I have a stock '90 convertible,l98/auto,that was running fine.Just the other day I started climbing the 6% grade with the cruise at 68on my way to work whenit started bucking and the "shift" light started flashing.I hit the brake to release the cruise and the pedal was rock hard.Being easy on the throttle I made it over the hill @ 45 mph,(would stumble if more throttle given),then it seemed to run fine but a little down on power.Ihad a local garage change the fuel filter and the only code that came up was the O2 sensor.Tried driving up the hill again and got the same results plus an occasional stumble as the check engine light came on briefly.No "check engine" shown while driving.I will check fuel pressure and there doesn't seem to be a vacuum leak anywhere.Any other ideas where to look?Coil,electrical?The only change I have made is a high flow air filter in the stock housing and the plugs,wires etc are only 1000 miles old.

 

And it only happens on the steep hill?

 

First thing... if the garage guy said change the fuel filter, then go get your cash back.

There's a small possibility that your air filter oil has fouled the MAF sensor. That can lead toother more serious problems.

BUT, the rock hard brake pedal (why did you press it so hard to release the cruise on an uphill? - just a tap will do it), but the hard pedal, and no codes makes me think brake booster diaphragm leak.

Getcher' noggin' down low near the pedal, and push the brake, with the motor runnin'. I betcha' hear the hiss...

 

Only interferes with driving when I hit the hill,power is down if I try to pass but liveable untill fixed.Didn't plan on hitting pedal hard to release,gave it the normal tap and found it hard.The 90 is speed density so Maf should be ok but other sensors?Even broke down and sprayed wd-40 around all leak points looking for silent vacuum leaks(worked on my s-10 throttle body)and didn't get a change in motor idle but will have someone hit the brake while listening.Thanks for the replys.

 

You can get a listen to the bb diaphragm in the driveway. Push the pedal with your hand. If it's what the problem is, you'll hear it under the dash - I'm pretty certain...

How DOES it idle, anyway?

And just for reference:

https://www.corvetteforums.com/m_37283/tm.htm

print it out, put it in the library that has the porcelain chair with a hole in it.

 

Its idling steady at 700 rpm.Nothing unusual when pushing on the brake either inside or out.Brakes only hard when stumbling which makes the booster suspect.Thanks for the thread,thought I'd looked everything over before I posted.

 

90 and 91 L98's do not have a MAF sensor. Have you had your catalytic converter checked. If it is getting plugged you will definitely start stumbling whent he engine is under any kind of load and needs to breath more. By the way, how many miles are on your 90. My car has 87k on it now and the converter is beginning to fail, I can hear pieces of the catalist vibrating around inside the converter. Mine will have to be replaced very soon.

 

It has 88000 miles and the california 3 converter setup,seems to still have plenty of flow,am not smelling anything and hear no rattles.I plan on changing it but want to wait for all mods so I only have to program it once but will probably look it over this weekend and may perform a convertectomy and find out if the horror stories of changes on a speed density system are true.

 

check with propane torch for vacuum leaks[:@], and check fuel pressure