail pressure to 30PSI.
The main reason for the change was to overcome fuel vaporization in the fuel rails in summer, being located in Brisbane QLD., Australia 32 plus degrees C ambient.

1 Like

As I understand it, the pre-H.E. D-Jetronic used FPR’s that held the rail pressure at a fixed value regardless of throttle or engine speed. That’s pretty easy to replicate, I’m sure it could be done with a single regulator. What you’ve got to watch out for, though, is that all 12 injectors see the same fuel pressure. If there is a pressure loss due to flow through the rail that results in lower fuel pressure at one injector than another, that’d be bad.

Jaguar chose to make sure that all injectors were seeing the same pressure with two tactics. First, they made the rail for each bank a loop, so fuel was feeding to each injector from two directions. Second, they provided a separate FPR for each bank. Of course, it’s up to the mech to ensure that both FPR’s are adjusted to exactly the same pressure.

It’s worth noting, though, that for the H.E. Jaguar ditched both of those tactics and went to a simpler tactic: Make the rail using fat enough piping that there’d be no measurable pressure loss. Yes, there are two regulators, but it’s not a one-FPR-per-bank thing, it’s one FRP controlling rail pressure and another FPR just to make the engine compartment look pretty.

You could reasonably expect to do something similar to the D-Jetronic, but you wouldn’t be connecting the FPR to the intake manifold as on the Digital P; that’s a difference in the way the Digital P works. You’d just want the FPR vented to atmosphere. I’d choose an adjustable FPR so I could set the rail pressure according to Jaguar specifications.

Somebody did all this a few years back, basically replicating the H.E. “square tube” rail on a pre-H.E. IIRC, it was claimed that it fixed a hot start problem.

Thanks John and Kirby

I think either Ed S or Bernard E used a single FPR too- with a HE style custom rail.

It seems that the FPR is up to the job

BTW - the HE has a crossover near the front end- might that defeat the ability to purge vapour locks?

I also wonder about having a push switch that operates the fuel pump relay directly- if you thought vapour lock might be about to be a problem, you could give it a few seconds of pump flow before attempting to start