Fuel pressure?

[TG Goose]

Can I get a definitive on how the FPR works? I have the current issue, car leans out on top end. Car is an 86 ETA body with 90 325i motor swap, has in tank transfer pump, inline high pressure pump.

I have a full time fuel pressure gauge on the car. If I jump the relay, I get just shy of 50psi. Car running I get a steady 38-40psi at idle. If you give it throttle it jumps to 45+ then dives to 30-35psi. FPR holds 25psi vacuum with my tester, no leaks.

Should the FPR increase pressure as RPM's increase? Or is it working fine and I have something else causing a high RPM wall?

[Gregs///M]

The pressure at idle should be around 43psi. The return line to the tank should show around 15psi at the same idle.

Disconnect the FPR while connected inline fuel rail inlet and the pressure should increase. The FPR also has a check valve to prevent leakage back to the tank.

FPR pressure is directly proportional to intake manifold vacuum pressure.

Check out the fuel section of the Bentley repair manul (free download, google search) and it will answer all your questions and provide thorough testing procedures.

[jlevie]

The pressure at idle should be ~37psi. As soon as the throttle is opened or the vacuum line disconnected the pressure should rise to 43-45psi and remain there. The numbers you cited could be from a bad FPR (less likely), a clogged fuel filter, or a weak pump. It it only does this when the fuel tank is at or below 1/2, the transfer pump is probably dead and the high pressure pump is worn. But start with a fresh filter.

[TG Goose]

Excellent, thats what I figured. Didnt think pressure should drop on Acceleration

I have a new FPR i'll try tonight, might as well do filter (old one has 2k on it). I can hear whining from inline pump, figure I might as well do the chevy vega pump swap. Inline pump was new about a month ago, purely preventative maintenance as the old one was looking rough (and had to be original 1986). New pump will put 50psi at the rail with engine off (jumping the relay)

[jlevie]

A whining high pressure pump is one that is having to work too hard or one that is worn. A clogged filter, obstructed return line, or dead transfer pump could be possibilities. But in this case we can rule out an obstructed return line as that would cause higher than normal rail pressure. A filter with only 2k on it should be not be a cause unless you have a rusty tank. Coincidentally, a rust tank can destroy a new high pressure pump quite quickly.

So the first things to check would be to see if the transfer pump is working and to check the tank for a layer of rust sediment in the bottom. If the tank is rusty, it needs to be replaced or acid cleaned and sealed before it destroys any more pumps and filters.

[TG Goose]

Roger that. This is my project tonight, currently I am forced to work today to earn mortgage money :-(

I tried a new FPR on my lunch break, jumping the relay gives 45psi. Car running gives 38psi. Blipping throttle gives 45psi then instantly falls to 35psi. Fuel pump(s?) it is!

[TG Goose]

Interesting update tho, if I unhook the vacuum line to the regulator it gives a steady 45psi. If i rev, rev and hold etc it STAYS ON 45PSI and does not move. As soon as I hook the vacuum line back up, PSI drops to 35 under load.

I just gave it a smoke test last weekend, held 8psi for 15min with zero leaks. (I own a Redline Smoke detection pump).

[jlevie]

Connect a vacuum gauge to the nipple on the manifold. There should be a high vacuum t idle and it should be much less when the throttle is opened.

[TG Goose]

I'll have to hook a vacuum gauge up tomorrow.

Got home tonight, transfer pump is mint, works great. Tank is clean inside, like absolutely spotless, sock on transfer pump is clean.

Main fuel pump is still whining tho, going to swap that this weekend. Gave it another smoke test, small leak at valve cover gasket, RTV siliconed and fixed.

[TG Goose]

Car makes 15in HG at idle. R3V'ing throttle drops to under 5, then climbs to 25in HG. Letting off throttle goes back to 15in HG.

I followed the test on this site: http://www.gregsengine.com/using-a-vacuum-gauge.html
I probably have worn rings, but otherwise car is functioning as it should

[jlevie]

Check your air filter.

[TG Goose]

Has a 3" pipe with massive K&N filter on end. DIY Cold Air Intake.
Car is basically 2.5" straight pipe with Dynomax race bullet muffler(where cat was) and Dynomax UltraFlo welded muffler

[jlevie]

I just can't see intake pressure dropping from 5 to 25 as engine rpm stabilizes with the throttle open unless something is restricting air flow into the intake. But that is what is causing the rail pressure drop.

[TG Goose]

Well it passes the test in the link I posted. Car is actually running much stronger since swapping the FPR, new fuel filter and filling the tank with Chevron 94. I might have had a tank of bad gas?

I think the system is running properly, I'd like to see someone else's set up and see if the FPR acts like mine or not with a gauge hooked up.

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