94 surging/bucking problem

On May 15, 1:40 pm, charlie > wrote:
> On May 1, 9:52 am, "charlie" >
> wrote:
>
> > hello
>
> > i have a problem with my 94 that is puzzling me.
>
> > 6speed
> > 140k miles
> > no ses light
> > service ride control (z07 option) light is on, but that is a bad servo that
> > i know about
> > no other indicators on
> > new battery
> > alternator normal output
> > all other temp indicators in normal ranges
>
> > the symptoms started a few months ago. in top gear at high way speeds, it
> > will seem to miss when a demand is placed on the engine (going up a hill,
> > passing, etc). in the ensuing months, it seems to have gotten worse, in that
> > it now:
>
> > at idle (which used to be around 800rpm), it would start to hunt, going from
> > 800-2000, with no throttle input. during around town driving, say in 4th at
> > 45, the engine would speed up so the car would accelerate, with no throttle
> > changes being done by me. the pedal doesn't move under my foot, so it's not
> > a spring, i believe.
>
> > i replaced the iac valve and airfilter at this point. when the old one iac
> > was removed, it had a lot of crud on it. i cleaned out the opening to the
> > throttlebody, but didn't take it off for a complete cleaning. i also
> > replaced the water temp sensor, as i noticed the harness connector was
> > cracked.
>
> > the problem has now changed, in that the idle has dropped to an abnormal low
> > of 400 and seems to almost stall. it still has the missing and bucking at
> > constant speeds, and also misses pulling away from a stop.
>
> > i took it to my local mechanic, whom i've had for about 20 years. he noticed
> > that every time it surges, the service ride control light changes intensity.
> > he seems to feel that the computer is bad and is affecting the timing
> > somehow, and said i should take it to the chevy dealer.
>
> > could this be a fuel delivery problem (fuel pump, filter, bad injectors)?
>
> > are there any other sensors that this could be, or does this sound familiar,
> > and if so, are there tests for them so i don't have to start throwing
> > expensive parts at the problem?
>
> > thanks,
> > charlie
> > cave creek, az
>
> short story: optispark.
>
> long story: i took it into the dealer after my mechanic didn't find
> the problem. he didn't want to start just throwing parts at it, and
> didn't have any onhand to do any testing with.
>
> the dealer at first diagnosed the missing problem as a couple of bad
> spark plug wires, and possibly bad plugs. replaced all wires/plugs.
>
> the surging problem he claimed was a MAS with baked on oil from the
> k&n air filter. solvent cleaned the MAS.
>
> turned it back to me, and within 1/2 mile was doing the same thing.
> went back, took the service guy for a test drive and nearly put him
> through the windscreen it was missing so bad.
>
> they kept it for a day and said they couldn't reproduce the problem.
> went back and took the manager for a test drive, and bucked him around
> some.
>
> i don't know what they did to diagnose, but called and said the
> optispark was bad. took a couple of days to get one. took out the
> water pump and found that it was on it's way to selfdestruction,
> missing some bearings and a rusty optispark. replaced both. wonder
> where the bearings will turn up in the future?
>
> runs like a champ now, as far as i can tell with 20 or so miles on it.
> lots lighter in the wallet, and i can't recommend midway chevy in
> phoenix for any service at all.
>
> thanks for the help,
> charlie

to followup on my followup: i finally found the surging problem

there is a hard plastic tube that goes from the top of the (valve
cover?) to the throttle body underneath the thin plastic engine cover
on the right side. one end of this tube has a 2" rubber hose adapter
(with 2 different inside diameters!) that had a crack in it. this
evidently opened up when it got warm and closed when it slightly
flexed, letting additional air into the throttle body. since this is
after the MAF and IAC, it tricked the computer into increasing the gas
delivered.

regards,
charlie
cave creek, az