angry power cut out and back in

[accident]

88 325is

okay, so for the last little while my car has been basically dying and restarting itself randomly while driving. sometimes the check engine light comes on when i let go of the gas, sometimes it goes on while i'm on the gas. when this happen the tach drops and i lose all power to the drivetrain, exactly as if the car has stopped running. i've replaced the main relay, the fuel pump relay, and the O2 sensor relay. The main relay is brand new from the dealer, the other two are used from a running car. I don't know what the fuel pump is like. The Crank Position Sensor is used, but in good shape.

i'm at a loss here, i dont know what the fuck is wrong with my car :(

stomp test no worky, pretty sure my TPS isnt sending WOT.

[jlevie]

The abrupt cut off and drop of the tach strongly suggests that the DME is shutting down from a loss of power or start signal, or that it is loosing the CPS signal. If there are fault codes stored in the DME they might provide a clue as to what is happening. If this is a CPS problem you'll likely find 1251, 1252, or 1261.

Since you have a new main relay, the other possible causes of loss of power would be the fusible link, ignition switch, harness wiring fault, or a problem with C101.

[accident]

if it was the ignition switch the CEL would most likely not come on, right? I've never seen it work with the key in accessory. If i pull back my trunk carpeting, what would I look for in the fusable link? What tests can I do on the harness and C101 to rule those out?

[jlevie]

If the fault was in the ignition switch, the CEL probably would not be illuminating on a cutout while the car is in gear and moving. But it could. So the first thing I'd do would be to interrogate the DME for fault codes. A bad TPS will prevent the stomp test from working, but the DME can still be interrogated with a diag system.

A simple test of the fusible link is to chech resistance from the bulkhead end to the battery end. Both ends of the cable need to be disconnected for the test. And have the codes read out before this as disconnecting the battery will reset the DME.

Diagnosing an intermittent is very difficult unless you have some form of data logging. Lacking that I'd do resistance checks on the power and ground leads to the DME and if nothing was found I'd replace the CPS with a new unit. Also take a look at the harmonic balancer with the engine idling. It it looks like it wobbles, or wobbles when you blip the throttle, replace it.

[StereoInstaller1]

Did you check the injector harness plug under the manifold?

[jlevie]

Did you check the injector harness plug under the manifold?

Unless the car has an automatic transmission, the tach won't take a big drop if C191 is the cause.

[nando]

Unless the car has an automatic transmission, the tach won't take a big drop if C191 is the cause.

whys that? automatics don't have different injector harnesses?

[StereoInstaller1]

Unless the car has an automatic transmission, the tach won't take a big drop if C191 is the cause.

yup, good catch

[jlevie]

whys that? automatics don't have different injector harnesses?

The torque converter can allow the tach to take a drop if the engine stops producing power. It won't be a huge drop, but it will be noticeable. A fuel cut from a flaky C191 on an auto can cause a tach drop. But on a car with a manual transmission a fuel cut won't cause a tach drop because the engine is directly connected to the drive train.

[accident]

in a non-moving state, if i floor it to redline and then let off the gas, the CEL will usually illuminate, and the tach needle flails like it's wagging it's finger at me. :(

i need to get a diagnostic tool.

[TwoJ's]

Did you ever solve this problem? Did you also have hard cold starts when this was going on?

[Kristeun]

^same question.. any other suggestions?

[ak-]

It was a bad DME if I remember what he told me.

[TwoJ's]

Yep, I bet that's what mine is too. I've narrowed pretty much everything else out.