loss of power after warm start

'93 geo metro / standard / 95,000 miles.

The car failed emmisions for hydrocarbons so we brought it to a dealership for a full tune up and to make the car pass. I don't have the invoice next to me to know exactly what they did but it was an exhaustive (expensive) tune-up.

The car passed the emmisions test with flying colors, but after the tune up a new problem that I have never experienced before has cropped up. If you drive the car just long enough for it to warm up, shut it off for 5 minutes, then start it again, it starts up fine (catches almost immediately) and idles fine, but the acceleration is jerky and unpredictable. If you hold the accelerator at a fixed position (just barelly depressed) in any gear, the car bucks forward, then stops accelerating in little spurts. On two occasions I drove it long enough (about 20 minutes) that the engine got weaker and weaker until finally it stalled. At this point trying to start the engine is impossible. The starter turns over the engine, but it never catches.

If the car is allowed to cool down for 2-3 hours, it starts up and runs as if nothing was ever wrong with it.

The fuel pump has been replaced to no avail. It has been in and out of shops and they can't seem to find anything wrong with it.

I'd like to start poking around under the hood myself... where should I start looking?

I think I would try to elliminate the fuel delivery system first. Put a fuel pressure guage onto the larger of the two fuel hoses that attach to the throttle body and route the guage up to the outside of the windshield and hold it there with a windshield wiper. Drive the car until it starts acting up and then check fuel pressure. If I'm not mistaken, the pressure should never drop below 13 psi.

I brought the car back to the shop to try and demonstrate the problem. The foreman there had made up his mind before test driving it that it's probably a problem with the wiring harness. He said he's seen that happen to alot of metros and it isn't worth trying to diagnose the problem because it could take 2-3 days to trace it and it would be thousands of dollars to repair once they locate the problem.

Do the gurus here concurr?

The two things I find troubling: he didn't seem to be interested in checking for the obvious things, like bad O2 sensor, other diodes, or other fuel mixture/pressure problems. The other problem is as my previous post stated, the car was working beautifully before it was serviced at their shop.

If the car is dead, the car is dead. Now I am trying to decide if I should try to recover the money we put into the original tune up.

Is it even plausible that the wiring harness would only misbehave after a warm start?

Thanks for the second opinions.

Sounds to me like either the ignition coil, distributor, or throttle postition sensor. I would have the TPS adjusted, and tested first.

if the car stalls and wont start again, wouldn't that rule out the TPS? Shouldn't it at least start and idle ok?

Yer not gonna like hearing this, but I had the same problem...here goes.

I purchased a 93 Geo Metro Convertible last year, and it ran great. A few weeks later, I decided to degrease and power wash the engine compartment. After the power wash, it drove the way you described, with the same problems you are having.

It was frustrating as hell, but I drove it with that hesistation problems for a couple of weeks...but lost patience and went at the problem.

First, I thought a spark plug was mis-firing, so I got new Bosch 4 platinum plugs ($6 a piece). Still hesitating.... Then I thought the spark plug connection was bad, so I purchased a tube of spark plug connector gel (it's an electo-conductive gel). That didn't work. I then tried a new distributor and points. That didn't work. I was about to give up and sell the car.

Finally, I replaced the spark plug wires. That fixed the problem. The wires I purchased were about $25 bucks at Pep Boys.

Afterwards, i think I believe I know what happened...

After the power wash, I prolly got water in my under my distributor cap, and had trapped moisture. That either did something later to the wires, or while doing something to the wires (washing or reinserting onto plugs), I cracked the older wires, which led to a short.

Since wires expand and contract with temperature, a crack in the copper will still allow electricity to jump the wire and pass thru the crack...sometimes. So, it runs like shite, sometimes. Sometimes, the bad wire fires the plug just fine, sometimes no.

It's possible that it's some bad spark plug wires, and even new wires installed by the dealership could end up being faulty.

thanks for the advice. this isn't the first time that shop screwed up my geo. i'm not sure i'm personally up to the task of changing sparks & wires, but i'll pass your advice on to whoever works on it next.