Overheating/coolant loss on long trips
 

Hey guys let me break it down for you.

I have an 86.5 Supra n/a automatic. It currently has JDM engine in it as I replaced the old one after it seized up on me.

As of a couple months ago I've had to periodically replace coolant with no visual signs of loss. The more I drive it, the more I notice it mostly occurs when going over mountains up hills etc on long trips. I thought BHG already?!?! and looked for milky solution under oil cap, dipstick and during oil changes and everything was good there. I did a compression check and it was perfect (200-202 psi all cylinders on warm engine), also no white smoke out of tailpipe so I set my attention at other probable causes.

Before my next two hour trip I bypassed my heater core all together, no more hoses going in or out of the firewall. The reason I did this is because my heater would sometimes out of the middle of nowhere start blowing cold air... even though it was just blowing hot and I am still on the same trip yada yada... so I bypassed it just to make sure it wasn't the cause, and that next trip proved it wasn't, overheated again as soon as I hit a hill. I let it cool down then I took the thermostat out and put more water in and it got me the rest of the way.
Next week I had to come home and it overheated again. So bypassed heater, removed thermo, did common BHG checks, and replaced radiator cap to no avail, any suggestions would be greatly appreciated guys. Feel free to ask as many questions as possible.

NOTE: It was overheating around town at one point as well and that's why I replaced the radiator cap cause I heard air and saw water coming out of there. So no more around town over heating but still mountain/long trip overheating. Also the coolant loss might just be from the coolant overflow after it is running hot... cause I'm not sure where it is going.

EDIT: NOTE: Also think it might be important to know that my temp gauge would raise up slowly then plummet back down to normal several times in a row before it would get hot and stay hot.

The temp gauge fluctuating and the heater changing from hot to cold sounds like you have air in the cooling system (which makes sense if it's been purging coolant).

After it overheats, is there any hot or cold spots in the cooling system? For example, is the upper hose hotter than the lower hose? One side of the radiator hotter than the other?

A possibility is that there may be some sort of blockage restricting the flow of coolant, causing it to overheat

I will hand check multiple places next time it occurs, I don't have a temp gauge that I can use to test it. Also I will try to see if the rad is blocked up at all... Any good ways to test that? I will try taking both hoses off of rad and full pressure garden hosing in the top and see if it backs up at all...

if the rad is original, might be a good idea to look for a new one. to purge the system of air, put the front of the car on jack stands, high as you can get em safely. then take the cap off the rad, and start the car. make sure the coolant is circulating, and while it is doing so, top off the rad. it may take a few mins as the car will warm up and open the thermostat. to help the process along, squeeze the lower and upper rad hoses and heater hoses periodically.

another thing you can do it replace your thermostat. if its a stock on, get a new one, as it may be malfunctioning. also check if you fan is working properly, you could have a slipping clutch. this would not attribute to you losing coolant, but might have to do with your temp fluctuations.

You are aware that 200+psi is WAYYY too high.... right? The N/A is supposed to be mid 150's.

cre, ive been curious about this too. the two motors i picked up for my project (88 jdm 7mge, 90 cressida 7mge) both tested high compression while cold! the jdm motor ranged 185-193, the cressida ranged 20-195 (3 cyls were completely boned). when i disassembled the motors, they both had the same type of pistons, but when i compared them to my new pistons, they had considerably less dish in them, almost flat tops. i was concerned that id gotten turbo pistons, but compared them with my freind john's pistons, and mine were definitely for N/A. so somewhere, toyota got a wild hair up their bum and put high compression n/a pistons in some motors.

i had no luck finding these pistons anywhere.. but that seems to be in the same compression range as mine, since his was warm.

sorry that was a lil off topic...

I'm going to hazard a guess that no one's bothered to get the part number and manufacturer off of one of these? It's cast inside.

Have either of you measured the dish volume?

i did a ruff measure and the flat top is almost 1/16th deep, not counting valve relief. my new pistons are 1/8th. i cant recall what the dish is on turbo pistons.

i can get the part # this weekend and post it up sometime next week. im ashamed to say, i never thought to get the number and cross reference it from the factory...fail.

i will certainly check it out though!

I took out the thermostat to make sure it wasn't that. The rad was replaced a little over a year ago, although I haven't ruled that out yet. The fan clutch I replaced this weekend because the checks I did on mine seemed like I should replace it... but still overheating :(
When I took the rad off I flushed it best I could and it seemed to be flowing just fine. Also felt the upper and lower hoses and noticed the bottom was cooler than the top, but still warm.

after looking more into it, it seems on long trips is only time it occurs, around town I'm golden.

Stock dish is 3mm below the crown. I don't know what's up if you've actually got different pistons; Usually the high readings are the result of oil draining into the cylinders from the plug galley.

Danny, what's your timing set to? Retarded timing (both mechanical and ignition) will cause higher engine temps.

I thought it was supposed to be 185ish... what does it mean if the compression is higher than it's supposed to be?

Well I have the JDM engine so I have no clue what pistons are in it. I haven't checked timing since I put it in, I'll have to check again. I might just take it to a shop instead of dealing with this headache, I'll deal with a wallet ache instead...

156psi optimum, 124psi min. If your engine was PERFECT and just had a lot of carbon buildup you still wouldn't see much past the lower 160's.

JDM uses the same pistons.

That's GE by the way; GTE is supposed to be about 15psi lower.

piston dish that on the pistons i bought was exactly as you said cre, 3mm, just under 1/8th inch (if my math is correct...??). i do remember gauging the other pistons and they are just under 1/16th inch, prolly around 1.5mm. so ill have to figure out what the serial number is today. ill get one and post back soon.. im very curious about these two sets of pistons i have...might have to use them in another build if i can find a good block. high compression 7mge:love:

so just an update, block tested it today and it is blown :( I hate life

Meh, still better than a hole in the side of the block with a mangled rod protruding.

my buddy john did that when he sunk his gte powered buggy bout a month ago. hydrolocked #6 cylinder and shot the bent rod through the block. quite impressive actually!

so i got one of the pistons yesterday, and the serial number is 42090. theres some in my album, linked below.

http://www.toyota-supra.info/forums/...albums198.html

hope someone has some light to shed on these pistons. i have two sets. but one set is pretty well screwed (person overheated the engine and welded the piston rings to the cylinder wall, as well as scraped the bejeezus outta the pistons).

btw danny, sorry to hear about ur block thats a bummer man :/

It's an oversized piston for a Toyota Crown 7M. 0.50 over.

Which also means that block and head have probably been shaved at least once before they got to you.

The part number for the Supra 7M-GE in that size is 42070.

hmm. well they spec out at stock bore (83mm...) so idk how they could be .50mm/.020 over. both sets are stock bore for a 7mge, so now im really confused.

as far as i could find, the crown only used the 7mge for 4 years, 87-91, then they switched over to the jz series or various other 4 cyl and diesel motors. doesnt say whether they were high compression, but since it would be a jdm motor, its very possible. either way, its too bad we cant order pistons like that for our 7ms. id buy a set. might strip these down and use them on another block.

It's possible they're from a different engine altogether and as such the first five digits are different. But out of the 7M's family, that number would be for 0.50 over sized pistons (yes, 0.50mm). I can't find anything else and nothing pertaining to the MKIII Supra or any North American vehicles. If you research it and find more I'd love to know. I'd speculate about it being a superseding p/n if not for the fact that it's already taken.

BTW - Toyota liked that bore... there are a number of pistons which will fit the 7M but aren't meant to.

Are they Aisin pistons?

yep they are aisin, its cast onto the opposite inner piston skirt. yea im curious too. ill see what i can dig up, might have to see about calling a few dealerships, or tomoco themselves.

the last few dealers i called didnt know jack shi* bout anything..:squint:

if i find out more ill post it up tho.

It may also be used in the Cressida too, but the difference in compression ration is news to me... Then again, the Crown was an interesting case; It even had traction control available for the 7M.

yea its news to me too. the fact i got two motors with the same pistons is kinda crazy.

but yea, never seen em b4 with a stock motor. then again, out of the 3 ive seen apart, two have had the high compression bits. :nuts:

Well, if they're JDM they're most likely from a Cressida or a Crown... the latter being less common. I don't know much about the Cressida, perhaps the JDM ran a higher compression.