RPM During Starter Cranking?

Mark V

Member
Oct 12, 2007
60
Lake Travis, Texas
Boat Info
2006 240 Sundancer
Engines
350 MAG MPI, Bravo III
2006 Mercruiser 350 MAG MPI/Bravo III.
800 hours on it, including original starter.

This only pertains to cranking an engine that won't start:
Has anyone ever watched the max RPM achieved when cranking their engine, when it has a problem and won't start? What did RPM you get?

On a related note, has anyone ever heard of a minimum RPM needed to start the engine?


I confirmed mine is only reaching 120 to 150 RPM with all the spark plugs in it and no sputter or signs of starting. (tach at the helm agrees with a calculated RPM I got counting rotor turns for known number of seconds and doing the math)
With all the spark plugs out and a compression gauge on one cylinder at a time, it peaks and holds at 180 to 190 RPM.

Mercruiser Tech Support indicates there is no specification for minimum RPM required to start the engine, nor could they provide a "typical" value.
Various internet boating forums mention RPM values "required" between 250 and 350 RPM needed to fire it up.
 
I don't know the values but my starboard starts at a slow speed from the starter. The port has a new starter and it spins real fast. Huge difference in starting speeds.
 
How old are the batteries?
I replaced mine this year.
Before I did the second battery, I tried starting it a few times on each individual battery. There was a noticeable difference in cranking strength/speed between the old and new battery.
 
Thank you all for the input. Sounds like I might be replacing the starter based on the low cranking RPM, not to mention the desire for added reliability to avoid getting stranded. will decide that soon.
Batteries are about 3 years old. When this problem started I had checked battery voltage at the helm's smartcraft gauge while cranking and it hadn't dropped below 10.5 volts while turning the 150 RPM. Going to to try and get a probe right on the starter positive post and get a cranking voltage there as well, but it is hard to get to. Stay tuned....
 
0BC4D527-269F-4B55-88E3-736494CFE8F2.jpeg

Slightly different take on the RPMS. You can jump start a car by pushing it about 1 mph and popping the clutch. Why would a boat engine be any different? Just asking. Don’t know the answer.
 
But with a car, your popping the clutch at 1-2mph with it in third or second. So it will spin the motor faster and start easier.

And I had a guy I worked with that pull started his auto. Put it in gear and was pulled to 35mph before it fired. Flat spotted the rear tires on that half ton pick up real fast. Not something I would do. o_O This same guy with the same pick up was hot roding one night and capatulted his ass end when the drive line broke loose at the trans. It dropped and hit a joint in the concrete road way. Funniest thing I ever seen him do.
 
But with a car, your popping the clutch at 1-2mph with it in third or second. So it will spin the motor faster and start easier.

And I had a guy I worked with that pull started his auto. Put it in gear and was pulled to 35mph before it fired. Flat spotted the rear tires on that half ton pick up real fast. Not something I would do. o_O This same guy with the same pick up was hot roding one night and capatulted his ass end when the drive line broke loose at the trans. It dropped and hit a joint in the concrete road way. Funniest thing I ever seen him do.
0A426FF1-95AE-45A2-9037-526B87328AA3.jpeg

I never did it second or third gear. Just put it in first and bam, it would start right up within a second or two. All it took was compression and a spark.
 
I always used second or third. It was just easier. I had a Jeep CJ-5 that 1st was a granny low. Anything over .35mph and the gear in the transfer case would come apart. In first and the transfer in low with the overdrive in low, I could get out and walk faster than it would move. But that thing would walk over a house. I was forever leaving that thing with the key switch on. Probably be easier to just replace the switch now that I look back.

Most of the time I was the one pushing. So it was always easier in the higher gears on me.
 
Better late than never-just sharing what I learned to close out this thread.
Hopefully it helps someone rule their starter in or out as part of the problem if their engine cranks but doesn’t start.

Starter Cranking RPM for a 2006 350 MAG MPI Engine with a 555 ECM;
(and other engines, but I did not try to identify them):

These numbers are calculated and assume ignition system is disabled and are most accurate without spark plugs and without the serpentine belt installed, and no unusual friction in rotating the engine.
167 engine RPM minimum spec with a good starter
272 engine RPM maximum spec with a good starter

These values only pertain to the RPM health of the starter motor assuming the rest of the starting system and engine is healthy. (battery, solenoid, ring gear, etc)

How I came up with those numbers:
For my 2006 Mercruiser 350 MAG MPI:
-starter gear has 11 teeth, confirmed visually on my starter. (Merc/GM offset bolt style)
-flywheel ring gear has 168 teeth per Mercruiser tech support based on their engineering drawing that showed the GM part number, then googling that GM part number (10105832).
-No load starter motor RPM range: 2550 to 4150 RPM from Mercruiser Service Manual 31.

11 teeth/168 teeth = 0.06548
0.06548 x 2550 =167 engine RPM
0.06548 x 4150 = 272 RPM engine RPM

With my original 15 year old starter on the engine I measured:
120 to 160 RPM with all the spark plugs in, cranking with the ignition system disabled.
180 to 190 RPM with all the spark plugs out and a compression gauge on one cylinder at a time.
Same 180 to 190 RPM without the compression gauge, no spark plugs and no serpentine belt.

New Mercruiser starter yielded
200 RPM w/o spark plugs, w/serpentine belt.
150 to 160 RPM during no start cranking w/plugs and belt.

Misc Notes:
My Smartcraft tach RPM matched a Rinda Diacom’s RPM.
The Diacom confirmed that my 555 ECM was still commanding the ignition, injectors, IAC, etc. and without any activation of the Guardian system even at the lowest RPM I observed during no-start cranking when the Diacom was connected: 150 RPM.
 
I never really paid attention until recently when my starboard engine was cranking significantly slower than the port engine. So I'm seeing 160 on the port which is my normal cranking engine. It turns out batteries are tired and when I leave the charger on starboard engine cranks at 150 but much slower 120 if I don't plug the boat in for a couple of days.
 
An EFI engine will start at 10 RPM if you wanted.... All they need is enough current and voltage to pressurize the fuel system, fire the injectors, fire the ignition coils, operate the idle air motor, and detect crank position. They need to turn over a couple of times so the computer knows where the crankshaft position is. If your engine is cranking slower than normal and it's EFI then probably there is too much of a voltage drop to operate the EFI to get it started (the current to operate EFI isn't incidental) - Battery or battery cable problem. I would measure the battery voltage at the EFI B+ during cranking.
 

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