I'd like to install a fuel pressure gauge on my rail. I have a gauge with a fitting for modern vehicles with a valve. Has anyone installed a fitting, like modern cars have, on an efi XJ?

Either way, permanently installed or fitting installed, where should I tap into the fuel system for accurate readings? My 82 has the loop style fuel rail.

 

What car? (6 or 12 cyl)

 

I usually connect on the inlet side, but anywhere on the rail before the regulator will work.

The easiest way to install the gauge is to use a t piece.... Which is a straight piece of pipe that you screw the sender into. Then cut a rubber fuel line and install the straight piece.

On my XJS I connected inline on the rubber hose that goes between the fuel rails....

 

Sorry, I have the 6.

 

I bought the brass fitting from Max Tow Performance.. $10.00. I know my regulator is different from your fuel rails, but finding a good fitting was hard.

This one worked perfectly for my installation as I wanted to have both the little FP gauge and the ability to hook up a large one by using a Shraeder valve fitting. The Shraeder valve and FP gauge use the same standard FP gauge fitting size.

 

Well that looks very nice!

I went with a tee at the return between the top of the fuel rail loop and the fuel pressure regulator. I also tee'd the vacuum at the FPR and hooked my vacuum gauge up there.

What I saw was a wildly fluctuating fuel pressure reading. The FPR responding to minor fluctuations in the vacuum level with large changes in fuel pressure. I assume that is not expected or desired behavior.

I tapped the vacuum supply at the first tee after the vacuum reservoir and connected the FPR to that. Fuel pressure stabilized down to 1-2 psi variation rather than the 8-10 that I was getting before.

Idle, stabilized.

Beer, drank.

Is my regulator trash?

 

I hope I don't offend anyone if I reply to my own reply. I have found myself with additional info and questions.

I'm currently running the fuel pressure regulator with a vacuum feed from the reservoir. The difference in the idle is too good to switch back to manifold vacuum. But I wonder about the performance impact this relocation could have.

What has me concerned is that my fuel pressure will not respond to low vacuum with an increase of pressure in this configuration. The reservoir provides much more consistent vacuum and largely modulates temporary fluctuations.

But why the heck did Jaguar connect it to manifold vac if they didn't want higher fuel pressures under low vac conditions? With this series 3 efi there is no way for the efi to know the fuel pressure so it does its injection thing and if pressure varies then the air fuel mix varies from the expected mix. Even under heavy load I wouldn't think that adding a random percent more fuel ( based on low vac raising the fuel pressure ) would be a proper solution. Does anyone know for sure what the intended behavior is?

I only know that the seat of my pants say that it's running better than I've ever seen and doesn't seemed starved at high rpm or wot ( thrilling! ). If it really should vary with vac I will acquire a vacuum delay and use that inline between the FPR and manifold.

 

The source is from the TB, but it is NOT manifold vacuum. Is there another source of ported vacuum you could try?

My FP gauge has a rubber "tip" on it and it had to be "cut" or opened up to the atmosphere...liquid filled and works very steady...

Have no idea what Jaguar did... have never owned one of their engines.

 

My gauge is not liquid filled, but its fairly steady. Currently it's pulsing between 35-36 lbs. I trust that's actual pressure fluctuation but even if it's gauge error I'm fine with that.

I could use the ported vacuum on the throttle easy enough but that would raise the fuel pressure at idle. This regulator delivers 45 lbs with no vacuum and 36ish lbs under full vacuum. I believe the spec is 32 or 34lbs.

EDIT:
"Fuel pressure is specified as 2.5 bar (36 p.s.i.) without vacuum applied but with the engine running this will fall in proportion to the prevailing manifold vacuum."-http://www.jagweb.com/aj6eng/42efi/page6.php
"The fuel pressure will normally be about 32 psi at idle."-http://andthatsjazz.org/jaguar/tuneup.html

I just wish I knew what Jaguars engineers were slide ruling when they gave the fuel pressure regulator variable output pressure. "Put some more gas in it. It's fine."

I'm going to pick up a Bosch regulator and a vacuum delay. If the delay smooths out the manifold vacuum then I'll reconnect the FPR there. If the new regulator is steady regardless of the un-delayed manifold vacuum all the better. Seems the logical way forward given my lack of info.

 

jigjag, never know till you try it!

i run 45psi fuel pressure, at idle,vac applied, mine only goes up about 3 psi ,no vac.

but it is very smooth needle movement, maybe atributed to my electric F pump.

run vac line right off the inlet manifold,at the end.

 

Do you have your AFM mixture screw opened way up? It seems like you're running 10+ psi over spec. I'd expect a very rich condition from that much fuel pressure.

I often feel like an idiot after resolving problems on this car. In this case, all the fuel lines and vacuum lines under the hood were too large for their fittings. Apparently the POs mechanic was out of the correct sized hoses but had good forearm muscles so he just clamped the injector connections down until they held pressure. With worm gear hose clamps for good measure! The vacuum lines, which are all in great condition and thus didn't get replaced immediately, have no clamps so they just had to try their best. It was not very good.

The loose vacuum connection at the manifold caused high fuel pressure, and exacerbated the vacuum fluctuations caused by the occasional miss I was getting at idle. The vacuum fluctuations caused wildly varying fuel pressure which made the misses more frequent and that feedback loop would just drive the idle mad. Once I gave it a steady vacuum the whole thing settled down and stopped fighting with the ECU fuel mix corrections. Happiness, in automotive form, was achieved!

Correct diameter hoses all around and the vacuum settled down pretty well, but not completely. In the end I installed a ball-style vacuum reservoir from NAPA in-line between the manifold and regulator. I drilled out the one-way valve so that it's just a pass-thru vacuum buffer. Output vacuum is steady like a rock and the transition to no vacuum on open throttle is smooth as well resulting in a nice smooth transition on the fuel pressure gauge.

I suspect that a few other XJs out there with a rough idle could benefit from a better managed regulator vacuum supply, but perhaps this isn't the case. Maybe a new unit will not have the same frantic reaction to vacuum changes. I'm still waiting for the NOS Jaguar regulator I ordered to find that out. The available "replacement" options, including Bosch units, either don't even state a pressure spec or are the wrong pressure for this vehicle.

The Series III service manual says that the fuel pressure MUST be 35.5 - 37 psi without vacuum attached. In that manual they do not specify a psi with vacuum attached. But the regulator must vary the pressure as expected by the ECU when vacuum is applied or again, mix issues are inevitable. Since the D-Jetronic system has no inputs about fuel pressure it just assumes you're feeding it properly. Change the supply pressure out of spec and you're going to have mix issues.

I'm now running at 30-31 psi with vacuum and 40-41 without. Out of spec at 108% of no-vacuum pressure, but damn close ( to expected 32 psi listed in Tune up Procedure - XJ6 4.2 Liter Fuel Injected Models) under vacuum.

Here's a short vid of it idling, in-gear and brakes applied ( worst case for my car before this mod ) and it's quite happy. I'm thrilled!

 

Not too elegant but accurate - tee in the cold start injector as shown below: