I'm the proud owner of a '91 Pontiac Sunbird convertible, which I
bought in '93 and have generally maintained myself with a few
exceptions. 120k miles, 4 cylinder (not EFI).

About 3 years ago, I noticed that the car would hesitate when I first
started it, more often than not when it was cold and rainy outside (but
occassionally, it would hesitate when it was warm and sunny). It never
stalled, it would just hesitate when I tried to take off.

By "hesitate," I mean that I would be pressing the gas, but for about
half a second it was as if no gas were getting to the carburetor. This
only happened when I tried to take off right after start-up, though;
once the car warmed up, no more problem.

Since the car never actually died, I didn't give it a lot of thought.
Now, it's 3 years later, and the problem has gotten a little worse.

In the last few months, the hesitation at start-up is a little more
noticeable. Instead of hesitating for a half-second, it usually goes
for a few seconds. Now, it also happens when I take off from a stop,
too, regardless of the temp of the engine.

When I'm accelerating, I can usually feel the engine hesitating for
microseconds at a time. This isn't horribly noticeable, but you can
feel it if you think about it. Usually, the passenger doesn't notice
it.

And the last symptom is that the check-engine light comes on every once
in awhile, but this may be related to a separate brake problem (a seal
is leaking in the rear brakes).

Over the years, I've replaced pretty much everything I can think of
that would be related to this problem, with no impact. I've replaced
the following:

1. spark plugs and wires (not the distributor)
2. PCV valve
3. fuel filter
4. air filter
5. fuel injector (not EFI, just the single sprayer)
6. timing belt

The first 4 are replaced regularly during a regular tune-up that I do
myself. The fuel injector really needed to be replaced since the spray
was pretty erratic, but I really thought that would solve the problem.
The timing belt was replaced by a mechanic that knew "for a fact" that
it would solve the problem...

I'm really doubting that the fuel pump is the problem, since it's been
progressive over such a long time, but I really can't think of anything
else that would have these symptoms. I've only replaced a fuel pump in
2 cars (both foreign cars with a hatch in the back seat), but in both
cases they pretty much went out without a warning.

Can anyone think of anything else that would cause these symptoms over
a 3-year period? How do you perform a diagnostic test on a Sunbird?

Thanks, all,

Jason