Hope someone can help!!
I have an 88 S10 Blazer with a newly built 355.
Just got it finished and have noticed when driving
the temp runs consistently at about 180.
When I come to a stop the temp gauge climbs very quickly to around 190 or 195 then goes right back down to 180.This is the only time it does this.
At idle it runs 180.
I have a 195 thermostat,all new parts including water pump,gauge,Etc.
The temp sensor is located in the top of my intake.
This is the most frustrating feeling not knowing what the heck is going on!!
Thanks for any ideas!!  

do you have a by pass circut hooked up??? so the closed thermostat does not cut off all the circulation completely...

when the stat is closed.. the coolant is supposed to go through the block, up the back of the block and into the heads.. forwards through the heads to the coolant passage in the front of the manfold.. .. there you usually find the heater hoses... this allows some of the coolant to go through the heater core and back into the water pump to get sent through the block again...  this keeps the coolant circulating past the thermostat and the temp sender...  if there is not a bypass...  it will have heat surges like you describe... ..  cars with bypass heater control valves cause weird overheating problems when the heater core gets clogged..  so when you call for heat..  there is no circulation past the thermostat and the engine overheats... ..................


you might try drilling a 1/8" to 3/16" hole in the thermostat so some coolant is always circulating..

what also happens is there are hot spots in the motor that under load are above the boiling point of the coolant.. this releases steam bubbles into the cooling system.. if they collect at the back of the stat .. steam does not transfer enough heat to expand the wax to open the stat... so you also get  surging temp readings..


one last thing... get the casting number off the water pump to verify in a water pump catalog if it is a regular rotation or reverse rotation pump.
...smooth water pump pulleys take a reverse rotation  pump.. grooved take a regular rotation pump...  

I dont have a bypass on it that I know of,And the motor is a 010 4 bolt block out of a 75 Chevy truck.
So I put a long water pump brand new on it.
It has the v belts on it to run power steering,water pump,alternator.
I was wondering if the stat might be sticking,or if there was possibly a air pocket in the block somewhere.
I did try burping the system,by while the motor was warm taking the plug on the intake slowly loose and releasing what seemed like steam then slowly took the plug completely out.There was coolant in the passage.
I then released the rad cap,and coolant flowed out of the passage for a few seconds then stopped,I then resealed the plug with teflon tape and topped off the rad with the motor running.
Still the same results.  

drill the stat....  that allows the coolant to bypass....

while the stat is out... put the housing back on and start the motor and see how fast the coolant circulates around through the radiator...

do you have a 30 to 50 pound pressure gauge you can put on the intake to see how much pressure is being built up behind the thermostat....only if you have the stuff there...


what year did you order the pump for...is it possable that yo get a reverse rotation version... i have not had both in my hands at the same time.. there may be a differnce in the fan flange... so i may be totally wrong...  

I ordered the pump for a 75,as far as the pressure gauge I dont have that available.
The thing is this is the second time this motor has been in the S10.
I had built this motor about a month ago and installed it and had some issues with the heads and rings,so I pulled it all along with the trans,and rebuilt it AGAIN!!
This issue was never present on the first install.
Same exact parts I'm running now.  

more info... hmm how fast does it build pressure in the radiator?????

lets try something else...

do you have any clear vinal 5/16 or 3/8 id hose??? so you can hang the open end from the hood.. and fill the radiator and it part way up..this will allow you to watch the coolant level.. you will need to wedge something under the radiator cap center  brass disc to allow pressure to escape to the overflow tube...

take the fan belts off...  fire up the motor with the radiator filled.and the clear hose partially filled. see if it starts to raise  the coolant as soon as it starts...

since there is no water pump circulation any coolant expantion will be slow to raise the level up the hose.. if it raises too fast.. you might have a leaking gasket or crack somewere...

with this set up in place...  a compression tester hose with the check valve core removed, an pressurised air supply..(compressor or truck tire) and a remote starter switch.. pull one plug at a time...  put the tester hose in the spark plug hole... bump the motor over a little at a time.. till you get that cylinder to tdc...then pressurize that cylinder... it that cylinder is leaking compression the clear tube coolant lever will raise..      

tapered drilled rubber plugs are available at some hardware stores to wedge into the filler neck... so a hose can be attached easier.. ...


by the way... i try to use hylomar gasket sealer on all the headgaskets i install.. permatex no longer sells it but a few tubes may be left over..  it is not like anything else ever used on head gaskets... never gets hard... good for 600F.. gasoline resistant..  

Well if your in California maybe its the wacky weather


Just Kidding  

Wish I was in California!!
I'm in Good Ol North Carolina.
I think I found the culprit on the funny temp readings.
I moved the temp sensor to the other side of the intake and put another stat in it,and it seems to be working fine now!!I had the sensor on the side where the heater hose hooks to the intake,I dont know if this has anything to do with it or not,but I will keep my fingers crossed.
Thanks for all the advice!!