My good old 83' GMC Pickup has had a funny symptom for 2 years that is
getting worse, and I hope you guys have a clue...
On a long high-speed highway uphill, the temperature gauge goes up a
little, maybe to 190 from about 180, then suddenly pins full scale
hot. After running slower with less load, it suddenly pops down to
180 or so. The truck never actually overheats.
Engine runs well, doesn't burn oil at 175,000 !! Replaced water pump
for a bad bearing recently, and the internal water jacket looked nice
I replaced the temperature sensor, and it didn't change the behavior.
Again, the truck runs strong and doesn't actually overheat or
What do you think is going on here?
Terry King ...In The Woods In Vermont
On Fri, 01 Jun 2007 06:36:05 -0700, " email@example.com"
I suspect that your radiator core is becoming plugged up and is on the
threshold of being able to properly cool your engine all the time. You
might check clutch fan operation as they tend to get soft with age and
not engage properly. One thing I want to stress here, that is not a
engine you want to overheat at all because while it is a sturdy motor
it can have a nasty habit of warping the head and blowing head gaskets
when it is overheated enough. A trip ot two to the around the 240 or
250 mark can do it sometimes. You want to keep it cool.
Thanks for the replies, guys!
Here's some more to help me think about:
- I discovered that this engine has an additional temperature switch
located in the head near the back. It is just a switch so when it
activates, the gauge goes to fullscale. I temporarily disconnected it
so I can watch the coolant temperature. Also, I recently replaced the
temperature sensor and the new one reads slightly higher than the old
one; probably just a calibration issue.
Basically, the cooling seems to regulate OK, over a small range.
The gauge has major markings at 100, 210 (1/2 scale) and 260
(fullscale). My rough calculation is that a "needle width" is about 8
or 10 degrees F.
The indication goes up to 210 over 5 mins or so at startup, and drops
to about 180 quickly, which I assume is the thermostat opening. I
drove 250 miles back yesterday, watching the gauge. Hot (80+) day, 60
to 75 MPH highway. The indication stayed within 210 and a small +- 1
needle width range. Even with a long uphill, and heavy throttle, it
didn't go any higher. (This is the condition where it previously went
fullscale before I defeated the second switch).
Snoman, I want to make sure I don't damage this engine, per your
advice! What do you think is happening here? This engine seems to
have the intake manifold and the top of the exhaust manifold cast
integral with the head, is this right? Could this be aging of that
switch? Could it be less-good circulation in the rear of the head,
with age?? What else??
I plan to run some serious cooling system cleaner soon; do you think
that's a good idea?
I'm thinking of running a cooler thermostat; 210 (if that's accurate)
seems too high. What do you think about that?
It seems the basic radiator capacity must be OK, but I see some
deposits lower down in the cross-flow radiator; cleaning may dissolve
some of that..
The fan clutch seems pretty loose; can it be adjusted??
Thanks for the suggestions, and I hope to keep this oldie but goodie
going for some more years here!
I'm reading thru your great site. I'm on an unmaintained dirt road in
rural Vermont, and THAT'S a subject for me!!
On Sun, 03 Jun 2007 01:20:21 -0700, " firstname.lastname@example.org"
THese temps are a bit high but not in the range that can damage
engine. Sounds like a weak clutch fan. Contray to popular beleif, ram
aitr cooling is not always enough to do the job.
Intake is cast as part of the head that year engine but exhaust should
not be as I recall
Never a bad idea but a raditor of that era used a brassed core that
was soldered in and the lead in solder forms slats with time the can
block the coling tubes/fins internally and flushing does not ususally
remove this. Look inside the tanks and look for deposits on ends of
tubes. If present, they use a process called rodding it out at a
radiotr shop where they remove end tanks and run small rods through
core passages to clean them out. Vinegar also can work well to remove
these salts sometimes. Drain and flush system then drain it conpletely
and fill it up with vinegar and start it up to circulate it and get
air out of system and let it sit overnight then flush next day. If it
still look "dirty" in tanks repeat. When done, flush vinegar out and
replace with fresh water and about 1/3 cup of baking soda to
neutrailize the acid (vinegar is a weak acid) then flush and refill
with fresh coolant
Cars of that era usually used a 190 to 195 Tstat some even 205's. The
problem with using a lower Tstat is that it can actually reduce
cooling efficency because when you lower coolant temp, the tempature
differentail between air flow and coolant is reduced and so it the
rate of heat exchange.
You can flush as mentioned above
Yes, as long as it does not wobble on shaft. You will find more info
on this in the link below.
I'm confused as to what this second switch is? Do you have an aux
electric fan on your truck? If so this switch could be to control
that and if so should complete a ground when a certain temp is reached
and should not be inline with the gauge. A new fan clutch made a big
difference in running temp on my truck when I replaced, get a decent
I am not 100% sure if it applies here but they used to have a high
temp switch on some models that would increase idle speed to increase
when engine reached a certain tempature or some other function as the
ECM's back then where very primative and limited on what they could do
so they depended on switches and such.
I got a wiring diagram finally, and it shows a separate switch in
parallel with the temperature sender. So that would send the gauge
fullscale if it actuated, which I think is happening here. I'm going
to do the serious acid clean/flush and see if that makes a difference.
The switch is labelled "Metal Temperature" and is in the metal of the
head between cylinders 5 and 6, quite close to two exhaust ports. So
this could be:
-- The switch has moved to be more sensitive in 175,000 miles, or
-- The rear of the head has some deposits and is getting less cooling,
-- The exhaust temperature is higher than normal in heavy loading.. ,
Thanks for the info and suggestions! It's been a big help...
...and I'm checking the fan setting.. thanks SnoMan..
On Fri, 01 Jun 2007 06:36:05 -0700, " email@example.com"
Had much the same thing happen with a farm tractor.
The *piddle* valve hole (for want of a better name) in the thermostat
was plugging up. Would seem to overheat right away but after shutting
down the engine for a few minutes the thermostat would open and then
all would be well until the next time.
The piddle valve allows heated coolant to bypass the thermostat
(really small hole) so that heat could get to the thermostat to cause
it to open.
My not be the same thing here but a thermostat is cheap insurance. By
the way, the dealer sells good ones. If you chose to purchase
aftermarket you will likely get what you pay for.
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