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Old 22-08-2020, 13:44   #1
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Carbon buildup in air intake and smoke from engine bay

Hello there,

This is my first post on this forum.. I've looked far and wide on the internet but cannot seem to find the cause/solution..

I was doing some troubleshooting on my Renault marine couach rc16d because the first time I took the boat out on a longer engine trip I noticed that there was black gunk and lots of smoke coming from the exhaust.. also the engine is smoking inside the engine bay and I had a feeling it was lacking some power.

I read that it might be a couple of things like clogged exhaust, air/fuel ratio not correct and some other things.

I checked the exhaust elbow and didn't find anything too bad, looked pretty clean. I didn't take the complete exhaust manifold off so there might be some buildup there still..

So then I took off the air filters and I noticed that on one of them there is a lot of carbon buildup in the intake.. while the intake on other cylinder looks normal to me.
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Any thoughts on what my problem might be and how to resolve it?
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Old 22-08-2020, 13:56   #2
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Re: Carbon buildup in air intake and smoke from engine bay

Most likely a leaking intake valve, again most likely one that is adjusted too tight so it never quite closes. Hopefully it hasn’t been damaged, but first check valve adjustment, but It could also be a stuck intake valve.

On edit, if it’s adjusted too tight, there will be no clearance, if it’s a stuck valve, there will be excessive clearance.

A leak down test will also confirm or deny a leaking valve too.

If it’s not the valve, then I don’t know what it is,
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Old 22-08-2020, 14:12   #3
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Re: Carbon buildup in air intake and smoke from engine bay

Thanks!
I did check the valve adjustments today and I did notice that on the cylinder with the buildup the rockers had quite a lot of play. I adjusted them but I did notice some smoke still from the engine itself, couldn't check the exhaust too well because it was dark already..
I will check tomorrow morning with the leak down test if it is actually stuck.. and maybe take the exhaust manifold off to see how it looks there.
Thanks a lot for the quick response!
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Old 22-08-2020, 14:24   #4
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Re: Carbon buildup in air intake and smoke from engine bay

If there was a lot of play, that means the valve is either stuck, or bent. It’s a misconception that valves wear loose, they don’t, the valve wears into the seat and that tightens up the clearance
I would either verify with a leak down test or just pull the head. A leak down test pressurizes the cylinder while it’s at TDC compression, any leakage is readily apparent, you’ll hear it leaking out of the intake port if the valve is leaking.

The other thing to check when you find loose clearances is for a bent push rod, a stuck valve can bend a push rod, remove the push rod and roll it on a flat surface, if it’s bent you’ll see it immediately.

My bet is your pulling the head and getting a valve job, I’d have the guides replaced by a machine shop and reamed to size, I like bronze guides myself.

If you go that route, do NOT let them talk you into “cleaning” up the head surface unless it’s warped, shaving the head on a Diesel can get complex

On edit, I should state that I know nothing about your particular motor, I’m saying things that are the average Diesel, I’m assuming some things, like your motor is not an overhead cam for instance
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Old 22-08-2020, 14:26   #5
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Re: Carbon buildup in air intake and smoke from engine bay

I don't think it's carbon, more likely dirt. If the engine smokes, that's probably oil from the breather mixing with dirt in the air with some of the mix coating the intake. Even with air filters, really small dust particles will get thru. I've seen a lot of intakes that look like that. Big marine diesels usually don't have air filters, just air silencers unless they're operating somewhere getting a lot of dust blowing off the land.
The intake on my PU turbo looks like your intake after a few thousand miles, and it's filtered air.
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Old 22-08-2020, 14:29   #6
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Re: Carbon buildup in air intake and smoke from engine bay

Yea I was hoping I wouldn't have to pull the head but I guess I'll have to.. first thing tomorrow that's what I'll do!

Thanks a lot for your wisdom!
I don't have a lot of experience with engines but I love learning more about it.. very good skill to know if you ask me.
I'll post if I know more
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Old 22-08-2020, 14:31   #7
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Re: Carbon buildup in air intake and smoke from engine bay

Another way is to run it with the filters off, see if one cylinder blows air and or smoke out of the intake, put something near the intake, but not on the intake, a Diesel can pull incredible vacuum, your looking to see if a lot of air is pushed backwards.
I bet there is, my reasoning for that is it looks like dry soot to me and crankcase vent is oily and greasy, plus what the real indicator to me is you saying the engine compartment is smoky. I think thats burned diesel coming out of the intake.

On edit, if you can do a leak down test, that will tell you for sure if you have a leaking valve or not
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Old 22-08-2020, 16:05   #8
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Re: Carbon buildup in air intake and smoke from engine bay

Thanks I'll try that as well.
I'm going to pull the head off anyways because I want to replace the gasket, noticed some oil leeking from the head.. and probably clean under the head as well(?) since I bet it looks bad inside as well.
I hope everything is still generally okay and fixable..
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Old 22-08-2020, 17:03   #9
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Re: Carbon buildup in air intake and smoke from engine bay

Assuming the top picture is the intake where’s your belt located and does it have excessive wear? Could very well be belt dust and cause the carbon on the exhaust as well. Seen whole engine bays lined with belt dust. If you suspect the belt check the pulleys for alignment, might just require a few washers. If not then pull the head and check your valves. Start simple.
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Old 22-08-2020, 17:47   #10
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Re: Carbon buildup in air intake and smoke from engine bay

If you pull the head, the valves are checked shade tree fashion with alcohol. Pour rubbing alcohol into the intake and there should be no leakage from the valve, I say a wet ring after several minutes is OK, but no leaks.
Same for exhaust side.

If I had the capability of a leak down test, I’d certainly do that first, even a compression test ought to tell you something.
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