Questions and Answers

BMW 528e Power Cutting Out Q. Hi, Vincent. The problem I'm having is this: After my BMW is completely warmed up, I'll begin to feel hesitation (loss of power) in small bursts - usually after about 20 minutes of driving. These bursts will eventually worsen to the point where the engine will almost feel like it's about to die completely. It literally feels like I'm running out of gas.

Feathering the gas pedal will keep usually keep the engine running and often the problem will intermittently go away, but it will not go away completely until the engine has cooled down again. Unfortunately, the cycle repeats itself the next time I take the car out. I've noticed that the problem usually starts on after slowing down from one constant speed at about 55 to 60 mph and is worst in the 45 to 50 mph range. I don't get any "check engine" lights and the tach does not drop off sharply indicating a loss of electrical power. I do see a slight increase in water temperature just before and during the periods of power loss.

I've checked out my fuel pump relay and it appears good (I haven't replaced it). My fuel pressure regulator is brand new. I've run Chevron Techron fuel injector cleaner through the system. I've thoroughly cleaned my air bypass valve (which helped to smooth out my idle). I've replaced my temperature sensor and temperature switch (I've also completely bled the thermostat housing). I've installed brand new spark plugs, distributor cap, rotor, wires, and a new fuel filter. During the engine rebuild (about 1,700 miles ago), I adjusted the throttle position sensor (it was WAY off).

My next troubleshooting project is primarily to check out the two fuel pumps (the Robert Bentley shop manual has a reasonable plan for this) for pressure and current draw and to test the pressure regulator under varying vacuum. I'm looking for other suggestions (like how to check out the DME or how to ensure I've got my fuel lines connected to the injector rail properly - in and out).

I'm going broke installing new "Easter Eggs" in to the car that don't fix the problem. The car is unreliable to the point where I'm almost afraid to take it out - not because I'm afraid of getting stuck somewhere, but afraid of the engine dying in front of a truck and getting run over. I've only recently seen this problem (two to three weeks). It never happened during the 1,200 mile break-in period following the engine rebuild. I didn't own the car before that. Please help.

1988 BMW 528e (10/87 production date)
2.7 liter with Bosch Motronic 1.1 electronics
Automatic transmission: (ZF 4HP22)
200,600 (about 1,700 miles on completely rebuilt engine and transmission - including cylinder head refurbishment)
ABS brakes
P/S - A/C - Cruise control
Rack and pinion steering

Thanks in advance!!!
Tim

A. The Motronic is a nice system but the only problem with it is you need a scan tool to do any checking on it. I think you're plan of testing the fuel pumps is the next logical step. If the auxiliary fuel pump is starting to go bad, it will display these same symptoms.

I don't know how the Bentley book tells you how to do it, but here is the procedure from BMW with the proper readings.

1. Connect suitable pressure gauge inline between cold start valve and supply hose or to main fuel supply hose, ensuring that gauge is properly installed to prevent fuel leaks. Nominal fuel pressure with engine running should be 2.5 bar (36 psi).
2. Disconnect and plug vacuum hose to pressure regulator, start engine and observe pressure gauge. Reading with hose disconnected should be approximately 2.7 bar (38 psi).
3. Reconnect vacuum hose to regulator and observe pressure gauge. Reading should drop to approximately 2.3 bar (33 psi).
4. Accelerate engine several times while observing gauge. Pressure reading should rise briefly as engine is accelerated.
5. If reading obtained in step 2 is too low, briefly pinch off return hose from regulator while observing gauge.
6. If pressure reading rises with hose pinched, pump, tank and lines are satisfactory.
7. If pressure readings observed in steps 2 and 3 are not as specified and pump, tank and lines are satisfactory, or if regulator fails to alter pressure in response to engine vacuum, regulator should be replaced.

You have to keep in mind however, that the car must be displaying the symptoms when you do the tests. Otherwise everything will check out good.