[BMC 1.8] Pretty sure its time for a head gasket replacement. Thoughts?

[thematt]

Hey folks,

 

Yet another looming project. After a successful rear crank seal replacement, I noticed some lumpiness in how the engine ran when cold. When I was cleaning up with soapy water I noticed air bubbling out of the head bolts near cylinder 1 and 3.

 

No oil in coolant or vise versa. Engine fires up easily and after at temp the mis firing stops bubbles reduce.

 

I'm ready to jump in with a head gasket replacement, but please stop me if it sounds like something else!

 

What I've confirmed so far:

Fuel is good, clean and air free, injectors all tested good, injection pump is newly(ish) rebuilt, retorqued headbolts to spec - that reduced the lumpiness for ~30 seconds.

 

I've attached a photo of the bubbling for what that's worth.

[Tony Brooks]

Did you re-torque the head bolts under the rocker shaft - rhetorical question.

 

First of all, apart from the lumpiness, that might just be cold air/gas between the stud and hole in the cylinder head expanding when heated and escaping (bubbles reduce when hot) so the first thing I would do is try to give it a good high speed/power run to see if it overheats or the bubbling gets worse or stops. 

 

I think this is a sea boat so needs to be reliable, so if it does overheat or the bubbling does not stop, then I think I would change the head gasket, but such bubbling can go on for years without problems. You can also but liquid that you mix with a sample of coolant that changes colour of hydrocarbons are present - that indicates a head gasket leak.

 

If you do change the gasket use a straight edge and feelers to ensure the block and head faces are absolutely flat and not distorted. Also (if you don't know) ensure that you stick to the head nut tightening sequence shown in the manual.

[thematt]

Hey Tony,

 

I torqued bolts per shop manual including the ones under the rocker assemble with a crowfoot. 

 

Coolant and oil were sent in for analysis and no mixing was shown. slightly high chromium from what I hope was the new lift pump and injection pump introduced to the oil.

 

You've kinda caught my math here. I'll be 100+ miles offshore. It's starting to feel like i should do the gasket to be sure. I'll send the head into a local show for cleaning and resurfacing and make sure the block is good before re installing.

 

Great call on the runnung her hard to see if it stops outright. I didn't even think about trapped gasses. Right now i don't have the transmission or shaft on so I can't put a lot of load on but i can get her up to temp. Worth a shot. If the bubbling goes away what would be the cause of the misfire when cold? Timing?

 

In the past season if I ran her hard I could get an over heat as i'm overpropped a bit, but generally when not overloaded she didn't overheat.

 

Thanks again for your help!

[Tony Brooks]

43 minutes ago, thematt said:

If the bubbling goes away what would be the cause of the misfire when cold? Timing?

 

No idea, it could be timing, either valve or injection, but I would expect other problems like difficult starting, exhaust smoke, and lack of power. It might be an injector that is sticking when cold, but, again, I would expect other symptoms. Remember that the base engine was designed over 80 years ago and then converted to diesel, so I would not expect it to run a well as modern engines where the design is helped by computer modelling. I would not worry about a bit of lumpiness when cold, as long as it runs well when hot and starts easily from cold.

[thematt]

13 minutes ago, Tony Brooks said:

 

No idea, it could be timing, either valve or injection, but I would expect other problems like difficult starting, exhaust smoke, and lack of power. It might be an injector that is sticking when cold, but, again, I would expect other symptoms.

 

You mean if the issue was timing or if it was a head gasket you'd expect those other issues?

 

I'll at least get her up to proper running temp temperature and check to see if air bubbles continue. If so, feels like a strong indication of early head gasket failure. If not it could be something benign like trapped air and maybe a total red herring. Is that a fair statement?

 

I've done a compression test when the engine is warm and get good compression across each cylinder of roughly 480 psi ±10 but I don't know if it's compression stroke is consistently that good.

[Tony Brooks]

3 minutes ago, thematt said:

You mean if the issue was timing or if it was a head gasket you'd expect those other issues?

 

 

Yes.

 

3 minutes ago, thematt said:

I'll at least get her up to proper running temp temperature and check to see if air bubbles continue. If so, feels like a strong indication of early head gasket failure. If not it could be something benign like trapped air and maybe a total red herring. Is that a fair statement?

 

 

Yes, I think so.

 

4 minutes ago, thematt said:

I've done a compression test when the engine is warm and get good compression across each cylinder of roughly 480 psi ±10 but I don't know if it's compression stroke is consistently that good.

 

You could always do it again cold, I would expect it to be a little lower, but that looks very good to me.

[thematt]

Cheers Tony, I appreciate it. I'll report back in a day or so.

[thematt]

Bubble tested at temperature. bubbles much reduced but not absent. Then I got smart. I have a very sensitive CO detector. It was able to pick up 4ppm over the bolts that were bubbling. Other Areas of the engine, no detectable CO2. Basically confirming combustion gases. Wish me luck for my first head gasket replacement!

Edited April 15 by thematt
grammar

[MtB]

8 minutes ago, thematt said:

Bubble tested at temperature. bubbles much reduced but not absent. Then I got smart. I have a very sensitive CO detector. It was able to pick up 4ppm over the bolts that were bubbling. Other Areas of the engine, no detectable CO2. Basically confirming combustion gases. Wish me luck for my first head gasket replacement!

 

Point of Order M'Lud...

 

CO is carbon monoxide. CO2 is carbon dioxide. Which were you testing for? 

 

If you were testing for CO2, then 400ppm is about the level of CO2 in normal atmosphere!

 

 

[thematt]

Testing for the molecule stated :)

 

And yes I picked up 4ppm, not 400ppm. The CO2 sensor on the boat is reading 750ppm right now because I was jumping up and down the companion way.

 

Attached a photo about 5 seconds after moving it away from the bubbling bolt.

 

[thematt]

Okay head is off! things looks pretty good to my inexperienced eye. First photo with the old gasket in place and second with the gasket removed and first round of cleaning. 

 

I'm planning on cleaning some of the carbon of the top part of the cylindars (the rest is smooth with evident cross hatching), but do I need to worry about any gunk getting between the pistons, rings, and cylindar wall?

 

I had assumed the oil/combustion will take care of anything small and large debris I'll vacuum up. But I have just seen some folks recommending to put thick grease on the cylindar / pistons to block anything from getting between the piston and cylindar. Have I endangered my engine by missing this simple thing?

 

 

[Tony Brooks]

Do not clean the full top of the piston, leave a ring of carbon around the top, say about 1/4" wide. This stops damaging any carbon build up between on the side piston crown that is said to improve the seal. 

 

I have never greased the bores, but when using abrasive cloth to clean the head face, I always stuffed the bores with rag.

 

Edited to add: Within the details I can see in the photo, I can't see any indication that the bubbling around the stud was a failing head gasket, although photos give limitations. On the 1.8 the most common failure point is between cylinders 1 & 2 or 3 & 4, between 2 & 3 is less common. So well away from the studs.

Edited April 18 by Tony Brooks

[MtB]

 

On 15/04/2026 at 20:50, thematt said:

Testing for the molecule stated :)

 

 

CO2 then eh?! 

 

 

 

On 15/04/2026 at 20:32, thematt said:

Other Areas of the engine, no detectable CO2.

 

[thematt]

10 minutes ago, MtB said:

 

 

 

CO2 then eh?! 

 

 

 

 

🤦I did not catch my typo twice. and then fully foot in mouth replied. Apologies,  and nice catch. I meant CO for all above statements 😅

1 hour ago, Tony Brooks said:

Do not clean the full top of the piston, leave a ring of carbon around the top, say about 1/4" wide. This stops damaging any carbon build up between on the side piston crown that is said to improve the seal. 

 

I have never greased the bores, but when using abrasive cloth to clean the head face, I always stuffed the bores with rag.

 

Edited to add: Within the details I can see in the photo, I can't see any indication that the bubbling around the stud was a failing head gasket, although photos give limitations. On the 1.8 the most common failure point is between cylinders 1 & 2 or 3 & 4, between 2 & 3 is less common. So well away from the studs.

 

Thanks Tony, 

 

Well I hope there's nothing bad stuck between the cylindar wall and piston. I'll do my best to remove any debris and cover them for further cleaning.

 

I'm taking the head to a shop to get professionally cleaned and resurfaced so I'll let them worry bout the head cleaning parts :)

 

I agree there's nothing obvious but those bubbles had to be coming from somewhere. I think this has to be the last thing I check for my sanity lol.

[MtB]

47 minutes ago, thematt said:

🤦I did not catch my typo twice. and then fully foot in mouth replied. Apologies,  and nice catch. I meant CO for all above statements 😅

 

Lol. I don't highlight these things to score points, but to save future readers trying to learn and finding themselves deeply perplexed like I did years ago at the start of the internet, when I spent literally HOURS trying to work out why some people wrote kW and others wrote kWh for apparently the same thing. I could NOT figure out why until I realised they were a mixture of typos and posters who didn't understand there was a difference! 

 

Nowadays I see the same thing on here from time to time, and also with CO being confused with CO2....

 

 

[Tony Brooks]

FWIW, I am not sure checking for CO above an engine as opposed to in the exhaust is a particularly reliable way of diagnosing head gasket problems, especially on a diesel and with such a low concentration. The vast majority of diesels, in effect, run a very weak mixture that limits the amount of CO produced in combustion, BUT at idle the amount is likely to be higher than at moderate speeds. As the 1.8 is an indirectly injected engine it should produce less CO at low speeds than a direct injection engine of a similar design vintage.

 

As I said up thread, the usual way is checking the coolant for hydrocarbons, not CO.

 

However, if it gives the OP peace of mind the time/cost of changing the gasket seems minor to me.

 

 

[Peugeot 106]

I had a problem where a tiny amount of oil was escaping though a bolt thread on a crankcase. The tapping was a through tapping and the oil was following the threads. Threadlock solved this. Bearing in mind that I have no idea how combustion gas could get onto the head studs or familiarity with BMC engines and can’t claim much experience could putting a small bit of thread lock on the offending stud (the bit that threads into the block) be worth a try when you reassemble. I’m assuming you could use a low strength threadlock so that you can get the stud out again if you need to 

[Tony Brooks]

1 hour ago, Peugeot 106 said:

I had a problem where a tiny amount of oil was escaping though a bolt thread on a crankcase. The tapping was a through tapping and the oil was following the threads. Threadlock solved this. Bearing in mind that I have no idea how combustion gas could get onto the head studs or familiarity with BMC engines and can’t claim much experience could putting a small bit of thread lock on the offending stud (the bit that threads into the block) be worth a try when you reassemble. I’m assuming you could use a low strength threadlock so that you can get the stud out again if you need to 

 

It certainly would not hurt to do that, but the drilling for the oil feed to the rocker shaft is at the opposite end of the block, I think, so that seems less likely, but never say never.

 

The offending stud is close to the pushrod hole, but unless the crankcase breather was horribly blocked, I can't see that having any bearing on the bubbles.  

Edited April 18 by Tony Brooks

[MtB]

Focusing back on the original problem reported, i.e. lumpy running when cold, I cannot imagine the microscopic volume of gas leaking up that cylinder head stud being even slightly responsible, CO in it or not. Also, the OP says "misfiring" which is an odd term to use for a diesel relying on compression ignition. I'm no diesel expert but I'd imagine a diesel 'misfire' would either men compression so low that the fuel doesn't ignite at all, or more likely, the spray pattern or fuel pressure is wrong until the engine starts getting warm. 

 

 

 

[thematt]

6 hours ago, Tony Brooks said:

The offending stud is close to the pushrod hole, but unless the crankcase breather was horribly blocked, I can't see that having any bearing on the bubbles.  

Small correction the offending studs are close to cylindar 1 and between 2 and 3 on the opposite side of the push rod holes. There's not much over there except for some small coolant channels that seemed all much blocked. I gentally cleared these and will do a quick coolant flush when I'm done with this project.

 

8 hours ago, Tony Brooks said:

FWIW, I am not sure checking for CO above an engine as opposed to in the exhaust is a particularly reliable way of diagnosing head gasket problem

 

True, however it is a very sensitive sensor and only picked anything up over the offending bolts. Even moving the sensor to a different bolt or to the exhaust the reading when back to 0.

 

This may be a big peace of mind project, but with the soapy bubbles that reduce as the running evens out combined with picking up possible CO, I had to check. This 1.8 is powering an offshore ocean going vessel. If I were staying coastal this season I probably would have just waited and observed if anything got worse.

5 hours ago, MtB said:

Also, the OP says "misfiring" which is an odd term to use for a diesel relying on compression ignition

 

I ended up paying for some mechanics ears to tell me if I'm crazy. The lumpyness is very obvious in person but doesn't come through in video well. Mechanic called it a misfire and reconned it's air in the fuel line somewhere. I bled the snot out of the fuel line for hours and nothing changed. Got my injectors tested and they showed good too. Injection pump is on 1 season after a rebuild. The last things I can think of are the head gasket and timing.

 

The engine runs with no smoke after the initial puff when started and once at temp smoothly so that seems to rule out timing. So I was left with head gasket and the possibility that I caught it really early and was lucky no oil / coolant seemed to be mixing and no gases detected in coolant. 

 

At the very least? I'm learning a ton. And mad respect to folks who do / have done this day in and day out. It's hard work and I'm very sore!

[MtB]

24 minutes ago, thematt said:

I bled the snot out of the fuel line for hours and nothing changed.

 

Run that by us again?

 

You should be getting bright, clear diesel or air. Not "snot".

 

Sounds to me as though your problems might be diesel bug rather than anything mechanical. Or just water in the fuel, even. Water in the fuel definitely makes the engine sound like its misfiring. DAMHIK! 

 

 

Could you bung up some photos of the snot, perhaps?

 

 

Edited April 18 by MtB

[thematt]

Just now, MtB said:

 

Run that by us again?

 

You should be getting bright, clear diesel or air. Not "snot".

 

Sounds to me as though your problems might be diesel bug rather than anything mechanical.

 

 

Ha! Well bled the air out of the bleed points past when no bubbles were come out (There were non to begin with).

 

Doubt it can be a bug for me. Fuel is 2 weeks old on a brand new tank. The problem existed before and after tank replacement and fuel is treated with bioside. Filters are in good shape and vacuum guage show good vacuum on a fresh filter. I can replace the secondary filter for the hell of it. Worth a shot at this point.

[MtB]

2 minutes ago, thematt said:

Ha! Well bled the air out of the bleed points past when no bubbles were come out (There were non to begin with).

 

Doubt it can be a bug for me. Fuel is 2 weeks old on a brand new tank. The problem existed before and after tank replacement and fuel is treated with bioside. Filters are in good shape and vacuum guage show good vacuum on a fresh filter. I can replace the secondary filter for the hell of it. Worth a shot at this point.

 

Hopefully the filter will be full of snot, paradoxically. That would be an easy win!! 

 

 

[thematt]

6 minutes ago, MtB said:

 

Hopefully the filter will be full of snot, paradoxically. That would be an easy win!! 

 

 

I don't even know what my feelings would be if it was just that after all this work. 🤣

Edited April 18 by thematt
autocorrect

[MtB]

9 minutes ago, thematt said:

Filters are in good shape and vacuum guage show good vacuum on a fresh filter.

 

As a professional fault tracer myself, this sort of statement troubles me. Say how you know the filters are "in good shape" and state what the vacuum measurement was that you class as "good"....

 

"Never assume, always check" is my own personal maxim.