First post and first corvette*

hey guys, I finally got my dream car, a c3…. But I’m having an issue. (1981)

It runs amazingly for about 30-45 minutes then stalls. It’s definitely a heat related issue. I’ve gone through all the post I could find here and can’t figure it out. I’m no mechanic but I can take things apart and put them back together but diagnosis on this one has me pulling my hair out.

recently replaced: ignition control module, coil, fuel pump (I didn’t replace it but it looks new), sending unit, thermostat and vacuum sensor, distributor cap

car will crank and die immediately when warm so I don’t believe it’s a fuel issue

no percolation inside the bowls

quadrajet is clean

fuel lines are metal

engine is running at 200 degrees

recently learned how to pull a code using the paper clip method… I *believe* it was 21 but it was hard to tell exactly. So theoretically my throttle position sensor is bad? Well I ran it till it stalled and then unplugged the sensor thinking it would start right back up (probably running rough but still running) and it cranked and died immediately.

there is a slight pressure release when opening the gas lid but releasing pressure doesn’t solve the issue. Everything is stock so I don’t believe it to be vapor lock. I’ve got through the lines and added some heat shielding tape and applied the heat silicon to the ignition control module.

I find no difference in the AC/fan being on or off

it starts and runs fine for another 30-45 minutes after about 3-4 hours.

rpm gauge isn’t working but it does fluctuate slightly at idol. Doesn’t run rough or rich from what I can tell.

changing coil did improve the performance slightly but didn’t solve my issue.

thermostat was replaced bc it did overheat slightly on my first trip home while sitting in traffic. Since then temp has been great. The thermostat that was in it was a 165 or 185 (couldn’t really read it) but I replaced it with a stock 195.

transmission is a 3 speed automatic.

I don’t fully understand the vacuum system yet so I’m not sure if this matters but headlights go up and down so I assume vacuum isn’t an issue. I only replaced vacuum sensor on thermostat when changing it bc I broke it.

should I go ahead and replace throttle position sensor even though the problem still happens when it’s unplugged? I don’t want to take apart the carb if I don’t have to….

fuel filter is also on my agenda, but haven’t looked at it yet.

I feel like I’ve ruled out all the obvious things, I have a couple local shops but the wait is insane. Any help would be appreciated! I hope I’ve been thorough enough here.

 

Sounds like the ign module. Did the engine do this before the module swap? Replace the original module and see what happens.

 

Notice it says adjust or replace.
Do you have the 1981 factory shop manual?
https://www.corvetteforum.com/forums/c3-tech-performance/145466-1981-comuter-codes.html

 

yes it was doing it before I changed it. that was one of the first things I did to try and fix the issue.

 

I don’t have the manual yet. I guess I’ll have to order one from Haynes. Wouldn’t the engine just run rough by bypassing the throttle sensor?

 

No not Haynes, it doesn't go into sufficient detail for any specific year. You need the factory manual for your year available from most Corvette vendors.