overheating...

[deadhead]

So what could be the most likely cause of overheating in a 1977 620 with a l20b motor?

 

Here are a few more details on the issue....

 

The truck sat for around a year due to lack of funds to correct a carb issue, I finnally got that corrected, changed all the fluids, wires and other tune-up parts, and fired it up. Ran a little choppy at idle, but just fine under load, (i'm thinking it has vacuum/advance issues). Ran it for a couple of weeks with no problems, so I decided to try the highway. After about three days of using the highway on either end of my commute (in CO the limit is 75 mph, and I am running a stock 4 speed), the alt. took a dump. I got the altie fixed up, and went back to running her. Well, fast foreward a couple of two weeks or so, (with limited or no highway driving), to this morning. I had some stuff to do near the highway before work this morning, so I figured why not, and jumped on the highway to get to work. After about 10 miles of driving 60-75 mph I took my exit, and watched the cab start filling with steaming coolant. I pulled off the road, and waited for the temp to come down, then pulled the cap and filled the radiator. It got me the rest of the way to work without even budging the temp guage. (only about 3 more miles...) At the end of the day I checked to make sure that the radiator was still full, and tried to fire it up. It was real reluctant to start, but did anyways, and I headed home on the back roads. The temp needle climbed slowly to operating temp. and hung out there for most of the ride home. It started to edge up towards the hot side, so I stopped at the store, popped the hood and had a peek, all looked normal so I left the hood up and went in to grab some dinner stuff. When I got back out the needle was in the middle of the guage again, so I ran over to the post office to grab my mail. Left the hood down, and ran in for 2 seconds, (litterally), came back out and the needle was waaay up at the H again, and the truck tried real hard not to start. I got it fired up, and coasted to the house. Popped the hood, and checked to make sure that the fan was working, everything appeared to be working, and no fluid was coming out the overflow, but there were damp spots under the hood. Gonna give it a while to cool off, then I will re check the fluids....

 

Any thoughts?????

 

Thanks,

Dave

[Z-train]

First replace the T-stat.I've had SIX in a row(4 bad out-right and and two that were out of spec)that were NFG.

 

3 were Mr.Garbage

1 was a Stant

1 was a dealer part

1 was Robert Shaw

[hang_510]

always replace the cheap / easy parts first.

 

next would be to have a close look at the rad. also the water pump.

sitting for so long may have clogged it.

 

going any longer will likely cause the HG to fail. its what happened to me :mellow:

[datzenmike]

Cab filled with steam?

 

Check the floor under and around the heater. It, or a hose may be leaking. While at it check all the engine hoses and clamps. Hard starting may be coolant on the distributor or wires.

 

Turn the clutch fan by hand. It should turn but offer firm resistance.

 

Does your truck still have the fan shroud on it?

 

Alt./fan belt snug and tight?

 

Leaves or plastic bag stuck to front of rad?

 

For all the bother of taking the thermostat out to test it.... don't, replace it with a new $12 one. If it fixes the problem throw the old one away, if not keep as a spare.

[yello620]

For all the bother of taking the thermostat out to test it.... don't, replace it with a new $12 one. If it fixes the problem throw the old one away, if not keep as a spare.

 

I would recommend that the new thermostat be checked for operation in a pan of water.

I too have had two in a row that just didnt work right. They both seemed to open too late and then very slowly.

Also recommend getting one with the steam bypass hole.

 

Good Luck

[sssr20det510]

steam in the cab is heater core or hoses if they come inside cab. when you filled it back up might have air in the system thats why it didnt steam back up I would check heater core first

[yello620]

Steam in the cab could also be multiple missing firewall plug and harness grommet seals. I guess that all depends on how much steam he is talking about.

 

Steam and a hot guage makes me think that it is more than a heater hore/hose problem.

[deadhead]

Steam in the cab could also be multiple missing firewall plug and harness grommet seals. I guess that all depends on how much steam he is talking about.

 

Steam and a hot guage makes me think that it is more than a heater hore/hose problem.

 

As far as the steam goes, I think this is right on, as there was extensive aftermarket wiring at some point, and I can see into the cab in a number of places from under the hood. Sealing the cab rewiring are on the list of things to get done this summer, for now I just need operational, so I am going to try to flush the hoses and water pump, as well as change the thermo this weekend, hopefully that takes care of it....

 

Dave