Weird Surging problem-93 w/ 90 engine

HELP!!

Have a '93 2-door Geo Metro Automatic 3 cylinder. We found out after purchase that it has a '90 engine. The car was manufactured in Canada, if that makes any difference. (Don't know where the engine was manufactured.) I only had this car 2 or 3 weeks before this problem started.

Engine begins surging while holding accelerator steady. Doesn't seem to do it until it warms up (A couple of minutes). This is a steady pattern.. up,down,up,down.. about 1 second or a little longer for each cycle.

When you look down the fuel injection port when it's doing it, it doesn't spray during the pulses.

We have replaced the following with new parts:
replaced the thermostat for the antifreeze.
spark plugs & wires
distributer cap and rotor
fuel filter

Also, we replaced antifreeze temperature sensor with a used one. (noticed at the junk yard that there was another sensor right behind that one, which we did not replace)

A lot of repair shops don't want anything to do with this car, because of the body/engine mismatch. No one seems to be able to test the computer except for Chevy, which charges $94 just to diagnose the problem!!

The computer (ECM??) looks like a BITCH to get to, under the dash and behind fuse box and more, if we were going to try to switch it out for one from a junk yard.

Can anyone help, PLEASE! Anyone had this problem themselves?

Thanks so much..

Martha and Mike

:banghead:

Hey _ I am interested in hearing what kid of advice you get as I have a similar problem in a 93 except it doesnt surges it simply drops to idle when holding gas pedal steady at hyway speeds. Sometimes it does do it when accelerating. Im hoping its not the TPS as they are way over 200 bucks at Autozone. But I havent taken the time yet to try to measure it which could be hard to do as it doesnt seem to do it when car is sitting still.

Good luck

You might have a leak at the base of the throttle body or at the intake manifold.
I think you can confirm by spraying card cleaner (or something that burns) on the interface sections of carb/throttle body. If the engine RPM change, you have a leak. Tighten the bolts that hold down the throttle body

A good way to test for this is to sniff with a propane torch (unlit of course). A change in RPM will show where the vacuum leak is coming from.

agreed on the intake air leak also check the fuel pump if you take off the gas cap and stick your ear to the hole you should be able to hear a hum if it pulsed it will most likely be the pump (sounds crazy but thats what ive been told is the best way to tell) if those arent it check the ecm but that should be fine if you are still able to drive it, good luck :)