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Discussion Starter #1
I am at a point that I'm going to pull out this 350 and just wait for the machine shop to be done with the 498 so I can stuff it in there. This freakin 350 has me going crazy trying to find the problem. The air dam is in tact, the seals around the radiator are there, put in a 2 row(1 1/4 cores) aluminum radiator , dual electric fans, timing verified. I am running a mechanical and regular temp gauges. This thing gets to 230 degrees in a hurry. The car runs fine, no misses but just can't get it to run cool. The reason I am worried is because if I can't cool this 350 , then imagine with a hyd. roller big block. GRanted this 350 has some high comp. Whoever did the engine put 305 heads on a 350 block (verified block casting# and heads) . I'm thinking this engine has got to much compression. I'm just pissed I can't drive my baby , and I just installed c5 wheels :( .
 

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I'm cooling a 12-1/2:1 compression BB with the stock fan. Compression is not your problem. I'm thinking maybe a thermostat or water pump problem. Hoses in good shape? And last but not least, trapped air.

Once upon a time, I had cooling problems too. Pulled the radiator and stuck a garden hose in it to see if it was plugged- it was- the water washed one of those old IBM punch cards out of the upper neck. GM parts had put it in there for tracking and it got stuffed way down in there..
 

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Another thing that you should look at is water flow. If the water is flowing to fast through the motor, it won't have enough time to cool before it cycles. Thus, it won't matter how big your radiator is, hot water won't keep a engine cool.
 

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whats your t/stat like ? If its old try removing the guts of it so theres some flow control and try that, if it just takes longer to get hot then it could be a sludged up block(power flush it) or its airflow (aircon condenser blocked with leaves crud?)
My 79 has never got hot except when the fan hub started to leak its oil out , another hub all fixed(the motor was rebuilt/flushed before i got it):D
 

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How does the engine run?

Any chance the verified timing was verified on a slipped balancer ring? Or on a mismatched balancer/tab combo? The 305 heads are making me wonder, half original parts half 305 parts is a recipe for a timing light that lies.

Every time I've seen chronic overheating to that degree the engine had fixed timing or some kind of timing problem.

good luck with your car
 

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make sure your radiator hose isnt collapseing ..if the spring isnt in it it can pinch off all your flow fast ..then look normal once the motor is off ...just a thought (the lower hose is more prone to this issue if i remember right ..been w ahile since i had this issue )
 

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have you verified your fans are spinning the correct way, sounds crazy but I have seen guys add wire fans so they are pushing when the should be pulling the air which will not cool the motor correctly.

I was running a similiar 355 to your 350 with no cooling issues (355, large cam 60 cc 305 heads and forged flat tops making around 10:5:1 compression) radiator is a 3 core gm cooled by a flex fan, runs all day long at 180-190. So that said I don't think its the compression giving you problems but more likely something simple like fan direction, thermostat, bad radiator cap , worn pump or clogging somewhere.

Try pressure testing the cap and system as well as a power flush if the fan check out ok:cheers:
 

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I had similar issues with my 77' It ended up being the heater core. It was not clogged, and I still had heat in the car but it wasn't flowing threw the heater core fast enough to cool the motor down. You could bypass it to see if this helps. Also another idea, if this has been an issue from the begining you may want to look into making sure the water flow passages on the 305 heads are the same as the 350. I'm not sure on that one, maybe another forum member would know better. Good luck.
 

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Water passages are the same 305 versus 350, and SBCs don't rely on the heater core for bypass circulation, that's what the extra hole in the water pump mounting flange is for.

The mention of a reverse flow pump is a good one, especially since the engine is a mismash.
 

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Discussion Starter #10
The only thing I can think of at this point is water flow or lack of it and maybe the jetting(lean condition) .
 

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Use an IR gun or get one (pretty cheap) and test the top water inlet at the rad tank and the bottom water outlet at the rad tank. You should have anywhere from 20 to 30° lower at the outlet. If so the radiator is working and you have sufficient flow.
BTW, you can't have too much flow.
Don't know what tstat you have, but the tstat housing should read pretty close to that temp rating.
You can have a defective one, try changing it and make sure it has an air bleed hole in it.
You didn't mention the age of the engine, but pull the two drain plugs on each side of the block. It will give you a good idea how clean the coolant passages are. I've seen a lot that are completely plugged.
Make sure your vacuum advance is connected and your timing is around 34 at rpm.
You can always plug the internal bypass in the pump to get more flow thru the rad and less hot coolant recirculating.
A very lean fuel mix will also make it a little hotter.
Make sure you're at or above 15psi in the cooling system.

What kind or pump have you, new, rebuilt etc?
 

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Discussion Starter #12
Use an IR gun or get one (pretty cheap) and test the top water inlet at the rad tank and the bottom water outlet at the rad tank. You should have anywhere from 20 to 30° lower at the outlet. If so the radiator is working and you have sufficient flow.
BTW, you can't have too much flow.
Don't know what tstat you have, but the tstat housing should read pretty close to that temp rating.
You can have a defective one, try changing it and make sure it has an air bleed hole in it.
You didn't mention the age of the engine, but pull the two drain plugs on each side of the block. It will give you a good idea how clean the coolant passages are. I've seen a lot that are completely plugged.
Make sure your vacuum advance is connected and your timing is around 34 at rpm.
You can always plug the internal bypass in the pump to get more flow thru the rad and less hot coolant recirculating.
A very lean fuel mix will also make it a little hotter.
Make sure you're at or above 15psi in the cooling system.

What kind or pump have you, new, rebuilt etc?
I currently have a reman waterpump that is about 1yr old. I had overheating issues last year and stopped driving the car until winter( kind of backwards for some folks). I don't know the exact age of the engine but appears like a recent rebuild . Plugs come out a little tan in color , no issues with them. As far as thermostat , I am not running one until I figure this stuff out.
 

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First, reinstall the thermostat. It will not aggravate overheating and in some instances will CAUSE it. Bare minimum you need a flow restrictor disc. The slight restriction of the thermostat hole forces complete circulation. So while it's true you can't have too much flow, you do need that single point of restriction to keep the back cylinders from running hot.

What application was the water pump looked up for? Is it a 70s vette pump or something late 80s? Does your engine use V belts or a serpentine?

Does your distributor use both mechanical and vacuum advance, or are they locked?

Old V8 Vega guy here. Totally know about cars that can't be driven during hot months, they make C3s seem like igloos. :cheers: Throw some info into this thread and we'll put ya on the bullseye
 

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Discussion Starter #14
The application I looked it under was fora 68 vette and running v belts. I am currently looking into the water pump impeller to see if it's the correct one. Let's hope I found the solution.
 

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Do you still have V belts on the car or is it a serpentine conversion? If it's V belt, you're OK. If it's serpentine you're probably onto something.
 

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Discussion Starter #16
I just verified the impeller and it's the correct one. It's still a v belt setup. I installed another water pump and still the temps creep up to 235 and I shut er down. This is has been a very frustrating problem to deal with. In the past I was kind of willing to put up with not driving the vette but not anymore. This is my baby and I refuse to store it for the summer!!:bang:bang:bang:bang:bang
 

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Discussion Starter #17
Ok, this is what I noticed. The temps kept going up , so I open the pressure release and noticed the temps went down. I am currently running a 16 lb cap an wondering if I should put a 4-7 lb cap . That way it will open sooner on it's own and cool the engine down.
 

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Did you try it with a tstat yet?
Did you check the two block plugs and flush the block out?
Here are a couple pics of what you can get out of a block that was also recently rebuilt. It would be wise to do that, you may not find a lot, but there is almost always something.
As far as the system pressure, the higher the pressure the better it will cool, not lower pressure. By opening the rad cap, you just released steam. Steam won't cool your engine.

This is about 1/10 the amount that came out.


You gauge how big the pieces ae by the size of the wrench.

 

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Have you verified your fan is spinning correctly is it set up as a pusher or puller?

have you tried swapping in a 180 degree thermostat

also have you done a leak down test on your cooling system?

There are several things that can cause a car to run hot.

Heres the short list, let us know what you have eliminated





Fan Switch (is it working properly)

Fan Direction (is it set up to push air from the front or pull air from the engine compartment)

Thermostat have you replaced it

Radiator cap is it working properly (if it is not holding back the proper pressure you will overheat)

Coolant is it a 50/50 mix or straight coolant (to much or to little can cause overheating issues)

Have you pressure tested the system (most autozones have a cooling system pressure testing system you can borrow )

Do you have after market under drive pulleys that have changed the ratio of the waterpumps pumping ability

do you have an ac condensor or oil cooler if front of the radiator that may have dirt or debris between it and the radiator



if all the above checks out pull the radiator and have it flow checked if it passes have the system power flushed


If all that fails to provide the answer you may have a bigger problem like a cracked block/head or head gasket issue:cheers:
 

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Discussion Starter #20
Have you verified your fan is spinning correctly is it set up as a pusher or puller?

have you tried swapping in a 180 degree thermostat

also have you done a leak down test on your cooling system?

There are several things that can cause a car to run hot.

Heres the short list, let us know what you have eliminated





Fan Switch (is it working properly)

Fan Direction (is it set up to push air from the front or pull air from the engine compartment)

Thermostat have you replaced it

Radiator cap is it working properly (if it is not holding back the proper pressure you will overheat)

Coolant is it a 50/50 mix or straight coolant (to much or to little can cause overheating issues)

Have you pressure tested the system (most autozones have a cooling system pressure testing system you can borrow )

Do you have after market under drive pulleys that have changed the ratio of the waterpumps pumping ability

do you have an ac condensor or oil cooler if front of the radiator that may have dirt or debris between it and the radiator



if all the above checks out pull the radiator and have it flow checked if it passes have the system power flushed


If all that fails to provide the answer you may have a bigger problem like a cracked block/head or head gasket issue:cheers:
The fans are coming on and working properly as pullers . I already did a pressure test and it holds pressure at 15-16lbs . I am at about 60/40 water/anti freeze. Water pump pulley is stock diameter and the trans cooler is in front of the rad but everything is clean. About 10 mins ago I check the water flow on my chevy blazer and at idle I can see a good flow of water moving and if I rev it a bit it get splashing all over . Now I go do the same to my vette ,first at idle and you see a little bit of movement compared to my truck . I rev it a bit and it flows more but not compared to the truck. I think I am on to something here.:surprised That second water pump that I tried was a used one I had in the garage , so maybe I can go with a new one. The griffin rad I put on there is also not a "brand new" one . I had it in my garage and took it to a rad shop to get it clean before installing it. I asked the shop to verify if it was flowing properly and they said it was good to go. Don't know if they actually flow tested tested it or not. I'm thinking it might be the rad and/or the water pump . When I took the other radiator out it was because it was also overheating with that one.
 
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