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Cummins 550 QSB 6.7 Turbo paint turning yellow

Old 08-23-2020, 11:59 AM
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Default Cummins 550 QSB 6.7 Turbo paint turning yellow

After running the engines pretty hard in the Emerald Coast poker run last week, I noticed my white turbo paint looks yellow, like it got way too hot. Is this cause for concern?
These engines run with 40lbs of boost at WOT, that is about double of my F250 diesel truck.
Old 08-23-2020, 12:24 PM
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It is normal for the turbo paint to discolor. It shows you are loading the engine. As long as all sensors work and read in the correct range, no problem. Marine Diesels do not like to be run over loaded. They need to get above WOT rating, fully loaded. My Yanmar 4LHA-DTE was rated at 3,300 rpm. Recommended WOT is 3,450 to 3,500. Your QSB is rated at 3,300 rpm and needs to reach 3,450 rpm's WOT.
Old 08-23-2020, 12:31 PM
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well maybe. its an obvious sign of overloading. high air temp. this could. make for very short-term life. hot air , hot heads, very hot valves. my QSB is 11 years and 1100 hours. paint is same white as when new
Old 08-23-2020, 01:02 PM
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Originally Posted by fireisland1 View Post
well maybe. its an obvious sign of overloading. high air temp. this could. make for very short-term life. hot air , hot heads, very hot valves. my QSB is 11 years and 1100 hours. paint is same white as when new
Good point. I guess the question is? Did the paint suddenly dis color or is it an old issue.
Old 08-24-2020, 04:27 AM
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Default cummins

I have 420hp QSB6.7's, have the same issue on the top of port engine (not on starboard) my mechanic says prob from moisture sitting on engine and then getting baked on as engine heats (paint discolors) (this engine is more exposed to the opening in engine room door/gutter when washing boat) He says not a serious issue- just cosmetic.I run these engines mostly at 75% load, sometimes at 80% load and occasionally 85% load.(at end of trip a short run at full load)..so not overloaded by any means

engines have never overheated, they run at 169 degrees at 65-80% load and at 85% load 171 degrees.

let me know if a diesel expert tells you something different.
Old 08-24-2020, 04:57 AM
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I don't have the numbers in front of me but the EGT curve of the 6.7 is quite different than the CAT, MTU and MAN engines I've used before. Their high temps are midrange rpms as they are just coming on boost and cool down considerably approaching cruise speeds.

Even water jacketed, you have hot, 750 degree gas flowing and paint just can't take that for long. Add to it the dirty little secret that many distributors will use rattle cans to touch up chipped paint during prep when they assemble gears, coolers, mounts AND risers. The factory Imron holds up well but the rattle can paint yellows quick!

Edited to add:

I just checked back through some sea trial data and confirmed the EGT's were hottest right at the hump as the turbo's are spooling up at 1600 rpm (905 degrees) and actually quite a bit cooler at 2800 (650 degrees). They did creep up from 2800 but only to 870 degrees at WOT.

Last edited by bills106; 08-24-2020 at 07:51 AM.
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Old 08-24-2020, 05:01 AM
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Originally Posted by saltwatersteel View Post
After running the engines pretty hard in the Emerald Coast poker run last week, I noticed my white turbo paint looks yellow, like it got way too hot. Is this cause for concern?
These engines run with 40lbs of boost at WOT, that is about double of my F250 diesel truck.
So what is pretty hard? The engine use parameters for an E-rated (pleasurecraft) 6.7, which is what we assume you had, "suggest" to keep the load at 80% or less for 10 out of every 12 running hours, and wot no more than 30 minutes every 12 hours and that assumes that at wot (100% load) the engine turns above the 3,300 rated rpms. If you went beyond that suggested load, then you may have done harm to the engine; if not, it is probably just discolored paint.
Old 08-24-2020, 05:01 AM
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My Cummins truck peaks at about 37psi ... i think ~40psi is normal peak boost for a cummins, imho (at least a little above stock/normal, i do have a mild tune on my truck, nothing crazy)

Old 08-24-2020, 05:33 AM
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I think you are correct on the overloading, I can only get 3200 RPM at WOT and I also noticed my fuel consumption at any given RPM ( except WOT) is much higher than what is shown on the Cummins chart. I now have 450 HRS and the turbos always had a little discoloration but last week they look like they were really over heated.
For this poker run, I ran the engines at 80-85% load , 50-55MPH where normally I cruise at 45MPH 70% load.
Old 08-24-2020, 05:40 AM
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How long did you run at 80-85% load ?
Old 08-24-2020, 07:40 AM
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80-85% load several hours at a time. I normally run 70% load but at the poker run you gotta to keep up!
Old 08-24-2020, 08:07 AM
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My turbos have yellowed, which I think is normal. However if you were running over 80% load you were pushing the engines pretty hard. I typically set in a cruise around the 75% to 77% load range. Your 550's should definitely hit 3300 rpm.
Old 08-24-2020, 08:25 AM
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Air side of turbo runs hot, just from the heat of compression. Turbo compresses air and that means heat. Just about every B or C marine engine has some browning on the air pipes and turbo before the aftercooler. Really dark brown and that thing has been run hard and probably over propped.

You do want to make sure you operate below the prop load line on your particular engine curve. Most of the time if you can reach rated rpm with load below like 95%, you are good to go at cruising speed. But each boat/engine combo is different.

No way is a common rail Cummins rated for 3300 going to reach 3450. Computer starts backing off fuel rate at about 3350.
Old 08-24-2020, 09:13 AM
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Originally Posted by Corndog38 View Post
No way is a common rail Cummins rated for 3300 going to reach 3450. Computer starts backing off fuel rate at about 3350.
This is a great point. All computer controlled motors are going to have a built in RPM limit.
Old 08-24-2020, 09:26 AM
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Yellowing is very normal

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