Trying to HELP - GMC owner with overheating issue when going up mountains.

LQQKatJon

LQQKatJon

Well-known member
Oct 22, 2010
3,210
530
113
St. Cloud, Mn
lqqkatjon.blogspot.com
1st thing - does not need to be "upgraded" a good stock coach will not overheat.

but, there are a few things he needs to do...

BAsics - check timing with a timing light. make sure the advance mechanism is working on distributor.

know the thermostat is working. 195 degree in my experience is the one to have. Some say 185, I highly disagree.

know that the fan clutch is good. Some say new fan clutch's are random on how well they work, but I installed a Napa part 2747 fan clutch
this past summer, and i know about 4 others that have as well and so far all are workers.

after that, it seems our 45 year old radiators can be not working as well as it should. myself, and i know others had engines that tended to get a
little warm under load, and a new radiator fixed that problem. mine was pretty gunked up.

dont be trying to drag heavy cars up the mountains either.... a good coach should have no issue with that, but the amount of extra load that creates
is crazy big, and if you have other mechanical issues it will only compound trying to isolate the issue.

if they still have problems after making sure the above is all in perfect order, let us know for further possible issues.

--
Jon Roche
75 palm beach
EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
St. Cloud, MN
http://lqqkatjon.blogspot.com/
 
I should add. an IR temp gun is a very handy tool to help figure out what is going on with cooling system, but it is not perfect. STill a tool
one should be getting out and using if you are having temp questions.

you can check the upper and lower hose temps, the thermostat housing. you can compare to what gauge readings are.
--
Jon Roche
75 palm beach
EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
St. Cloud, MN
http://lqqkatjon.blogspot.com/
 
> Hello everyone,
>
> I'm trying to help someone that has an overheating issue with his GMC. (I believe it's a 455 and I will confirm as I've gotten since last night.)
>
> Every time they use it up mountains going to higher altitudes it overheats badly.
> (FYI - I'm a retired ASE and US Army mechanic and I DON'T want him to just throw parts at it.)
>
> My hope is that other GMC owners have experienced this with their coach and that there are some proven upgrades he can do; as I want to make sure
> that he does the right and best upgrades to his cooling system and NOT simply for him to replace parts until it is finally fixed.
>
> Your help is appreciated.
>
> Sincerely, Tony

Tony,

I am glad that your friend has you on his side. Please advise him to join this forum at his first opportunity.

Jon had things very right about the first checks. A coach should not have any difficulties such as that, but they are all forty+ years old. As per
Jon, ask him if he hears the fan roar on all cold starts. He should. If there is doubt, if he should try to spin the fan cold. It should not spin
easily. That would be my first check. A failed fan clutch is not easy to detect unless you know to look for it. Passcars don't seem to have this
problem because they typically don't get run at these loads and are seldom this age. If there is goo in the thermal spring on the front. Clean it
with brake cleaner before you replace the clutch. But it there is, plan to replace it in the future.

The coaches cooling system does need the ram air through the front, and if there have been any mods up there, you should look them over carefully. My
great joke there is that a contact that tows a 5er, had heat issues and so he put a fan in front of the radiator and it got worse. Well his fan
clutch was bad too.

Is the fan shroud intact? Some people like to take it out, well, GM put it there for a reason....

The last thing I will add is that the stock carburetors do not do well over about 9KMSL - they can go lean and that makes for hotter. This is my
experience, but if you step into it more, you can "flush" that old quadrajet and dump in enough fuel to cool things at the expense of fuel.

Run Jon's list first.

Matt
--
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
 
Tony,
Your line referring to higher altitude makes me question the radiator cap. It needs to maintain 11 psi (or AT LEAST 9 psi as some replacements do) to
prevent boiling at altitude. Of course 50% anti-freeze is required.

Other than that, the replies about fan clutches are good. The ONLY time my GMC overheated was on a 100 degree day, with the A/C on, after about 1/2
hour at full throttle climbing into the mountains west of Denver. That's how I discovered my fan clutch, which had always been unusually quiet for 20
years, was bad.

A clogged or limed up radiator is also a good possibility. Rather than just replacing it, take it to a radiator shop and have it "rodded out" if
possible. (Remove tanks and physically clean the passages in the core.) If you can afford it, a new aluminum radiator from Jim K is good, IFF you
need it.

HTH,

Rick Staples

--
Rick Staples, '75 Eleganza, Johnstown, CO

"Advice is a dangerous gift, even from the Wise to the Wise, and all paths may run ill." -Tolkien
 
I suspect the Fan clutch.
By the time the engine is getting hot, you cannot help but hear the Fan
cranking up, should that not happen,then replace it.

On Mon, Nov 9, 2020 at 2:23 PM Richard H Staples via Gmclist <

> Tony,
> Your line referring to higher altitude makes me question the radiator
> cap. It needs to maintain 11 psi (or AT LEAST 9 psi as some replacements
> do) to
> prevent boiling at altitude. Of course 50% anti-freeze is required.
>
> Other than that, the replies about fan clutches are good. The ONLY time
> my GMC overheated was on a 100 degree day, with the A/C on, after about 1/2
> hour at full throttle climbing into the mountains west of Denver. That's
> how I discovered my fan clutch, which had always been unusually quiet for 20
> years, was bad.
>
> A clogged or limed up radiator is also a good possibility. Rather than
> just replacing it, take it to a radiator shop and have it "rodded out" if
> possible. (Remove tanks and physically clean the passages in the core.)
> If you can afford it, a new aluminum radiator from Jim K is good, IFF you
> need it.
>
> HTH,
>
> Rick Staples
>
> --
> Rick Staples, '75 Eleganza, Johnstown, CO
>
> "Advice is a dangerous gift, even from the Wise to the Wise, and all paths
> may run ill." -Tolkien
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.gmcrvparts.com
1-800-752-7502
 
Keep ion mind, the OEM radiator cap is 9PSI. Follow the suggestions above, checking the fan clutch cold being the easiest. Sending the radiator to a
shop for cleanout is the hardest but may be necessary. Timing and distributoe weight movement is in the middle. Also check for vacuum leaks.

Tony, how about a name so we know who you are when we meet ata rally, and a location so if there's a GMCer in striking distance s/he'll be aware.

--johnny

--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
 
Jon’s summary was very good. What about operator error on gear choices? I find downshifting sooner than later is a better way to go. Up and down
the grade.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
I live in a mountain state and what worked for me a good radiator is a must.I my case an aluminum one from jimmy k. Second a good fan clutch 2797
that roars when it comes on at around 210 degrees 180 or 195 thermostat. Down shift to 2nd when the grade starts getting steep. Most long grades
around here are 6 or 7% . I always tow a Honda CR-V so 3:70 gearing is my choice of final. On 100 degree day the roaring fan going on and off will be
music to your ears as you climb a steep mountain grade don’t forget a good stacked plate trans cooler for the tranny.

--
Roy Keen
Minden,NV
76 X Glenbrook
 
> I should add. an IR temp gun is a very handy tool to help figure out what is going on with cooling system, but it is not perfect. STill a
> tool one should be getting out and using if you are having temp questions.
>
> you can check the upper and lower hose temps, the thermostat housing. you can compare to what gauge readings are.

Another good use of the IR gun is you can check various points on the rad to see if there are dead areas due to clogs, corrosion, etc.... Also a good
way to confirm coolant is flowing through the radiator effectively. Should be hot on the drivers side top to bottom, and cooler as you move across to
the passenger side. Once you confirm the coolant is moving through the rad, then look at things like airflow, fan clutch, etc.... If not enough
coolant is moving through the rad, there may not be enough heat being rejected by the rad to even activate the fan clutch....

--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX
 
I fought overheating problems for a long, long time and after trying everything else I finally determined that it was due to a cracked head on the
driver's side in the front-most cylinder. It might be a cheap enough test to have some of your friend's coolant analyzed by a lab for the presence of
combustion products so that this particular problem can be either confirmed or ruled out.

--
Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
 
Has anyone tried the coolant analyzer from Harbor Freight to detect combustion gas in the coolant? Just wondering about its accuracy for detecting
cracked heads or failed head gaskets. I have had an overheating problem, but when pressurizing my cooling system to check it, there is no drop in
pressure over 2 hrs. However, others have said that a combustion leak can be temperature sensitive and only show up when the engine is hot.
 
> I have had an overheating problem, but when pressurizing my cooling system to check it, there is no drop in pressure over 2 hrs. However, others
> have said that a combustion leak can be temperature sensitive and only show up when the engine is hot.

I would not interpret a no-leakdown pressure test as proof that there were no combustion leaks based on my personal experience. My overheating problem
caused by a cracked head was indeed pressure sensitive as cruising the interstate at 55 MPH was no trouble but speeding up to 65 would produce
overheating in short order.

--
Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
 
Not Harbor Freight, but I bought a kit online and tested it on a friend's late 90's Northstar Cadillac which had overheating problems and I had my
suspicions. It did pick up combustion gases in the coolant. A known issue with those Cadillacs.

Now I need to figure out how to MacGuyver a way to connect the combustion tester to the fairly well hidden GMC radiator neck...

Richard
--
'77 Birchaven TZE...777;
'76 Palm Beach with 18,477 verified miles;
‘76 Edgemonte
 
> Not Harbor Freight, but I bought a kit online and tested it on a friend's late 90's Northstar Cadillac which had overheating problems and I had my
> suspicions. It did pick up combustion gases in the coolant. A known issue with those Cadillacs.
>
> Now I need to figure out how to MacGuyver a way to connect the combustion tester to the fairly well hidden GMC radiator neck...
>
> Richard

Richard,
Maybe pickup one of these to bring the filler out where you can easily access it.
http://www.gmcmhphotos.com/photos/radiator-install-2c-radiator-split-shroud/p66549-radiator-filler-extension.html

Also, I had some issues with a blown head gasket on an Olds 4cyl that I had many years ago. Following the directions on the bottle, I used Bars Block
Seal. It sealed it up and my daughter drove the car for another 3 college years. Still OK when the car went to the great bone yard in the sky. JWID
--
Larry
78 Royale w/500 Caddy
Menomonie, WI.
 
Thanks for bringing up the problem with getting the combustion gas tester into the well hidden and obstructed radiator fill neck. I'd better work this
out before wasting the money on the tester. The fill neck extension probably doesn't have to be perfectly air tight. Seems like any combustion gases
in the coolant would rise up through any fairly tight rubber hose extension to be tested.
 
> Thanks for bringing up the problem with getting the combustion gas tester into the well hidden and obstructed radiator fill neck. I'd better work
> this out before wasting the money on the tester. The fill neck extension probably doesn't have to be perfectly air tight. Seems like any combustion
> gases in the coolant would rise up through any fairly tight rubber hose extension to be tested.

Robert,

I have never done this because in engine labs, we had other equipment, but you could create adapters to put it in the coolant overflow line. If the
cap is right everything going out of the system has to go there.

Matt
--
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
 
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