barryweber

C-4 Corvette 1993 700 R 4 trans

I purchased this vehicle about 1 year ago with 42,000 miles and paid $5600 New paint job, interior work. I have put on about 1800 miles. Fuel injection wizards needed! Starts fine, Idles at about 1200-1400 rpm. cold, no missing. Will continue to run at fast idle no matter what one does. Fast idle 1200-1400 rpm. Half hour later still at fast idle.

When the motor gets warmed up: 180 degrees and still at fast idle 1200-1400 rpm. When I shut the car down, restart it, idles down to about 800-900 rpm, which is ok for an automatic trans car. Put it in gear rpm drops to about 700 rpm and idles ok. The fun now begins! When I move the throttle just off idle a little: surge, stumble like trailer hitching. If you drive the car, it stumbles with just a little throttle. Driving in city traffic is awful. It feels like you are pulling a trailer (Jerk, stumble, jerk jerk. Hit throttle, it revs up fine. Back off throttle to a steady speed and the stumble jerk stumble jerk.)

This is what we have done.
ECM sent to Cardone for a rebuild or whatever they do
Replaced, plug wires, plugs, map sensor, throttle position sensor, air charge sensor, engine cooling sensor, two O2 sensors, knock sensors, EGR valve, checked rocker arms. Valves seems to be ok. Compression test 180-185.

Described here is the only way we have been able to get it to run at a steady speed/rpm
Unplug the TPS, Take a different TPS plug it into the harness and leave it laying loose beside the throttle body. Seems to run at a steady speed. Throwing a TPS code hooked up this way. When TPS that is in proper place and hooked to the wire harness, there is no code, but runs like garbage stumbles and jerks.

My Mechanic is pretty sharp, but it has got him stumped. I'm 68 years old and anything with a computer should be left out of a car. My next step is to pull the motor, computer, sensors, relays and anything that is in anyway connected to a computer. Set it off in the barn and got to a speedway motor, order a 383 stroker, carburetor motor and put it in so I have something to drive.

FrankieD

You need to get a scanner hooked up and see what the IAC is doing. That will give some clue as to what's going on. At idle the IAC should be around 30-40. If it is reading 160 it is at it's max opening and possibly stuck open.

barryweber

Mechanic scanned the IAC and it was very steady at 38 no up or down.

FrankieD

your video shows you unplugging the TPS and using a spare TPS to plug in . I would test the TPS that's mounted on the throttle body. Check to see that it sweeps / increments linearly. Check to see if the voltage reading from the TPS is linear when opening the throttle blades fully Suspect the TPS is causing issue.

barryweber

The TPS on the throttle body is new the one that is off to the side is the one that was in it. (originally) Both TPS units were calibrated before installation and tested to be in working order.

FrankieD

Even though the TPS is new, can you explain what you mean by calibrated and tested. Was this issue happening as well with the older TPS?
I have also had experiences where light throttle tip-in stumble was caused by carbon buildup behind the throttle blades and Throttle body mouth. Try using brake clean or throttle body cleaner and a rag and get in behind the throttle blades and throttle body mouth.

There have also been instances where the CTS ( coolant temperature sensor) is defective. I believe it is the one located on the water pump that the ecu uses . This causes the ECU to think it is always in open loop/cold start up mode.

Cliff Harris

The most common cause of a high idle speed is a vacuum leak.

Many times the vacuum check valve under the passenger side fuel rail cover gets brittle and breaks, causing a vacuum leak.