New mini-"upgrade"-fuel pressure gauge install

[IronicBenzTech]

So I was having start up issues with my '87 325E. And thought it would be a good idea to have a fuel pressure gauge installed to check for fuel pressure drop when engine is off and for any future diagnosing. A quick $60 order from JEGS and I got a
5/16 fuel pressure gauge adapter and a 0-60 psi water-filled gauge. I know it's not awesome but as a mechanic it seemed like a good idea. How does 45psi sound? I think alldata says 38psi? :)

[FLG]

IMO, leave it stock and use the gauge only if your having issues or put it in a better spot. You now have a potential for 4 points of failure all sitting on top of the valve cover near not only an ignition source but a very high temperature heat source as well.

[IronicBenzTech]

Point taken.....

Maybe on the fuel line near the brake booster? My first thought was that if it does leak it would go down the left side of the valvecover and down the back of the block/trans bell housing .... I may reconsider... Won't be driving it til Monday afternoon anyways

[jlevie]

45psi is way too much pressure for an M20B27. Normal pressure would be 32-34psi at idle and 36-38 above idle. The FPR might be the wrong one (one for an M20B25), the FPR could be bad, or there could be an obstruction in the fuel return path. If you can blow through the return line, the problem is the FPR.

[nando]

My bet is you have a 3 bar regulator instead of the proper 2.5 bar version. I think it says on the regulattor.

[IronicBenzTech]

I figured...... I knew my fpr was bad before..... So I replaced it with an autozone one cuz I was in a hurry..... autozone says its operating pressure is rated at 38 psi.....Now I'm gonna see if I can return the pos or exchange it

[TobyB]

Wait, wait, wait... the pressure regulator is referenced to the intake manifold.

So you have to know the absolute pressure in there, too. At 2.5 bar (37 psi) fuel pressure at idle, if there's -.7 bar in the manifold, the pressure should be
1.8 bar above atmospheric. Or 27(ish) pounds.

if you unhook the vacuum line to the back of the regulator, it should hop up to 37.

Or 45 if it's for the i/is.

t

[IronicBenzTech]

ok... so i pulled the vacuum hose to the fuel pressure regulator and the pressure shot up to 55 psi... seeing this had me realize and remember and that I smoke tested my engine a week ago and found a pretty decent vacuum leak from the valvecover gasket, oil cap, and tps switch. I took care of the oil cap right away because i was at work. would all these leaks cause the FPR to "over-compensate"?

[jlevie]

ok... so i pulled the vacuum hose to the fuel pressure regulator and the pressure shot up to 55 psi... seeing this had me realize and remember and that I smoke tested my engine a week ago and found a pretty decent vacuum leak from the valvecover gasket, oil cap, and tps switch. I took care of the oil cap right away because i was at work. would all these leaks cause the FPR to "over-compensate"?

Nope, the wrong FPR is is installed.

[IronicBenzTech]

NOTE: Fuel injected engines also rely on intake vacuum to regulate the fuel pressure behind the injectors. Fuel delivery cannot be accurately metered unless a fairly constant pressure differential is maintained. So the fuel pressure regulator diaphragm is connected to a source of intake vacuum. Vacuum working against a spring-loaded diaphragm inside the regulator opens a bypass that shunts fuel back to the tank through a return line. This causes the fuel pressure in the injector rail to rise when engine load increases (and vacuum drops). Thus, the regulator uses vacuum to maintain fuel pressure and the correct air/fuel ratio. A vacuum leak changes the equation by causing a drop in vacuum and a corresponding increase in line pressure.

from http://oldfuelinjection.com/?p=93

i think I may have learned something today :)