Okay folks, here's a big six challenge

[Big Dog]

My '91 535i has developed a strange problem. Most of the time it will run very smooth, but occasionally, about once a month, it will start just fine, idle for about 5 seconds and then start to surge heavily and "hunt" for idle, the swings getting bigger and eventually it dies. Start it again and same thing, smooth for about 5 seconds, then surging and increasing roughness until it dies.

When it's in this mode it has no power, won't accelerate at all and acts as if it's starving for fuel but the fuel pressure guage doesn't move and show 38 psi with no movement.

Put a vacuum gauge on and while the idle is smooth it gets 16 inches of steady vacuum, but once the surging begins it starts moving all over the place.

Strange thing is, sometimes you can just shut the car off and start it again and it will run, idle and accelerate just fine and you won't have the problem again for several days.

Already cleaned the idle control valve, checked the secondary ignition, cleaned the contacts on the main relady and ECU and checked the fuel pressure regulator and crankshaft sensor.

Any ideas?

thanks,

Big Dog

[richard v]

Idle

[dusty21]

Intermitent problems hard to find, an over-rich mixture
also feels like the engine is starving, but not from
too little fuel but to much fuel amd insufficient oxygen in the mixture.

Cheapest Suggestions:

1) Check the time/temp sensor connection on top of the
engine for corrosion - one of the two sensors commands the extra rich
starting mixture, and if it does not cut off within a few
seconds, the engine will be flooded and act as you described.

(Temperature Sensor - Water - Goes to the ECU)

2) Carefully examine the vacume line to the fuel pressure
regulator. If it leaks intermitently, the fuel pressure changes, something the ECU and idle control valve cannot handle..., they
count on a constant fuel pressure.

Most Expensive:

Replace the above parts.

[Big Dog]

Great call, I pulled the temp sensors from top of the thermostat housing, cleaned them and the connectors and the problem is gone.

thanks,

dog