| $225 radiator experiment revisited [message #371375] |
Fri, 26 May 2023 10:06  |
Tilerpep
Messages: 404
Registered: June 2013
Location: Raleigh, NC
Karma: 7
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Senior Member |
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Back in 2020 I posted some details and pictures about my $225 radiator experiment on my 1975 coach.
Old thread if you are interested:
https://gmc.mybirdfeeder.net/GMCforum/index.php?t=msg&th=43326&goto=352412&rid=3615#msg_352412
I've put 5000 miles on it in front of the 455 I rebuilt at the same time. I still do not have the cooling where I want it. As another thread today was asking about the Yukon cooling parts experiment, and I am actively trying to iron out mine to 100% for summer season, I am starting a new thread. As much to sort my thoughts and ongoing troubleshooting as current and future readers. If you have solutions, explanations, engineering insight, practical experience, etc. please chime in. There is still need/desire/reason to figure out a local, off the shelf, option to the $1000 custom/oem style radiator.
Short version: Advance Auto radiator listed for a c3500 454 that has a built in tranny cooler, with an RX-7 (huge) external oil cooler. Started with a stock style 195 thermostat, went to a 180 Mr Gasket high performace, and now running 195 Mr. Gasket High performance.
Part I don't like - my fan clutch stays locked on to keep the coach cooled over 60 mph if ambient over 70. Also, my temp oscillates between 185 and 210 depending on expected variables like temp, load, duration, etc. My 1978 with 403 goes to temp and stays there like a modern car.
The coach never problematically overheats as the fan engages to correct it. In fact goes down t0 185 within a minute or two of load removed or speed reduced (for example, hot (210) and loud (fan locked) at highway speed and I pull off down exit ramp and engine will be cool and fan turns off by time I have entered the side road).
John Lebetski and old threads on this topic from Richard Denny have proposed a high pressure in the engine bay that effectively stops ram air flow through the radiator and bay. This possibility explains more of the behavior than anything else I have come across. Especially since some of this heat struggle was normal for this coach with a oem copper huge radiator before my rebuild (I tried four fan clutches and three thermostats with that setup. I had the oem tanked and tested several thousand miles before the eventual engine rebuild and radiator experiment). I don't know if some coaches are more susceptible to this. I have tried fan clutches, with and without wheel well liners, different thermostats, with and without oil cooler entirely as well as oil cooler placement in the air flow.
My list of "Why Hot at Speed" includes 1. air flow 2. water flow 3. radiator BTU capacity 4. ignition timing 5. carb too lean 6. oil cooler heat in the wrong place for fan clutch response (remember the rx7 cooler).
I have done my best to confirm timing, bought a AEM air/fuel meter which shows about 16 at highway speed (I am learning A/F as I go and I understand this to be lean but should not be overheat lean). Changed multiple configurations of air baffles and where the oil cooler is placed. Same behavior.
Todays experiment is going to be to take the thermostat out completely and see if the temp osciallation changes. My next experiment is likely a new water pump. This pump is the GMB recommended years ago when I first bought this coach ten years ago to replace leaking pump. That pump was moved to the rebuilt engine, and if it never moved enough water to begin with may explain ongoing issue.
Coach is very usable, but all ideas for improvement or testing appreciated. Goal is: stable temp at 70 mph with no fan at 90 ambient.
1975 Glenbrook, 1978 Royale rear bath
Raleigh, NC
[Updated on: Fri, 26 May 2023 10:14] Report message to a moderator |
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| Re: $225 radiator experiment revisited [message #371386 is a reply to message #371375] |
Thu, 01 June 2023 11:27  |
Keith V
Messages: 2337
Registered: March 2008
Location: Mounds View,MN
Karma: 0
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Senior Member |
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My biggest concern with GMC's and cooling is the utter lack of ducting. Look at any modern car and the rad is sealed to the grill, or under if a bottom feeder.
No radiator will cool if it doesn't get airflow.
I'm adding ducting to mine, as it's pretty good, until you climb a hill in hot weather, then the fan comes on.
The ducting I've added seems to help, but I'm going to add more.
Keith Vasilakes
Mounds View. MN
75 ex Royale GMC
ask me about MicroLevel
Cell, 763-732-3419
My427v8@hotmail.com
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