Fuel pressure problem

[Seagull81]

I need a little help with a fuel issue. I have a stock injected 302 (stock FPR) with a Mustang tank and pump. While working on the engine I noticed the fuel pressure at 30 pounds. It used to be 39 or 40 pounds. If I take the vacuum line off the regulator the pressure goes up to 39 pounds again. Is that the pump or the regulator that is not working correctly? I am guessing the regulator, but I thought I would get y'all to comment.
Thanks,
Steve

[fkemmerer]

Originally Posted by Seagull81

I need a little help with a fuel issue. I have a stock injected 302 (stock FPR) with a Mustang tank and pump. While working on the engine I noticed the fuel pressure at 30 pounds. It used to be 39 or 40 pounds. If I take the vacuum line off the regulator the pressure goes up to 39 pounds again. Is that the pump or the regulator that is not working correctly? I am guessing the regulator, but I thought I would get y'all to comment.
Thanks,
Steve

It may be just that your regulator is manifold vacumn compensated. The idea behind the vacumn connection between your regulator and the manifold is to alllow the regulator to keep the pressure difference across the injectors constant. When your intake is under vacumn, the injectors need less fuel pressure to deliver the same amount of fuel. When you took the previous readings at ~40 lbs, were you using the same regulator? Were then taken with the engine running at the same vacumn level as the more recent ones?

[Ronbo]

It's working correctly, might need adjusted though. If your using a liquid filled gauge under the hood don't trust the reading when the engine is warm. (the fluid expands and makes the gauge read low)

[olddog]

For every 2 inches vacuum the fuel pressure should drop by 1 psi. See what the pessure is with the vaccum line disconnected. Measure the vacuum. Then see if it drops as discribed when you reconnect the line.

PS
Factory fuel pressure regulators were set for 39 psi with the vacuum line disconnected

[olddog]

Originally Posted by Ronbo

It's working correctly, might need adjusted though. If your using a liquid filled gauge under the hood don't trust the reading when the engine is warm. (the fluid expands and makes the gauge read low)

Now that explains why my fuel pressure reads lower when hot than cold. Thanks Ronbo!

[Ronbo]

Can't really take credit for this, Patrick demonstrated this issue in another thread.

The low pressure (15psi) gauges drop a lot. (around 50%)

[ByronRACE]

The chassis of the fuel pressure gauge should be vented to atmosphere.

This is likely a detail that the knock-off manufacturer failed to copy. Drill a pinhole in the gauge body somewhere below the fluid filled gauge face and it shouldn't do that anymore. That is, unless the entire gauge is filled with fluid...if that's the case, it's a design flaw.

The mechanical gauges have metal bellows in there that uncurls with pressure and moves the needle. If the pressure inside the gauge housing builds with temperature, the uncurling effect will be lessened because the differential pressure is lessened.

Typically just the face is filled with fluid; to dampen the needle movement.

Byron

[Ronbo]

Actually, the entire guage needs filled. The bourdon tube and linkage have much more mass than the needle does so they have to be submerged in the glycerine / silicone fluid. The gauges aren't vented to atmosphere as the glycerine would weep out as it expands, that's why they leave the air pocket so the seals don't pop.

Easiest fix would be to use a non-filled gauge mounted to the fender.

Or of course mark the gauge reading when the engine's hot.

[ByronRACE]

I have taken apart gauges that are not filled completely in the manner described, but I can see why they'd make them that way...lots less precision involved.