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BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

Hey guys,
i did a fuel pressure test the other day and got 3 bar on tick over but a drop when the engine was revved.
Is the FPR faulty or is there a vacuum leak, if so where do i start to look for one.
is there air being blown into the vacuum pipe to the FPR to cause the drop in fuel pressure
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BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

Today whilst the engine was running at tick over, i took off the vacuum pipe to the FPR and there was no fuel leaking and the vacuum pipe was sucking.
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BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

can someone please explain if the fuel pressure regulator should drop in pressure when revved or increase as i am being given conflicting views.
one person said that fule pressure will drop when the engine is revved as the fuel is going throught the injectors.

I thought the purpose of the FPR is to maintain a constant pressure. When the engine is revved, more fuel is used and pressure might drop, at the same time the vacuum to the FPR decreases, allowing more fuel to flow and an increase of pressure, otherwise you run the risk of running lean at high revs.
Am i right or wrong?

Thanks
Alan
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BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

anyone care to clarify this for me please???
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Guest · Posted: Fri Sep 18, 2009 10:18 pm Post subject:

basically the FPR ensures constant pressure difference across the injectors, so that the fuel quantity does not depend on the manifold pressure.
So if the FPR is preset to 3 bar this means :
fuel pressure - manifold pressure = 3 bars, hence FP = MP +3

at idle (closed throttle butterfly), MP= -0.5, which results in a gauge reading of FP = -0.5+3 = 2.5bar
at wide open throttle MP = 0 which results in FP = 3bar
at part throttle you are somewhere in between.

Also after you stop the engine MP=0 hence FP=3bar and the pressure will drop slowly as fuel pressure leaks across the FPR valve. Typical drop time is between 5minutes and 20minutes. If lower your FPR is leaky and should be replaced.

Also beware of cheap gauges sometimes their accuracy is somewhat low. I've seen some sourced at a plumber's supply shop where the pressure was wrong by about 1 bar.

regards,
zp

BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

thanks for trying to explain ZP.
I read you post a few times and am still confused, sorry.
If the vacuum pipe to the FPR is removed the fuel pressure should increase, if the vacuum pipe is connected whilst the engine is at idle the fuel pressure drops and rises when the engine is revved?
At idle mine is 3 bar, when revving it drops, is mine faulty?

I think you are saying that it should be 2.5 on idle and increase to 3 when revving.
Thanks again.
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Bobbber nl · Guest

First thing to get your head around... when the engine is at idle, the vacuum in the fpr vacuum pipe is MORE than when revved. Got it?

LDA · Alfa 33 Joined: 28 Nov 2004 Posts: 276 Location: France

no i don't get that.. Confused

an engine is an air pump.. the harder it pumps the more vacuum you get?

i think i know this from having a vacuum gauge in a mates mr2. at idle it's about -.5bar when on full chat it's about -1bar.

now this is not the case in my turbo powered car. at idle it's around -.5bar, as the revs rise the pressure reading goes up due to the turbo.. so on full chat i'm getting 2.1bar.

is that not how it works?
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RFlower · Posted: Sun Sep 20, 2009 11:27 am Post subject:

The manifold pressure ("vacuum") connection is on the engine side of the throttle butterfly.

When the butterfly is closed (idle) the pressure at the connection is much lower than atmospheric (more "vacuum") as the butterfly is allowing very little air in.

When the butterfly is opened, more air gets in and the pressure rises towards atmospheric. At full throttle the pressure is almost atmospheric (it will not reach full atmospheric pressure because of the restriction of the filter and inlet piping).

The fuel pressure to the injectors needs to be a constant pressure above the manifold pressure, where they are situated, otherwise they would deliver fuel at a different rate as the pressure around them varied.

The pressure you read on a gauge should rise when you open the throttle or remove the connection to the FPR.

If it falls there could be a restriction in the fuel filter or lines, the pump could be delivering insufficient volume, or the FPR could be stuck open.
_________________
Dick Flower, Nr. Carcassonne. '94 Trofeo 1.4 ie (F), '93 Imola 1.3/1.4 ie (now for breaking) (F), '91 1.7 ie (GB)(spare car), '86 Sprint 1.5 QV (F). '87 VW Syncro camper (F), '73 NSU Ro80 (F), '99 Fiat Seicento (F)

BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

when i first did the test i put the tester on the return from the FPR and got around 7 bar, does that show that the fuel pump is ok?
I have fitted a new fuel filter.
So on idle should it be 3 bar or 2.5 bar and it should rise when revved, yes?
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LDA · Alfa 33 Joined: 28 Nov 2004 Posts: 276 Location: France

RFlower · Posted: Sun Sep 20, 2009 1:11 pm Post subject:

I'm not sure you have the gauge connected at the right place. It should be on a tee on the injector side of the FPR. I don't know how you could get 7 bar on the return side, as this is the return to the tank and should have very low pressure - unless your return pipe is blocked???

Did you actually put the gauge on the return line without a tee? That's the only way I can see you could get 7 bar, as you would block any flow from the pump, which could also damage it.

I connect my gauge with a tee to the pipe from the pulsation damper, as this is the most accessible point.

Yes, once again, it should be about 2.5 bar at idle and RISE about 0.5 bar when the throttle is opened. Unless you have a very good quality gauge I wouldn't bother too much about the actual values you see on it.
_________________
Dick Flower, Nr. Carcassonne. '94 Trofeo 1.4 ie (F), '93 Imola 1.3/1.4 ie (now for breaking) (F), '91 1.7 ie (GB)(spare car), '86 Sprint 1.5 QV (F). '87 VW Syncro camper (F), '73 NSU Ro80 (F), '99 Fiat Seicento (F)

BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

Yup i originally put it on incorrectly, on the bottom of the FPR "the return" i only mentioned it to show ask if the fuel pump is up to the job because the fuel pressure was getting that high.
My tests have now been done with a Y piece after the dampner.
The guage is a mechanics snap on one.
So based on you guys it looked as if my FPR is fubar, or is there something up with the vacuum to cause it to drop when engine is revved and be 3 instead of 2.5 on tickover?
Do i need to do more tests, as in remove vacuum pipe and take reading of fuel pressure and possibly test the vacuum pipe for measured vacuum, what should i get on idle and revving?
Thanks for your help guys.
Why do they say fuel pressure should be 3 bar when on idle, when based on what has been said it is supposed to be 2.5, do they mean at idle with no vacuum connected it should be 3 bar?
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RFlower · Posted: Sun Sep 20, 2009 5:22 pm Post subject:

The fuel pressure is 3 bar above manifold pressure, that is 3 bar above about minus 0.5 bar when idling, i.e. about 2.5 bar.

If you remove the hose from the FPR you should get the same result as opening the throttle, i.e. the pressure on the gauge should go up.

You don't seem to have done this, all you have to do is pull the little rubber suction hose off the FPR while idling with the pressure gauge connected.

If neither makes any difference and the pressure drops when you open the throttle the FPR is probably stuck open or its diaphragm is damaged.

If pressure drops when you remove the suction hose but not when you open the throttle, the hose or its connection at the manifold is probably blocked.
_________________
Dick Flower, Nr. Carcassonne. '94 Trofeo 1.4 ie (F), '93 Imola 1.3/1.4 ie (now for breaking) (F), '91 1.7 ie (GB)(spare car), '86 Sprint 1.5 QV (F). '87 VW Syncro camper (F), '73 NSU Ro80 (F), '99 Fiat Seicento (F)

BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

I have done a fuel pressure test with everything connected and only revved the engine and noticed the fuel pressure dropped which is why i thought i should ask, as that seemed incorrect to me. I have not done any tests with the vacuum pipe removed, i have only removed the vacuum pipe whilst the engine was running with no test equipment attached, to see if there was a fuel leak, which there wasnt and to check if there was vacuum on the vacuum pipe, which there was. I have also blown and sucked on the vacuum pipe with the engine off and there is no blockage.

Now thanks to you guys, i will retest and i think i understand what i should expect and that is;

On idle with vacuum pipe connected, 2.5 bar
on idle with vacuum pipe removed 3 bar
when revved or holding fast idle, with vacuum pipe connected, an increase of fuel pressure to 3 bar.
on fast idle or revving with the vacuum pipe removed, 3 bar

I still dont know what value i should see from the vacuum pipe when testing vacuum from vacuum pipe at idle and when revved.

Am i now thinking correctly?

Thanks for your time in clearing things up and ed u ma kating me Laughing

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RFlower · Posted: Sun Sep 20, 2009 7:30 pm Post subject:

You got it! - I think Smile

You should only see 3 bar with the vac pipe on with full throttle. It's good enough to see an increase with pipe on and about 3 bar with it off.

Good luck.

Dick

I've been under my VW most of the day. Changed all 8 cv joints (it's 4wd), front dampers & suspension balljoints, and adjusted camber and toe-in. A sod of a job, a lot heavier than working on an Alfa! Just have the back end to finish now.
_________________
Dick Flower, Nr. Carcassonne. '94 Trofeo 1.4 ie (F), '93 Imola 1.3/1.4 ie (now for breaking) (F), '91 1.7 ie (GB)(spare car), '86 Sprint 1.5 QV (F). '87 VW Syncro camper (F), '73 NSU Ro80 (F), '99 Fiat Seicento (F)

BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

good going Richard
Thanks again guys, anyone know what i should get on the vacuum test, as i am thinking that might be my problem i might be able to do a test tomorrow, see how Simon from Formula One feels.

If i can find the problem then maybe i can put it in for an mot and pass and can then drive it again.
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BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

I did a fuel pressure and a vacuum test on Tuesday.

This time I tested the fuel pressure with the vacuum pipe on and off and at idle and with the butterflies fully open, only for a brief moment. The pressure was at 3.5 bar and didn’t move when vacuum pipe was removed when at idle and WOT, the pressure also dropped after about 5 mins after the engine was switched off. I am thinking the FPR is fubar’d.

The vacuum gauge showed nothing on idle or WOT, not sure if a really sensitive gauge is needed but there is definitely a sucking force coming from the pipe. I didn’t suck on the FPR, as I didn’t have any spare pipe. I was thinking that maybe the vacuum pipe was blowing on the FPR causing the pressure to increase but now nothing is happening.

It has been suggested that maybe there is blockage on the vacuum pipes causing low vacuum. I did blow and suck on the pipe last time.
Should I remove them individually and investigate further?
Is there a 1 way valve on the vacuum pipe?
What do you guys think? Anything else to look for?
Thanks
Alan
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RFlower · Posted: Thu Oct 01, 2009 8:41 pm Post subject:

First, are you sure your vacuum gauge is good? If the pipe is sucking the gauge should have some reading. It is absolutely impossible to have no vacuum on the pipe unless it is blocked. Are you sure the pipe is connected to the right place??? It should go to a push-on fitting at the back of the manifold.

If you're sure the gauge is OK, I think the first thing to do is check for blockage of the vacuum pipe by removing it and blowing through it (with mouth). If you pull it out straight you should even be able to see through it.
You could also try blowing down the pipe into the manifold.

As far as I know the connection is just a hole in the inlet manifold, maybe you could try gently poking a small copper (soft metal) wire into the manifold via the connection.

If you remove the piping to the back of the manifold you should be able to see the connection from inside (maybe with a mirror and torch)

It sounds as if there is something inside the manifold blocking the pipe.
_________________
Dick Flower, Nr. Carcassonne. '94 Trofeo 1.4 ie (F), '93 Imola 1.3/1.4 ie (now for breaking) (F), '91 1.7 ie (GB)(spare car), '86 Sprint 1.5 QV (F). '87 VW Syncro camper (F), '73 NSU Ro80 (F), '99 Fiat Seicento (F)

BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

Hi Richard,
I don’t know the sensitivity of the vacuum gauge which has Kpa on it I think. I can feel sucking on the vacuum pipe as it sucks on the skin on my finger.

The pipe on mine, which is a 16v with cat, goes to a 6 legged plastic spider like looking thing. 4 pipes go to the throttle bodies, 1 pipe to the carbon canister and the other to the FPR. I think that this spider then has a pipe going to the ICV in the front of the plenum chamber.

I have already sucked and blown through vacuum pipe that goes to the FPR and it seems not to be blocked, but I need someone to listen in each throttle body.

I am a bit confused when you mention inlet manifold, are you referring to an 8v by any chance?

Thanks
Alan
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RFlower · Posted: Sat Oct 03, 2009 7:31 pm Post subject:

Hi Alan,

Sorry, yes I am referring to an 8v, but the vacuum pipe still eventually comes from the inlet side of the throttle butterflies and should show a suction of about 0.5 bar (50Kpa) when they are closed.

I don't understand the varying results you've had when removing the suction pipe from the FPR; first the pressure dropped, now it doesn't change.

I can't see all four suction connections being blocked. That really only leaves the FPR.
_________________
Dick Flower, Nr. Carcassonne. '94 Trofeo 1.4 ie (F), '93 Imola 1.3/1.4 ie (now for breaking) (F), '91 1.7 ie (GB)(spare car), '86 Sprint 1.5 QV (F). '87 VW Syncro camper (F), '73 NSU Ro80 (F), '99 Fiat Seicento (F)

BigAl · P4 Joined: 06 May 2003 Posts: 2992 Location: U.K Surrey

Thanks Richard, I will look at the scale on the vacuum gauge and check its working by sucking on it, as I should have done on the FPR but had no spare pipe, I believe we humans can suck at least 0.5 bar.
I know, it’s weird, when I first did the fuel pressure test I did not remove the vacuum pipe of the FPR as I didn’t know what I should expect, after being ed u ma cated by you lot, I did.
The only differing thing between the first fuel pressure test and the second one was a suck and blow on the vacuum pipe, now I get no fuel pressure change, I’m guessing there is no 1 way valve anywhere, could I have damaged anything?
I wanted to eliminate any other problems before replacing the FPR, like vacuum not being sufficient to operate the FPR, could that still be a problem?
I thought that the fuel pressure drop could have been caused by the vacuum pipe blowing on the FPR diafram which is why I tested the vacuum pipe.
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