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Question: Underhood Heat problem on my 81

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Hey all,
Been awhile since I've been on here but I've checked in once in awhile. In-between medical & work issues I've managed to do some work on my 81 since my last post. I removed all of the pollution crap including the air pump & air cleaner. Don't panic, I still have all of the original stuff! :)
I installed am Edelbrock 2101 intake & 1406 carb along with a non-computer controlled HEI dist. I installed new plugs, plug wires & thermostat. I wrapped the stock exhaust headers & the fuel line from the pump to the carb. I used an Edelbrock 14" X 2" air cleaner due to hood clearance. I also installed a new fan thermostatic temp control switch to run the original electric fan. The fan turns on at 180 degrees & pretty much stays on until the engine is shut off. It looks much better & runs great until it gets hot. The engine is not running hot, about 180 to 190. But after it's been in the 185-190 range for awhile the engines starts running bad & wanting to die. :mad I'm pretty sure it's under hood heat/fuel related but I'm not sure what else to do to correct the issue. Any ideas? I can't afford a new vented hood.

Thanks for any suggestions,

Don
 

Peer81

Well-known member
Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
The engine should run well even if the temp is up to 210. What temp thermostat did you use? Mine has a 180 thermostat and 190 fan switch and the fan almost never comes on. If its a fuel heat problem i would check the carb thick insulation gasket below the carb.

Greetings Peter
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
The engine should run well even if the temp is up to 210. What temp thermostat did you use? Mine has a 180 thermostat and 190 fan switch and the fan almost never comes on. If its a fuel heat problem i would check the carb thick insulation gasket below the carb.

Greetings Peter

I also installed an Edelbrock rubberized heat insulating gasket between the carb & intake. The fuel pump is still the original one. Only the carb, fuel line from pump to carb, intake & distributer were changed. The engine runs fine until it gets hot, then it wants to quit. It sounds like a fuel percolating/vapor lock problem. The engine usually runs about 185 to 190 degrees without the air on & about 195 to 200 with the air on. Those temps should not affect the running of the engine. I believe it has to be the under hood heat that is affecting the fuel. Is there a different fuel pump that might help?
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Any chance you downgraded the car with a 2 port fuel pump instead of the correct 3 port?

I'm not sure what you mean about 2 port or 3 port pump. It still has the original mechanical pump on it. I wrapped the fuel line (steel braided rubber line) from the pump to the carb with "cool tube" but it doesn't seem to make any difference. I've thought about an under carb heat shield but I don't know if it will actually make any difference or even if it will fit due to hood clearance issues. Any thoughts?
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
The engine should run well even if the temp is up to 210. What temp thermostat did you use? Mine has a 180 thermostat and 190 fan switch and the fan almost never comes on. If its a fuel heat problem i would check the carb thick insulation gasket below the carb.

Greetings Peter

It has a 180 degree thermostat & the fan is set to turn on at 180 degrees. I did install an Edelbrock rubberized heat insulating gasket between the carb & intake. It's about 1/3 of an inch thick.
 

Vettehead Mikey

Well-known member
Joined
Aug 26, 2003
Messages
3,486
Location
Not that far from Ottawa
Corvette
1973 Coupe
I'm not sure what you mean about 2 port or 3 port pump. It still has the original mechanical pump on it. I wrapped the fuel line (steel braided rubber line) from the pump to the carb with "cool tube" but it doesn't seem to make any difference. I've thought about an under carb heat shield but I don't know if it will actually make any difference or even if it will fit due to hood clearance issues. Any thoughts?

The stock pump has one inlet and two outlets. The large outlet goes to the carb, the small outlet is a bypass that returns a small amount of fuel to the tank. This increases the total fuel flow through the pump ensuring the vapour lock does not occur. Many owners get dazzled by shiny Edelbrock things and install a new pump that does not have the bypass line.
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Did the problem start when you made the changes or sometime after?

The problem was occurring before I made any of the changes but I guess was not as noticeable because the engine didn't run as well as it does now. The engine runs much smoother with more overall throttle response. It doesn't have quite as much kick when you initially open the four barrels but I'm assuming it's the difference in the 750 Quadrajet & the 600 Edelbrock. I do think the heat/fuel problem is worse since I made the changes or maybe it's just a lot more noticeable now. I will check the fuel pump but I never changed it, I'm assuming it is the original one.
 

Peer81

Well-known member
Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
Does the problem get any worse as you stand still for a while? (It should if its a vapor lock)
Try using a 160F thermostat to check if the vapor lock is less prominent.

In my opinion it isn't a vapor lock but something else. Runners ng lean for example. Is the choke valve fully opened when hot etc etc?

And what outside temp are you running in?

Groeten Peter
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Does the problem get any worse as you stand still for a while? (It should if its a vapor lock)
Try using a 160F thermostat to check if the vapor lock is less prominent.

In my opinion it isn't a vapor lock but something else. Runners ng lean for example. Is the choke valve fully opened when hot etc etc?

And what outside temp are you running in?

Groeten Peter

The outside temps have been in the high 80's - low 90's & quite humid. The choke is fully open & the fuel pump does have the return line connected. The problem definitely gets worse if I'm sitting still in traffic. It seems to run ok running 35+ mph at operating temps. But when you go slow or stop when the engine is at operating temps it starts running rough & wanting to stall. As long as the engine temp is slightly below 180 degrees it runs great even stopped in traffic.
 

Peer81

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Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
Running it without the hood should fix the problem! [emoji6]
But if it is really a tempature problem i would say begin with the 160F thermostat and go from there!

Greetings Peter
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Running it without the hood should fix the problem! [emoji6]
But if it is really a tempature problem i would say begin with the 160F thermostat and go from there!

Greetings Peter

Yeah, it'll idle all day even with the air cond on if I keep the hood open. I don't think changing the thermostat would change the top end of the temp range, just the bottom end. :ugh
 

LLC5

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Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Yeah, it'll idle all day even with the air cond on if I keep the hood open. I don't think changing the thermostat would change the top end of the temp range, just the bottom end. :ugh



Does it die only at idle speeds, or all rpm's when the engine is hot?
 

Midnight 99

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Joined
Nov 10, 2013
Messages
122
Location
Jefferson City, MO
Corvette
2019 GS Ceramic Grey A8 Conv. & 1969 Camaro SS
I had a fuel heating issue with my 69. When I put the crate engine in I installed a 4 row aluminum radiator with dual 12 inch fans and a full shroud. I also made sure all of the seals were installed to ensure the air was pulled through the radiator. The car never got over 205 even with the A/C on and sitting in traffic at 100 degree plus. With the L88 hood on the car we could always feel the hot air coming from the engine bay when we had the top down. The only issue was where the fuel line runs up the front of the block and the fuel filter were getting direct blasts of very hot air on them and the fuel would start to vapor lock in the line. I made a shield out of a sheet of aluminum that bolted to the A/C bracket and upper water pump bolt and it solved my issue.
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
Does it die only at idle speeds, or all rpm's when the engine is hot?

It only seems to want to die at idle or slow engine speeds when the car is not moving. Once the car is moving it seems to clear up. It only acts this way after it has been running at 180 degrees or above for a little while. Until it has been hot for awhile, there are no issues, it runs like a champ.
 

Don61

Well-known member
Joined
Jul 14, 2012
Messages
71
Location
Ohio
Corvette
1981 coupe white w/red int auto
I had a fuel heating issue with my 69. When I put the crate engine in I installed a 4 row aluminum radiator with dual 12 inch fans and a full shroud. I also made sure all of the seals were installed to ensure the air was pulled through the radiator. The car never got over 205 even with the A/C on and sitting in traffic at 100 degree plus. With the L88 hood on the car we could always feel the hot air coming from the engine bay when we had the top down. The only issue was where the fuel line runs up the front of the block and the fuel filter were getting direct blasts of very hot air on them and the fuel would start to vapor lock in the line. I made a shield out of a sheet of aluminum that bolted to the A/C bracket and upper water pump bolt and it solved my issue.

My 81 has the original clutch fan & the original electric fan. I think the fact that there is no good way for the engine heat to exit the engine compartment may the problem. My fuel line & filter does run in front of the passenger head & across the intake manifold like yours. I applied "cool tape" & "cool tube" over the line & filter but I will try to make a shield like you did & see if that helps. Thanks for the idea.
 

Peer81

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Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
That's it!! Great idea Don! I think the clutch fan isn't working when hot! Isn't a problem when moving but when your idling or standing still the clutch fan isn't moving air into the radiator. :)

Greetings Peter
 

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