Funky gauge fluctation

I replaced my thermostat last night and now, my engine temp surges to right at the 0 in 210 mark, when I brake. When I accelerate, it goes to alittle west of the middle mark. What gives? It is fine while driving. I basically have a new system: 9 month old GDI, water pump, uper and lower hoses, 1 year old clutch fan, radiator cap, and 1 day old heater control valve and t-stat.

Well, if it worked before you did the work and works funny afterwards I say you got something backasswards or if it is just the gauge then you might have a loose connection. Did you use a factory OEM Tstat, one with the hole in it installed at the 12'Oclock position, did you bleed/burp the system ?? If you used the tstat w/o the bleed hole you will have a heck of a time getting all the air out... If you reversed the hoses on the heating mixer valve it will work funny too. If you did all three then all bets are off :wow:

The hole in the 12 o'clock position? What is that for? I have a '91 so I don't think I have to burp my system. I'll have to go in and move the t-stat to the 12 position. Why would this make a difference?

t bell said:

The hole in the 12 o'clock position? What is that for? I have a '91 so I don't think I have to burp my system. I'll have to go in and move the t-stat to the 12 position. Why would this make a difference?

Click to expand...

thats so the bleed hole is at top-lets the air run out into the top of the radiator. If it were pre91, I would say you have an air bubble in the back of the block sitting on the temp sender. When you brake, it sends it forward, and you got water covering it-the true temp reading- pushing the gas down sends the bubble back-air reading/steam. Could easily be reverse of this, if the block sits level, or is down in front.

The factory OEM thermostat has a small weep hole in the flange, this allows air to get thru it and bleed out into the radiator and hopefully get pushed out to the overflow.
This is what I do with my 'open' system on my 98. Nose uphill in my driveway.
Service system and refill, start it up and let the thermostat open, the coolant level will go down as it sucks new coolant into the engine. I add coolant to bring the level back up into the radiator neck, when the new coolant hits the tstat will slam shut so you gotta wait, be patient, it will open again. You will see that it is kind of fizzy looking like green soda, thats air in the coolant. I keep running it and keep it topped off till the coolant is no longer fizzy looking. I keep the level right at the top of the neck, you can also control it somwhat by keeping your thumb on the throttle linkage, rev it and the fluid goes down and you can add more. Once I have all the air out I give it one final rev, quick dump of coolant and quickly twist the pressure cap on. From then on the over flow will work normally and any excess air will bleed out over the next few days. Oh, before you start fill the CLEANED over flow bottle to the cold mark, you don't want to loose siphon action thats used here.
What I just described takes about 30-45 min start to finish, 45 is closer if you replaced the water pump or drained the block with the block drain which I don't do...