Low RPM/Rough idle when in gear

[Jon1229]

Hi All,

I have a 2001 Mercury 150 saltwater series and about a month ago I replaced the lower unit (not sure my problem is related, but I have a feeling it is). When I shift into forward or reverse my RPMs drop from about 900 to about 600 and my engine seems very unhappy. It shakes a little bit and there is a little bit of knocking noise. Yesterday when I shifted into reverse the boat stalled. When I go up to 1000 RPMs it seems to run perfectly fine (forward and reverse), and when the boat is idling and not engaged with the prop it idles smoothly. It runs nicely from 1000-WOT but idling is concerning to me because it seems like it wants to stall. I don't believe the answer is to adjust the idle, I'm looking to solve it correctly. I'm not opposed to removing the lower unit again or anything of that sort. Thanks for any advice

 

[havoc_squad]

First thing, compression check cylinders.

If compression within spec, check spark plug condition and for spark using gap tester.

I think most mercs are 100 PSI minimum with no more than 10% difference on all cylinders.

Sounds like engine is either way overloaded, your idle is messed up, or dropping a cylinder.

My guess is you might be running lean on one or more carbs at idle speed since at higher idle at load does fine.

If all those things check out and you suspect lower unit, you might want to test it on land with prop OFF at 1500 rpm or lower in gear.

If it is an obvious gear box issue, you should be able to isolate the noise source on land running it on a hose.

 

[Jon1229]

First thing, compression check cylinders.

If compression within spec, check spark plug condition and for spark using gap tester.

I think most mercs are 100 PSI minimum with no more than 10% difference on all cylinders.

Sounds like engine is either way overloaded, your idle is messed up, or dropping a cylinder.

My guess is you might be running lean on one or more carbs at idle speed since at higher idle at load does fine.

If all those things check out and you suspect lower unit, you might want to test it on land with prop OFF at 1500 rpm or lower in gear.

If it is an obvious gear box issue, you should be able to isolate the noise source on land running it on a hose.

Thanks! I’ll start testing it in the morning and post an update when I have one

 

[Jon1229]

Hey havoc_squad,
i ran compression, all cylinders between 118 and 123, I replaced all spark plugs just as a precaution cause why not, tested all for spark and they’re all working fine. On land, the motor doesn’t make noise when the prop is off even at about 7-800 rpms. As soon as I put the prop on there is a rattling/knocking noise coming from the lower unit area. The noise disappears when the rpms are increased. What do you think? Thanks again

 

[Jon1229]

Just drained lower unit gear oil, it had a decent amount of water in it, definitely needs new seals, could this fine the problem or is the damage done from running it with water in it?

 

[Texasmark]

Just drained lower unit gear oil, it had a decent amount of water in it, definitely needs new seals, could this fine the problem or is the damage done from running it with water in it?

Seals and water won't "make" the problem. They already did!

 

[havoc_squad]

I am guessing your bearings probably got trashed due to water intrusion and the extra load is likely from excessive bearing play from that damage.

Lower unit has to come apart for diagnosis.

Unless the problem is only in the prop shaft bearing carrier, you'll probably want to pay a professional that has the tools and training for merc gearcases or maybe pay for an SEI lower unit if available for your model and does not require a lot of expensive changes to make it work.

If everything else is in good condition in lower unit but the bearings, I'd likely say all bearings need replaced as a precaution which would mean complete teardown and re-shiming affected areas to get them within spec.