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m939 overheated Exhaust Gas Temperatures

H

hozmonster44

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i have a 1991 931a2. 6cta. i ran the exhaust gas temperature too high on my last trip. went up to about 1400 to 1600 degrees. truck ran ok for a couple of days. truck was running noisy after . knocking more than normal. heating up EGT really fast after. has been running ok. i'm getting air coming out of the engine, through dip stick and vent hose ? but last night went for a test drive. lost all power, smokes too much. cloud of black smoke. EGT runs really high and no power. can not get any speed. i can only run at about 5 MPH . any sugestions ? where do i start ?. i have since changed the oil and filter. found junk (sludge) in oil and oil filter.
 
porkysplace

porkysplace

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Sounds like it's time to pull the heads , aluminum melts at a little over 1200* degrees you probably burned a hole in a piston.
 
74M35A2

74M35A2

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How did your EGT get that high? Assuming you turned up the fuel pump, and then held the pedal on the mat with a load or going uphill. If you have blow-by and such as you describe, your engine is done for and needs new cylinder kits, that is all there is to it. Order up an aftermarket overhaul kit. Probably about $1400. Should not be too bad to install, it is a simple engine. Used 8.3's go for about $2500 and up. You could pull the injectors and do a compression and leak down check to confirm, but it would kind of be a waste of time since it seems obvious, and those injectors are notorious for corroding into their wells, and needing a puller to remove each, after soaking with penetrant for days. This makes me want to put in an EGT buzzer alarm on mine when I wick the fuel up, as I could see how easily this could happen. The high EGT and black smoke you have now is diesel fuel burning in the exhaust manifold, because there is not enough compression to light it off in the dead cylinders. Sorry to hear about it, but great time to pick up a used Detroit Series 60 or Cummins ISX, both 500hp engines!
 
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74M35A2

74M35A2

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JohnnyBM931A2 said:
The idea of aluminum pistons in a diesel engine just sounds stupid to me.
Click to expand...
Nearly all engines run aluminum pistons, and without issue as designed. Steel would eat up so much more power throwing that much mass back and forth, and would require larger rods, etc.... I'm interested in hearing why this even occurred, but dollars to donuts the fuel rate was turned way up without regard to increasing corresponding air flow, and temps flew out of control. Many here have turned up their fuel rate and still can barely hit 1200F.
 
74M35A2

74M35A2

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You could have your new replacement piston tops coated with a ceramic thermal barrier insulation to help keep them from absorbing some heat. It is not that expensive, if you want to keep the throttle down on a wicked up motor. That, plus a full size charge air cooler up front, gutted aftercooler, and a larger turbo, and it should be able to handle all the fuel an MW pump can deliver.
 
WillWagner

WillWagner

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^^ This.

People think these things should accelerate like a sports car.
 
74M35A2

74M35A2

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@ 1600F, it probably did.....once. We love carnage pics !!! They make fabulous screen savers.
 
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74M35A2

74M35A2

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He had to have had a gauge, otherwise how could he know it was 1400-1600F range? A gauge won't keep it below 1250F unless your foot is synchronized with it (once you increase the fuel rate).
 
H

hozmonster44

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lol you guys crack me up !... i was on a 8 hour trip back from pheonix to la. it was a long hill that was higher and longer than i thought. when the guage hit 1200 i slower down a lot. when i looked back down it was at 1400. the problem i have which is my own fault is that i did not put the egt sensor where it should of gone in the exhaust manifold. i put ti after the turbo. so my readings are off by 100 to 200 degrees. the fact the i put my meeters down low didn't help. i turned the fuel pump screw up 2 turns. i know you can go a lot more but i did not want to take a chance on this very thing happening. the governor springs are turned in the 3 turns. i also bored out the inside of my muffler to lower the exhaust temps but i guess it was not enough... lessons learned !
 
98G

98G

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JohnnyBM931A2 said:
No.. I just prefer to have things that are overbuilt, rather than being built with components that sacrifice strength for efficiency. But I guess that's just me.
Click to expand...

Increasing reciprocating mass will shorten the engine life by placing more force on the rods.

In the case of the 600,000mile diesel engine, the aluminum is the better choice. It's more expensive than steel. It wasn't chosen for efficiency or cost, but rather for increased service life.

Thank you Hoz for posting this. I for one am glad to learn from your experiences.
 
WillWagner

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98G said:
Increasing reciprocating mass will shorten the engine life by placing more force on the rods.

In the case of the 600,000mile diesel engine, the aluminum is the better choice. It's more expensive than steel. It wasn't chosen for efficiency or cost, but rather for increased service life.

Thank you Hoz for posting this. I for one am glad to learn from your experiences.
Click to expand...
Untrue. Cummins as well as other manufacturers have been using pistons made of non aluminum for close to 20 years. Early ones were articulated with aluminum skirts and hardened alloy crowns, now they are one piece with just a small "foot" for the skirt.
 

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gimpyrobb

gimpyrobb

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74M35A2 said:
How did your EGT get that high? Assuming you turned up the fuel pump, and then held the pedal on the mat with a load or going uphill. If you have blow-by and such as you describe, your engine is done for and needs new cylinder kits, that is all there is to it. Order up an aftermarket overhaul kit. Probably about $1400. Should not be too bad to install, it is a simple engine. Used 8.3's go for about $2500 and up. You could pull the injectors and do a compression and leak down check to confirm, but it would kind of be a waste of time since it seems obvious, and those injectors are notorious for corroding into their wells, and needing a puller to remove each, after soaking with penetrant for days. This makes me want to put in an EGT buzzer alarm on mine when I wick the fuel up, as I could see how easily this could happen. The high EGT and black smoke you have now is diesel fuel burning in the exhaust manifold, because there is not enough compression to light it off in the dead cylinders. Sorry to hear about it, but great time to pick up a used Detroit Series 60 or Cummins ISX, both 500hp engines!
Click to expand...
I wouldn't do anything till I opened it up to look.

Then I'd probably order a take-out from Glen in Kansas for $500. Just my thoughts for fast and cheap.
 
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