re:heater core replacement

[rja]

...so i get the heater core put in; with new valve, new hoses; even put a "flush valve port" at the hight point in the system to burp the air out. i'm out sightseeing, just enjoying the "warmth" and i notice the temp gage is running a tad higher than when the core was by-passed. i decide-enough heat- and pull the lever to close the valve. the gage starts to rise and when it gets to about 3/4 scale, it suddenly jumps to full-overheat position! i pull over and feel the radiator hoses and they aren't even that hot. the fan is turning and the air from the radiator isn't hot either. thermostat? bad sending unit? air lock in system? no leaks in system and seems to build and hold pressure. any thoughts welcome....even just to ride a newbie a bit! '74 1800.

[dmwhiteoak]

I always like to start with the simple things. After you filled and burped it did you finish up by taking the cap off your newly installed T and top it off there ? Did you also check the wire connectors to your two tenp sending units on top of the head. If one of them have a bad connection your gauge will not read accurate.

[rja]

hey dennis-i started the car and let it idle while burping the cap until it ran w/just anifreeze coming out. the temp leads seem to be connected properly. the car idled on my carport and the heater core was blowing hot air before i ever took it out on the road. had the new hoses hooked to the core before i ever disconnected the old ones. only lost about a cupful of antifreeze in the whole process. i really expected to see it run a little cooler w/the new "radiator" in the loop. and it did ok until i shut the control valve. it didn't take it long to go crazy. i'm kinda leaning toward an air pocket or a sticking thermostat(even though i've had no trouble w/it before). guess i'm just looking for a logical answer.

[azruss]

an air pocket is a logical start. my guess would be you had some trapped air in the heater and closing the valve squeezed it out into the main system. one more bleed and you should be good to go.

[rja]

thanx; will be trying that first thing in the am.

[rja]

update-temp gage quit pegging on the hot side, but still "drifts" back and forth across the middle of the scale. like a thermostat trying to find its way! have never seen this before, but i bet the car spent years with a bypassed heater core and now is trying to run in a new "spot". and do the two sending units make the gage more sensitive? i'm assuming they both go to the gage because i have no hi temp lite that i know about.

[courtenay]

One of the senders is for the actual temp. The other one "pegs" the gauge - it would be the one that would turn a light on if there was one. Many of us have disconnected it. On my '80, it's the one closest to the radiator.

[htchevyii]

Some of the Behr t-stats seal so tight that it's hard to get the system filled. Small hole drilled thru the poppet in the stat can help.

[rja]

thanx for the info, guys. new toys can befuddle if you don't have the box they came in! i've been squeezin' on the radiator hoses, bleedin' the flush tee; anything i can think of to make this thing act right. gage swings about 1/4 of the scale right across the middle. you can sure tell when you put the heater core to work, 'cuz the gage drops right off. may end up swappin' out the t-stat eventually. feel like it's a 195 and would rather have a 180. got an extra one off my z car w/a little hole and a loose rivet in the poppet. a new gasket shouldn't be hard to make.

[BEEK]

ok, now i might have an idea.... are you running a thermostat in the head, under the coolant tee?
if so do you have a 3 way hose connection next to the water pump?
if you answered yes to both questions, you are running 2 thermostats. what made me think is you said there is a poppet in the thermostat, and the hose junction thermostats do not have one, or at least any that i have seen in over 30 years. if you have the 3 way tee in the lower radiator hose, that is your thermostat, check its operation and remove the one from the head. of you can do like i do and find a water neck from an early spider and eliminate the junction thermostat and run one in the head, as you have already made a burp spot. running 2 thermostats would cause some of the problems you are having

i have now read that you have a 74, this one only had a thermostat in the head, in my opinion its the optimal setup. the 74 should not need burping, the radiator cap is higher than the highest part in the coolant system. i would replace the thermostat with a 180 deg. buy a new one and the rubber seal you need, no gasket there.

[rja]

thank you for that insight, beek. yes, i have only one thermostat(in the head)and i feel it should be replaced. when i unplug the forward temp transmitter, the gage acts like it should. is there any downside to running this fashion? not sure i could get the rubber seal locally, and i want to drive the car NOW!

[courtenay]

I have been running with the forward sensor unplugged for a couple of years now. I think others have as well. You can get replacements (AutoRicambi sells them) and I have one - just haven't installed it yet.

[rja]

thank you courtenay; been out driving all day. with the forward sensor unplugged. gage still swings a bit, but no pegging high. i still think the car needs a new t-stat, and a by-pass line for when the heat isn't needed. a parallel line to the heater w/some kinda valve to operate. i'm sure this has been discussed before, and i don't wanna buy a new sender and have the same problem. i'm hoping to have a good working a/c in this car for the summer and it's gonna need all the cooling it can get.

[courtenay]

The sender costs about $9.00 so it's not a huge outlay. I do plan on installing mine - my middle name is "procrastinate". I'm learning that one of the curses of retirement is not having a lot of time to fully utilize all of my toys. I was only able to but about 500 miles on my Fiat this year, so the temp sender wasn't a big priority for me.

[rja]

yeah, been meaning to procrastinate also, but just keep putting it off! more satisfying to work on them than to drive them. for me, anyway. just a grease monkey at heart, i guess. if we don't keep 'em on the road, who will?