Fuel pressure mis(t)ery

[OrMar]

Car is '89 200 20v.
Until today, my fuel pressure was 33psi at idle, and would raise to about 39 psi if I apply throttle. As far as I know, that's not enough (should be around 46psi). So, i detached vacuum line from fuel pressure regulator, and now I have 44psi all the time, BUT car hesitates bad at WOT, and lambda goes totally lean (I have instruments for both fuel pressure and air/fuel ratio). At partial throttle, it's not so bad.
I would never expect that car will go LEAN if I increase fuel pressure; can someone explain me this ?

Thanks,
Mario

[UrS4boy]

The fuel pressure regulator (FPR) is designed to create a constant pressure between the fuel rail and the pressure in the cylinder, e.g. 4 bar, whether there is real vacuum or under boost. When you removed the "vacuum" line from the the FPR you took away the pressure signal to the FPR so it still thinks its only at atmospheric pressure, i.e. the pressure differential across the injector is lower than it should be, hence lean conditions, pinging (pinking), etc. You need to a) re-install the vacuum/boost line. You probably need to replace either the fuel pump or the FPR. The FPR is cheaper and easier to replace than the fuel pump so start with it. If the problem still persists, you probably need to replace your fuel pump.

Dave F.

[OrMar]

I have tried 2 FPRs with same results. However, both were used. Fuel pump is new, so next step is to buy new FPR. I will go for adjustable one, because I will probably tune this engine later. Any sugestions ?

Thanks,
Mario

[UrS4boy]

I am confused. You say your car is an 89 20v and you are in the US(?). If its a turbo (you don't say), either its an 89 10vt or its a 91 20vt (3B). Maybe I missed something.

[OrMar]

I'm in Croatia, and my car is Audi 200 20V (3B, turbo engine).
My profile is updated now.


Mario

[OrMar]

Vac line back on

I have put the vacuum line back to fuel pressure regulator, and readings are as follows:

Idle: 2,65 bar
partial throttle: 3,03 bar
full throttle: up to 3,65 bar

Can someone confirm if these values are ok ?
(sensor is attached to fuel rail)

Thanks,
Mario

[UrS4boy]

I have put the vacuum line back to fuel pressure regulator, and readings are as follows:

Idle: 2,65 bar
partial throttle: 3,03 bar
full throttle: up to 3,65 bar

Can someone confirm if these values are ok ?
(sensor is attached to fuel rail)

Thanks,
Mario

You need to give us the boost values at the same time. Throttle position doesn't tell us enough.

[OrMar]

You need to give us the boost values at the same time. Throttle position doesn't tell us enough.

Not sure for partial, but at WOT it's 1.9 absolute.

Mario

[UrS4boy]

Not sure for partial, but at WOT it's 1.9 absolute. Mario

Let's see if we can sort this using my AAN as an analogy:

Under WOT boost in 2nd, with RS2 hardware and MRC software, I get 26 psi relative boost (1.79 Bar relative). I know I need 84 psi at the fuel rail to get this (84-26 = 58 psi = 4 Bar). My fuel pressure regulator is a 4 Bar unit.

You have 3.65 Bar at the fuel rail (about 52.9 psi) and have 1.9 bar absolute boost. 1.9 absolute boost is about 27.6 psi which is about 12.9 psi relative boost (about 0.89 Bar relative boost). Using my analogy, the difference between the fuel rail pressure and your relative boost pressure should equal the pressure regulator nominal pressure. From your data, your regulator is at 3.65 bar - 0.89 bar = 2.78 bar. I think that is too low, even for a 3B. I imagine that your FPR should be a 3.0 bar unit.

Based on the above, I can only conclude that either your fuel pump is tired and is looking for a replacement or the FPR is leaking and cannot hold the required pressure in the fuel rail.

But that's just me. http://www.s2forum.com/forum/images/smilies/wink.gif

[OrMar]

Based on the above, I can only conclude that either your fuel pump is tired and is looking for a replacement or the FPR is leaking and cannot hold the required pressure in the fuel rail.

I have new (VDO) fuel pump, and with 2 different FPRs I'm getting exactly same results (ok, both are used).

Mario

[UrS4boy]

I have new (VDO) fuel pump, and with 2 different FPRs I'm getting exactly same results (ok, both are used).

Mario

Strongly suggest that you purchase a new and proper FPR for your car and install it (bleed the fuel rail pressure by running the engine cold and then pulling the fuel pump fuse and waiting for the engine to stop). I am quite sure that a new FPR will help. They are not that expensive in my experience.

Dave F.