overheating by vacuum?

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wild scallion
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My tercel:: 86 sr5 3ac "Virginia"

overheating by vacuum?

Post by wild scallion » Tue Jan 28, 2020 11:53 pm

Whale, I've read and read... I can keep trying things, but time to ask.

I just did my first engine swap, between two 86 sr5's. There was a lot above my head, but I did it, thanks largely to this group, and my 888 page bible, and it "worked" (after getting the distributor/timing in the rightish place at the very last panic phase).
Complexities included wild speculation on vacuum line relevance between the weber system I ended up keeping and the original carbed engine I used as an organ donor. lots of octopi deletions, danglers, and re-routes as logically as I could.

Now I'm overheating, I've tried the obvious stuff, but don't understand the implications of vacuum and am wondering if I should learn which lines are performance-critical and which ones are more in the idealist-fluff category.

Gauge rises slowly up to redline within 15 minutes, in subzero weather, before I get scared and give it half a day to cool. Fan does not come on.
Top hose is Hot and pressurized. Bottom is cold but not collapsing. Radiator top is hot. No fluid level change either direction in the overflow.
I reused the radiator and fan, which were fine before the swap.
I replaced the water pump and thermostat, which were also fine but I had replacements, so I did. I have since tried the old thermostat again to no effect. After writing this, my next plan is to reinstall the old water pump, unless someone here tells me I should play with vacuum (or timing, or wiring...) first?

Just for fun- The temp gauge goes UP if I turn on my headlights or the heater fan on high. It goes down again when I turn them off.

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dlb
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Re: overheating by vacuum?

Post by dlb » Wed Jan 29, 2020 10:45 am

None of the vacuum lines have anything to do with the cooling system.

Was the new engine known to be in good working order, ie had you driven that car recently before doing the swap and know that it worked without issue?

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NWMO
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Re: overheating by vacuum?

Post by NWMO » Wed Jan 29, 2020 11:17 am

wild,

Does the cooling fan come on if you unplug the radiator sensor at the bottom of the radiator?

Chris
Psalm 37:4 "Delight yourself in the Lord and he will give you the desires of your heart"

In remembrance of my friend ARCHINSTL:

T4WD augury?
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Petros
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Re: overheating by vacuum?

Post by Petros » Wed Jan 29, 2020 9:43 pm

turning on the heater should bring the temperature down. usually vac leaks will have little effect on temprature (running lean can cause it to run hot, but rarely overheat), head lights demand is not enough to make it go up much, and it should not rise right away. sounds almost like the temp gauge is wired to the wrong contact.

you sure the temp gauge is connected properly, to the correct sensor? also, try switching out the temp sensor from the old engine into the new one. the gauge is the same but presumably you now have a different sensor when you used the sensor that came on the new engine.

the gauge is operated by the senor on the coolant outlet on the front of the head (where the big hose goes into the radiator from the engine). to test the gauge you should ground that wire to the block, the gauge should rise to the top. if the gauge rises to the top while on the sensor even when the engine is not warm, the sensor is bad.

it is important to know you are actually measuring engine temperature properly, or else nothing else you do with fix the false overheat indication. if you are not sure you can buy a cheap after market gauge at an auto parts store and install it temporarily just to verify that the engine temperature is indeed going up. Usually the factory temp gauge is reliable, if it does not work it would be the sensor at the front of the engine.

good luck
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

wild scallion
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Re: overheating by vacuum?

Post by wild scallion » Thu Jan 30, 2020 1:38 am

dlb, the donor motor was in "good enough" working order (subjective, of course!) prior to the transplant. Something new is amiss.
Vac can affect timing, which can affect temp, correct? Vac is just such a genie situation to me...

chris, fan Does function, via the unplug test. Just never kicks on.

Petros, wiring could definitely be a wild card, whether intentionally deleted over time, or just bad wires here and there. I was pretty careful not to wire it back together any different than it was, for what thats worth, but I like the idea of swapping sensors. I should really re-swap the valve cover too, it has one different component which I haven't spent time to ID, the circular guy just aft of (and part of) the cover, with vac lines, the donor does not have this piece at all. I will try the sensor ground-test as well.

Thanks, all. will report findings for closure. And probably spend time reading about webers...

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dlb
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Re: overheating by vacuum?

Post by dlb » Thu Jan 30, 2020 9:20 am

Vacuum advances the timing at idle and partial throttle, but not enough to make the engine overheat. It would run like crap long before you'd notice any temperature anomalies.

I like Petros' idea of testing the gauge before doing anything else.

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NWMO
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Re: overheating by vacuum?

Post by NWMO » Thu Jan 30, 2020 11:35 am

I like Petros' idea of testing the gauge before doing anything else.
x 2

If you still have your old temp sending unit, you might give it a try as well (unless you already swapped it). It seems after my rebuild I experienced higher temps right out of the gate and found the cooling system was not full. I kept topping it off after shorter test drives and the problem took care of itself. Better safe than sorry though, but I would try to confirm you are actually running hot.

Chris
Psalm 37:4 "Delight yourself in the Lord and he will give you the desires of your heart"

In remembrance of my friend ARCHINSTL:

T4WD augury?
"Oh, do not ask, 'What is it?' Let us go and make our visit."
T.S. Eliot - "The Love Song of J. Alfred Prufrock"

"Now and then we had a hope that, if we lived and were good, God would permit us to be pirates."
Mark Twain

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marlinh
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Re: overheating by vacuum?

Post by marlinh » Mon Feb 03, 2020 2:32 pm

The fan switch in the bottom of the radiator has to sense a certain temperature to turn on the fan. From what you describe, it doesn't sound like coolant is circulating. The thermostat needs to open and hot coolant reach the sensor. You may have two defective thermostats. You didn't mention if the cooling system was bled after your engine swap. The best way to test your system is with a handheld heat gun. Then you can be absolutely sure of the temps you are seeing. You may also have a defective rad cap if you have not seen any coolant exchange in the reservoir. There is a separate valve in the cap for that, you did say it was holding pressure. I have had several cars over the years and have seen weird things happen with the gauges. Jumpy, sticky etc. Please keep us posted.

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