Farm Equipment - Agco 6690 engine trouble

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Agco 6690 engine trouble

Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
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URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=173627
Printed Date: 16 Dec 2025 at 6:37pm
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Topic: Agco 6690 engine trouble

Posted By: patf10
Subject: Agco 6690 engine trouble
Date Posted: 17 Aug 2020 at 10:11pm

I am helping a friend work on his 6690 agco. 4 cyl air cooled engine. He was running the other day when he noticed that there was oil pouring out from behind the starter. We investigated today and noticed that there is a crankcase vent on the back of the motor and it is coming from that. So it seems there is excessive blow-by in the engine. It seems odd that it would start so abruptly. He said it never overheated or anything like that. Is there something else to the crankcase venting system we are missing? Im thinking maybe a blown head gasket. There is some oil around the top of the cylinder on #3. Im not sure how these engines are if that could be the issue. We really need to get this fixed so hay can be finished mowing and baled.

Thanks

Pat

Replies:

Posted By: tbran
Date Posted: 17 Aug 2020 at 11:22pm

The engine has an oil separator … https://na.apb.agcocorp.com/p/agcoallis/79013783/l/46?tocGuid=95f4d4d9-75e8-e0a2-75a1-a4a6e1b9770f" rel="nofollow - https://na.apb.agcocorp.com/p/agcoallis/79013783/l/46?tocGuid=95f4d4d9-75e8-e0a2-75a1-a4a6e1b9770f

there should be a seal on the block / vent tube. "these engines" are like all diesels in that pistons sieze for the same reason - heat - lack of lube - or dirt. The 'head gasket' has nothing to do with oil - if it is 'blown' you will hear a loud popping noise. Oil on the top could only come from tappet cover or push rod seal leakage. IS the radiator clean ? it does have one - if you can't feel a hot blast of air coming from it - it is plugged - it is an air/oil cooled engine.

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When told "it's not the money,it's the principle", remember, it's always the money..

Posted By: patf10
Date Posted: 18 Aug 2020 at 9:47am

Well the thought is that maybe the head gasket is blown into an oil galley that would allow compression to enter into the crank case. The engine isnt very dirty. the oil cooler that runs along the engine and inside the ductwork is fairly clean as well. Just trying to figure out if there is an issue with the crankcase ventilation system. We took the "breather/separator" off and didnt see any issue with that i dont think. However we dont have anything to compare it to.

tbran, that link you sent didnt seem to work

Posted By: Amos
Date Posted: 18 Aug 2020 at 4:10pm

I think the only oil supply to the head is up the center of the push rod...but I may be mistaken, I have not taken apart a 6690, but a couple 5660 which I am thinking is the same engine, just smaller displacement. Oil return should be down the push rod tube. Head gasket should be just a fire ring.

Posted By: tbran
Date Posted: 18 Aug 2020 at 9:13pm

Sorry , a blown head gasket would be only a 'fie ring'. Blow by is going to be a bad piston.

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When told "it's not the money,it's the principle", remember, it's always the money..

Posted By: Rusty Allis
Date Posted: 18 Aug 2020 at 9:52pm

I actually had to look this engine up. kind of goofy, like a inline motorcycle engine.

I would suspect a stuck ring. seeing it's air cooled I would have to wonder if the fan sucked in a bunch of garbage and it's packed around the fins. I have zero experience with these engines, but seeing the size of the machine and the fact it's air cooled....It seems to me to be a WTF were they thinking kind of design

post photos if you can. curiosity is killing me

Posted By: patf10
Date Posted: 19 Aug 2020 at 5:12am

Well I was wondering about the fire ring thing. So maybe it’s not a gasket like I was hoping. I’m going to make an adapter and we are going to compression test it. I guess we’ll see what that gets us. If I have decent compression. Anyone familiar with the crankcase vent / oil separator system?