Yanmar 3GM hard starting after injector service

[skunther]

It’s my mistake - I meant IP, not lift pump. Sorry!

[skunther]

That 5-7 seconds of cranking was a maximum number, for example in the spring after a winter lay up. During the summer probably more in the range of 3-4 seconds when cold. If it had been run in the past few days, it would start up immediately. Now, after work performed, it won’t start in three consecutive attempts of 10 seconds each.

[Wotname]

Originally Posted by skunther

That 5-7 seconds of cranking was a maximum number, for example in the spring after a winter lay up. During the summer probably more in the range of 3-4 seconds when cold. If it had been run in the past few days, it would start up immediately. Now, after work performed, it won’t start in three consecutive attempts of 10 seconds each.

OK, that makes sense so please disregard my previous post. If there has been any leaks (as reported) on the high pressure side of the injection process (either at the pump or at the injectors), then such leaks could account for slow(ish) starting. This is especially confirmed by the fast starting if the engine has been used in the previous few days.

It really does sound like the service guys have messed up the injection pump service or the re-installation. It shouldn't be too difficult for a competent mechanic to find the fault(s).

As DIY guy, I would check the injection timing as well as confirming the injectors are popping fuel (and not leaking) when the engine is turned over. A professional might have a quicker / better approach though.

[skipperpete]

Originally Posted by skunther

It’s my mistake - I meant IP, not lift pump. Sorry!

Ok, that makes some sense…. But can you elaborate on the “low compression” that the pump tech mentioned? A savvy ol diesel mechanic might have opened up the maximum fuel stop to replicate a normal cold start ( volvo penta or ford Lehman style) if you have really low compression. The proper fix is a valve grind and a set of rings

[skunther]

I meant Injector pump, not lift pump. My mistake. I’m unfortunately not able to edit the post to correct it.

[skunther]

Low compression was in reference to the engine cylinders. 160-170-160.

[Wotname]

Originally Posted by skunther

Low compression was in reference to the engine cylinders. 160-170-160.

Whoa - are those numbers in psi?

If so, they are waaayyyy too low. They should be around 400 psi. Either they were measured incorrectly or there is a major leak somewhere. The only good thing I can see is that they are consistent across all 3 cylinders which might suggest incorrect measuring technique or the same issue with all 3 cylinders.

[skunther]

Yup, PSI. They said it should be ~250, but that spec for 3GM is vague. Either way, the engine started up fine with that compression before work was performed, so I don’t believe that’s the issue, though as someone else pointed out to me, low compression could be exacerbating another issue.

[Wotname]

Originally Posted by skunther

Yup, PSI. They said it should be ~250, but that spec for 3GM is vague. Either way, the engine started up fine with that compression before work was performed, so I don’t believe that’s the issue, though as someone else pointed out to me, low compression could be exacerbating another issue.

Well 160/170 is way less than ~250. Compression testing is a bit of a black art and there are many variables but I would seriously challenge the ~250 claim. Maybe the guy doesn't have a suitable gauge age or proper technique and gets ~250 when he does the test but 3GM30 will be hard (if not impossible) to start with reading of 250psi let alone 170psi.

The theoretical value is given by the compression ratio which we know is 23 for a 3GM30. Normally actual compression is between 15 and 20 times the compression ratio which gives us a range of 345 to 460. As a DIY guy, I expect to see consistent numbers around 390 to 420 dependent on gauge, rotation speed etc.


Some numbers from Yanmar for their industrial engine (3TNV88F) which is similar to a 3GM30 https://www.manualslib.com/manual/12...?page=1#manual

[Wotname]

I asked chatgpt for some guidance.

Q. What is the compression in psi for a Yanmar 3GM30 engine?

A. The compression pressure for a Yanmar 3GM30 engine should typically be around 400-500 psi (pounds per square inch) when measured with a compression gauge. However, it's important to note that actual compression values can vary depending on factors such as engine condition, wear, and maintenance history. It's always best to consult the engine's manual or a qualified mechanic for specific compression specifications and testing procedures.

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Was this the same mechanic who pulled the injection pump, had it serviced (by whom) and refitted it without confirming the timing?

I getting picture of this guy and it ain't positive...