350 Heating Up at Stops

[oldgold70c10]

What does your coolant look like? Are you running plain water or coolant mix?
Even if it does not get cold where you live to worry about freezing you need coolant mix for the corrosion protection. If the water is brown or the coolant is not bright green, flush the system before you do anything else. Scale and sludge in the radiator and engine water jackets will rob you of cooling capacity.

[Boog]

I agree with bumping the base timing up some. Then reduce idle speed and readjust carb. Use manifold vacuum for the dist too. This should help to reduce engine temp and actually make the truck more responsive as well and use less gas too. Reducing idle speed will lower the trans fluid temp thereby reducing the heat load on the radiator.

[cebra]

Originally Posted by 68 P.O.S.

Good deal on the shroud, that has a huge effect on cooling. Are you sure the rad is good to go? A 4 row should be plenty. Could be clogged just enough to reduce it's efficiency maybe. The rad is the main part of your cooling system, do you know anything about yours?

Bump your timing back up to 12 or 14 and readjust your idle. If you trust your temp gauge that's good enough for me. You can get an IR thermometer to double check it.

Yes, check your vac advance. Make sure the weights move freely & are lubed and make sure the vac can diaphragm isn't leaking or blown out. Easy check...hook it up to manifold vacuum and the idle will jump noticeably. If it doesn't, the vac can is bad or you already had it hooked up to manifold.

Perhaps on the radiator, it looks very new and clean and it never heats up besides when it is stopped. I may have to bite the bullet and have it pressure tested. I set the timing back up, no change their in temp less it may heat up slightly faster. The vac advance should be ok as I put a new distributor on it a few months back, can test that as well though just to make sure.

[cebra]

Starting to wonder about the radiator as I have tried everything I can think of and/or have read on here. My coolant is 50/50 and looks clean. I have emptied the radiator about once a month for the last 6 months battling this issue. The flex fan vs the clutch fan didn't change anything for me. The clutch fan is pretty far into the shroud where the flex was not but neither fixed the issue. How much does it generally cost to get a radiator pressure tested and will this tell me if the flow is sufficient or if it just has a leak (I know it doesn't)? Thanks for all the suggestions.

[Stocker]

Originally Posted by cebra

The clutch fan is pretty far into the shroud where the flex was not but neither fixed the issue.

It's possible that one problem was exchanged for another, and the results ended up about the same. What I mean is:

You started out with a crappy (my opinion) flex fan. It sat in or near the end of the shroud where it should, but its poor design allowed overheating.

Your new clutch fan can't properly do its job if the fan sits too far into the shroud. Instead of pulling air through the radiator, air is simply passing around the fan, between the fan and the shroud.

If you used a spacer (such as the one your flex fan had) with the clutch fan, that might be the problem. The fan clutches I have seen do not use a spacer. Remove the spacer and the fan will be better positioned in the shroud. Probably already posted but the fan should sit at the end of the shroud, roughly halfway in & halfway out.

Any chance you can post a pic or two showing the placement of the fan?

[Andy4639]

One thing that hasn't been mentioned. Seal off the radiator so the air has to pull through it and not around it. Air is like water it flows to the easiest path. If you have gaps between the radiator and support and shourd close them up with some type of sealer. I use the black foam to seal off around it. It will work wonders for the cooling effect.
Timing will also help if it's not set right.

https://www.usplastic.com/catalog/it...SABEgKt_vD_BwE

[cj847]

I also have a problem. I have a 350 crate engine, new water pump, new radiator, direct drive fan, etc. It is heating up when idling in 100 degree weather with A/C running. Runs cool at 70 mph regardless of weather.

At this point, my only guess is that I have an A/C radiator support and a non A/C shroud. The shroud has about 1/2" gap all the way around the sides and bottom of the radiator. The top is close to sealed. According to the LMC website there are different shrouds and different mounting methods depending on A/C or not.

Does anyone have pics of both fan shrouds installed and uninstalled? Thanks in advance.

[Steeveedee]

How hot is "heating up"? What temperature thermostat? How long does it take, idling, to get to what you feel is too hot? A direct drive fan, as in 4 blades, or a clutch fan with an eliminator installed? We can help if we have more details. Pics are always welcome!

[geezer#99]

And are the fan blades half in / half out of the shroud.

And retarded timing makes for idle heat.
What’s your timing like?

[cj847]

180 degree thermostat, heating up is the line just before the red mark on the gage (I.m guessing about 220 ish), 7 blade aluminum flex fan, fan is perfectly centered front to back with < 1/2 radial clearance.

Pics are hard to see:

[cab4word67]

Wow I have a Zz4 and had a 3 core out of a surb and never saw 200 with a 180 stat. then installed a new 2 core w/1" tubes and it still runs at 190 no more no less. just have a black Magic ele fan pulling. I run at 10 degs adv, headers and eldie 650

[Stocker]

Originally Posted by cj847

7 blade aluminum flex fan

This jumped right out at me. Very good chance that's your problem.

I've owned my '72 K20 for 44 years. The only time it has ever run hot was years ago when my fan clutch failed. I replaced it with an aluminum flex fan. It ran hot. I repaired the fan clutch and it was fine for many years until the shaft seal failed and fluid leaked out. I traded the flex fan for a half-rack of Bud, then helped my buddy drink it. Win-win.

My advice is get a heavy duty thermal fan clutch and an 18", 7-blade OEM-style fan. That's the combo that came with factory A/C on my K20. Hayden 2747 is a heavy duty thermal fan clutch made by Four Seasons in Lewisville, Texas. I bought mine at my local NAPA and it's a perfect replacement for trucks with factory A/C. More info here as to types of fan clutches. https://www.haydenauto.com/en/produc...-types/thermal

[cab4word67]

yup same clutch fan I put on the 66 corvette with a 7 blade. no more heat. problily didn't need the fan but we had it. the clutch was bad.

[tdangle]

My 350 with the air running in 100 degree heat just barely gets to 1/2 way into the operating range when idling for a while, such as a drive thru. I have a 3 core aluminum radiator, clutch fan and sealed all the gaps around the shroud to the radiator. That made the difference. The thermostat only sets the minimum operating temp.

[body bolt]

How is the fan positioned in the shroud? It should only be about half in.

[MySons68C20]

Here is mine. Stock 7 blade no shroud does not get hot.
I don't have a/c but I bet that would solve your problem as others have said.

[Stocker]

Originally Posted by MySons68C20

Here is mine. Stock 7 blade no shroud does not get hot.

Yeah, but you're in Wenatchee!

j/k -- I lived there a short time in the late 60s while working in Cashmere. I still remember Icicle Creek. Beautiful country!

[cj847]

Here is an update and future plans:

-I removed the A/C clips and screwed the shroud up tight. (pics below)
-Just in case, I enriched my idle circuit significantly.
-I have found that my old radiator was a heavy duty which I replaced with a 2 core Champion, which should be plenty for what I use the truck for. (I have ordered the appropriate mounting hardware) (pic below)
-I ordered a Hayden HD fan clutch and a seven blade fan.
-I added a pic of the gage at what I believe is 180 degrees (it will run at this position all day as long as I'm going over 20mph).

With the shroud up tight and rich mixture I am now getting warmer when idling but not hot (like 200 vs 220). I was hoping this was enough to truly "fix" the issue but apparently not.

FYI, once complete, I will have:
new 2 core Champion radiator
new HD water pump
new HD fan clutch
new 7 blade fan
new 180 degree thermostat
new outlet neck
new hoses
new coolant
new radiator mounts and pads

I don't think there is anything else that can be done.....

[Stocker]

Originally Posted by cj847

I added a pic of the gage at what I believe is 180 degrees (it will run at this position all day as long as I'm going over 20mph).



With the shroud up tight and rich mixture I am now getting warmer when idling but not hot (like 200 vs 220).

Have you verified temps with an IR temp gun? Maybe your sending unit and old temp gauge are not quite accurate.

FWIW, I run a 190* (or 195*, I forget) T-stat and my gauge reads lower than yours, nearly half-way to the next mark towards the left. Not saying mine is accurate either...

[Steeveedee]

At idle, heating is generally caused by lack of air flow. My truck was running hot both at speed (when towing) and at idle. I put a new severe duty fan clutch and 19" 6-blade fan. I removed the stock shroud because the fan was way far into it. I formed an aluminum strap into a circle and made a sheet metal shroud to mount it. After all that, as I was filling the radiator and seeing that the thermostat was opening, I could see that it wasn't flowing much coolant. So I drilled 3 #10 holes in the thermostat, and that brought the water flow up and the temperature down. But the holes allowed flow and it took longer for the engine to warm up, so I bought a high-flow thermostat at NAPA. I have yet to tow the trailer in the highway, but the temps are staying down, now. I put a water pump on it about 5 years ago. It is possible that the thing doesn't pump properly, though the running hot persisted after changing the pump. I changed it because it was leaking.