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Hello I recently purchased a 76 Custom Deluxe 10 for my son and it just isn't exactly my area of expertise. Just a few points I could use some opinions in. First off it has a newer 350 in it. I believe 5.7 from about 1985 or so. It has also been routed for a serpentine belt. It has edlebrock intake and valve covers. I bought it from an old timer neighbor who worked on it only as project it just sat on a farm for better of 10 years. Here are its problems #1 leaking oil from rear of engine I believe it is all over the bottom of oilpan and transmission. Rear main seal would be my guess. I would really appreciate confirmation and importance of repair also the difficulty of changing it. Leaks about a quart or more per week. Problem #2 It runs hot on temperature gage while on the highway needle is about 4/4 (all the way) up yet no visual signs of overheating. Otherwise in the city it runs about 3/4 of the way up on gage. We have flushed the radiator, changed thurmostat and the overflow tank. Please let me know everything I can do to get this engine up to top condition. Thanks very much. MIKE.
 

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On an SBC I would bet on the valve cover gasket or the back of the intake manifold (across the center where it meets the block) leaking the oil you are seeing. It could be a rear main, but you would be surprised how much oil can come out of a bad valve cover if its left for a while.

I would pull the intake and reinstall using new Edelbrock or Fel Pro Gaskets, and a 1/4" high bead of oil resistant sealer instead of the rubber end seals. Also make sure you put sealer around the center intake bolts that go into the oil gallery flange.

Check the valve covers to make sure they are straight, and then reinstall using rubber gaskets that have the metal reinforcement in them. Replace the bolts with studs so you can slip the gasket and covers over them and just torque down according to specs. Make sure you use the spreaders under the nuts so that it does not bend the cover flange.

I have a '75 GMC with an SBC and the stock temp gauge is not that accurate. There is a tapped hole near the thermostat on the Edelbrock manifold, and you can install a separate mechanical temp gauge probe or an electrical sender.

My truck also runs hotter on the highway than in town. It does not make sense to me since I am using an OEM 3 row radiator in decent condition and a Stewart water pump, but running hot is better than overheating.

Bruce
 

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Running hotter on the highway is usually due to flow restriction in the cooling system. The radiator may need to go to a radiator shop to have them clean it or I have had decent luck taking it out and using CLR to flush it at home. It also may be a bad gauge.
 

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It may also need a good tune-up/carb calibration. If the timing is retarded, or too far advanced, the engine will run hotter. Also, if the carb is lean at cruise rpm, your engine will run hotter.

Checkd to make sure you have the correct water pump for the serpentine setup(reverse rotation)

It would not hurt to pull all the plugs and do a compression test, this will give you a better assessment of the condition of the engine, and allow you to inspect your plugs for fouling, detonation, lean conditions, or just wore out.

Check the plug wires and firing order too, wires could be dry rotted from age, and never assume the previous owner/mechanic never made a mistake.

Install a quality aftermarket temp guage. the capilary tube type are less expensive, but somewhat more difficult to install. The newer electric guages work very well, and only need to pass 1 or 2 wires thru the firewall.

I agree with the possibility that your leak is from the back of the intake or valve covers. Very common issue. Wipe a rag back there (By the trans top), you'll find oil.
 

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Im thinking 2 things...Is there an oil sender behind the intake. And you said that was routed for Serpintine belt.....Is the water pump turning the right way, and make sure lower hose does not collapse when you rev it up. (I have seen that on C10's)
Okay 3 things!!
 

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Get a gauge with numbers on it so you can know the exact temperature.
The leak is probably the oil pan gasket. You can do it with the engine still in the truck. It's 4 pieces so you can just drop the pan, clean it up, and install. Make sure to use rtv where the gasket meets the curved rubber pieces. I doubt it's the rear main. If it is you may be lucky enough to have a 2 piece which is super easy to change.
Check the hoses at higher rpm like stated before.
What kind of fan setup are you running? Pushers or fans in front of the rad with make it run hot on the highway since they restrict airflow a lot.
To find the leak just clean up the back of the motor, run it for a few minutes, then let it sit. It shoudl be pretty easy to see. Just look for the highest oily part.
 
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