Fuel Pressure Regulator

[omera60]

Hi All,

I'm having some problems with my 1990 316i (M40)'s fuel pressure. For some reason the DME keeps the injectors open longer than necessary. The BMW repair manual states they should be open between 2-3ms, mine are around 4.52ms. Also, my load signal should be 2ms, mine is 1.80-1.90ms and finally the AFM Up/Uv variable voltage should be between 0.200-0.300 v but I get 0.179. The AFM is brand new from BMW, also swapped with another one does the same thing. DME swap also yields no difference. I made them run a smoke test, there are no leaks. All my injectors are new from Bosch as well as the fuel pump. I've tested the fuel delivery rate and the pump is working perfectly., however, I also tested the rail pressure and it is below the specified 3.0 bar +-0.06. I'm getting a 2.3ish bar. I swapped it with another one from my under restoration Touring (unknown FPR state), I'm now getting around 2.5ish bar at idle. If I pinch the return line the pressure rises dramatically to above 3.0bar. Also, If I disconnect the vacuum line it jumps up to 3bar. I'm looking for a brand new FPR but they are like gold for some reason nowadays, extremely hard to find and expensive. I'm assuming the Touring's FPR is also bad as it was in a bad shape when I got it. But do you think the culprit could be the FPR with the given info?

Best,
Omer

[tomstin]

I'm not familiar with the 318i, but I assume it has a O2 sensor? Have you checked that? If that is reporting bad readings to the DME, the DME could be trying to adjust the fuel trim.

[omera60]

I'm not familiar with the 318i, but I assume it has a O2 sensor? Have you checked that? If that is reporting bad readings to the DME, the DME could be trying to adjust the fuel trim.

Hi Tomstin,

Unfortunately no as my vehicle doesn't have a CAT or O2 sensor :(

[roguetoaster]

Sure, it could be the FPR, but that wouldn't really explain why the dwell time is longer on the injectors. Maybe try running a restriction on the return side to maintain 3bar, and see how that impacts it at different throttle states.

What are you trying to achieve anyway? Resolving an emissions issue?

[omera60]

Sure, it could be the FPR, but that wouldn't really explain why the dwell time is longer on the injectors. Maybe try running a restriction on the return side to maintain 3bar, and see how that impacts it at different throttle states.

What are you trying to achieve anyway? Resolving an emissions issue?

I can certainly try that but would it be safe to take it for a drive as if there is a restriction on it and under normal circumstances if I push the throttle it already rises to 3 bar? What I cannot solve is why these 3 parameters and the pressure is out of place when everything else is perfect including the emissions such as CO? I thought maybe the DME is increasing the dwell time as there is not enough pressure on the back of the injectors thus the amount of fuel that is normally required is not being sent in for combustion so by increasing the timing it is trying to get to that optimum amount? When cruising around the town or highway there is no problem however, on cold start if I blip the throttle it surges. If I come to a stop with the engine hot there isn’t much of a problem other than slight shaking. But if I use the throttle suddenly like for example reversing back up a steep hill and release the throttle in quick succession it will stall (Auto car btw), but will start back up again without any issues.

[roguetoaster]

It's probably okay to restrict return flow as long as you are monitoring fuel pressure in real time.

Remember, with the car only able to see air flow, air temp, engine temp, and throttle position there is no practical way for the DME to compensate, or know about an out of spec mixture, so I don't see it increasing injector dwell beyond spec. Consider that the specification may be incorrect for non catalyst cars, as I do know that this is the case for M42s in some ways.

Your issue sounds more like a small vacuum leak to me. If the heat exchanger/PCV conduit under the throttle body has not been serviced in some time there is a small o ring that is a problem for M40s, as are the mess of vacuum hoses. Also, have you checked the ignition system in any way?

[omera60]

It's probably okay to restrict return flow as long as you are monitoring fuel pressure in real time.

Remember, with the car only able to see air flow, air temp, engine temp, and throttle position there is no practical way for the DME to compensate, or know about an out of spec mixture, so I don't see it increasing injector dwell beyond spec. Consider that the specification may be incorrect for non catalyst cars, as I do know that this is the case for M42s in some ways.

Your issue sounds more like a small vacuum leak to me. If the heat exchanger/PCV conduit under the throttle body has not been serviced in some time there is a small o ring that is a problem for M40s, as are the mess of vacuum hoses. Also, have you checked the ignition system in any way?

Will try that sometime this week and report back on it.

The E30 repair manual gave these specs for my engine, this is what I am trying to achieve. I have no other ideas at this point. I know that the load signal (TL) is calculator as ms in: TL=Q/n x K where: Q is the number translated from the AFM door, n is the rpm and K the injector constant.



The problem is we performed smoke test probably more than I could count with no results, though it was my first idea as well. You are correct about the O-ring but That part has been replaced and also sealed. All those vacuum hoses have also been replaced as well. I have excellent compression on all cylinders but I also replaced the valve seals as well.

I've replaced every component of the ignition system; coil, distributor, rotor, cap, ignition wiring (all Bosch parts) to the correct BMW OEM F8LCR's spark plugs with 1K ohm resistance. Every component there when tested came out on spec.

I've also tested the manifold vacuum at idle

[omera60]

roguetoaster I did as you’ve suggested, it does indeed rise, went out and got myself a new regulator but unfortunately it didn’t solve the issue, with the new regulator the pressure is as in the picture, did a smoke test but couldn’t find any leaks, the manifold pressure is the same as above. Any more suggestions, what should I check now?