Tech tip: Thermostat replacement on CGT/2144cc I5

[Fisher, Scott]

As mentioned in another message, yesterday's main job involved replacing the
thermostat on my 1983 CGT.  Several things I learned may help other
q-listers with similar jobs, so I wanted to pass on what worked.

First, troubleshooting: In June, with temps over 100 F, I had a heater hose
blow on the CGT so I towed it to our shop, which replaced all the hoses
(those who do not learn the lessons of history, etc.).  But it continued to
run at the very top end of the range, bordering on overheating.  Not the way
I like a car to behave.

I first replaced the overflow-tank cap after Phil Payne mentioned they have
been determined to be single-use items, and since my car definitely
overheated in June it was an easy fitment.  It helped, but not as much as
I'd like.

My wife actually determined that it was the thermostat (or at least
confirmed my suspicions) when I pointed out that the radiator fan a) didn't
run any more than normal, even when the car was quite warm, and b) didn't
cause much of a drop in the gauge reading when it shut down.  Since I had
replaced the sending unit about two years ago and it seemed to work, I
reasoned that the problem must be that there was cool water in the radiator,
but not enough was flowing into the engine -- therefore suspecting the
thermostat.

I bought a 176-degree thermostat and went to install it.  To my surprise,
the bolts came loose from the block easily.  Getting the housing off took a
bit more patience.  It's a nicely made piece, designed so that the
thermostat housing actually fits into a machined space in the engine block
-- meaning that getting it out requires patience, determination, and a 4-lb
hammer used *very* carefully.  I applied penetrating oil to the
circumference of the thermostat housing, then tapped lightly using a long
screwdriver to apply vibration (not force) on varying portions of the
housing.  I alternated between vibration and leverage -- having removed the
grille it was a simple matter to slip the end of the long screwdriver into
the mouth of the housing and levering away from the block.  Eventually the
housing shifted, I was able to rotate it free and it came off.  Brake
degreaser, a brass brush and a succession of shop towels rendered it a nice,
clean aluminum color in a few moments.  But it was not something you'd want
to beat on too hard -- be careful as shattering it would have caused serious
unhappiness.

Removing the thermostat was easier, partly because I could see exactly where
it had failed -- one of the springs had become misaligned, in such a way
that it looked as though it would increase the force required to open the
thermostat -- and therefore I knew I was going to discard it.  It's a tight
fit to get the old one out of the block, but I was able to rotate it first
by tapping with the hammer/screwdriver aligned against the strap, then pry
it out with longnosed pliers.  I cleaned the block face with brake solvent
(after sopping up the coolant that remained) and installed the new
thermostat, with its O-ring.  (Wish I'd had metric taps & dies -- I usually
prefer, if I haven't got new hardware, to chase the threads of bolts and
nuts before reinstalling them, but at least I did go over the threads with a
wire brush before putting on a light coat of antiseize.)

Refitting was simple -- place it in the block with the strap aligned
vertically (the Bentley says "with the arrow pointing down," but this
thermostat had no arrow so I duplicated the alignment of the one I removed)
and screwed the housing cover back on.  I returned the hose to its spot and
set about adding coolant.

Which was the most frustrating part of yesterday's job.  Adding coolant to
the expansion tank does NOT get coolant into the radiator effectively, as
the air in the system prevents the coolant from rising.  And the Bentley
gives instructions on the 4cyl car but not the 5cyl -- or at least I
couldn't locate the thermo-time switch (which the Bentley says you need to
loosen to bleed air from the engine) from the picture in the book.  On my
second try, I removed the top radiator hose and poured about another quart
of coolant in, thinking this would let the coolant diplace any air in the
radiator, but I still wasn't getting water into the engine, as evidenced by
the rapid rise of the temp gauge on restart.  I shut down, waited to let the
engine cool, and pondered how to get water *into* a radiator that has no cap
or other mechanism for adding coolant.  At that point I had my brain flash,
resulting in the Tip of the Day.

Tip of the Day: To get coolant into the radiator on a CGT (and almost
certainly a 5-cylinder 4K, possibly a 5K as well), loosen the hose clamps on
both ends of the upper radiator hose.  Remove the hose from the cylinder
head's fitting, then rotate the hose on the upper radiator fitting so that
it's pointing up.  Pour coolant into the upper hose (with the cap off the
overflow tank so that water will rise as it displaces the air) till the
overflow tank reaches the mark.  Rotate the hose so that it aligns with the
head fitting, then tighten both hose clamps and start the car.  

That did the trick.  I let the car idle in the driveway for 15 minutes and
the needle barely touched the bottom of the thermometer icon on the gauge.
Result: happiness.  My son and I put the grille back on, wiped it down with
some plastic protectant spray to make it really black, then washed our hands
and went in for dinner.  I will check the level in the overflow tank again
tonight just to make sure that it's all correct after the coolant settles in
the block and head (and any air left in the engine has bled out), but it
worked perfectly on the ride to work this morning.

Oh, final fun comment: when we were wiping down the grille, Charlie asked
me, "Dad, what are these rings for?"  So I told him the Young Person's Guide
to Prewar Grand Prix marques.  "I will count them," he said, and when he
counted them all his face broke into a big wide-eyed grin.  "Dad!  There are
four rings, and *I* am four!"

--Scott Fisher
  Sunnyvale, CA
  1983 CGT/1993 100CSQ