Easy cold start, hard restart when engine is warm

[brentnsd]

Have a Universal M-25 in a 1984 Catalina 36. The engine was rebuilt, and had new instrument panel and wiring redone. The engine fires right up after 15 second glow plug preheat when the engine is cold. After the engine has run up to operating temp (~160f), and then shut down, it is won’t re-start. Sounds like dead battery, but battery meter reads good at 12.6volts. Alternator and starter are new.
If allow engine to cool down for 10-15 minutes, engine will start without glow plug preheat. Baffled 😕

[kev_rm]

Probably starter or solenoid. Monitor voltage when "trying" to start it when hot. If you get a huge drop, then starter, if not, then solenoid. Also, iffy starter mounted solenoids sometimes behave with a little corporeal punishment from a hammer.

[skipperpete]

I would spend some time eliminating the usual suspects in the electrical system such as bad connections and earths, possibly refit the original starter. Check the battery with a load tester if possible and if all this stuff checks out ok I suggest the possibility of tight pistons or perhaps rings not gapped properly on re assembly.
How extensive was the rebuild? Was it done by a reputable workshop? Have you done a seatrial and put some load on the engine?
Some time ago a similar thing happened to a boat in our marina, a newly rebuilt engine and it simply stopped on the sea trial , was towed back in with a suspected fuel problem and magically restarted when cool and ran perfectly again until about 75°C then stopped again. The problem was that the machinist had made the fit between the cylinder and the piston too tight and as the piston expanded , the friction increased so much it physically stopped the engine. The solution was to re hone the bores to the achieve the. correct clearance, a tedious job that involved stripping the engine to a bare block again and transporting it to the reboring guys workshop. The pistons were scuffed a bit and could have been reused but the owner decided on new pistons and rings( properly gapped) and the engine lived happily ever after.
How hard is it to rotate the engine using a wrench on the crankshaft pulley nut when the engine is hot compared to cold?
I'm hoping it's just an electrical problem

[brentnsd]

Thanks for the input folks. Starter and alternator were replaced after the complete engine and transmission rebuild. New Catalina instrument panel and wiring were installed at engine rebuild. Kev-rm - Will do voltage drop test again.

Sadly, I suspect Skipper Pete is correct about incorrect piston/rings tolerance and assembly being suspect. Will test how hard to manually turn over the engine while cold vs. hot.

I do ave a few hours on the engine now. (The engine oil is still golden.) Was wondering if after enough hours of break-in engine operation, the problem would resolve itself.

Thanks folks. Will post after executing these test ides.

[KC375]

For what it’s worth I’ve had this experience with two engines I rebuilt. They were VW engines (air cooled flat fours – one Westphalia one karman ghia). They would start well when cold but after warming up any engine speed below 1,000 RPM would result in a stall with no restart possible until after about a 15 minute cool down. I was warned of this behaviour before beginning the rebuilds. Apparently the biggest “tightness” was addresses as the rings seated themselves. There was a wide range of advice on how to do the “break in”. The common elements were – gentle use but cover the full range of anticipated engine RPM and frequent oil changes (at least two during break in). The break in period was as predicted but I don’t recall – I think it was 10 to 15 hours of use or about 500 miles. After break in both engines performed flawlessly for many years.

[mikeo16]

I would suspect the newly replaced starter. I have heard this before that the replacement starter ended up being the issue

[senormechanico]

Check to see how hard it is to rock the engine by hand just a tiny bit when cold vs hot. You don't need to turn anywhere near a full turn, just see if it moves as easily when hot as when cold.

[Scubaseas]

Check voltage at the solenoid when it's not starting. Should be the same as at the battery. If not the same voltage look at starter switch or wiring in a relay. If the same voltage I would suspect the starter brushes hanging up or a bad armature or loose shaft bushings or possible the solenoid itself.

[NYSail]

When you say sounds like dead batteries leads me to believe it’s elecrical in nature. Use meter and after warming engine up turn off..... assuming it won’t start like with dead batteries chech with meter. I suspect bad elecrical connection somewhere or new starter has defect... wouldn’t a bad rebuild still attempt to turn over as long as power supply was good.

Good luck

[basssears]

What happens when it won't start? You turn the key and... what? Engine cranks but doesnt catch, starter doesnt spin at all but you hear the solenoid click, or no sound at all?

Definitely check voltage while trying to rank, that will tell a lot.

[BruceS]

Bad rebuild for sure. Insufficient ring gap or oversized pistons in standard bore?

I had one at one stage. Rigged up a thermostat bypass & opened it to cool down the motor before switching off.
Don't ever switch off straight after a long hot run as the heat in the cast block continues to be 'released' but no cooling water flowing to remove it.
Watch the temp gauge rise after you kill the motor?