i have a very mildly buil 307. and it is in a truck that i pulled a hot 396 out of. i left the big block rad in the truck. now with my newly rebuilt 307 it gets a little hot in traffic. near the 200 mark and never really slows down when it is climbing. it shoots back down once i get moving again but it still kinda concerns me. the small block had a milodon water pump made of cast iron. and has a 180 degree stat in it. i don't have a fan shroud but the big block ran fine w/o a shroud. and i jsut got a new flex fan, which has a considerable amount of pitch to the 7 blades on the fan as opposed to the 4 bladded fly swatter that was on there before. even with the new fan it still climbs a bit in traffic but not as fast as before.

i have 2 thories going right now, which cancel each other out

one: the cooler stat will not allow the engine to get as hot when it is warming up and keep it overall cooler.
two: the cooler stat will just delay the overheating issue. and once the stat stays open , which would be sooner than the 180 stat, the coolant will jsut start to heat up.

I'd put a shroud on it first. How far is the fan from the radiator? Also I would drain the coolant and look down the radiator fill neck with a flashlight and see if there was corrosion or gunk in what flues I could see.

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interesting you would mention the fan distance from the rad. after i posted i went out and was looking at it and i think the fan on the small blocxk is further fromt eh rad than the big block. as i remember i had a difficult time getting the fan bolts out with the big block and it is fairly easy with the small block.

unfortunatly with the truck never coming with a shroud i think i'll have to make one. i figure just some sheet aluminum, tin snips, and pop rivets should to the job.

What year truck? If it is between 73 and now it had a shroud from the factory, and one can be had easily.

That is a symptom of low coolant in the radiator. That would be the first thing I'd check.

Seeing as how the engine appears to only get hot at idle and not while driving that pretty much narrows it down to lack of air or water flow at idle. A good fan is only as good as the shroud that funnels the air for it so I would start there.

Going with a cooler t-stat will only delay the overheating condition, the t-stat is just a valve, once its open the coolant will get as hot as the engine will heat it and as cool as the radiator can cool it. The t-stat just helps to get the coolant to a minium temp, after that it is irrelevant.

Put a shroud on, Throw away that flex fan and put the "fly swatter" back on. Flex fans are junk IMHO.

It sounds to me like the fan is too far from the radiator. The big block sits closer to the radiator, so with the small block installed the fan has been moved back.
A shroud will help, but changing the thermostat won't really do anything for you.
The best mechanical fans (an electric fan will be your best solution, but I get the impression you don't want to go that route) will be the stock-type fan with the viscous clutch. They actually pull more air than flex fans but use less power overall.

I second exactly what sniggle_t said.

the truck is a 70 c-10. so does that mean it didn't come with a shroud? after really thinking about what i asked i am kinda on everyone's side here now. except i'll prolly stick with the flex fan for now. or atleast untill the next swap meet. i know there are spacers that can be had to move the fan closer to the rad but seeing as i am trying to get rid of this truck i don't really want to spend much money with this issue. i think the most i will do is get a shroud, check fluid levels and timing and make sure it isn't too far advanced and making it run hot.

i drove it today and somewhat tested it in traffic and the new fan seemed to help it only got past 190 once but it still did so atleast i'm getting somewhere.

[This message has been edited by bryan1970 (edited June 17, 2005).]

Did you keep the same sending unit? There could be a variance in them between the two engines and it just shows up on the gauge as being too hot.

I have never used a thermostat? My car always runs 160-180. Never had any problems.

I do have a gutted t-stat in as a flow restrictor.

[This message has been edited by Mwilson (edited June 22, 2005).]

<font face="Arial,Verdana" size="2">Originally posted by GetMore:
It sounds to me like the fan is too far from the radiator. </font>

That is a good catch off the post.

Agree as well, if it runs hot the thermostat isn't going to do squat.

I also didn't read the whole post that well as I typically like caps at the beginning of a sentence. ANYWAY you need to sort out the shroud and make sure the fan is at least half way into it to pull cooling air through the radiator.

[This message has been edited by Rick WI (edited June 23, 2005).]