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Old 09-25-2011, 09:57 PM   #1 (permalink)
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NP205 Overheating

I recently sold my 70 Chevy K10 to a guy who was driving it home 1,000+ miles. I finished the rebuild a few years ago and have about 5k on the odometer prior to sale. The only item not replaced or overhauled was the 700R4.

I get the call a few days after the sale and the t-case has overheated melting the site tube I installed and thus puking oil out into the exhaust while traveling down the freeway. He stops gets out and the t-case is draining oil that is smoking and softening the asphalt as it drips on the ground.

Long story short I decide to take it back... I get the case out today and tear it down, I can not find anything indicating wear, no shavings, all the bearings appear to be in decent shape. The case was definitely hot as the paint on the back of the front output shaft had bubbled.

The trans oil smells of burning as well. I always had softer shifting and figured the trans was older but still in tact and chalked it up to the higher horsepower engine.

So the question is could the the trans slipping create enough heat to saturate the t-case and boil the oil? I find it hard to believe but was not around when the incident happened and the lack of direct evidence within the t-case is concerning. Or is there something I am not noticing?
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Old 09-25-2011, 10:03 PM   #2 (permalink)
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Edit- it took some re-reading, but I kinda get the jist of it.


My first thought is simply a plugged vent tube, or it was low on oil to begin with.
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Old 09-25-2011, 10:43 PM   #3 (permalink)
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I would have said bearings. But you say they are all good?
I would also suspect a pretty good drag on the engine if it was creating that much friction.
Any other symptoms he was mentioning that could have seemed odd?
And no chance the exhaust itself could have been a culprit of anything?
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Old 09-26-2011, 01:50 AM   #4 (permalink)
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1000 miles in low range maby? ha


any noises? blueing, gaulling of anything? what part of the t case was the paint bubbling, that would be a good place to start looking for the source of heat. i dunno man? the vent would cause it to puke, maby it puked most of the fluid out and then started to run low and over heat. maby that site tube burst due to the pressure rather than melted.
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Old 09-26-2011, 10:02 PM   #5 (permalink)
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i would suggest the wrong fluid...

why wrong?

well, if its a 700r4, then it isnt stock... so someone put a 205 behind it, and didnt change the seals on the input sleeve, which can lead to an internal leak of auto trans fluid into the case, which will possibly overheat during a 1000 mile drive..

so, the ATF boils out, the trans is sluggish because its low, and the transfer case survives....
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Old 09-26-2011, 10:38 PM   #6 (permalink)
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i would suggest the wrong fluid...

why wrong?

well, if its a 700r4, then it isnt stock... so someone put a 205 behind it, and didnt change the seals on the input sleeve, which can lead to an internal leak of auto trans fluid into the case, which will possibly overheat during a 1000 mile drive..

so, the ATF boils out, the trans is sluggish because its low, and the transfer case survives....
GM actually suggested atf as an acceptable fluid for the 205 during the 80's. I've been running my 205 on atf for the past year with no issues so I don't think that would be a cause for this drastic heat issue.
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Old 09-27-2011, 12:10 AM   #7 (permalink)
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Went through all the parts again and there is nothing indicating failure of a bearing.

I put all new seals in 2 years ago and have driven 5k with no problems. The exhaust H pipe does run behind the case but is 8 inches back which makes convection less then likely, radiation is possible...but the length makes me doubt that much heat transfer.

The bubbled paint was on the back side of the front shaft section, aluminum plate.

No atf was present in the case and the trans was still holding fluid.

Driving in 4 HI seems a likely source of heat, the tubing had a max temp of mid 200's, poor choice on my part, but if the vent tubing collapsed due to heat then I would have lost the vent function.
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Old 09-27-2011, 06:44 AM   #8 (permalink)
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Went through all the parts again and there is nothing indicating failure of a bearing.

I put all new seals in 2 years ago and have driven 5k with no problems. The exhaust H pipe does run behind the case but is 8 inches back which makes convection less then likely, radiation is possible...but the length makes me doubt that much heat transfer.

The bubbled paint was on the back side of the front shaft section, aluminum plate.

No atf was present in the case and the trans was still holding fluid.

Driving in 4 HI seems a likely source of heat, the tubing had a max temp of mid 200's, poor choice on my part, but if the vent tubing collapsed due to heat then I would have lost the vent function.
Possibly your change to a sight tube resulted in the case being overfilled, I have seen planetary gearboxes overfilled result in much hotter operation. The gears submerged in oil increase pumping action = more friction = more heat.
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Old 09-28-2011, 09:48 PM   #9 (permalink)
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Quote:
Originally Posted by geo in a K5 View Post
GM actually suggested atf as an acceptable fluid for the 205 during the 80's. I've been running my 205 on atf for the past year with no issues so I don't think that would be a cause for this drastic heat issue.
No, they didnt.

when GM went to the 208, it was acceptable to use as fluid. the 205 was never given ATF.

just because you get lucky doesnt mean its right for everyone to try it.

helical cut gears operated at speed need a thicker oil than ATF when the supply of oil is limited or not properly rated for the application. If you are using gl5 lube, you might be making a miracle happen, but based on the amount of tooth contact in a 205, its design could also be helping you.

Of course, I have seen late model transmissions designed to run with ATF burn up when the fluid gets hot, too... but what do I know?

This conversation has been had before. Dont start this argument again. An NP205 requires 80-90wt gear oil. just because youre getting lucky, doesnt make it right.


for clarification, read this thread
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Old 09-29-2011, 10:00 AM   #10 (permalink)
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Without an actual temp reading it's hard to do much about it but keep in mind that mid-200's isn't really hot for this case. It's not great for the oil but they run pretty hot.

Given the use we've put the 205 through, I would say put some good synthetic gear lube in it and send it on the way. You've pretty well proven there's nothing wrong in it with a visual inspection so eliminate the sight tube and fill 'er up. We've run the Amsoil 75-140 in 120 degree heat in 4 hi for hundreds of miles with no problem and our use is nothing compared to some others racing them. a glue on temp strip might be a handy thing if it's something you are going to have to keep an eye on.


ATF didn't kill it, I'd have to think that GM had some engineers on board making sure the cases weren't going to explode when they spec'd it. Their priority was probably fuel milage rather than longevity past 200K miles but they don't explode/overheat or other bad things. They just don't hold up as good as one with a good gear oil in it. If you find a source for 300+ degree clear tubing let us know, I'd like put a sight tube on some of our stuff but hardware store tubing probably isn't going to cut it.
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Old 11-29-2011, 09:57 PM   #11 (permalink)
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I don't know if this helps you, but I used some Polycarbonate tube which has a maximum continuous working temp of 240 degrees. I have a gauge mounted in the dash so I can keep on eye on the temp.

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Old 11-30-2011, 10:59 AM   #12 (permalink)
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Yes they did. Straight from the original book out of a 1983 k30.



For what it is worth though; gear oil is the far better choice.
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Old 11-30-2011, 01:15 PM   #13 (permalink)
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Perhaps a quick read might help you: http://www.machinerylubrication.com/...-helical-gears

GM introduce the 208 in 1981 or 1982, this might be why your owners manual has poor information. Plus, you believing it makes it true, right?
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