7A fuel pressure testing and figures

[steve briance]

Hello all!

I was curious the other day as to what the fuel pressure was on my 7A. It always feels like it could go faster and having fiddled, checked and adjusted everything else i realised that in the 14 .5 years of ownership - i have never tested fuel pressure.

I got myself a fuel line adapter and gauge and set about fitting it this evening to see what i could see.

Results make interesting viewing.

Book say 3.8-4.2bar. (idle with vacuum then WOT no vacuum)
difference of 0.4bar.
holding pressure after 10 mins 3.2bar.

mine has a difference of 0.5bar which seems near enough to me but the running figures are 3.1 and 3.6bar.

I'm missing quite a lot of pressure here but the enrichment action seems to be working consistently, just further down the scale. I also checked the holding pressure and it dropped to 2 bar within a few mins.

The car has the 034 bosch injector kit on it which might explain a running difference from factory if the injectors allowed more fuel through but with the engine off, they shouldn't and don't let any fuel through (never have fuel wet plugs or hot restart problems)

Has anyone ever had, diagnosed and fixed fuel pressure problems which turned out to be the FPr? I was just wondering if it could fail and just give lower pressures overall despite still appearing to 'work'. I think it is the original FPr on the car, I've never changed it...

fuel lines were replaced a couple of years ago as they had succumbed to the rust problem. All kunifer pipe now and leak free.

[GreenMachine]

Don't forget that the FPR lowers the pressure with vacuum (under idle conditions) so 4 bar (58psi) - 12 or 14 inches of vaacuum = 44 psi or roughly 3 bar...

Try removing the vacuum tube from the FPR:

A.) Apply a vacuum to the FPR (hand pump) does it hold? If not, the diaphram is toast.
B.) Test the fuel pressure with the vacuum line disconnected to see what the pressure is.

Those injecters shouldn't have any effect on the fuel pressure.

[steve briance]

I tested the FPR with and without vacuum, that's how i ended up with the figures of 3.1 and 3.6bar. disconnected vac pipe and blocked it.

With vacuum i.e the hose back on, the gauge was reading a steady 3.1bar accelerate briefly to 3000rpm and it moves up a bit though not to 3.6 bar and then back to 3.0 bar before settling at 3.1 again. Repetition of the test got the same results. I dare not go to WOT and hold on the driveway, neighbours might not like it! ;-)

I think from that the diaphragm is functioning, I am curious as to how it seems to be just 0.6bar down overall.

Don't suppose you happen to have the part number handy do you? I know other cars run other FPR at differing pressures, i just wonder whether mine has somehow come to have one of those instead of genuine 7A item?

[GreenMachine]

Part number is:

054 133 534

The 3B FPR looks similar but is lower pressure.

http://forums.quattroworld.com/s4s6/msgs/21108.phtml

[steve briance]

Hi, thanks for that!

I will take it off and double check the p/n this weekend.

The 3B item is lower but i 'think' it works the other way round as manifold experiences boost and is under pressure when most fuel is needed compared to almost atmospheric on the 7A. That's a whole new debate though!

cheers

[GreenMachine]

I doubt you have a 3B unit on there but who knows. The 3B unit works the same. It's the engine that experiences boost though. So the FPR will take away pressure AND add it with boost. Same design.

[steve briance]

Well, this weekend i have ascertained that i did indeed have a 7A FPR on my car (054 133 534)

I was given a different FPR by a mate from the forum (cheers Ed ) and swapped that in. It gave exactly the same readings +/- 0.1bar.

Either both are dodgy or the gauge is...

I found another gauge to try and screwed that in to find the figures were now 3.4 and 3.9Bar.

either way both gauges show a low figure so something is not right somewhere!

I did a fuel delivery test with the pump and it gave me 800ml in 15 seconds so no sign of lacking there.

what to do next?

[steve briance]

fuel pump removed and confirmed it is the bosch 040 pump. 102Lp/h and 6.5bar, its only 7-8 years old and not noisy, i have no reason to think it is lacking ooomph.

[mcandmar]

The pumps usually have a pressure relief valve built so it may be bypassing too early limiting the output pressure.

[steve briance]

would that be the round thing on top of the pump that looks a little bit like a FPR? If that is low, it could explain why both pressure tests and both regulators gave the same figures...

[mcandmar]

Its usually just a ball bearing and spring in the plastic top cap of the pump.

[steve briance]

not the big round thing that is screwed into the outflow union on top of the pump?

(pic not my car but looks similar)

[mcandmar]

That is some sort of pressure damper, i didn't know they fitted to 7A engines, it was usually the older mechanical injection motors that had them.

[steve briance]

that's odd. It has always been there as far as i know and i have had the car 14 years.

I wonder if it had an incorrect replacement early on? (the basket did seem to have some marker pen writing on it which i doubt was there from the factory...)

I'll pop the cover off and get a part number to research what it is and find out if its limiting the pressure or just damping it. I understand that the FPR has a damping effect as well, doubt i need to be damped twice!

cheers

[mcandmar]

Just checked the part system, they were fitted to 7A's also. Did you fit the fuel pump as i'm not sure its stock fitment.