I've been chasing a overheating issue for a few weeks on my 400ci, c6. It will idle all day long and just barely makes it to just below half on the gauge. If i drive the truck it will overheat and begin to run poorly. So far, I've replaced the thermostat with the correct style for this engine, replaced upper radiator hose, replaced fan clutch, and added the missing fan shroud. I've also run a hose through the engine until the water comes out of the radiator clear. There was some nasty scale in the thermostat housing, and no thermostat installed previously. I just rebuilt the top end, I never had the truck on the road before the rebuild due to a stuck valve. It does not smoke at all, no milkshake on the dipstick. I noticed it has a 7psi radiator cap on it, I know a higher pressure cap won't lower temperatures, but it is pushing coolant past the cap. If the truck does overheat I can sit and idle for a while and the temp will return to below the halfway mark.

Any help appreciated, I would like to be able to start driving the truck to work.

 

Get yerself a 13 lb radiator cap. It'll help with the boiling point of the coolant (you are running 50/50, yes?) and raise pressure.

 

When I have had hot rolling and cool idling it was indication of a plugged radiator. The radiator just could not handle the heat generated with "power on" yet did OK at idle. Rodding out (physically cleaning water passeges) the radiator may work if there are shops in your area that do this at reasonable cost. Or get a new one. I have had good luck with a Champion radiator on my dent.

 

X2 on the rad being worn out. I guess they scale up from using tap water?

 

Agree with fitting a 'correct' 13lbs cap, and that the radiator core is clogged.

Flushing the radiator with a hose does not unclog it.

 

I've replaced the radiator with a brand new aluminum 3 row unit. I had a few issues with fitment, the new radiator isn't quite as deep as the factory unit so I had to space it closer to the engine with washers. I think the core itself is the same size but the bracket that bolts to the core isn't the same size. Before I spaced it the power steering belt rubbed a hole in the hose and the fan blades were not in the shroud. I think it's pretty close to the factory location.

I took the truck for a ride on the same route I drove it last time it overheated. It was doing great, about the middle of the 'E'. Then I went full throttle off a stop light I let off at 55 mph. Temp got to the space between the 'E' and 'M' and the truck started bucking like it didn't to go. I was coming through the neighborhood and the needle got to the end of 'M' never hit the 'P' but still warmer than I want it to be. I'm starting to wonder if the transmission is getting hot and making the coolant temp rise. I have no carpet because I just replaced the floor pans, the floor gets very hot.

 

The state of tune can have an effect on engine temperature. If it's running leaner at speed it will run hotter. Timing will also affect it.


A lightly loose fan belt will provide enough drive to keep it cool at idle since there is less force required to move the water pump at low speeds. As engine speed increases, it will begin to slip, probably without making a sound.


Michael

 

The transmission will not cause your engine to overheat. Do yourself a favor and get a block test kit from an auto parts store. Follow the instructions on the kit and if the fluid changes from blue to green or yellow, you have head or head gasket problems. To change temps as fast as you say that is really the most likely problem.

 

Transmission High temp will definitely raise temp in the radiator. If one gets hot the other is going to.

timing at speed could be an influence. Too little or too much. Check for mechanical advance working.

Radiator cap for sure.

Pure water is a better coolant the 50/50 mix but doesn't have inhibitor or lube.

 

It will raise the temps a little, but will NOT cause an overheat of the engine. There just isn't enough heat in a transmission, especially unloaded and with the size of the cooler in the radiator, to heat four gallons or whatever of coolant that much.

Just because he did top end work does not mean that something was inadvertently done wrong or parts were not bad out of the box. Also, does he have the correct thermostat for the 400 with the collar?

​​​​​​Yes, timing can be an issue. Make sure that the initial base timing is correct, the mechanical advance is changing and returning all the way back with RPM, the vacuum advance works priperl , and the vacuum delay (if installed) is in the vacuum line the right direction and operating correctly.

However, the OP makes this sound like the overheat occurs very quickly. Hence my earlier recommendation. Exhaust pressure in the cooling system will blow the coolant out of the radiator, sometimes in a very spectacular fashion, sometime just a bit of loss. The manner that those gasses enter the cooling system makes all the difference as to symptoms such as overheating at cruise or idle.

The block test is a simple and relatively inexpensive way to rule this out. If the OP has a timing light, by all means, do those checks first as they will not cost anything but time.

 

Factory temp gauges are not terribly accurate. That said, needle anywhere on the "Temp" word is not overheating. If it gets to the line next to "H" that would suggest hot. If in doubt, install a mechanical gauge as a diagnostic. Running down the road in hot weather "normal" would be 200-205 degrees. It is also likely to warm up 10-15 degrees at an extended stop then cool down when you roll.

It is normal for the gauge to rise a little under heavy load. It has to get somewhat hotter to completely open the thermostat and the T stat takes some time to react.

If you take it up to speed and hold it there for a while (eg 10 minutes) the gauge will normally climb up a little then settle part way back as the T stat opens up and temp stabilizes.

 

Symptoms of overheating, but only on hard acceleration...

• Lean condition caused by ignition timing
• Cracked head/block
• Head gasket

Does the coolant smell of exhaust? There are tools to test for cracked head/block without tear down

 

I do have the correct thermostat for this engine. My '73 Mustang takes the same style.

There is no compression leaking into the cooling system, coolant is not forced from the radiator during cranking or running. does not smell like exhaust. Also no smoke at all, not growing oil, not milky, and coolant level has not changed since initial fill. New radiator came with a new cap.

The route I'm taking is roughly 9 miles, the truck began acting up on mile 7. I'm about 1 mile from the neighborhood entrance. It's a slow rise, it doesn't shoot up immediately.

I previously set timing by ear, advancing until idle stopped rising. I rechecked timing, it was 28 initial, I got the timing light out and got it down to 16 degrees. It doesn't sound like it idles as well as it did but feels like it runs slightly smoother. Took it back out, it hesitates off a stop and doesn't like light throttle. Ran it harder than I did on the last drive and did not have the bucking issue this time. Ambient temperature has gone down because it has rained here on and off for the last 3 hours. I don't have vacuum advance hooked up. While it's not getting hot like it was, idle and part throttle doesn't run as well as it did with more timing.

 

Does the mechanical advance work and what was the total at 2500-3000 rpm?
If you hook up the vacuum advance how does it run then?


Have you checked if the fan is installed right? If it is not pulling air but pushing air will not move thru radiator when moving and could over heat.


Have you pulled plugs to see what they look like? If lean they will look new and can cause issues like what you say it is driving like now and over heating. What carb you running?


Did you say the motor was rebuilt? If so was the block hot tanked?
Dave ----

 

Make sure the lower radiator hose has a spring in it, those can rust away over time. Without it hose can collapse and severely limit flow when rpm's are high. Definitely sounds like a coolant flow issue, airflow issues usually show up at idle/slow speeds.