'91 318is (M42) fuel pressure question

blalor · blalor's Garage

I've been trying to chase down some drivability issues with my E30. It's a '91 318is (first year for the M42 motor). The body has 185k+ miles and I replaced the engine a few thousand miles ago with one that had 145k miles on it. I replaced the fuel pump this week after testing with a fuel pressure gauge showed low, erratic pressure; the pump was also getting quite noisy. Before the new pump went in, I was barely getting 35psi, and it took quite a while to get there. It's looking much better with the new pump, but it still doesn't seem right.

With the car idling, the pressure is at about 35. When I disconnect the fuel pressure regulator's vacuum hose the pressure rises to 43.5psi and stays there no matter how much I rev the engine. With the vacuum hose connected, however, the pressure fluctuates between 35 and 44 psi, depending on engine load. I swapped in two different fuel pressure regulators today (including a brand new Bosch one) and they're all doing the same thing.

The Bentley manual says the fuel pressure should be 43.5 and doesn't mention any variations with load, so that leads me to believe that it should *always* be 43.5 (how else would the ECU know how much fuel gets delivered with each injection pulse?). Obviously, this isn't happening. It seems like the pressure regulator is letting the pressure drop too low when any vacuum is applied to it, but again, with three different regulators, I can't really blame them for this…

Is this a legitimate problem, or just the way the system works? If it is a problem, what are the likely culprits? I'm going to attempt to test the flow rate tomorrow to make sure there aren't any restrictions, but I'm otherwise completely stumped.

Thanks,
Brian

Mntltan · Mntltan's Garage

I would have to defer to Bostogrun in Minnesota. What he explained to me is that what you are describing is the normal operation of the fuel pressure regulator. With the vacuum line connected to the pressure regulator, excess fuel is allowed to return to the tank, hence the lower pressure. If you remove the vacuum line, the regulator senses low or no vacuum and closes the valve which allows the fuel to return thereby giving you the full pressure of the pump which is needed when you hit the gas. At idle when you removed the vacuum line, did the idle increase? If it did, which it should, it's because of the increased pressure in the fuel rail is delivering more volume. The DME doesn't know about fuel pressure, only RPM and throttle position (and a few other things), so the pulse of fuel to the injectors is times based on the engine speed essentially.

As I said at the start, I must defer to Bostogran because he REALLY seems to know this system.