The V8 Owners Forum -

hi, i'm thinking my fuel pressure regulator is throwing my 95 3.9 v8 discovery into limp mode...

my question is, can they be rebuilt, reset or in any way fettled. it would save me the time and expense of buying a new one and be a good trick to know

Phil, that does seem very unlikely, because the electronics functions as they might relate to the limp mode may not be affected by the purely mechanical/vacuum functioned FPR.

Also it entirely possible to test the FPR with simple procedures. so no need to replace unless it is toast.

Doubt if one could ever be fettled/repaired tho'.

Also just to understand how it works may lead you to agree, so have a read of the PDF available from this link.

http://www.vintagemodelairplane.com/pag ... ply01.html

Sure the Efi system is not exactly the same as your '95 car but the FPR principles of operation hold good.

thanks for the reply, i've been told that if the fuel pressure regulator was knack'd then the engine would be running so rich that the ECU couldn't reduce the injector timing any further, and the ecu would get totally confused and throw her into limp mode...

does that make sense to anyone???

I think the person who told you that was wrong.

The ECU under the control of various sensor inputs determines how long the injectors must be opened for in each engine cycle, to provide the exact amount of fuel needed for a correct Air/Fuel mixture, under any circumstance, speed, load, temperature, etc..

To achieve that, the fuel pressure across the injectors must remainabsolutely constant at all times.

But to remain constant at the injector pintle means that the ACTUAL pressure relative to atmosphere must vary according to the manifold vacuum.

That issue is fully explained how this is achieved in the referenced essay.

Ramon

welshphill wrote:thanks for the reply, i've been told that if the fuel pressure regulator was knack'd then the engine would be running so rich that the ECU couldn't reduce the injector timing any further, and the ecu would get totally confused and throw her into limp mode...

does that make sense to anyone???

The regulator reduces the pressure as vacuum rises. I'd guess the most likely fail would be where it didn't reduce the pressure. Making it run rich under light load. So the lambda sensors feedback would make the ECU attempt to compensate?

As others have said the FPR is unlikely to be faulty as if it is faulty then it would more likely show a lower pressure. This would be due to say a bit of crud holding the valve off the seat. This all assuming that the vacuum pipe is sound.
It would be worth checking the tank return pipe for restrictions / kinks as these would put the pressure up.

However I'm not sure if an over rich mixture would put the ECU into limp mode. If it reacts that way to a very rich mixture signal from the lambdas then possiblly it may do so if the designers considered a rich mixture to be detrimental to the engine. Unlikely, but I don't know for sure.

BTW have you tried disconnecting the ecu for a few minutes and then try again.
Regards Denis

DEVONMAN wrote:BTW have you tried disconnecting the ecu for a few minutes and then try again.

yep, tried that, but if i turn the ignition off, after about 10mins she'll pop out of limp mode, but with-in a min or two pop back in!!!

btw, this is only when she's running on petrol, on lpg she will run, pull, rev (when you 'kick down' the auto box, she will happily rev to 4,000rpm) faultlessly...

running on petrol, in park or neutral (4 speed auto box) wheels not turning, you can hold any rev you like, for as long as you like (but with petrol prices in mind, i've not taken her over 3,000rpm or done this for more than 1m30s)

When mine goes into limp mode it will still rev just it's sloppy rich

Rich enough to make your eyes water with the fumes - in the back garden - not even confined space

Ian

If it is going into limp mode, there should be a fault logged on the ecu, testbook/rovercom should show it.

Know anyone nearby with diagnostic kit? I woudl get this done as it should help isolate the issure to get it fixed.

I have it for my RRC but suspect your along way away

I would suspect Lambda sensor/s is down or a cat problem but could also be the airflow meter.

Ian Anderson wrote:When mine goes into limp mode it will still rev just it's sloppy rich

Rich enough to make your eyes water with the fumes - in the back garden - not even confined space

Ian

try taking her round the block...

it's when shes in gear or the wheels are turning that the ecu limits the revs to 1550rpm

Manual box and still revved like hell!

Ian

wheels turning????

welshphill wrote:wheels turning????

Try disconnecting the speedo sender wires.

Regards Deniis

where would i find it???

will she run ok without it connected...

welshphill wrote:where would i find it???

will she run ok without it connected...

The sender will be somewhere on the gearbox with either 2 or 3 wires coming from it.
There should be a connector nearby which you can unplug.

The vehicle will run without it but the speedo will not work if it's an electronic type. Also the engine will not switch off the fuel on over run so it will not be so economic. But, if it solves the problem you will be closer to a cure.
Regards Denis