My brother has an 89 corvette with the 4+3 manual tranny. he was out of town and it sat from friday to monday. it ran perfectly fine when he left. today when he drove it to work any time he put a load on the motor (really showed in 3rd and 4th) if he free revs the motor he says it runs just fine. moving along at 1500rpm if he gets on it at all he says i "shakes" he compared it to a washer on spin cycle that just isn't balanced. i'm trying to diagnose this from 700 miles away so i need all the help i can get. BTW he did check the timing and it's right on. thanks.

 

check the spark plug wires, could be crossfiring causing erratic firing, im at a loss, ive never seen a SBC shake like that.

 

It sounds like it is dropping a cylinder, that year motor is tune port injected, it is not uncommon for an injector to plug if the car is not driven frequently. Try putting a couple bottles of Chevron techron fuel injector cleaner or Valvoline synpower total fuel system cleaner in the gas tank and drive the car, if that does not work then check the following.


-Fuel psi- it should be in the mid 40psi range. If not at spec then change out fuel filter and retest.

-Cap, rotor, wires, plugs.

Repost what you find, good luck

Tim

 

he refuses to run high grade fuel all the time. for a while he was running mid one tank and high the next, now i believe he just runs mid or maybe even mid and low swapping. i will tell him to try the fuel system cleaners i already told him to stop messing with the lower octane fuels. cap rotor wires and plugs were all done by me less than 3-5K ago. the car is his daily driver. he commutes probably 30 min in either direction.

 

Concentrate on the fuel system since you already did the ignition work to it.

Good luck,Tim

 

Even though you did the ignition, have him look for rubbing of the spark cables. Had one last week that would drop a cyl under hard acceleration, one wire was rubbed through but shorted to ground only when there was load on the engine and it was a little twisted in the mounts.

When did you do the spark plugs?

AlpineRAM

 

AlpineRAM the plugs were done with everything else approx 3-5K ago... and man i've never done a harder spark plug replacement. i had to cut a ratchet in half just to get in where i needed to! even then it wasn't easy.

 

I agree, sounds like a fuel issue. Run some injector cleaner through it and see if it helps. Check fuel pressure, should maintain about 40psi at idle. Those engine compartments are pretty tight, it couldn't hurt to check out the plug wires to make one didn't come loose or something. I would also check the vacuum lines and MAF sensor.
Is the check engine light on?

 

My wife had a similar issue with her Pontiac minivan. I ran injector cleaner through it and it went away then came back a few months later. I replaced the fuel filter on the advice of a Chevy mechanic and the problem completely went away.