68 c-10 250-6 overheating

[dwmurry2006]

Heres the problem. I got a 68 c-10 2wd longbed with the 250 6 cylinder and 3 speed saginaw. Factory 2 core rad. and the old 4 blade fan. Just within the past month it started overheating on the highway. The dummy light would come on. I had a 195* t-stat at the time.

Heres what i've done so far...
Drain and flush the cooling system and put fresh coolant 50/50 mix in it.
threw a new thermostat in there to please myself. was told to put 160* in it Overheating really never quit. Checked to make sure water was flowing well- a ok.
Bought a temp gauge to satisfy myself. I could start it up and idle it and it never would get over 185-190. Take it for a spin and pretty quick it made it to 240 while driving. So i pulled off to a exit and it climbed to 280 and sat there. Idled it up and brought it down to 240. Drove it home and never went above 240 but never any lower.

So I moved on... Then found out vac adv. quit on distributor. Was told that it would cause my overheating not having enough timing advance while driving.

found skip white performance had a hei dist for chevy 6 cyl. to upgrade from points.
called and talked to tech support and they said it would drop in and run just fine in my truck.
so i went ahead and bought it. 85 bucks shipped to my house.

drop it in and set timing at 10* btdc like tech told me to do. and plugged vac adv. in fired right up and purred like a kitten.sounded great idled so much smoother. but when you crack the throttle it would start bucking and stumbling then let out a huge backfire, not cool at 2 am when everyone around is sleeping. tried to drive it and its impossible. bucking and stumbling and the backfires...oh my lands.
next morning i call and tech told me it was from my old rochesters port vacuum applies full vac when you barely crack throttle instead of supplying gradual vacuum with throttle. told me to just leave vac adv. off and advance timing a few more degrees.

well dont i need the vac adv for mpg and drivability?

told him i wanted it to work with the vac adv. or i didnt want the dist. since other tech told me it would work just fine. he then told me how to use the allen wrench to adjust the amount of vac adv. well that didnt help at all. so i disconnect and plug vacuum line and advance the timing some more and well.... the truck has a miss at idle now.

IT STILL OVERHEATS. It wont if you let it sit there and idle but when you start driving it goes up to 240 it will go up to 280 when you stop then you take off and back to 240 and stays there.

is this from not having enough timing advance? from the lack of vac adv. or does my problem lie elsewhere. My overheating problem started when the vacuum advance died on my points distributor.

sorry for the long post just wanted to be thorough. please help this is my daily driver.

[cdowns]

take a thermometer and put it in the coolant in the radiator to see if you have an actual or percieved problem

[68gmsee]

First, the vacuum advance is primarily used at low idling speeds. Once you open the throttle to a certain rpm, it has no bearing on the timing. At higher speeds the mechanical weights on the rotor shaft control the timing. So, yes, you could conceivably learn to run without the advance.

Now all that being said, there should not be a need to disconnect it. Try to find the problem first. Make sure you have the required full 12VDC going to the HEI. See the electrical section for details. And check for vacuum leaks in the carb, hoses, etc..

Now, re. the overheating, if you really have an overheating problem and not a gauge problem as cdowns mentioned, I would start by checking the lower radiator hose for collapsing under higher rpms. It will restrict the flow without you realizing it. Then you will have to do a pressure test on the radiator or leak down test on the cylinders. If you have a head gasket leak it will cause symptoms like you describe.

[68gmsee]

Here's one of the posts in the electrical section: (searched "HEI wiring")

http://67-72chevytrucks.com/vboard/s...d.php?t=148346

[gregg72340]

I had a head gasket issue on the same motor, did the same things. Never acted like it had a bad head gasket except for the overheating. Ran a can of head gasket sealer in it and havent had a problem since. I know its the cheap and "Wrong" way to fix it but for $8 it beat doing a head gasket right now

[DenimViking64c10]

I am having the same problem (250 I6). Mine overheats at idling and cools down maybe 10 degrees while hitting the throttle (it hovers around 210-230). It also has a new water pump, thermostat, headgasket, hoses, rebuilt carb and distributor, temp. gauge and radiator. radiator does have a small pinhole leak which i dont think should make it overheat within ten minutes of operation. PLEASE HELP! losing my darn mind over this problem. i just want to enjoy my truck!

[DenimViking64c10]

Oh and one more thing. Could somone tell me the correct way to set the valve lash on the 250 I6? i got them set somewhat but still got some chatter.

[68gmsee]

Originally Posted by DenimViking64c10

I am having the same problem (250 I6). Mine overheats at idling and cools down maybe 10 degrees while hitting the throttle (it hovers around 210-230). It also has a new water pump, thermostat, headgasket, hoses, rebuilt carb and distributor, temp. gauge and radiator. radiator does have a small pinhole leak which i dont think should make it overheat within ten minutes of operation. PLEASE HELP! losing my darn mind over this problem. i just want to enjoy my truck!

Since everything is new, fix the pinhole first. Depressurizing the cooling system will cause you problems. Now as for the rest, make absolutely sure that it in fact is overheating and that you don't have a gauge problem. Also make sure you don't have a head gasket problem. Heat from the cylinders gets into the cooling system and causes overheating. A shop can check for exhaust contamination i the system or do a leak down test for you.

My 68 had an overheating problem which I cured by replacing the cap and installing an overflow tube. No more overheating.

[68gmsee]

Originally Posted by DenimViking64c10

Oh and one more thing. Could somone tell me the correct way to set the valve lash on the 250 I6? i got them set somewhat but still got some chatter.

More than likely you have hydraulic lifters. I like to adjust mine while engine is running. Messy but does the job for me. I loosen the nut until I barely hear a tic than tighten slightly until it just stops ticing than 1/2 turn more in 1/4 turn increments. Some people adjust it 3/4 turns in. If that doesn't resolve, you may have worn parts. Either bad lifters, worn rocker arms or even the cam.

I start with adjusting than replacing the lifters before looking at the rest of the stuff. Always do the easy things first.

[DenimViking64c10]

Appreciate all the advice guys!
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