Another Overheating Mystery - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**Oregon 1967 GMC** · 12-06-2022, 10:05 AM

After 5-10 minutes 400 sbc is heating up to 220° I turn off and notice the radiator feels cool. Have replaced thermostat, (180°) got air out by filling and running even jacking up front end. Temp sensing unit from classic instruments does not have adapter that can trap air. Laser thermometer shows temps are reading correctly. I plan on replacing water pump next. I just finished rewiring entire truck and installed 4 row radiator because I installed air conditioning
Thought I had air pockets but I've done everything to get the air out. Any suggestions?

**Oregon 1967 GMC** · 12-06-2022, 10:07 AM

It was running fine before so it's not the head steam holes or anything like that. Can someone flip photo? I tried.

**kwmech** · 12-06-2022, 10:13 AM

What was/has changed from before?

**Oregon 1967 GMC** · 12-06-2022, 10:29 AM

Quote:

Originally Posted by kwmech

What was/has changed from before?

just finished rewiring entire truck and installed 4 row radiator because I installed air conditioning

SRU1436 · 12-06-2022, 10:35 AM

Quote:

Originally Posted by Oregon 1967 GMC

just finished rewiring entire truck and installed 4 row radiator because I installed air conditioning

Are you reaching these temps while idling or driving? Is the AC on or off when you reach these temps? What kind of fan you you have? Fan shroud? How big are the cores on the radiator? I had the same problem on my chevelle after AC install. The AC condenser blocks a lot of air flow, it should be (I believe and if I remember right) based in vintage air directions about 1.5 inches away from radiator. My fix was add a HD fan clutch to my mechanical fan.

**Oregon 1967 GMC** · 12-06-2022, 10:54 AM

Quote:

Originally Posted by SRU1436

Are you reaching these temps while idling or driving? Is the AC on or off when you reach these temps? What kind of fan you you have? Fan shroud? How big are the cores on the radiator? I had the same problem on my chevelle after AC install. The AC condenser blocks a lot of air flow, it should be (I believe and if I remember right) based in vintage air directions about 1.5 inches away from radiator. My fix was add a HD fan clutch to my mechanical fan.

Idling and driving
AC on and off
Clutch fan with shroud half in half out as direction say.
I'm thinking some air possibly left in system and tired water pump.

**MikeB** · 12-07-2022, 05:59 PM

Quote:

Originally Posted by SRU1436

The AC condenser blocks a lot of air flow, it should be (I believe and if I remember right) based in vintage air directions about 1.5 inches away from radiator.

FYI, Vintage Air says to mount it as close to the radiator as possible without touching, so condenser and rad don't rub each other.

The closer the better, otherwise the fan(s) will draw air from the gaps between rad and condenser because it's the path of least resistance.

**truckster** · 12-06-2022, 10:13 AM

Here you go.

Just because you changed the thermostat doesn't mean it's good. Have you tried running without one to see what happens?

**Oregon 1967 GMC** · 12-06-2022, 10:28 AM

Quote:

Originally Posted by truckster

Here you go.

Just because you changed the thermostat doesn't mean it's good. Have you tried running without one to see what happens?

No but I will. Was just hoping it would fix itself 😄

72SB · 12-06-2022, 11:33 AM

The fan is not on backwards...is it?

how many blades?

IMHO fewer (2 row) but larger row (1 1/4" to 1 1/2") is better than smaller 4 row radiators

**Oregon 1967 GMC** · 12-06-2022, 12:51 PM

Quote:

Originally Posted by 72SB

The fan is not on backwards...is it?

how many blades?

IMHO fewer (2 row) but larger row (1 1/4" to 1 1/2") is better than smaller 4 row radiators

5 blades and I wish it was on backwards

**Stocker** · 12-06-2022, 01:10 PM

Quote:

Originally Posted by Oregon 1967 GMC

5 blades

My K20 has a factory 7-blade fan (18") on a HD Hayden 2747 clutch and it does not overheat. If purging air from the system doesn't do the trick, maybe consider upgrading to a 7-blade on a HD clutch (if yours is standard duty).

**Oregon 1967 GMC** · 12-06-2022, 02:05 PM

Quote:

Originally Posted by Stocker

My K20 has a factory 7-blade fan (18") on a HD Hayden 2747 clutch and it does not overheat. If purging air from the system doesn't do the trick, maybe consider upgrading to a 7-blade on a HD clutch (if yours is standard duty).

Thank you

samert111 · 12-06-2022, 02:26 PM

Does it run cooler and stay steady temp with the heat control on hot but without the fan running?
If so it needs the heater circuit open as a bypass loop to keep constant flow of coolant past the sending unit
I just went thru this on my 68 with the vintage air and 350 CI. Temp would climb up to 240 then bounce back down to 200 then back up again repeatedly. Once I turned the temp control to hot it would drop to 185 and stay there all day.
I tried everything including drilling holes in the stat, burping the system repeatedly but no luck.
I ended up adding a bypass loop from the unused coolant port in the intake to the unused port on top of the water pump and problem solved.
I can't explain why it works but it does.

72SB · 12-06-2022, 01:35 PM

Quote:

Originally Posted by Oregon 1967 GMC

5 blades and I wish it was on backwards

My 5 blade would not keep up when I put in my built 357...7 blade fixed that

Also if you don't have a HD fan clutch, you need one

RichardJ · 12-06-2022, 03:52 PM

I was thinking the same as grumpoldman and samart111. Vintage Air sells this heater hose bypass. It's intended mainly for fnord application. Check the heater hose temperatures as grumpy suggested.

KQQL IT · 12-06-2022, 09:41 PM

I put a high flow water pump on my 400 and it held the T Stat closed.
Took it out ran cool, ended up back with a stock water pump and the same Stat. Never an issue since.

rockyrivermark · 12-08-2022, 10:31 AM

Maybe try pulling stat completed and see what happens.
Get IR temp gun and get temps at stat housing.
Both top and lower radiator hoses hot?

**Oregon 1967 GMC** · 12-08-2022, 01:35 PM

I'm going to be doing that this coming weekend and post an update
Thanks

**Accelo** · 12-10-2022, 01:34 AM

Did you get the heads and gasket for the 400ci? The ones with the extra steam holes in them?
Cheers

**Oregon 1967 GMC** · 12-15-2022, 08:12 PM

****UPDATE*****
The culprit was this plastic cap that lodged itself inside top radiator hose. I'll take credit for doing that whoops! Put a heavy duty water pump in anyway. Runs really cool now. It's about 40° here in the Great Northwest. Yes I'm happy.

**Stocker** · 12-15-2022, 08:38 PM

Quote:

Originally Posted by Oregon 1967 GMC

****UPDATE*****
The culprit was this plastic cap that lodged itself inside top radiator hose. I'll take credit for doing that whoops! Put a heavy duty water pump in anyway. Runs really cool now. It's about 40° here in the Great Northwest. Yes I'm happy.

Oops!

Glad you found it and all is well now. Kinda curious how it got in there, though....

**Oregon 1967 GMC** · 12-15-2022, 09:40 PM

Quote:

Originally Posted by Stocker

Oops!

Glad you found it and all is well now. Kinda curious how it got in there, though....

I probably couldn't have thrown it down the hose if I tried but somehow it got down there. I more than likely got distracted and it somehow fell in there.

geezer#99 · 12-15-2022, 10:41 PM

Some new rads come with a plug in the inlet/outlet to keep varmints and crap outta the insides.
Maybe that’s what it is!

**Oregon 1967 GMC** · 12-15-2022, 11:14 PM

Quote:

Originally Posted by geezer#99

Some new rads come with a plug in the inlet/outlet to keep varmints and crap outta the insides.
Maybe that’s what it is!

I think you're right

12-06-2022, 10:05 AM	#1
Oregon 1967 GMC Senior Member Join Date: Jan 2014 Location: Portland, Oregon Posts: 448	Another Overheating Mystery After 5-10 minutes 400 sbc is heating up to 220° I turn off and notice the radiator feels cool. Have replaced thermostat, (180°) got air out by filling and running even jacking up front end. Temp sensing unit from classic instruments does not have adapter that can trap air. Laser thermometer shows temps are reading correctly. I plan on replacing water pump next. I just finished rewiring entire truck and installed 4 row radiator because I installed air conditioning Thought I had air pockets but I've done everything to get the air out. Any suggestions? Attached Images __________________ Hey man 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603

12-06-2022, 10:07 AM	#2
Oregon 1967 GMC Senior Member Join Date: Jan 2014 Location: Portland, Oregon Posts: 448	Re: Another Overheating Mystery It was running fine before so it's not the head steam holes or anything like that. Can someone flip photo? I tried. __________________ Hey man 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603

12-06-2022, 10:13 AM	#3
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,628	Re: Another Overheating Mystery What was/has changed from before?

12-06-2022, 10:13 AM	#8
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,976	Re: Another Overheating Mystery Here you go. Just because you changed the thermostat doesn't mean it's good. Have you tried running without one to see what happens? Attached Images __________________ I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson

12-06-2022, 11:33 AM	#10
72SB Registered User Join Date: Apr 2022 Location: Moorpark, CA Posts: 774	Re: Another Overheating Mystery The fan is not on backwards...is it? how many blades? IMHO fewer (2 row) but larger row (1 1/4" to 1 1/2") is better than smaller 4 row radiators

12-06-2022, 02:26 PM	#14
samert111 Registered User Join Date: Jul 2022 Location: Rockford, Mi Posts: 76	Re: Another Overheating Mystery Does it run cooler and stay steady temp with the heat control on hot but without the fan running? If so it needs the heater circuit open as a bypass loop to keep constant flow of coolant past the sending unit I just went thru this on my 68 with the vintage air and 350 CI. Temp would climb up to 240 then bounce back down to 200 then back up again repeatedly. Once I turned the temp control to hot it would drop to 185 and stay there all day. I tried everything including drilling holes in the stat, burping the system repeatedly but no luck. I ended up adding a bypass loop from the unused coolant port in the intake to the unused port on top of the water pump and problem solved. I can't explain why it works but it does.

12-06-2022, 03:52 PM	#16
RichardJ Registered User Join Date: Apr 2010 Location: So Cal Posts: 1,466	Re: Another Overheating Mystery I was thinking the same as grumpoldman and samart111. Vintage Air sells this heater hose bypass. It's intended mainly for fnord application. Check the heater hose temperatures as grumpy suggested. Attached Images __________________ '67 GMC 2500, 292, 4spd, AC

12-06-2022, 09:41 PM	#17
KQQL IT At the body shop. Join Date: Aug 2009 Location: Land of fruits and nuts. Posts: 5,254	Re: Another Overheating Mystery I put a high flow water pump on my 400 and it held the T Stat closed. Took it out ran cool, ended up back with a stock water pump and the same Stat. Never an issue since. __________________ " That didnt make it any newer " " Dont antique the equipment "

12-08-2022, 10:31 AM	#18
rockyrivermark Registered User Join Date: Apr 2011 Location: Fairview Park, Ohio Posts: 1,031	Re: Another Overheating Mystery Maybe try pulling stat completed and see what happens. Get IR temp gun and get temps at stat housing. Both top and lower radiator hoses hot? __________________ My build page http://67-72chevytrucks.com/vboard/s...d.php?t=764662 My cabinet building site http://www.cbcabinets.com

12-08-2022, 01:35 PM	#19
Oregon 1967 GMC Senior Member Join Date: Jan 2014 Location: Portland, Oregon Posts: 448	Re: Another Overheating Mystery I'm going to be doing that this coming weekend and post an update Thanks __________________ Hey man 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603

12-10-2022, 01:34 AM	#20
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,295	Re: Another Overheating Mystery Did you get the heads and gasket for the 400ci? The ones with the extra steam holes in them? Cheers

12-15-2022, 08:12 PM	#21
Oregon 1967 GMC Senior Member Join Date: Jan 2014 Location: Portland, Oregon Posts: 448	Re: Another Overheating Mystery **UPDATE*** The culprit was this plastic cap that lodged itself inside top radiator hose. I'll take credit for doing that whoops! Put a heavy duty water pump in anyway. Runs really cool now. It's about 40° here in the Great Northwest. Yes I'm happy. Attached Images __________________ Hey man 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603

12-15-2022, 10:41 PM	#24
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,731	Re: Another Overheating Mystery Some new rads come with a plug in the inlet/outlet to keep varmints and crap outta the insides. Maybe that’s what it is!



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