A2K1

I am getting check engine light on idle when car warms up. It goes away when I lightly press the accelerator pedal and comes back on about 2 seconds after I release the pedal and RPM drops to normal idle.
Idling is very rough, I had to increase idle RPM to prevent engine halting.
Few weeks ago it used to appear time to time when idling too long, but now it always appears after engine idling for several seconds.
It feels like car lost some power and sometimes engine halts when I press gas pedal to the ground.

Other symptom that may not be related to this problem: tachometer shows way too high value, on idle it is about 5000 and max value when car is moving. It used to show 2x actual RPM before.

According to GM documentation:
DTC 21 is Throttle Position Sensor voltage too high.


This is a 1980 California car, the first Corvette model that had computer controlled carburetor. Computer does not have permanent memory, so all errors go away when conditions that trigger them are gone. It also does not have diagnostics port, the only way to read error code is to ground a special wire when check engine light is on.

I tried:
Replaced spark plugs and ignition wire – no change
Cleaned the carburetor with spray and fuel additive – no change
Cleaned ECC contacts – no change

Is there some way to test TPS?
Is there some way to test that ignition distributor reports correct RPM value to ECC?
What else can cause this error code?

terry82

I do not know much about a 1980 from cali .on my 82 if I had a tps that was giving me a code ,I would take it off and check it with a ohm meter.you check it buy watching the numbers go up and down on the meter .if the numbers don't go up and down smoothly then that is your problem.

wajulia

The throttle position sensor is on the front left of the carb. It is attached to the connector on the front of the carb. The sensor is still available in parts stores and on the web. It measures the movement of the accelerator pump lever. There are test and adjustment proceedures in the service manual. I am not at home so I can't access mine, but maybe someone else on this forum can get them for you

To replace it you have to take the top of the carb off, a little fiddly, but it can be done

Don't waste your time on all the other stuff until you address the TPS

A2K1

Thank you, terry82 and wajulia!

terry82, can you please describe testing process in more details?

Can someone please share the part of service manual about troubleshooting the TPS? I suppose it should be the same procedure for all computer controlled Quadrajets.

hugie82

Sounds like an EGR valve sticking open a little. Carbon builds up and holds the valve open a tiny bit letting exhaust gases into the intake.

terry82

http://www.bing.com/videos/search?q=...3B1D8E955D0C4B

A2K1

Thanks for help, everyone!

I have finally had some time to spend working on the car this weekend. Here is what I found:

Rough idle was caused by two malfunctioning spark plugs. Spark plugs problem was caused by too rich idle mixture. Mixture was too rich because carburetor had incompatible choke vacuum pull off installed, so I had to adjust it's arm to open the choke correctly.

Engine error code 21 is probably caused by bad bad calibration of TPS. I could not find any tool to adjust that small TPS adjustment screw (it's a hex head about 2/32"). I managed to turn it a little bit counter-clockwise with two small flat screwdrivers, now I am getting code 21 error almost all the time when I am not pressing accelerator pedal. So it seems like I just need to get a correct tool, connect a multimeter to the signal wire and simply adjust TPS to correct voltage on closed throttle.

Is there some special tool for TPS adjustment? Does anyone know the exact size for this screw?

wajulia

I have attached the service manual fault tree for code 21 as well as the adjustment procedures for the TPS. Hope they help

A2K1

wajulia, thank you very much!

A2K1

I was able to adjust TPS using a 2.5mm roll pin. If someone needs this tool, you can buy a set at O'Reilly.

My carburetor did not have that aluminum plug (probably removed by previous owner), so all I had to do was to connect voltmeter to TPS signal wire (it's a middle wire on the connector), turn on ignition, set throttle to fully closed and turn the TPS adjustment screw till voltmeter read exactly 0.56 volts.

I was getting error code 21 because TPS was sending just 0.48 volts on idle, which is too low.

Thanks for help!

A2K1

I am getting code 21 again and TPS is acting very strange. Pressing on accelerator pump makes it report about 3 volts, while pressing on accelerator pedal makes it read just 2 volts. On idle it jumps from ~0.4 to ~0.7 volts, very rarely it jumps to 3 volts for no reason.
TPS is definitely malfunctioning, I am going to replace it.

Rough idle was caused mostly by incorrect spark plugs.

A2K1

Check engine light is gone after installing new TPS.

wajulia

Congrats on sorting it out. Always feels good to find and fix a problem like that

A2K1

Thanks!