671 engine, oil in coolant - diagnosis help request

oil in the coolant and slowly loosing coolant, otherwise engine performing normally

We thought to check the oil cooler, pulled it, took it to a shop, they opened it, did their thing and sent it back without a report of a problem. Our symptoms persist.

Cruising these forums there is clearly a lot of knowledge here, so what I'm looking for is ideas on things for us to check, in order of likelyhood. Our maybe even check some not as likely possibilities before we get into spending the big money, :) as oil in the coolant sounds like there could be a serious issue. I'm not exactly sure what that issue is. Educate me if you are inclined, please and thank you.


Regular Contributor
The heat exchanger is the usual cause. I'd call the shop and ask if they did a pressure test. Less likely is a cracked head/block, bad head gasket. But since you're also loosing coolant, it looks a little more like a head gasket. Do a compression test. There's an adapter made to go in the injector cup that gets connected to a regular diesel compression gauge. Sometimes you can find them on ebay.
Heat exchanger is probably due for a cleaning, we'll get the pressure test done at the same time. That's a good idea. Thanks Lepke. You think I should do the compression test first? I'm leaning toward the heat exchanger just because I'm more familiar with getting that done. Clearly not the best way to approach it, but...


Regular Contributor
The heat exchanger is shared by raw water and coolant. In the event of a leak, coolant is under higher pressure than the raw water, so coolant would leak into the raw water. And It wouldn't put bubbles in the coolant. Bubbles usually mean combustion gases getting the coolant. That means head gasket, cracked head or cracked block. A compression or leak down test probably would confirm a head gasket problem. There is a test kit to test for combustion gasses in the coolant.
There is a coolant pressure test tool that connects to the filler cap. It has a hand pump and a gauge. You look for the coolant system to hold pressure. Sometimes if there is a leak you can see it drip, looking thru the cylinder ports with the air box covers off. Engine has to be rotated so each piston is at bottom dead center.
Well damn, I was hoping not to have to get into the head. Commerical boat and rolling into the business season through Jan 3rd or so. The boat is running on average 3 hours a day (not that busy). What do you think about just watching the oil level through the holiday season and getting into the head in January, too risky?


Outstanding Contributor
You don't say how this system is configured so I am letting you know that my comments are based on a pressurized, closed cooling system with raw water heat exchanger.

I think that your oil cooler is leaking and your shop didn't pressure test it or they would have explained that to you before giving it back. The test results should be on the work order.

You need to ask them. If they reply "yeah sure", you need to ask them what test pressure they used and if they wrote that down somewhere. If the cooler core won't hold ABOVE normal oil pressure (see below) oil will contaminate coolant.

Oil pressure = 40 to 60 psi, depending on engine and oil pump condition. While the coolant pressure on these old gals is much lower...4-10 psi depending on the cap mostly.

THAT'S where you START the investigation.

If you don't get PRECISE, CRISP responses from that shop about your cooler I wouldn't, couldn't trust them.

If the cooler IS leaking, the oil pressure is much higher while the engine is running and, of course, oil will enter the coolant.

However, when the engine is shut down the oil pressure goes to ZERO while coolant pressure remains UP while the engine cools. This can actually cause the coolant to enter the oil system. In this case, because the coolant is contaminated with oil, this "reverse leakage" might not be immediatly obvious. But IF this is occurring, it could lead to engine bearing failure. Especially if you are running an antifreeze solution.

So...NO...don't run the boat until you can be positive that NO coolant is getting into the oil.

You could have two completely separate problems causing the leaks or, as Lepke points out, block, head or gasket issues. But you really need to keep in mind that, if they didn't reliably check the oil cooler core then you're sort of back to square one without further testing.

Good luck.
Thanks jgmo,
I will check with the shop on the results of the pressure test, or do it again. I will also find out about how the system in configured. I did not consider coolant in the oil. Would a oil test sent to a lab detect that?


Outstanding Contributor
Yes, an oil analysis would detect antifreeze if that's what you're asking. But simply pressurizing the system with a hand pump would tell you if there is an internal leak.
You would;
Pressurize to about 17 psi and see if and how long it takes to leak down.
If it holds, there's no internal leak
If it leaks down, you would first inspect for external leakage.If you don't see leakage, drain engine oil and remove the air box inspection covers. Pressurize again with the oil drain plugs removed as well and see if you can pinpoint where the coolant is escaping.
As Lepke pointed out, the engine would need to be slowly rotated to find leakage in an individual cylinder.

Good luck.
Okay, update.

I found the receipt/report from the gear oil and transmission oil cooler. Says they were pressure tested from 20 -170psi, no leaks.

I got the hand pump kit and found this today: pressurized to 12 psi, lost one psi in 35secs, and was down to 7psi after 5 min. I did not find external leak with soapy water.

I also pulled oil for a lab test (mailing off tomorrow) to ensure no coolant in oil. But the oil from the fill cap and dipstick doesn't not show any visible signs of an emulsification. Unlike the coolant tank, which is pretty ugly.

So from the advice here next step is 1) remove air box inspection covers 2) pressurize with oil drain plugs removed while rotating the engine so each cylinder is bottom dead center.

I am a little confused as to what this is going to tell me beyond "there is your leak, take the cylinder head off". It seems like I'm headed to taking off the cylinder head so that I can inspect/replace the head gasket and cylinder heads. Even if I had found an external leak, I'd still be doing that to replace the gasket yes?

Here is the next question. I'm probably bringing in some outside help as this is above my pay grade, since I realize taking the cylinder head off is a pretty decent sized job, what other checks on the engine should I be making while I've got it apart?

As always, please correct me if I'm saying something stupid, I'm pretty ignorant and got no ego in this game, just want to fix it right.


Outstanding Contributor
A couple of things that you listed were confusing to me so, a couple of questions:

You say "gear oil and transmission oil cooler" but what about engine oil cooler? Is that all a single unit?

And then;
You say say that you checked for external leaks with soapy water. This test should be done with the cooling system full and you observe for coolant leaking. Did you drain the system prior to pressurizing and look for escaping air?
If so, I guess that could work but it won't when you look for internal leaks.

Sorry I didn't make that clear before but that's the problem in information exchange like this. It's just too easy to miss something or "garble" the message.

I should have asked for more information from you before offering any advice. Such as:
Is this an inline or v block engine?
Is it turbocharged or naturally aspirated?
Does it have a coolant expansion tank or recovery bottle?

I should have also told you to pull the valve cover(s) before pressure testing to look for leaks at injector tubes.

It appears that you have a pretty significant coolant leak someplace but that still doesn't explain the oil in water.

It is encouraging that you don't see water in the oil though.

You're correct about finding an internal leak this way. You see coolant leaking from the oil pan drain...so what then? Well, it just indicates that tear down and inspecting is probably your next move to find out why.

On the other hand, you see coolant coming into the air box it still will probably lead to taking the head off but if you find one particular cylinder is the source as you rotate the engine, it gives you a specific area to scrutinize.

Also, if it's turbocharged and aftercooled, it could just be the cooler leaking.

I think it's wise that you're going to get some local help with this. Just do your very best to vette them before getting started.
Thanks for the reply.
I guess I should have said transmission oil cooler and engine oil cooler, they are separate units.
I checked for external leaks with the coolant tank full. I haven't drained it yet. But that leads me to another question. How do I clean this sucker when I have reached the end of the project, I need to get the remainding oil out of that sucker.

It is an v6 inline
No turbocharger
Coolant expansion tank.

I think I'm good with taking the value cover off and testing again, that's a good idea.

Ignorance comes up here with this question. Is there a part of the head gasket that if blown could lead to oil in the coolant? Some of my online research seems to suggest that is possible, but I'm inexperienced enough to not really know.

I also did not check for external leaks where the oil pan meets the engine block, only where the cylinder head meets the block, so I've got two things to do with another pressure test. I'm also going to replace the coolant and try and do a little cleaning to give myself a little more time to address the issue.

Again, the advice here is appreciated. And yes, I've had local mechanics out that have billed the hell out of us, couldn't find the problem, and we ended up fixing on our own. Different boat, different time, but that problem ended up being a blown injector O ring that should not have really been that hard to find, and the clues where there, air the the diesel only in the return. Once we figured it out, it was straightforward work. Pfttt, the good people here in Oahu are so busy they rarely even return phone calls. The local Detroit dealer is hit and miss, mostly miss in my past experience. I end up calling Mike in Parts at Tacoma Diesel in WA for parts.


Outstanding Contributor
First off, you said; "it is an v6 inline". Nope!

It's either a V-6 (3 cylinders per side) or it's an in-line with in-line meaning all 6 cylinders are in a row. I suspect it's an in-line.

Anyway, yes....finding good help to repair an old 2 stroke Detroit gets harder every day as all the old timers drop off the planet. I haven't worked on one in many years and, even though I still have most of the special tools and manuals, my recollection and skills have drastically faded. But IF you have the CORRECT engine manual and are willing to read it, these engines can be repaired by anyone with reasonable mechanical ability. Detroit manuals are very thorough....as engine manuals go.

Answering your question about if the oil can get in the coolant via the head gasket, the answer is YES if it's an inline engine.

And probably NO if it's a V-6. The V block engines don't use one piece head gaskets.

The oil intrudes via the one piece gasket if the seal between an oil passage and a coolant passage is compromised. And, just as I explained about the "reverse leakage" in the cooler taking place after engine shut down, the same can occur with a failed head gasket.

There's no reason to monitor the oil sump to block for coolant leaking. It can't happen. I recommended draining the oil and leaving the drain plug(s) out to see if coolant is entering the sump.

In a shop setting you would use regulated shop air to test as it will keep your 15 or so psi on the system without leaking down as long as you want. If you're just using a hand pump it leaks off pressure too swiftly so you will want to pump it up several times in order to detect internal coolant loss.
This applies to watching the air box inspection holes as well.

Taking the head off is an opportunity to do a complete repair on one of these "modular" engines and is something that should always be considered. If the engine is high hour, low compression or unknown internal condition, this is where you would, ideally, choose to do a "semi-overhaul".

With the head removed the pistons and liners can be replaced along with new rod and main bearings. In the long run, this could be a labor and cost saving way to go but might not be very attractive to a diy'er. Lots of measuring and additional special tooling required. Not to mention additional parts cost. Just sayin'.

As far as finding major damage, as in a cracked head or engine block...
Most folks these days choose to repower with a modern platform since the costs can get up there in finding and purchasing these replacement parts.

As far as cleaning up the cooling system, there are commercial degreasing and flushing solutions available. I have also used Tide detergent and Dawn dishwashing liquid in my "storied" past. I didn't like the Tide as it was so "sudsy" that it was difficult to get IT out. Makes a big mess too that wouldn't be very welcome around some marinas these days.

Before I retired, I experimented with a microbe based cleaner called Oil Eaters that was very effective and easy to clean up. It actually seemed to work like magic! Too good to be true sort of magic! But...that was several years ago so I'm not sure if that option still exists.

One last question:
Are you taking heat from this engine into the house for any reason? I know some set-ups "T" off coolant supply and return lines, usually through an electric "diversion pump", to provide hot water for space heaters or windscreen defrosters. If so equipped you have to also consider that for coolant loss.

Good luck with all this.
yeah, inline, not a v. Sorry, duh.

no heaters or the like, but with oil in the coolant I wouldn't put that high on the probability list anyway

The former owner of this boat did the rebuild in his garage, so I might be the owner of that special tooling (as well as all the spare parts that came with the boat and business). I will look into that once I know what I'm looking for. I watch some videos on rebuilds.

Probably not repowering, as with a commercial boat that comes with a new stability test that might reduced the passenger count. At least as expensive and time consuming as a rebuild itself.

Your not looking for a Hawaii work/vacation are you? :)


Outstanding Contributor
I won't be taking any vacations. Going to the neurosurgeon Wednesday to see if anything can be done about my failed lower back. My days crawling around engine rooms are pretty much done.

Hopefully the engine doesn't have so many hours on the overhaul that you would need to do pistons and liners. Bearings can always be rolled in from the bottom....if there's room.

If you have good measuring equipment....straight edge, calipers, torque wrench, bore gauges, micrometers etc...you might only need a high quality ring compressor (I prefer the Kent-Moore "funnel") to pull off an in frame. Well, those and money.
Again, if there's room in there.

Try not to assume anything about troubleshooting the problem or problems. It's like crime detective work...go where the evidence takes you if at all possible. Although, a S.W.A.G. is sometimes all we get. If you know what I mean.

You have my best wishes and moral support but that about empties the chamber for me.

Please don't hesitate to keep us updated or to just brainstorm with us. Sometimes just voicing a subject will lead to solutions.

Good luck Matt,

Think we are going to start with the top end rebuild. I'm actually hoping that I will see a blown seal when we get the head off. It does look like I would have to pull the engine to do the full rebuild, there isn't much room under that oil pan, though it might be possible.

I am waiting for the engine oil lab test to come back before I run a degreaser through the coolant system. I think I'm right when I say that if there is any coolant getting into the oil that would be a bad idea. I am planning on using a Zep engine part degreaser. Any concern there? I haven't pulled out the manual yet, but I assume there must be an impellor in the coolant pump somewhere. Probably ought to change that since it has been seeing oily water. Thoughts?

Thanks for the tip on the ring compressor.

Sorry about the back, got a buddy dealing with that, its a bitch. I'll post the progress as it happens.
It occurred to me the other day that my entire problem could be a mistake by a "helpful" part time captain that added oil to the coolant tank, though I think this unlikely since the coolant tank is losing pressure. I want to clean that sucker and observe accumulation of oil again before I get in deep.


Outstanding Contributor
Like I said before, don't assume anything. If you can imagine it, somebody has probably done it somewhere, sometime, some place:>).

Your thoughts about holding off the degreaser until the oil lab results are in are probably wise.

As far as the Zep, I don't know having never used it. But keep in mind that you don't want to use anything caustic that could etch metal or deteriorate seal material. Many commercial engine degreassers contain acid. You don't want to use anything inside that engine that you wouldn't put on the paint of your pickup.

Yes, the water pumps have impellers but they are all brass as far as I know. All the 2 cycle Detroits I ever worked on had rebuildable pumps as much as I can remember. But there are plenty of older models I've never seen so that may or may not be entirely true.

I wouldn't be too concerned about the pump unless it is leaking and, if it is, you can probably replace the water seal without removing the pump from the engine. It should have a removable cover under an internal snap ring if you're curious about what it looks like inside.

I can't remember ever finding a failed impeller on a 6-71 but you could manufacture a whole new universe out all the stuff I've missed. My experience with the inline engine was in my very early days as a mechanic. I worked primarily with the V-71 and 92 engines. Fnished up with with the Series 60, 50 and 40 4 stroke DDEC engines.

Bon vente.


Outstanding Contributor
Well, that report looks encouraging at any rate.
Hope you have guessed right about the "helping hand". ;>)
New information:
Now that I am monitoring closely it seems apparent that the oil loss is coming from the transmission. The engine oil level has not changed noticeably. That has to the transmission oil cooler right? I can't think of any other place the coolant and transmission oil could be in contact.
We did get the entire coolant system flushed and got 99% of the emulsion out of there.
I was told the oil cooler tested okay, but I'm guessing that was not correct. I'm not getting oil in the bilge.
I'm thinking just get a new transmission oil cooler.
top down on transmission.jpgbottom of coolant tank.jpg
Not what I expected to find. One pictures shows lines running out of the coolant tank. The other shows same lines running into transmission. So is the oil cooler installed inside the coolant tank? I was expecting a tube shaped heat exchanger in line with coolant and oil, but that is not what I found. I'm thinking there is a plate oil cooler inside the tank, is that right?


Outstanding Contributor
I don't know but that sure does look like a plate cooler that the lines are connected to.

I've never seen a set up like that but whole new worlds can be made from things I've never seen before.

Ron Sparks

New member
Matt: The 6-71 inline engines were manufactured with two types of cylinder head to block sealing, the Low Block which used a single gasket, and the High Block, which uses individual seal rings for each cylinder, oil and coolant passages. A leaking cylinder seal will not affect coolant seals, and will not cause loss of coolant, or coolant in the oil.
Low Block engines have not been built since about 1950, thus virtually all engines now in service are the High Block configuration.

If you are serious about rebuilding your engine, I would be interested in working with you. As a Detroit Diesel factory trained and certified Tune-Up and Overhaul Technician, I am experienced and qualified for the job. Also, see the boatdiesel forums for my contributions since 2002 regarding Detroit Diesel engines. Should you be interested in talking, my telephone number is (321) 216-6618, located in Florida.

Ron Sparks
Matt: The 6-71 inline engines were manufactured with two types of cylinder head to block sealing, the Low Block which used a single gasket, and the High Block, which uses individual seal rings for each cylinder, oil and coolant passages. A leaking cylinder seal will not affect coolant seals, and will not cause loss of coolant, or coolant in the oil.
Low Block engines have not been built since about 1950, thus virtually all engines now in service are the High Block configuration.

If you are serious about rebuilding your engine, I would be interested in working with you. As a Detroit Diesel factory trained and certified Tune-Up and Overhaul Technician, I am experienced and qualified for the job. Also, see the boatdiesel forums for my contributions since 2002 regarding Detroit Diesel engines. Should you be interested in talking, my telephone number is (321) 216-6618, located in Florida.

Ron Sparks
left phone message
and thanks Ron, I didn't know about the Low Block configuration, I haven't ordered any parts yet, but that is good information. I've only owned the boat for 3 years and haven't been into the block. However, I'm expecting now that the oil cooler is the problem and I may not need to go into it all all.

Ron Sparks

New member
Hello Matt: Received your message, thank you. If solving any further issues would benefit from a telephone conversation, feel free to call.

My best,

So I thought I'd close the loop on this. jgmo was right, who tests your equipment is critical. It wasn't until we removed the oil transmission cooler the SECOND time and had it tested that we found the leak there. The first part we got had too many cooling fins and didn't fit in the tank, the second one was right. Apparently the coolant tank was changed from the original on the engine serial number. Anyway, we finally got it done and transmission oil level is holding and we are pretty sure the leak in gone. There is still quite a bit of emulsified oil in the coolant system, despite a couple of rounds of degreaser going through it.
Thanks for all the help gentlemen. It was very helpful to have a sounding board of the more experienced.


Outstanding Contributor
Great news!! So glad that you sorted it out!

Now maybe you can ENJOY firing the old girl up!

Getting a coolant system degreased is pain...
...I know!

But keep in mind that she will cool much better the cleaner you can get it.
The oil film on everything acts as an insulator that inhibits heat transfer probably more than you may realize.

Way to hang in there and GETTIN' 'ER DONE!

Good luck