Persistent Annoying Timing (Or Something) Malfuntion

I've been dealing with a persistent misfire/timing issue for weeks now.

The car stumbles badly at idle and sounds like it is misfiring. A drop cylinder test revealed nothing, however. The car will seem to run normally then suddenly stumble and misfire causing what I think are small backfires.

I've changed the sparkplugs, wires, rotor & cap.
I've cleaned out my PCV hose and replaced the PCV valve.

There is no oil or other evidence of problems on the sparkplugs, no oil in the air intake.

The spray pattern of the throttle body seems fine and does not coincide with engine stumbling.

When the car is cruising down the expressway, there are no obvious power problems or hesitancy. But when the engine spins down to idle, the car stumbles and misfires repeatedly.

Any suggestions?

Just a dumb, stupid question from a novice....

Does the Metro Engine Distributor have a vacuum advance?

Would this possibly be a vacuum problem?

Just thought I would ask.

DoctorBill

I don't think it's a dumb question at all. I'm not skilled at diagnosis. I can wrench pretty good and follow instructions.

My car is a 1991 Geo Metro LSi Convertible. So that means I do, in fact, have a vacuum/centrifugal advance unit. The hoses themselves are in excellent condition but the unit itself could be faulty...that hadn't occured to me.

I guess I can fetch a vacuum tester and check it out.

Could all this sputtering and cantankerous behavior be caused by something so simple?



Just a dumb, stupid question from a novice....

Does the Metro Engine Distributor have a vacuum advance?

Would this possibly be a vacuum problem?

Just thought I would ask.

DoctorBill

Okay....

It's not the vacuum advance. It works perfectly as does the centrifugal advance.

The guys at the auto shop suggested it might be a knock sensor? I'm pretty sure the metros don't have knock sensors.

I just thought of something from previous conversations on this forum....

You say the car acts up when you idle down......

Possible relation to throttle - The Throttle Position Sensor (TPS).

As I understand the TPS, it is a potentiometer (pot) that is connected to the
throttle shaft and tells the ECM the throttle's position (rotational place).

I took mine off while I went thru the Throttle Body and checked it for
continuity.

Potentiometers can wear out and have sections where there is no carbon
(if it is a carbon type) or bad contact (if it is wirewound).

I know not what types these Geo Metros have, but you might put a VOM
on yours and slowly go thru the entire range of rotation of your throttle.

I, myself, would remove the connector and test the resistance as I very slowly
rotated the throttle or had someone very slowly press the gas pedal.
Watch the ohms meter (determine if ohms, kilo-ohms or meg-ohms setting rqd)
and if it suddenly shifts - bingo - bad TPS....

If you have a discontinuity near the idle (where the throttle would set most
of the time - max wear!), then this could cause the ECM to freak out at idle.

What do you think...?

If yours is bad, a used TPS can be obtained from any "Pull-n-Save" or from
wrecking yards - cheap....

DoctorBill

I dunno about the TPS being bad...I could check it just to see, but it's been my experience (with a Suzuki Samurai w/ TBI) that when the TPS fails the thottle just opens up and revs the engine like crazy. I don't think it can kind of fail or function properly intermittently. But I could be wrong about that.

I'm concerned that maybe my exhaust is plugged up and perhaps that is causing some problems at idle. How much pressure is supposed to be felt at the tailpipe? When I put my palm against the pipe, I barely feel anything.

Note, I haven't actually tested the compression outside doing the drop cylinder test but shouldn't compression problems be evident with blowby in the air intake or on soiled spark plugs?