Diagnosing Starting Failure Post Oil Leak / Possible Overheat

[Alex Alvarez]

Hi all,

I'd love help in diagnosing an engine issue Yanmar 2GM20F, injector nozzles with ~80hrs runtime.

Backstory:

I started the season strong with my oil filter spinning loose and leaking oil until the pressure alarm went off and engine stalled out on the 2nm trip back to the marina upwind in 25knt winds. I tightened the filter back on, filled the oil, and got her to start again. She ran perfectly find for about 15 minutes before stalling out again, and not being recoverable. No oil leak the second time. Engine was hot, but not completely scalding to the touch.

Notes of interest:

- Inspected fuel tank, and no growth (have previously cleaned her after fuel gunk in filter stalling issue, no RPM oscillations indicative of fuel issues)

- Banjo bolt on Injector #1 high pressure fuel line sheared when attempting to inspect fuel delivery at injectors (maybe it was already compromised, and the system can't hold pressure because of this?)

- After cranking attempts we get wispy light grey smoke that smells of light rubber burning from air intake (with silencer removed). see https://drive.google.com/file/d/1boQKOglVvbXpfPRrYgP2sWyTl-jAiR3z/view?usp=drivesdk

- Attempted to start with decompression lever open on injector #1 post shear, with no dice.

Please share any ideas to help diagnose before I attempt troubleshooting with a new banjo bolt, and then later take her down to the block to see how much damage was done.

Example cranking video (post banjo bolt shearing)

https://drive.google.com/file/d/1boQKOglVvbXpfPRrYgP2sWyTl-jAiR3z/view?usp=drivesdk

Thanks

Alex

Patica, P 31-2, Hull #120

[Dave Cole]

Nothing will happen until you replace that banjo bolt.  Can you get the stub out of the injector?

You said the engine was hot, but not scalding?
How hot?  Do you think it overheated?
The engine originally stalled? 
Due to low oil pressure or overheat?

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[Michael Cyr]

Can you crack open the other injector's bolt?  If so, release both compression levers and spin the motor (10 seconds max with 30 seconds wait between) and see if you are getting fuel pumping out.  If not, loosen the bleeder nut on the top of the fuel filter on the engine. Pump the lever on the little fuel pump until you get fuel from the bleeder.  Then open the bleeder on injection pump if there is one, otherwise crack open the banjo coming into it. PUmp again. Fuel?  If not, the hose/line between the filter and the injector is clogged. I've seen it happen. If you get fuel before the injection pump but not before the injectors then the issue is the injection pump or lines to the injectors. My issues tend to be between the tank and the first pump, e.g., water separate, fuel pickup.  I installed a polishing system last spring and things have been good since.

Mike in Maine

[Dave Cole]

I made a mistake.  The banjo bolt you broke is the return line (drain) from the injector.  The pressure line from the injection pump is on the side of the injector.  So your engine should run with that banjo bolt broken, but it would leak fuel.  Follow the procedures to check for fuel pressure/ blockage and/or bleed the fuel system.  

[Alexander Alvarez]

Thanks for the ideas all. 

I dont think the engine overheated but I am not putting it out of the question. It ran for 10min at ~2000rpms, and later 5-10 min at 2600-2800rpms before the oil pressure sensor went off and then stalled out by the time I went down below to inspect the engine room. The engine stalled presumably because of lack of oil pressure? I feel ok about it since it recovered and ran fine after topping off the oil. I could not tell you why it stalled the second time.

I wasn’t getting fuel leaking from the injector return as I expected, but yes from the pressurized fuel from the secondary filter bleed knob. When I get the parts ordered in I’ll do a more thorough bleed and fuel delivery test.

Assuming we get fuel delivery, but no starting then the issue would have to be with compression right? Piston rings or gaskets both creating a good seal.

I’m in for a fun adventure.

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[Michael Cyr]

Yes, but if the engine just stalled, and you can still turn it, then it's likely a fuel issue. Compression usually doesn't just disappear. However, if it overheated, or in other rare cases, you may have damaged a valve or the head gasket. There's a guy that makes compression tester adapters for the GM engines. Not expensive and you can pickup a diesel compression tester at Harbor Freight. The adapter goes in place of the injector. Easiest way to get injector out is just to loosen, but not remove, the retaining saddle on top and then spin the engine with the starter with the compression levers closed (after removing the fuel line to the injector). The compression "should" pop the injector free.  But I'm still betting on fuel.

Mike in Maine 

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[Dave Cole]

I agree with Mike.  

Also, make sure your cranking battery is in good shape.  You need sufficient rpm while cranking to fire the engine.

Dave

10M #26

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[Last Resort]

That's a very good point. I had an old YSM8 and it stalled out in rough water one morning.  I had replaced the original starter with a knock off one and it seemed to be working ok, but it took a bit to get things started once in a while.  After the boat stalled, I towed it to a marina and was finally going to let someone else see what they could find, and I "never" do  that :)   I had to wait, so while I did, I decided for "some" reason to put the original starter back in so the mechanic wouldn't have to see the knock off.  Once replaced, I just hit the start button for a second to be sure it was working, and DAM!!!!  the engine started!!!   It turned out the knock off didn't have the same kind of starting power that the OEM did....lessen learned.  I now get the OEM's rebuilt when needed and use a Bosh knock off when necessary.  I had left the last one in being lazy and why, it was working.  But for the second time with a knock off, the end covering the starting wheel broke off.  Both times I was lucky enough to get the piece out without any other issues. Sot OEM in good shape for starters is the way to go!

[Guy Johnson]

I'll share that I was having trouble starting my atomic 4, sometimes it just clicked the solenoid other times it started right up after checking all of the connections and still having issues I decided to replace the starter. The starter bolts were loose, preventing the starter from making a good ground. 

Guy

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Subject: Re: [pearson ] Diagnosing Starting Failure Post Oil Leak / Possible Overheat

 

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[Joe May]

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[Dave Cole]

It doesn't take much.  

I had a truck that didnt want to start.  It would crank but not full speed.  Its a diesel.

Load tested the batteries, they were fine.  Cleaned all the cable connections, looked ok.  Got underneath the truck and the starter, which was a replacement, had a steel stud for the electrical connection on the solenoid.  It was copper plated steel and it had rusted.  The stud and cable connection was hot from cranking the engine.   I had a spare starter from the same manufacturer and it had a solid copper stud! (Argh!).  Installed the spare starter and all starting problems vanished.  There was just enough resistance in the starter stud connection to slow the cranking speed so the engine wouldn't start, even with a jump.

Dave

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[Alexander Alvarez]

Alrighty. Banjo bolt fixed, fuel lines bled and attempted start.

Almost start without both injectors. Smoke from air intake.

Rear injector decompressed, no start. More smoke.

Front injector decompressed, start and runs. Gray smoke and black water exhaust. Some fresh deisel in exhaust water. As soon as we close front decompression, engine stalls.

Seems like we have issues in one cylinder.

Any insights before I go deeper into inspecting the head?

Alex

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[Michael Cyr]

Wow, that is VERY interesting. The only thing the decompression lever does is lock the valve in the open position and hence no compression in the cylinder.  Nothing else.. which means that even with the level compressed (compression disabled) on cylinder one, the injection pump is still trying to send fuel to both cylinders - hence some diesel in the exhaust water. 

Does it stall slowly and quietly or does it just slam to a stop or make any other strange noises? If the injector is popping prematurely, and it stops suddenly, then it might be a case of too early ignition. I can't think of a reason why simply having compression on cylinder one would interrupt fuel delivery or compression on cylinder 2. Hence I'm thinking that compression and/or partial combustion on cylinder #1 is causing a pressure issue that is preventing cylinder #2 from moving properly.  

First thing I might try is to swap the injectors between one and two and see if the behavior follows the injector. 

Here's an AI's opinion.  Lots of helpful info here.

Based on your detailed description, the Yanmar 2GM20F engine starts and runs on cylinder #2 alone, but stalls or won’t run when cylinder #1 is allowed to compress. That strongly suggests that cylinder #1 is introducing a mechanical or combustion problem that actively interferes with engine operation.

Here’s a breakdown of likely causes:

---

🧩 Diagnosis Summary: ✅ Confirmed:

• Fuel system bled, fuel reaching injectors

• Engine runs on cylinder #2 alone

• Adding compression to cylinder #1 kills the engine

• Cylinder #1 cannot run the engine alone

---

🚩 High-Likelihood Causes 1. Severe Valve Timing or Valve Train Issue on Cylinder #1

• If the exhaust valve isn't closing properly, or the timing is off, cylinder #1 may:

  • Build little or no compression

  • Cause a reverse pressure pulse in the intake or exhaust

  • Act like a "brake" when allowed to compress

👉 Clue: Since the decompression lever lifts the exhaust valve, a stuck-open exhaust valve (or one that’s not sealing) could let the engine run when lifted — but choke it when forced to seal.

Check:

• Valve lash/clearance on cylinder #1

• Rocker arm, pushrod, valve spring integrity

• Bent valve or worn cam lobe

---

2. Stuck or Damaged Piston Rings / Scored Cylinder

• If cylinder #1 has very poor compression due to:

  • Worn piston rings

  • Broken ring

  • Scored cylinder wall

…it won’t contribute power and may create drag when compressed.

👉 If compression is very low, the engine won't fire that cylinder — but now has to work against its resistance.

Test:

• Do a compression test or leak-down test on both cylinders.

• If compression on #1 is much lower, that’s your issue.

---

3. Fuel Injector Issue on Cylinder #1

• Even if fuel reaches the injector, the nozzle may be:

  • Clogged

  • Dripping (not atomizing)

  • Not firing at correct timing or pressure

This can cause misfiring or fuel wash, contributing to loss of power and even stalling when the cylinder loads the engine.

Test:

• Swap injector #1 with injector #2 and see if the problem follows the injector.

---

4. Internal Mechanical Damage

• If cylinder #1 has:

  • A bent connecting rod

  • A damaged piston crown

  • Or timing gear damage, it may allow it to move but not contribute power — or worse, resist it.

This is less likely than valve or injector issues but possible if the engine overheated or hydro-locked previously.

---

🔧 Recommended Next Steps:

1. Check valve lash and verify the valve train moves freely.

2. Run a compression test on both cylinders.

3. If compression is low on #1, do a leak-down test to isolate valve vs ring issue.

4. Swap injectors to rule out injector failure.

5. Inspect the pushrods, rockers, and valve springs on cylinder #1.

---

🧠 Conclusion:

The fact that enabling compression on cylinder #1 kills the engine points to a mechanical or combustion fault in that cylinder — most likely a valve issue or very poor compression, not just a fuel delivery problem.

Mike in Maine

[Alexander Alvarez]

Tomorrow’s plan of attack:

Swap injectors to determine if injector or cylinder issue.

If not injector, remove rocker cocer and inspect rockers.

If no problems there, go deeper to inspect cylinder, piston and gaskets.

Alex

[Michael Cyr]

Yes, that's a good plan IMHO

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[Alexander Alvarez]

It stalls within 3 seconds of dropping the decompression lever. Nothing too agressive, but definitely as if there were some mechanical forces saying nope, time to stop running.

Alex 

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[Michael Cyr]

I'm very interested to hear whether the problem follows the injectors after you swap them. Mike

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[Michael Cyr]

It occurs to me that the compression, without ignition, of cylinder #1, could just be robbing cylinder #2 of enough inertia/power that it can't keep running. That's why it runs with cylinder #1 decompressed. Anything that would prevent cylinder#1 from normal operation, valve issue, broken ring, mistiming, injector popping at the wrong time or not at all, could be the root cause.

Mike

[Dave Cole]

I think Id pull the valve covers and see whats going on.  Turn the engine over slowly and make sure things are moving properly.

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[David Lidrbauch]

https://youtu.be/qp056LYvdRc?si=l0YCfapfzZNcPAng

This video may be helpful 

-David Lidrbauch

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[Alexander Alvarez]

Thanks all..

AM experiments revealed rocker timings are all out of whack. The exhaust of one cylinder is opening with the intake of the other but vise versa is not happening. Intake stays closed and opens randomly sometime later unsyncronized with the other cylinder. Oddly enough it’s on the cylinder that is working

Tappet getting stuck or something more dire. Taking the head off after lunch to take a peek.

Thank you all

Alex 

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[Alex Alvarez]

Here’s an upload of the rocker valve  timings after pulling the head and lapping the valves. The rear exhaust valve took a lot of effort to get to budge and had some corrosion on it. 

I’m still not liking the way the valve timings aren’t symmetrical but I haven’t been able to find a video of valve timings for this engine to compare against

https://drive.google.com/file/d/1ixyNoYAX2NUTKGvPRwoBtW-pXks8Q-0C/view?usp=drivesdk

[Michael Cyr]

I think it looks weird (valve operation symmetry) because the valve positions are swapped, i.e., the inner two valves are both intake, the outer two, exhaust. So when it looks like the 2nd and fourth valve (from the left) are operating in tandem, that's actually an intake valve on the left and exhaust on the right. I watched your video and focused on one cylinder at a time and the sequence looks good.

Mike in Maine

[Alexander Alvarez]

Mike,

I like the tandem, but we only get it once. Wouldn’t we expect the backmost valve (exhaust), to open in tandem with the second from the front (intake) of the other cylinder as well?

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[Michael Cyr]

It's a little hard for me to visualize so I have to follow it through in my head. We know that the left cylinder's intake is opening at the same time as the right cylinder's exhaust. So that means that the left piston is at the top ready to begin its downward intake stroke.  The right piston is at the bottom, ready to begin it's upward exhaust stroke. On the next 1/2 turn, the left piston will be traveling up on compression to TDC, then firing and moving down on its power stroke. However, the right piston, once it got to the top of the exhaust stroke, starts down with the intake valve open for its intake stroke. Remember, the left cylinder's valves are closed during this time for compression and power.  So, no, they should not be in sync.

Mike in Maine

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[Dave Cole]

How hard is it to pull out the camshaft?  Also check the cam timing to the crankshaft?  You might have a sheared key someplace.  

You're this far into it.  Cant be much more to pull the camshaft.  You might have some wiped out cam lobes as well.

A few more hours of work and you can verify everything instead of guessing what else is, or could be wrong.  

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[Alexander Alvarez]

It’s easy, if the engine is on its side… I suspect cam troubles as well.

Plan is to finish reassembly and attempt to start tomorrow (maybe it was just a stuck valve), and if no success, hoist the engine out and do a proper teardown and service.

As always thanks for everyone’s inputs

Alex

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[Dave Cole]

This engine was rebuilt recently, right?  

Did you replace the camshaft? 

If they dont get the heat treatment right, you can get a soft camshaft.  I had one in an old Chevy van.  The cam lobes were a mess.  

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[Michael Cyr]

Curious to know what you find out.  Do you have a diesel compression tester?  They can be had fairly inexpensively at Harbor Freight in the USA but you'd need an adapter to go in place of the injector. There's a fellow who sells them on ebay and he is very good to deal with. I would do that before commencing with a full tear-down, so as to give you intelligence on the state of the rings.

Mike in Maine

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