Overheating Issue only under boost

JDMMA70

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Dec 4, 2006
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So guys before anyone says it I am not in BHG denial ive put that as a possibility but im not 100% sure yet. And like many others id rather be sure before i tear apart half the motor AGAIN haha.

So heres what going on. Two weeks ago i was taking a friend for a ride and after a few pulls the temp started to rise, not all the way but about 3/4. I turned on the heater and drove home, the temp came back down. When I arrived home i noticed that my overflow tank was full and was bubbling (assuming gas from the radiator). At this point I thought well damn I probably blew the HG, so i started to do the normal test compression test (fine), decided to try and burp the cooling system, and got some massive air pockets but that was it not constant bubbles or anything. Decided to try a suggestion from a neighbor to crank the car with the rad cap off to see if i get bubbles. I got nothing. Then proceeded to perform a block test, I recieved no indication of exhaust gasses present in the system. Changed tstat (OEM Toyota), rad cap (OEM Toyota). Water pump and Fan clutch are both brand new Toyota (A year and half old). So heres the issue I can drive all day get stuck in stop and go traffic on the freeway with the A/C on and it wont overheat a single bit my overflow wont fill up or anything. The moment i start boosting after about four or five pulls the temp will start to rise again. Im looking for opinions.

I was even able to replicate what happens after i do a couple of runs.

http://smg.photobucket.com/albums/v436/Cobra281s/?action=view&current=20120624_100516.mp4

So only things left i can think of maybe a bad radiator, or of course BHG. Now if it is a BHG is it possible its blown without having those crazy symptoms people talk about all the time. i.e. it blew only on the exhaust side and im pushing boost into the cooling system? Ive never gone through one before so if this is it this will be my first. I replaced the HG in 2008, I did it by the book and the forum and the only thing i wasnt that happy with at the time was the machine work on the head.
 

Sside

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May 20, 2008
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One of the guys I met a few months ago was having the same problem as you... The car kept overheating but only after pulls. He changed the stock radiator for a mishimoto radiator and the overheating went away..... So was it a bad radiator? No it was not..... The radiator he replaced is now in my car and I have driven all the way to spring with the temp never rising above normal. So lets look at the three cars and see what is different. All three are 7M's, yet only one of them has a stock 57 trim that does not overheat with the stock radiator. The other two have bigger turbos and one of them was proven to overheat with the stock radiator when on boost. I say, radiator.....
 

JDMMA70

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Dec 4, 2006
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The car does not have the undertray, and I suspect it was lost sometime by the previous owner before the millennium lol. It does still have the two plastic trays under each fog light housing.
 

IJ.

Grumpy Old Man
Mar 30, 2005
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There are also a bunch of plastic blockers around the Rad with foam seals, most Mk3's are MIA, undertray is good for 3>5c at highway speeds, if your Rad/AC Condenser are blocked/damaged this could be enough to tip you over the edge, 3/4 on the stock gauge is DAMN hot!
 

JDMMA70

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The condensor doesnt have anything blocking it from the front other than the pusher fan, and stock IC. The Radiator i havent looked behind it in a while, but my Auto trans cooler is mounted to it, its been there since Aug 2010. The foam seals im assuming would have gone around the entire radiator top and sides correct? And if so yes they are no longer there. Is it common for the stock radiators to clog up over time? I cant see any obvious signs of damage or leaks. It got a 50/50 mix of Toyota Red and Distilled Water. Its probably 30/70 now with all the water ive added.
 

IJ.

Grumpy Old Man
Mar 30, 2005
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Blocked= leaves, bent fins, Air will take the path of least resistance, rad/ac get blocked/damaged the air looks for the easiest path which is now down and out under the car and around the sides due to the plastic/foam blockers missing, at low speeds the fans flow enough to draw the air through the rad so it doesn't overheat, get up to speed and the fans stop and the air you need to cool the engine bypasses the rad and up goes the temp..

Sad part is this becomes circular with a 7M as they BHG so easily...
 

JDMMA70

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Dec 4, 2006
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I have a few bent fens behind the radiator the part facing the fan, but these are the only bent fins i see. I just checked in front of the rad and all i see is dust a few pieces of grass, a small flower, and my trans cooler which is still attached for the radiator. At this point if theres any problem with the radiator im going to assume its internal. I really dont know what else to check at this point. Maybe i overlooked something but Im just out of ideas.
 

radiod

Supramania Contributor
Dec 13, 2007
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Do you notice the small electric fans on the radiator staying on constantly or do they go off? I had a bad radiator and it had similar issues in warm weather, but not to the degree of moving the stock gauge up to 3/4. Stock gauge wouldn't budge on mine, but my autometer showed it would run up to ~220*F after a few pulls. The electric fans turn on at 210*F which is the upper limit of "normal operation". If your electric fans are staying on your not really getting under that.

When I changed my radiator, the old one weighed significantly more than the new. Definitely didn't need a scale to be able to tell the difference.
 

jonahs_supra

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May 17, 2011
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i once encountered a problem like this
customers 87 vette kept overheating when he was running it hard
found the radiator fins where plugged up with leaves, dirt etc...

after cleaning the fins out...he's been driving it ever since with no issues(about 2 yrs now)

a good trick i use on the busses at work is take a piece of paper, start up car then place paper infront of condensor
if the paper pulls back to the condensor then you have some air flow
 

cmoser18

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Jun 2, 2012
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Same exact problem here. I'm not sure if it's the same with you but I have black particles in the over flow tank when the tank fills. Thinkin blown HG
 

jonahs_supra

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May 17, 2011
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JDMMA70;1851847 said:
When I arrived home i noticed that my overflow tank was full and was bubbling (assuming gas from the radiator).

bubbles either indicates a BHG(even though you did a block test) or you have a coolant leak somewhere under pressur
and when engine cools down its sucking air in????
 

Turbo Habanero

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Apr 28, 2009
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I couldn't imagine good compression numbers with a bhg.

Have you noticed any smoke yet? Do you thinl the reason your adding coolant is just because its been pushing out the overflow?
 

Turbo Habanero

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Apr 28, 2009
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cmoser18;1852100 said:
Same exact problem here. I'm not sure if it's the same with you but I have black particles in the over flow tank when the tank fills. Thinkin blown HG

I have those same little particals very minor but my car does not over heat and has good compression. So what those are from I do not know possibly just a dirty take and radiator.

I also was thinking maybe just some dirt and particals from when i scraped the block surface clean when i did the Hg
 

JDMMA70

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Dec 4, 2006
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Turbo Habanero;1852148 said:
I couldn't imagine good compression numbers with a bhg.

Have you noticed any smoke yet? Do you thinl the reason your adding coolant is just because its been pushing out the overflow?
No smoke I am adding coolant because its being pushed out.