overheating. Not sure what the issue is

[kaysboat]

Yesterday the temperature light came on in =my 1980 C10, 250 straight six. I cranked the heater to max heat and did a bit of running around and stopped and started it before it had a chance to boil over. Each time I re-started it, the light came on after a few minutes of normal city driving.

This afternoon, I got out the shop manual and went to try to diagnose things.

1. The light came on as soon as I started the truck, even though it had been sitting for 24 hours.

2. I took the rad cap off and looked for water flowing through the rad with the engine running. Never seemed to flow even after a few minutes of running.

3. After it warmed up, I tried squeezing the upper rad hose. According to the shop manual, you should feel a surge of pressure if the water pump is working. I really didn't feel any surge.

4. I shut it down and felt the back side of the rad. It was hot in the upper corner of the driver's side, and cooler as I moved my had down the rad, and as I moved my had towards the passenger side.

I'm thinking my water pump may be pooched. However, before buying and replacing random parts, any thoughts?

Thanks.

[kaysboat]

Also...

inside the rad there is a beigish goop with black streaks in it. To me that says there is oil in the cooling system (cracked head gasket, cracked head or cracked block. However, I've never seen any water in with the oil when doing an oil change, and the coolent level is always fine). I noticed it before and did a complete coolant flush last year, but the goop was still there after the flush.

Thanks again for any thoughts.

[Getter-Done]

I would start with a Pyrometer to see if the engine is really getting hot.
The sending unit could be bad. A pressure test (on the coolant system of course) would not hurt anything if you have a tester or don't mind buying or borrow one.

[Danmcg]

That goop you mentioned make me think of Bar's Leak. That stuff will clog a radiator bad sometimes.

[Dead Parrot]

Check the heater return hose. It should get warm before the upper radiator hose since it bypasses the thermostat. If the heater return dumps into the radiator, you can check for flow there.

If your heater returns to the water pump, as a test, remove the thermostat, and recheck your flow. Should see lots of flow as soon as you start it. If you have good flow, replace your old thermostat with the new one you bought before performing this test. The old thermostat could be the cause of your problems.

If no flow, you probably have a dead pump.

If the light is still on when the engine is cold, replace the sender as well. Be sure to get the one for an idiot light as the sender for a gauge is NOT interchangeable.

[kaysboat]

Based on the comments, I think it may be the sending unit. I had a look on google and performed a basic test. I ran a jumper wire from the wire going to the sending unit to a ground (the engine block) and the light was still on. Based on what I found and this test, I think it may be a broken sender unit/temp sensor.

Thoughts?

Thank you in advance.

[kipps]

If the light came on immediately on a cold engine, you definitely have a defective sender, circuit, or light. Not saying that is the only problem, as the engine might still be overheating, but it is definitely defective.