Overheating question

[jaybee]

Dad has a '93 Chevy WT1500 (that's the "work truck" model) with 4.3L V6.  He's having trouble figuring out why it gradually overheats.  Radiator has been removed and flushed at a shop.  Thermostat has been replaced and checked to make sure it's in right.  Fan clutch shouldn't be an issue on the road but it kicks in as the engine gets warm.  Antifreeze is bright green rather than polluted with combustion product and coolant loss is gradual, only when it's been long enough to get good and warm.  Cooling system maintains pressure and pulls coolant from the overflow when shut off.

Basic behavior is that it warms as normal, cools a bit as the thermostat opens, then starts climbing again.  Turning the heater on full blast cools it slightly, but it continues to climb.  Air conditioning is out of the question, temp shoots right through the roof.  It's also down on power.

Other details;  This is a very low mileage truck for its age with only 67,000 miles (real miles, it's been in the family since new).  It has throttle body injection and a computerized distributor (no vacuum can).  

Any suggestions on what's been missed?

[Fat Cat]

Impeller on the water pump?
Hose collapsing internally?

[Bruce Dorsi]

Dad has a '93 Chevy WT1500 (that's the "work truck" model) with 4.3L V6.  He's having trouble figuring out why it gradually overheats.  
Any suggestions on what's been missed?

I agree with Fat Cat's thinking.  .....Although the truck has low miles, it is 18 years old.

Does it have the original water pump?  

There is the possibility that the pump impeller has corroded away, or is slipping on its shaft.

[Arnold]

Fat Cat and Bruce have spent waay too much time on older vehicles to know about slipping impellers. They are dangerous signs when one knows that. The first one I encountered one and figgered it out..was almost institution time for me(still is). I REALLY liked the one that slipped..chewed up the shaft..then stuck on pretty good..crooked.
  Collapsed hose..what is that lol..I would think that a quick squeeze when cold would rule that out.
 Internal engine friction..I knew a guy that had an old Buick that overheated when warmed up and was really hard to start when warm. Bad oil.
 Too lean?.
  GM had a problem..recall..years and years ago with collapsing ex.pipes.

[wayne petty]

does it loose coolant?? when it overheats...

have the intake manifold gaskets ever been changed???

has the heater hose quick connect ever been changed?

what...

all of these have a quick connect on the back of the intake manifold  passenger side.. it breaks off from engine vibration and broken motor mounts allowing the engine to rock against the heater box under the hood..

there is a tool to extract the broken section from the aluminum intake manifold...   warning..  the factory sealed it with lock tite.. so you might want to warm that part of the intake to get it off...

gm replacement quick connects are STEEL bodied...   some of the aftermarket versions are aluminum castings.. but the casting is thicker in the weak area..


if you intend on removing the intake manifold....  you should be aware that there is a huge amount of coolant stored in the intake manifold   i have taken to using a wet or dry shop vacuum that has been washed out and the filter removed to pull all the coolant from the intake via the open heater hose opening and then the thermostat opening with the outlet and stat removed...

you will also want to remove the distributer cap and preset the crank so the reluctor and pick up coil align with the rotor pointing toward number one... makes it easy to install the distributer..  dead stick time it..  so it starts on the first try...


why am i describing all this... i have had to do this a bunch of times ...


the special broken section removal tool is

Dorman 800-408 .

http://contentinfo.autozone.com/znetcs/product-info/en/US/drm/800-408/image/4/


the quick connect is a dorman : 800-401  Hose Connector
do you see the sealer on the threads.. the OEM stuff is really tough..

http://contentinfo.autozone.com/znetcs/product-info/en/US/drm/800-401/image/4/



this may also help... shows where the coolant hoses all go... including the quick connect i described.. has a great picture...

http://www.compnine.com/index.php?u=1&year=1993&artnbr=TC08-019&artsfx=NULL&supplgroup=NULL&catcode=53C&modelcode=C&makecode=LC&modelseries=NULL&uid=1&modelid=6551&capuid=1&majorgroup=08&grouptype=B

[jaybee]

Thanks guys.  We'd been discussing the possibility of a bad impeller, think that's where we'll go next.  No coolant loss as long as it isn't driven far enough to get REALLY warm.  He's been driving it on 12mi trips to the farm and it doesn't get so warm that it pushes coolant until you get beyond that...but trying to make it a round trip without some cool down time in the middle & it's a problem.

[wayne petty]

i have been thinking about hooking a pressure gauge to the water crossover behind the thermostat for a long time...   see how much in block pressure the water pumps actually make...  

it could be an easy test to remove the temp sender and screw in a series of bushing reducers to install a 1/4 inch or even a 1/8 pipe pressure gauge on the intake for a few minutes of tests..  i did say minutes...

this is just a though...

if there is little pressure built up behind the thermostat... the impeller must be not working as well..

the only motor i have to test on currently is the 460 Ford in my van i am about to do a water pump on..   i already have a stack of gauges...

[jaybee]

Interesting concept, Wayne.  I presume you're going to do it that quickly in part to take out the variable of pressure build up due to heat?  Obviously you don't want to go without a temp sender, but as the engine warms you'd almost need 2 guages--one to collect block pressure, one to collect radiator pressure.  Difference between the two is your water pump effect.