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Thread: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

  1. #1
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    Default 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    Hi everyone. We have owned this car for a year now. Had a very mild overheating issue when we bought it. Only got hot on long runs, coming home which is uphill. Engine finally threw the water out big time and it tried to seize.
    I had it fully rebuilt, top and bottom. New Radiator, thermostat, water pump, fan clutch etc etc. Added 2 x Davies Craig 10" thermo fans. Set to come on at 90 and off at 85 so they are on most of the time. But,,,,, still got hot. 30 minute casual drive uphill guage shot up, overflow bottle bubbling like mad and threw some water out. Only about 100ml but if I kept driving would have been worse. First "warm" day and A/C was on. Done 2000 other trouble free short and cool weather runs.

    Any ideas?

    Last idea I have is the bottom hose may be sucking itself flat. Have ordered some new hoses but they are weeks away.

    Thank you.

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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    Some things to try and isolate the issue:
    https://www.toymods.org.au/forums/th...eating-radomly

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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    Thank you mate

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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    It's back at the shop today. First feedback is I am headed for a bigger radiator and a big thermo fan and remove the fan clutch altogether. Believes the engine is fine it just runs hot and the cooling system can't handle it. I know some V8s years ago couldn't handle hot summer days and traffic etc. Don't understand why this thing would be doing that.

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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    What was completed for the Engine Block preparations? Was the Block hot tanked and flushed afterward? Were new freeze plugs installed? Those things performed will aid in troubleshooting your cooling system. Any restrictions or buildup should be removed with that hot tank procedure and the flushing of the block to check flow.

    Replacement of the old radiator sized properly should correct any other issues, However, if all of the trapped air is not removed, that would cause you issues also.

    Also, check your heater system to make sure that it is not leaking which will also cause you issues!
    Last edited by 73GTV; 03-11-2021 at 09:07 AM.

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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    Thanks 73GTV. I know everything was crack tested. Not too sure about the rest. Cost a bomb and used a local guy known for high HP quality builds etc. They still have it so hoping they have found something. They are convinced the head etc is 100% fine.Have done leak down tests etc. New radiator same size as OEM and air definitely removed. Always got hot on long runs before the rebuild, especially coming home which is up hill. Since the rebuild has been generally good but twice randomly has shot up and it's not a faulty gauge because it chucked water out.

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    Junior Member Backyard Mechanic jondee86's Avatar
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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    When I hear the phrase "these engines run hot" I understand this to mean that the factory cooling
    system does not have adequate capacity for all operating conditions. All things being equal, the ability
    of the radiator to reject heat to the air is governed by DeltaT... the difference between the
    temperature of the coolant inside the radiator and the air entering the radiator. When it is cool out
    DeltaT is higher and if the thermostat is operating correctly the cooling system will be able to control
    the engine temperature. But when it is hot out (first warm day) DeltaT is lower and there is extra
    load on the engine (A/C on).

    Under these conditions, the thermostat opens as far as it can, but the coolant temperature continues
    to rise as the radiator cannot reject heat fast enough. The coolant temperature is now "floating" at
    some equilibrium point where the increase in DeltaT manages to balance with the constant engine
    heat produced cruising on the flat. But then you hit the uphill grade which adds more load, the
    coolant temperature is pushed even higher, until it reaches boiling point.

    This will happen with a functioning thermostat as it is the result of an undersized cooling system.
    However, there are many other factors that can affect the performance of the cooling system, and
    some of them (clogged radiator, broken thermo, collapsed/kinked hose etc) can cause the same
    symptoms as an undersized system. So you need to eliminate those possibilities before diving into
    a bigger radiator and more fans. In my experience, if the cooling system on a RWD car is working
    properly, there is enough air passing through the radiator at cruising speeds to cool the
    engine without any fan at all.

    Since you have replaced the radiator with a new standard size one, replaced the thermostat and
    water pump, there are not many possibilities left to check. If you can lay your hands on an infra red
    temperature gun, try checking the temps of the top and bottom radiator hoses straight after a drive.
    The top hose will be hot (leave the engine idling) and the bottom hose should be substantially cooler.
    If the bottom hose is getting close to the top hose temp, then you need a bigger radiator

    Cheers... jondee86

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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    Thanks jondee86 really appreciate it. I was thinking the same but in much less technical terms! It's my sons car and he is on his P's. I need it reliable to he doesn't end up stranded in a dangerous place or destroy the engine. I will see if I can try the temp gun that makes good sense. I have ordered a new hose set mainly to replace the bottom hose in case its sucking in at all. I understand the mechanic is thinking along similar lines to yourself. The car cam from Victoria somewhere so probably didn't see the heat we see in NSW and perhaps they lived on "flat" ground. I live in the mountains. Going anywhere is fine, coming home is uphill and an issue if it's hot. If he can't find any issues, I think it's headed for a big fancy thermo fan of some type. He's not happy with my 2 x Davies Craig ones for some reason. Perhaps a bigger radiator too. Small price to pay if it works and the temp gauge doesn't move anymore. Soul destroying! Drove it for a year with the temp getting high from time to time before it went bang. 8k rebuild and,,,,,,, still getting high.

  9. #9
    Junior Member Backyard Mechanic jondee86's Avatar
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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    There is a better than linear relationship between the quantity of air passing through a radiator
    and the amount of heat that can be dissipated to the air. To be effective the fan should be enclosed
    in a shroud along the lines of the fan shroud originally fitted in an 1986 Toyota See the attached
    pic for why a flat plate with two small fans is not the best solution.



    Also, small high speed fans with straight blades are more noisy and less efficient than fans with
    wider nicely curved blades. See if you can find something more along these lines...



    Fundamentally, you are looking to drag more air through the radiator than you are getting now.
    Without any figures to work with, it is hard to say if a decent shroud and fan combo is going to
    be a big enough improvement to solve your overheating problem. At highway speeds the forward
    motion of the car will be pushing pretty much all the air through the radiator... the fan is just there
    for the ride. An electric fan is really provided to help cooling when the vehicle is stuck in traffic
    or moving at low speeds on hot days towing a caravan uphill

    I'd be inclined to take a look around your local wrecking yard to see if you can score a complete
    fan and shroud off something older with a V8 engine that could be adapted to fit your car. That
    way you could see if it helps without outlaying for new of expensive custom items. If they don't
    do the job then you will need to starts looking for a complete radiator/fan/shroud assembly that
    can be made to fit. Talk to your engineer as he might have some ideas for an upgrade.

    Cheers... jondee86

  10. #10
    Forum Member Grease Monkey
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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    I had the same problem on a diesel estima and chased my tail for a while trying to find the issue, found that the replacement thermostat was a piece of crap. What I mean by that is the factory jap toyota thermostat has a much bigger opening than the replacement. It tested correctly but would not flow sufficient coolant on a hot day uphill. don't use a Ripco or super crap brand get a proper Toyota or jap one. Its hard to pick the size difference unless you have them side by side.

  11. #11
    Junior Member Backyard Mechanic jondee86's Avatar
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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    Good point about the thermo.

    One of the suggestions made above (or in the linked thread) actually addresses the issue of cooling
    system capacity, and would be worth trying when all the likely problem areas have been cleared. You
    will essentially have a complete new cooling system, and I assume you will have checked for blockages
    in any of the associated heater and bypass hoses. While I am not familiar with the MA70 layout I know
    that smaller Toyota engines from that era had a bypass to direct some coolant from the back of the
    head onto the thermo to make sure it would start to open when the engine warmed up.

    Regardless, when you replace the bottom hose it would be worth running a chack by removing the
    thermo for a while.. If you run the car like that there should be nothing restricting the flow of coolant
    and you will have available all the cooling capacity that the system has to offer. The temperature will
    be floating and will go up and down according to engine load and ambient conditions. If it still overheats
    on a warm day at highway speeds when there is plenty of air flowing through the radiator, then you
    don't have enough cooling capacity.

    One other point that may be relevant... ignition timing (either late or early) can affect cylinder head
    temperature and the amount of heat that the cooling system has to deal with. Likewise, an engine that
    is running leaner than normal at cruise may also run hotter, although this should not be a problem with
    an EFI engine that uses an O2 sensor. So basically just check your ignition timing and tailpipe colour

    Cheers... jondee86

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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    Thanks everyone. Thermostat hole size for full flow interesting. Before the engine died I actually tried adding an extra hole in one thermostat for some extra flow and tried on thermostat at all - zero detectable difference it stall ran hot. (yes I know what a thermostat does) It does have the original shroud and new fan and fan clutch all standard fitment + the 2 x 10" Thermos which are front flush mounted so pushing air directly through. Mechanic still has it and I haven't heard anything so starting to wonder if he's found an issue with the rebuild after all or waiting on delivery of some monster new radiator! There's 3 core radiators on Ebay for about $150 that was always on the cards as extra insurance if in doubt but at the moment there is no doubt, it just does not stay cool enough to be trusted to drive 30 mins on warm day. I'll keep you posted. Searching the web amazing how many cooling issues there are with these cars especially ones straight after full rebuilds. That does suggest to me the cooling system is simply a bit small. Ironic, I must have read 50 posts and NONE asked about the bottom hose sucking in OR the ambient temp. Of course we assume a car should handle any reasonable ambient temp, especially a Toyota that is designed to be driven sportingly but,,,,,,,. As soon as I got this thing it ran hot and yet the owner from Victoria said it never ran hot on him. Worth noting, he offered to drive it up to NSW from Victoria as part of the purchase so he seemed confident there were no issues. Just a few degrees warmer may have been enough to tip it over.

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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    Turns out part of the bottom half of the fan shroud was missing and the top was a bit damaged so new shroud on the way. Twin 10"thermos are on the front so I would have thought they would more than make up for what the engine fan was losing so I also have a new 3 core radiator on the way and a set of hoses. Will all go in at the same time. I really can't see the shroud fixing it, not with the thermos running almost all the time. I actually wondered if with the thermos, no shroud at all would be better so the air can be pushed through easier, without it hitting the shroud on the other side. About 2 weeks till it will be done and test it once again. Weather should be warming up by then too.

  14. #14
    Junior Member Backyard Mechanic jondee86's Avatar
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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    Correct.... if there is no fan mounted to or running in the neck of the shroud, then the shroud
    is redundant. The effectiveness of the two fans on the front of the radiator will depend on the
    percentage of the radiator face area that is exposed to the airflow from the fans, also to where
    the fans are located on the radiator. Near the top is best as that is where the hot coolant enters.

    Post a pic of the setup when you have it installed.

    Cheers... jondee86

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    Default Re: 7MGE Supra 1986 Manual MA70 OVERHEATING - After Full Rebuild

    Will try. Thanks. You can't really see the thermos at all as they are in between the rad and the a/c. Tight squeeze in there. I think it's all down to the new radiator really. Does a 3 core with the same amount of air going through it make enough difference to the 2 core to keep it cool? I will know in 2-3 weeks. If it still gets warm it goes for a second opinion. The engine fan is still there, but as noted that fan + shroud arguably hinders the thermos blowing straight through a bit. Air passes through and hits the shoroud but should still exit OK same as ait coming in via the grill I guess. 1 more trap for young players. The P Plate attached to the car was hanging down and blocking some air flow. Moved that this morning. Bummer is new plates on the way which will hang down too. Number plates I mean. No where on these to remote mount as it has driving lights and no other space. Will have to custom mount a bit higher up on the bumper to maintain air flow and side mount the P Plate. Ahhh

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