Hey, guys. New to this forum. Havefound some articles from here helpful in the past, so I thought I'd run this by you guys.

94 Camaro - Girls car (v6 auto). 200k miles. Bought it cheap and have been fixing it up enough to be a decently reliable car. New A/C, starter,battery, Plugs/Plug Wires, IAC, TPS, EGR, Fuel Shraeder valve, Alternator, Tires.

Motor mounts and shocks will need some work, but not dire.

Couple weeks ago I noticed that once car is warmed, and up to speed, if traffic slows to say 50 MPH and I give the car enough gas to accelerate, but not enough to drop out of Overdrive, there's an intermitent, quick vibration to the car. The sensation is like the same thud you feel hitting a decent size rock with your tire while at speed (and yes, I'm sure I'm not just hitting rocks). It's very intermittent. Yesterday it happened more regularly and significantly. Acceleration dropped to non-existent, although the engine was running, ( I accelerated 5 MPH in a quarter mile) and the Check Engine light came on. To make it to a service shop I shifted down to Drive (instead of Overdrive) and the problem pretty much went away (though I was running 3000 RPM's and burning through gas like an H1 Hummer).

I had a Chevy dealer look at it a month ago because the Service Engine light came on and I could not pull codes (both via scanner and shorting the OBD I connectors). They said I needed a new computer, new EGR (which I did) and new motor mounts.

Yesterday drove the car to a mechanic I generally trust and he said the computer showed no codes and the motor mounts weren't too bad. He said the only codes he got from the computer were for the EGR, but, since it's new he wasn't sure if they were old stored codes or not. Otherwise he said they were fine and that the EGR shouldn't affect the performance too much.

A family friend who doesn't live local, suggested maybe a sensor was going bad, which was feeding wrong information to the computer. I am also stuck in the throes of having one mechanic say the computer is fine and a dealer saying it's bad, although I trust the mechanic as much as anyone. I still have not been able to get a store bought scanner tool to read the codes (it's one of those cheap $30 deals where it shorts the ALDL pins for you and the service engine light blinks.) So I don't know if it's a wire that's loose or what.

So in lieu of all that I have already changed, and many of your considerable experience, do you guys have any ideas? Appreciate any help.

 

The dealer just wants $$$. I can't say I've really heard of a PCM going bad. If the EGR is stuck wide open, it can run like poo. Do you know how long the catalytic converter has been on the car?

 

Thanks for the reply stan. Incidentally I was reading my initial post. It's not that I don't have the cheapo code scanner, it's that it doesn't seem to be flashing the service engine light as it should when connected to the ALDL connector.

As to catalytic converter, I have owned the car a couple months. Before that my girlfriend owned the car for 4 years and it has not been changed.

Considering what all I've done to the car so far, I can think of a few other options:

1) The car's computer could in fact be bad. That could presumably throw off the timing (timing is controlled by PCM with a distributorless car, right?) I have never ridden in a car with a "misfire", or a timing issue but that's sort of intuitively whatI think this might be. If the computer is screwed, then it might not be sending the firing signal at just the right time which might result in improperly timed plug which I'm thinking could feel like I described, (like running over a bump or rock). It might also be the reason I can't pull codes from the ALDL.

2) Upon more thought, I realized that I neglected to clear the codes after installing the EGR valve. In this case the EGR valve is not the issue, and I'm looking for an issue which would light the "Service Engine Soon" but not throw a code.

3) Something completely unrelated could be wrong. Since I've changed the spark plugs and wires, the only thing that occurs to me (beside the PCM) would be to check the ignition coils to make sure none are damaged. None of them have been checked or changed in the 4.5 years we've owned the car.

4) Wildcards. Anybody know of weird sensors that would throw off the ignition, timing, or performance of the car and result in an intermitent "bump" (can't hear it but you can definitely feel it in the seat/floorboards)?

 

The bump you hear in the floorboard is the transmission or the torque arm hitting the floorboards when under heavy load, probably because of bad transmission mount or torque arm mount. Also possible that the center mount or the 2 piece drive shaft is loose.

You cannot flash codes on 94 and newer Camaro, & a most code reader do not work on it, for the 94 you need an OBDI scanner. Also same late 94s came with a OBDI PCM, but with a OBDII connector (like the 95 models) requiring an adapter to read the codes.

As Stan said a stuck open EGR can screw up how the engine runs under load. Also check the ignition coils and the ignition control module under the coils. Make sure the plugs are gapped properly and of the right heat range , and that the new plug wire are run per factory specs to avoid inductive cross fires and the insulation damage. Check the fuel pressure and pressure regulator.

The PCM is probably the most reliable part in the car, and seldom go bad.

 

My vote would be carrier bearing on your 2 piece drive shaft. i had the samething goin on with my 300z, i had a 1 piece made. Why do you v6 guys have 2 piece?

 

I'm not sure who's genius idea it was for the 2 piece driveshaft. It shaved quite a bit of weight when I swapped it out fora 1 piece aluminum.