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#1 |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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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?
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Hey man ![]() 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603 |
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#2 |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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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.
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Hey man ![]() 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603 |
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#3 |
Senior Member
![]() Join Date: Mar 2003
Location: Colfax-California
Posts: 8,554
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Re: Another Overheating Mystery
What was/has changed from before?
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#4 |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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Re: Another Overheating Mystery
just finished rewiring entire truck and installed 4 row radiator because I installed air conditioning
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Hey man ![]() 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603 |
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#5 |
Registered User
Join Date: Mar 2019
Location: San Ramon, CA
Posts: 227
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Re: Another Overheating Mystery
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.
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#6 | |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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Re: Another Overheating Mystery
Quote:
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.
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Hey man ![]() 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603 |
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#7 | |
Senior Member
![]() Join Date: Jul 2000
Location: North Texas
Posts: 3,514
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Re: Another Overheating Mystery
Quote:
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.
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Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! |
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#8 |
Senior Member
![]() Join Date: Jan 2001
Location: Orem, Utah
Posts: 7,923
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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?
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I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson |
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#9 |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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Re: Another Overheating Mystery
No but I will. Was just hoping it would fix itself 😄
__________________
Hey man ![]() 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603 |
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#10 |
Registered User
Join Date: Apr 2022
Location: Moorpark, CA
Posts: 759
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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 |
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#11 |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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Re: Another Overheating Mystery
5 blades and I wish it was on backwards
__________________
Hey man ![]() 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603 |
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#12 |
20' Daredevil (Ret)
![]() Join Date: May 2003
Location: Jefferson State
Posts: 13,650
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Re: Another Overheating Mystery
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).
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- Mike - 1972 K20 LWB 350/350/205 RIP El Jay |
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#13 |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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Re: Another Overheating Mystery
Thank you
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Hey man ![]() 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603 |
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#14 |
Registered User
Join Date: Jul 2022
Location: Rockford, Mi
Posts: 71
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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. |
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#15 |
Registered User
Join Date: Apr 2022
Location: Moorpark, CA
Posts: 759
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Re: Another Overheating Mystery
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#16 |
Registered User
Join Date: Apr 2010
Location: So Cal
Posts: 1,442
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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.
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'67 GMC 2500, 292, 4spd, AC |
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#17 |
At the body shop.
Join Date: Aug 2009
Location: Land of fruits and nuts.
Posts: 5,223
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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.
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" That didnt make it any newer " " Dont antique the equipment " |
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#18 |
Registered User
Join Date: Apr 2011
Location: Fairview Park, Ohio
Posts: 1,031
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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?
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My build page http://67-72chevytrucks.com/vboard/s...d.php?t=764662 My cabinet building site http://www.cbcabinets.com |
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#19 |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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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 |
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#20 |
Senior Member
![]() Join Date: Mar 2015
Location: washington
Posts: 2,256
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Re: Another Overheating Mystery
Did you get the heads and gasket for the 400ci? The ones with the extra steam holes in them?
Cheers |
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#21 |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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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.
__________________
Hey man ![]() 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603 |
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#22 | |
20' Daredevil (Ret)
![]() Join Date: May 2003
Location: Jefferson State
Posts: 13,650
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![]() Quote:
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__________________
- Mike - 1972 K20 LWB 350/350/205 RIP El Jay |
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#23 |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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Re: Another Overheating Mystery
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.
__________________
Hey man ![]() 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603 |
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#24 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,638
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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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#25 |
Registered User
Join Date: Jan 2014
Location: Portland, Oregon
Posts: 436
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Re: Another Overheating Mystery
I think you're right
__________________
Hey man ![]() 67 GMC Short Stepside 400 small block, turbo 350. Build thread:http://67-72chevytrucks.com/vboard/s...d.php?t=650603 |
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