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Help! underhood heat problem

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Hey everyone, Hope everyone is doing well. I haven't been on here in awhile due to a lot of life circumstances. But I've got a problem with my 81 vette that I've been working on for a year & still can't get fixed. I posted about it on here a long time ago but couldn't find a cure.
Anyway, my 81 vette, original 350 motor & 350 trans. I changed the HEI distributor to a non-computer controlled HEI, changed the computer controlled quadrajet to a 1406 Edelbrock 600cfm carb & installed a thermostatic switch for the electric fan. I removed the smog pump & related hoses & all of the plastic ducting for the original air cleaner. I installed an Edelbrock 2" thick air cleaner. I installed a new carter fuel pump, a inline fuel filter,fuel pressure regulator & gauge. It easily holds 6lbs of pressure. I used heat reflective shield on the fuel line from the pump to the carb. I installed an under carb heat shield & even coated it with heat reflective material underneath. I also used a thermo-spacer between the shield & the carb. I even wrapped the headers with heat wrap the best I could without removing them. It has a 160 thermostat & the engine never runs over 185 degrees. Runs well until it gets hot, especially when it's hot outside. Then it stumbles, backfires & stalls, especially at idle. Once it starts running badly you have to let it cool down & it will run ok again once it's cool. The timing is set at about 10 degrees & I've even tried rebuilding the carb. With all of this work it still stalls when it gets hot. When you open the hood it's unbelievably hot under there. Touching the air cleaner will burn you. It had a similar problem when it was all stock but not as bad. It ran worse all of the time stock but didn't stall. It runs much better now until it gets hot.

Any ideas?
 

Hib Halverson

Technical Writer for Internet & Print Media
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71 04 12 19
If you removed the computer-controlled HEI, how are you controlling the spark curve and what's the timing at idle?
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Is it a mechanical fuel pump? If so, does it have two ports or three ports like the original.

It is a mechanical fuel pump & has the return line as the original.
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
If you removed the computer-controlled HEI, how are you controlling the spark curve and what's the timing at idle?

I removed the computer controlled HEI & replaced it with a normal non-computer controlled HEi with a vacuum advance that runs off the carb. The initial timing is set at about 10 degrees at idle. The timing does advance as you rev it up but I haven't checked how much. The thing is, it runs well until the under hood temps get hot. The engine never overheats & all seems well until it gets hot. If the weather is cold outside, like 60 degrees or less, it doesn't have the problem & runs well.
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
Ok...so coolant temperature is not an issue?

From your description, it sounds like the spark curve is working...sort of.

The only other issue which could cause high under hood temperature is lean air-fuel ratio at idle and part-throttle.
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Ok...so coolant temperature is not an issue?

From your description, it sounds like the spark curve is working...sort of.

The only other issue which could cause high under hood temperature is lean air-fuel ratio at idle and part-throttle.

Coolant temperature is not an issue. I don't believe spark is the issue either, it starts well & runs well until it gets hot. Every time I check the fuel pressure, it's at 6 psi. I haven't tried re-jetting the carb but don't really believe I should have to do that. The problem occurs mostly at idle,
part-throttle like stop & go traffic or setting at a traffic light. Again, it only occurs after it has been running awhile & is well warmed. I've been told that under hood heat is a common problem with these early 80's vettes. Since this is my first one, I don't know if it is or how to fix it. I can't afford to put a new hood with better airflow on it. I am getting so frustrated that I'm seriously thinking of taking a loss & selling it.
 

Antz81

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Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Just read through your old thread. I assume no symptoms have changed. Which means it will run fine as long as the hood is open, or if travelling at about 35+mph.
The only difference that your getting is the air flow. Which makes me think could it be your fan clutch is a bit loose?
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Just read through your old thread. I assume no symptoms have changed. Which means it will run fine as long as the hood is open, or if travelling at about 35+mph.
The only difference that your getting is the air flow. Which makes me think could it be your fan clutch is a bit loose?

Symptoms are pretty much the same, maybe a little worse. I think that air flow has a lot to do with it. It seems to get awful hot under the hood. I've never seen a car get that hot that wasn't overheating. I never thought about the fan. The fan is turning & so is the electric fan but I supposed the clutch could be bad. I don't remember it getting that hot when I first bought it. I'll check out the clutch fan. I'm at a loss so anything is worth checking. Thanks.
 

Vettehead Mikey

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Joined
Aug 26, 2003
Messages
3,487
Location
Not that far from Ottawa
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1973 Coupe
If the fan was a contributor, the coolant temps wouldn't be 185 max. All Corvettes have elevated underhood temps, yours don't sound much different than anything I've experienced- nor would it explain the rough running even if it was that hot.

HEI units can sometimes work intermittently when hot. I'd first check fuel flow down the carb throat while idling to see if it's normal and not flooding. If OK, look at the ignition module.
 

dirtypants

Well-known member
Joined
May 14, 2015
Messages
69
Location
Spokane
Corvette
81 4spd Corvette Blue dark met
runs pooly when hot but coolant is not hot

interesting...but don't sell yet. I have an 81, most of the same mods..all smog is gone, Edelbrock carb, intake and Performer cam, 160 deg thermostat with the Hypertech 160 degree module..do you have that? The cooler therm requires the Hypertech mod that kinda goes with it.
if your coolant is not "hot"..and it us just the under hood heat..it is affecting air heat..and it is affecting fuel heat, both can change the mixture and fuel heat can expand and cause vapor locks or flooding.
With my similar mods I do not have any problems..i do have a new fan clutch. You can take out the rubber weatherstrip air dam at the cowl end of the hood..this was in place to help keep cabin air cooler so engine heat did not escape to the cabin vent areas..it has nothing to do with rain water etc..so removing it will allow some engine heat to escape off the top toward the rear. Your side vents help a bit lower down. Seems you have done a lot to shield heat sources. So i kinda don't think it is the heat itself..it is probably no diff than other cars and maybe is lower..you could check it with an oven thermometer under hood that records hi temp with a tell tale...just to see what hi temp is..but again,I don't think the heat itself is the issue. There is something not quite set up right to manage the "normal" heat. Also are you using the correct heat range plugs..it is a pain to check plugs..but maybe right when it runs poorly you can shut down..and check a plug...to see if runnning lean..or too rich. Since it runs well again after cooling I would doubt way to rich which would foul the plug, so possibly too lean. either due to air, fuel, or the module chip not being correct for your mods. Possibly the fuel delivery changes when heated. Sorry to be repetitive..but I do not think it is directly the underhood heat..something else is reacting incorrectly to what is probably normal heat and no amount of cooling the air a little will be enough. With the mods you have made (did you change the intake manifold too?) and if you add the performer cam..with all the smog crap gone (no need for catalytic either)..your 1981 350 should run with a ton more power !! Even with the 8.5 compression..which will allow current lousy gas without pre-ignition..tho I use only no-ethanol, with added NOS off road racing octane boost (3 numbers..so gets 90 back to 92 or 93) and some Techron or Lucas fuel system cleaner. As said nearly same mods you have..runs fine. My timing is a bit more advanced at idle at 14-16deg, ..a little too much but it yields the 36 deg at high revs I want. I have the exhaust analyze figures...you might test that at normal when running ok..and then when running poorly. I am still thinking..Hypertech module for the 160 deg.
 

Vettehead Mikey

Well-known member
Joined
Aug 26, 2003
Messages
3,487
Location
Not that far from Ottawa
Corvette
1973 Coupe
I think the OP has removed all the computer stuff on his car, so playing with chips will make no difference.

I believe the compression ratio in '81 was 8.5:1 so there is no need for high octane gas, never mind doctoring it with expensive additives. Regular (87) octane is more than enough. Octane rating of fuel has no bearing on the present issue in any case.
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
interesting...but don't sell yet. I have an 81, most of the same mods..all smog is gone, Edelbrock carb, intake and Performer cam, 160 deg thermostat with the Hypertech 160 degree module..do you have that? The cooler therm requires the Hypertech mod that kinda goes with it.
if your coolant is not "hot"..and it us just the under hood heat..it is affecting air heat..and it is affecting fuel heat, both can change the mixture and fuel heat can expand and cause vapor locks or flooding.
With my similar mods I do not have any problems..i do have a new fan clutch. You can take out the rubber weatherstrip air dam at the cowl end of the hood..this was in place to help keep cabin air cooler so engine heat did not escape to the cabin vent areas..it has nothing to do with rain water etc..so removing it will allow some engine heat to escape off the top toward the rear. Your side vents help a bit lower down. Seems you have done a lot to shield heat sources. So i kinda don't think it is the heat itself..it is probably no diff than other cars and maybe is lower..you could check it with an oven thermometer under hood that records hi temp with a tell tale...just to see what hi temp is..but again,I don't think the heat itself is the issue. There is something not quite set up right to manage the "normal" heat. Also are you using the correct heat range plugs..it is a pain to check plugs..but maybe right when it runs poorly you can shut down..and check a plug...to see if runnning lean..or too rich. Since it runs well again after cooling I would doubt way to rich which would foul the plug, so possibly too lean. either due to air, fuel, or the module chip not being correct for your mods. Possibly the fuel delivery changes when heated. Sorry to be repetitive..but I do not think it is directly the underhood heat..something else is reacting incorrectly to what is probably normal heat and no amount of cooling the air a little will be enough. With the mods you have made (did you change the intake manifold too?) and if you add the performer cam..with all the smog crap gone (no need for catalytic either)..your 1981 350 should run with a ton more power !! Even with the 8.5 compression..which will allow current lousy gas without pre-ignition..tho I use only no-ethanol, with added NOS off road racing octane boost (3 numbers..so gets 90 back to 92 or 93) and some Techron or Lucas fuel system cleaner. As said nearly same mods you have..runs fine. My timing is a bit more advanced at idle at 14-16deg, ..a little too much but it yields the 36 deg at high revs I want. I have the exhaust analyze figures...you might test that at normal when running ok..and then when running poorly. I am still thinking..Hypertech module for the 160 deg.

On 81's the computer only controls the carb, the distributor, the electric fan & the lock-up converter, correct? I have changed the carb & distributor to non-computer controlled units & installed a thermostatic switch for the electric fan. The lock-up converter just no longer locks up. So what is the 160 degree hypertech module? Is it the distributor module or a computer module? If the computer no longer controls anything, why would I need it?
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
I think the OP has removed all the computer stuff on his car, so playing with chips will make no difference.

I believe the compression ratio in '81 was 8.5:1 so there is no need for high octane gas, never mind doctoring it with expensive additives. Regular (87) octane is more than enough. Octane rating of fuel has no bearing on the present issue in any case.

Thanks, my thoughts also.
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
If the fan was a contributor, the coolant temps wouldn't be 185 max. All Corvettes have elevated underhood temps, yours don't sound much different than anything I've experienced- nor would it explain the rough running even if it was that hot.

HEI units can sometimes work intermittently when hot. I'd first check fuel flow down the carb throat while idling to see if it's normal and not flooding. If OK, look at the ignition module.

It could be the HEI I suppose, the one I'm using isn't brand new. I've thought about buying a whole new distributor just to be sure, but haven't yet. But the way it acts, it really makes me think it's the carb. It's like it's running out of gas but usually only when it's hot & at idle or just off idle, like when you're sitting at a stop light or taking off from one. But the carb was new when I put it on & I've since rebuilt it myself with no real change in the results. The fuel pump is also new (with a return line) & holds 6 psi. The only things mentioned so far that I haven't thought about were the clutch fan & the ignition module. I will try replacing those when I get a little throw away money again. But I'm not confident that either will fix the problem.
 

dirtypants

Well-known member
Joined
May 14, 2015
Messages
69
Location
Spokane
Corvette
81 4spd Corvette Blue dark met
runs poory when hot

Hope you find a cure. let us know. yes the 8.5 comp ratio allows lousy gas..but lousy gas also has (some brands) a lousy additive formula..designed for low performance cars: Modern cars that are computer controlled "tune down" to eliminate detonation, and manage air temps and a lot more. Excess octane does no good but also does no harm other than cost..not much cost on a car that is not driven many miles, and cheap insurance as the additive package in premium fuels is..well...premium. The other issue is the ethanol fuel, attracting water,being corrosive to older metals, and going bad in as little as 30 days. So, if one is using corn gas, then the ethanol additive stabilizer should be used in older cars and in any low use car where fuel may not be used for a time. Might be like a belt and suspenders..but then again your pants don't fall down. For me anyway, same as oil..there are cheap oils that "may' be just fine..but they are not what I put in my collector cars..nor my daily drivers. If all the computer mods are out of the car, then the changes that the modules "would" have made are of course not being made, so manual adjustments have to be correct, air, fuel, spark...correct for start up, correct for warmed up. Cars have run well that way, no computers, for about 80 years. Back to the poor running 81 vette...Is the computer mod either gone or disconnected? Is there a module next to the battery in the box behind the seat? Is it hooked up or disconn?? Is there a carb choke,either manual that is not operational,or electric? I my 81, with similar mods, my Edelbrock carb has electronic choke. Something is certainly changing the way the car runs when it heats up. I am out of ideas..anyone have a fix ?? Maybe it is time to call in the experts that have testing means to find out what is going on?
 

Vettehead Mikey

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Joined
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Messages
3,487
Location
Not that far from Ottawa
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1973 Coupe
Not to hijack but some of your ideas about gasoline are misguided. The octane rating of gas has no connection whatsoever to the quality, formula or quantity of additives. The octane rating is a measure of resistance to detonation (pinging) and absolutely nothing else.

I've been using E10 (10% ethanol) fuel in my Corvette (and all my other toys) for 20+ years with nary a problem. The sky-is-falling stories on the internet are out of control. Let's not beat that issue to death again.
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Hope you find a cure. let us know. yes the 8.5 comp ratio allows lousy gas..but lousy gas also has (some brands) a lousy additive formula..designed for low performance cars: Modern cars that are computer controlled "tune down" to eliminate detonation, and manage air temps and a lot more. Excess octane does no good but also does no harm other than cost..not much cost on a car that is not driven many miles, and cheap insurance as the additive package in premium fuels is..well...premium. The other issue is the ethanol fuel, attracting water,being corrosive to older metals, and going bad in as little as 30 days. So, if one is using corn gas, then the ethanol additive stabilizer should be used in older cars and in any low use car where fuel may not be used for a time. Might be like a belt and suspenders..but then again your pants don't fall down. For me anyway, same as oil..there are cheap oils that "may' be just fine..but they are not what I put in my collector cars..nor my daily drivers. If all the computer mods are out of the car, then the changes that the modules "would" have made are of course not being made, so manual adjustments have to be correct, air, fuel, spark...correct for start up, correct for warmed up. Cars have run well that way, no computers, for about 80 years. Back to the poor running 81 vette...Is the computer mod either gone or disconnected? Is there a module next to the battery in the box behind the seat? Is it hooked up or disconn?? Is there a carb choke,either manual that is not operational,or electric? I my 81, with similar mods, my Edelbrock carb has electronic choke. Something is certainly changing the way the car runs when it heats up. I am out of ideas..anyone have a fix ?? Maybe it is time to call in the experts that have testing means to find out what is going on?

I do not run the cheap gas in the vette. I usually rotate between the middle grade (89) & premium (93) when I fill it up. The computer module is still in the car next to the battery & still hooked up but doesn't run anything to my knowledge. I was hoping it would still work the lock-up converter but apparently it doesn't. I can unhook it with no problem & see if anything changes. I'm kind of grasping at straws now.
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Thanks

Hey everyone, thanks for your time & responses. I really appreciate every one of them. I will continue working on the vette as money, time & health allow. I'm always open to suggestions if anyone has more.

One question, can you change the cam on these vettes without removing the engine or does it have to come out?
 

bill81vette

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Messages
4,268
Location
Troy,NY
Corvette
1981 dark blue metallic
You don't have to remove it but you will have to remove the Radiator and a/c condenser.
 

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