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post #1 of (permalink) Old 03-01-2003, 07:48 PM
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Overheating with Chevy 350...(long)

OK, I've checked through the archives and have tried everything I can think of, with no luck. I've worked as a mechanic for the last year and a half, and have never seen a problem like this. Before I pull the heads and check for a failed gasket, I wanted to solicit as many opinions as possible.

The pertinent specs on my jeep:
-'81 CJ7 with a NEW GM Goodwrench 350 ('69-85 truck engine)
-Weind water pump
-March serpentine pulley kit (not underdrive pulleys)
-Mepco 3 core heavy duty conversion radiator
-Flex-a-Lite dual electric fan (rated at 2700 cfm)
-MSD 8360 distributor and 6al ignition
-Edelbrock Performer intake and 600 cfm carb
-Stock Chevy "rams-horn" exhaust manifolds
-195 degree t-stat
-16 lb rad cap
-50/50 coolant/water mix with overflow tank

I've only put a couple hundred miles on the new engine, and have developed an overheating problem. The engine will idle at 200-210 all day long without overheating, but once I give it gas (even at a standstill), the temp rises slowly to 240-250. Keeping the throttle at the exact same position, the temp stays at 250 for about 3-5 seconds, then slowly falls to around 180, then comes back to 200-210, where it stabilizes. Eventually, the temp starts to rise again, while keeping the gas at the same position, and the process repeats.

Here's what I've done:
-replaced rad cap and t-stat
-replaced upper and lower rad hoses
-burped coolant system of all air
-adjusted carb mixture many times
-adjusted timing from 0 degrees up to 20 (detonation)
-installed new mechanical water temp gauge along with Autometer Phantom electrical gauge to verify correct temp readings
-replaced vac advance on distributor
-changed advance springs (all 3 sets)
-disconnected vac advance hose
-hoses do not collapse under throttle
-cooling fans will hold a shop rag on the front of radiator without falling
-looking down radiator, I can see the coolant moving
-changed spark plugs
-verified tdc

I can't imagine I have a problem with the water pump, cooling fans or radiator, because this condition occurs even at outside temperatures of below 15 degrees. I'm not losing coolant and I don't have coolant in the oil or a sweet smell from the exhaust. When I put the intake on the engine (years ago), I did a stupid thing and used alot of blue RTV along with the intake gaskets, so I'm thinking of pulling the intake to make sure I don't have any blocked cooling passages. My next step will be to check the head gaskets.

Anybody have any suggestions at all???? This has been a 5 year project, with countless dollars spent...I hate to see it just taking up space in my driveway for much longer.

Thanks alot and sorry for the length of the post.

Greg J.
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post #2 of (permalink) Old 03-01-2003, 09:16 PM
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Re: Overheating with Chevy 350...(long)

1. Is there any expelling to the overflow?
2. Is the engine running too lean?
3. Is the exhuast restricted?
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post #3 of (permalink) Old 03-01-2003, 09:38 PM
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Re: Overheating with Chevy 350...(long)

1-No coolant is transferring to the overflow. In fact, other than the temp gauge, no other signs of the engine running hot have occurred (coolant spewing, bubbling, etc).

2-Plugs have a nice brown/tan color to them and no signs of running a too lean mixture. I have tried both leaner and richer combinations of jets/metering rods in the carb.

3-No indication that the exhaust is restricted. Exhaust from the tailpipe is full and consistent. It is brand new 2 1/4 pipe, joining into a single exhaust under the oil pan, running into a Series 40 Flowmaster. No catalytic converter installed.

I've got a infrared temp gun at the shop that I might bring home and experiment with. I might be able to narrow down some hot spots, and see if my coolant flow is free.

Thanks for responding

Greg J.
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post #4 of (permalink) Old 03-01-2003, 10:14 PM
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Re: Overheating with Chevy 350...(long)

Something to think about,
If you are using the stock Jeep gauge and the sender from the 350, you might not be getting accurate readings from the gauge, because of the different resistance readings of the senser.Try an aftermarket mechanical temp gauge for a true temp. reading.

post #5 of (permalink) Old 03-01-2003, 10:31 PM
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Re: Overheating with Chevy 350...(long)

Good idea, but I've got all new Autometer gauges with an electric water temp unit. I thought it might be reading wrong, so I bought a cheapo Sun mechanical temp gauge to compare and got the same results. The electric sender is plugged into the intake and the mechanical is plugged into the driver side head.


Greg J.
post #6 of (permalink) Old 03-01-2003, 11:46 PM
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Re: Overheating with Chevy 350...(long)

OOPs, I missed that part.
You say you can see the coolant moving, but could the t-stat be cycling? Like, what else would make the temp fluctuate like that? Have you tried a different stat, or a 165 ?

post #7 of (permalink) Old 03-01-2003, 11:57 PM
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Re: Overheating with Chevy 350...(long)

I have gone through the same problems with my `62 and the 383 ci Chevy. Are your inner fenders tight to the frame? If so use a 2" hole saw on the bottom edge above the frame. The engine makes so much heat you need to have an exit for the heat. One of my friends has a `70 CJ5 that had fenderwell headers, he needed to cut the fenderwell. When they rusted through he put rams horns on. But, he left the fenders cut. He never gets over 180 degrees even in rush hour stop and go in 100 degree weather. I tried drilling some holes in mine and it cooled down. Now, with a 195 thermostat I run about 190. Keep cool,

post #8 of (permalink) Old 03-02-2003, 12:50 AM
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Re: Overheating with Chevy 350...(long)

I agree with bigugly. open up the inner fenders a bit to give the hot air a place to go.
if that doesnt work you may have to pony up for an aluminum rad.
I would also use a 180* t-stat. Just a suggestion. Wade
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post #9 of (permalink) Old 03-02-2003, 07:59 AM
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Re: Overheating with Chevy 350...(long)

Cutting holes in the fenders is an interesting idea. I've got a 'glass body and fenders on the jeep, so it wouldn't be hard to cut some material from the inner fenders. Yesterday while I was working on the jeep, my Dad commented on how much heat was coming from the engine compartment. Most of my work has been with the hood open, so I would think that the heat would be escaping pretty quickly.

There was quite a bit of discussion in the archives on aluminum radiators and how they helped operating temperatures. I seem to recall at least 2 posts where Mepco conversion radiators were used, with similar overheating problems at highway speeds or travelling uphill. Might be something to try.

I've thought about changing water pumps, ditching the electric fan and going with a belt driven unit, and maybe getting an aluminum radiator, but I honestly hate to put more money into it. Especially when I can't find concrete evidence that one of the items is failing or not performing properly.


Greg J.
post #10 of (permalink) Old 03-02-2003, 09:03 AM
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Re: Overheating with Chevy 350...(long)

Just a thought, is your bottom hose collapsing? My motor seemed to put off a LOT of heat when we first crancked it. We had the electric fan blowing from in to out through the aluminum rad. and you could not hold your hand in front of it very long. Can you feel that your rad. is dissapating heat? I don`t think the head gaskets would be the problem.
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