ok guys. After a few "failed to proceeds" Here is what I've done. Put a small hole in fuel cap. Installed a fuel pressure gauge under the bonnet and a low pressure "idiot light" in the cabin.
Everything seems to going along great (fingers crossed).
On a test run today, the light (set to come on below 30psi) stays off most of the time. The only time that it came on was whilst going down steep hills (foot of accelerator). Get to the bottom and put the foot back down on the accelerator and light extinguishes. WHY?

 

WHY???, HAHA its a Jag, just joking.

You have the TPS set JUST RIGHT, so congratulations.

The ECU is set to "fuel off" when some things align (as in the planets).

With the accel pedal OFF, and engine speed above about 1200rpm, as in down hill running, the ECU cuts fuel, (TPS in the 0.32- 0.36v range), so when re applying the gas there is no backfire due to unburnt fuel. More of an issue with CAT cars than ours of that era down here.

Same applies at highway speeds, and the throttle is lifted off to slow for a lower speed limit.

Ours is quite audible, sports type exhaust, and the "burble" heard is a sign of confidence that all is OK.

The high vac in those situations would also have the FPR wide open, thus lowering a pressure drop to I "think" about 28psi, but that is memory only so no flame if I am wrong.

Nothing to worry about in my opinion.

 

According to the ROM fuel pressure should be between 28.5 and 30.8psi unless it has been adjusted higher.

Maybe the reason pressure drops on closed throttle down hill is the pump is turning off as the pressure builds then not turning on again until pressure drops below 29psi - just a guess

 

I think it's exactly as Grant says: high vacuum = open pressure regulator = lower fuel pressure.

Cheers
DD

 

I'm not following you, Grant.

The fuel cut-off is achieved by the ECU taking injector pulse down to zero....switching the injctors 'off', basically.

But that doesn't have anything to do with fuel *pressure*.

Am I missing something?

Cheers
DD

 

Warren, I think you're reading from the "pre-HE" section of the ROM !

The HE fuel press spec is 36 psi and it doesn't have the adjustable regulators as the pre-HE did.

I'm not aware of the fuel pump being commanded 'off' under any circumstances. Am I missing something?

Cheers
DD

 

Yeah silly me

The fuel pump does not run continuously when pressure is built to a point the pump stops. Like when the ignition is turned on you can hear the pump run until fuel rail pressure is up and it turns off (maybe this is ECU priming the rail). Thinking out loud here - but running FPR controls pressure only if this is closed will the pump turn off, so it is not likely with a running engine.

I do love that burble on trailing throttle with my sports exhaust.

 

Maybe its my ability , or lack of, to put into words whats in the brain.

The HE, and PreHE fule pump is a constant, except for the timer thingy at ignition ON.

With closed throttle, and engine revs sensed as above 1200rpm (approx), the ECU cuts the injector pulse (Doug is right). Once revs drop below this value, and assuming the throttle is still closed, the pulse returns, so the beast dont stall. Obviously if the throttle is opened, the pulse is returned.

I believe this engine speed is sensed via that shielded wire.

The fuel pump never stops running, and excess fuel is returned to the tank as normal.

The TPS setting at the correct split voltage at idle position is critical for many running issues, and this is only one of them.

 

I did a small video of my fuel pressure yesterday.

It's around 30 PSI at idle, rising to around 38 PSI when vacuum is low (throttle activated or vacuum line removed)

 

A tad low in my opinion.

Mine was 36ish at idle vac connected, and 41ish without vac.

Tired pump, maybe.

 

Looks Ok to me.

I believe Jaguar specifies 36 psi 'when cranking'...with no other details. I guess the presumption is if the pressure meets spec here then it will be Ok through the entire range of operation.

I've never heard or read anything from Jaguar describing how far out-of-spec the pressure must be before it causes a problem. From experience with a 4.2 Series III (which has the same pressure spec) I know that 55 psi will cause gross over-fueling. From others' experience (also Series III) I know that 18-20 psi isn't enough to make the engine run. There's a lot of room for pondering in between.

But I don't see a fuel pressure problem in your case.

Let's hear what others have to say

Cheers
DD

 

Pump is definitely tired, but if it can generate 38 PSI with vacuum off, it must be the regulator that reduces pressure to 30 PSI with vacuum connected.

 

Correct fuel pressure must be essential to keep fuelling correct on an open loop fuel injection. I'm pretty sure there are no oxygen sensors on my car. Engine and hardware around engine is from an 1989 UK XJ12.
I will connect my eBay - made in China - fuel pressure meter to a calibrated air source tomorrow. It's nice to know how much the offset is...

 

Agreed.

The LH FPR may be getting tired.

There seems to be many reports of failures lately. Not all the diaphragm, but the needle seat, thus allowing bleed off, and improper pressure.

 

Diaphragm is OK, no fuel in vacuum hose.

I'm about to order parts and just wanted to know if I should replace the FPR as well.

The HE engine was mounted in 1996, 57.000 km ago, and fuel pressure at that time was noted in the log book to be 2.5 kg = 2.5 bar = 36 PSI.

There are no later notes about the FPR, so it is most likely the original 128.000 + 57.000 km old FPR I have

 

If it were mine, yes.

They are not that expensive now from what I hearing.

I got mine 20+ years ago, and they were about $100.

 

Just FYI the injectors are specced and rated at 3 bar by Bosch. That's 43.5 psi. If that is the rated pressure I'd assume that they should operate at that pressure when the FPR is not seeing vacuum.

My OEM FPR was at 42 no-vac before I replaced it while grasping for straws. Grants made 41. Some correlation there.

Seems a likely enough assumption that Jaguar would have designed the system to the injectors published specs.

This doesnt suggest a on-vac FPR rating but I bet those Bosch FPR specs are out there too.

 

Of course, but in my mind a question remains as to whether or not your 30psi @ high vacuum is actually causing a problem. I wouldn't think so, but.....? Some increase and decrease in accordance with vacuum is normal...but there's no spec to guide us.

How quickly does the pressure increase when vacuum drops? That might be important....but again I'm not sure.

Cheers
DD

 

It reacts instantly on vacuum changes, check my video a few posts earlier in this thread.

I started looking at fuel pressure due to the engine was running poorly some weeks ago.
Major issues turned out to be bad sync of butterflies due to a broken throttle axle bushing and bad connections at power resistor pack, but it is still missing a little in idle. Idle is by the way too high (around 1000 RPM), but I suspect the AAV needs a service.

 

I will buy a new FPR. I just felt it would make sense to check if it was off spec before doing so...