Strange RPM/Speed problem

randys58

Rinker 232/Mercruiser 5.0/Alpha drive. I expect this is going to take the mechanic to answer but figured I'd ask anyway. My boat is docked about 1/2 mile up a fairly shallow river (at low tide have near zero depth below prop). Anyway, several times, I've taken the boat out to the bay, and across to Fire Island (4 mile trip). Upon reaching the bay (and end of the 4 MPH zone) I find that maximum RPM is only about 3000 and speed about 15 mph (normal full throttle is 4500 RPM & 40 MPH). All other gauges are normal (oil, temperature, etc). No warning tones and engine sounds normal.

I'll spend a few hours at the beach and expect the same for the return trip only to be surprised the boat is running normal again!

This has happened 3 times, always on the outbound leg of the trip! Also, this may be coincidence but all 3 times the bay was fairly rough on the way out and less so on the return trip. And one other observation; one time while on the bay I was experiencing this 'low rpm/low speed' issue when it suddenly corrected itself mid-trip. About the time that it 'self-corrected' I had been playing with the trim (tried moving it up and then down part way thinking it may be impacting the RPM) but future attempts to trim the drive up or down have failed to 'fix' the issue.

Anybody have a theory what could cause low rpm/mph even when engine otherwise sounds normal, and for it correct itself a few hours later when I restart the engine and head back to my home dock?

The only common factors seems to be it occurs on the outbound part of the trip (where I travel 1/2 mile in fairly shallow muddy river before reaching the open bay) and it occurs when the bay is 'rougher' (waves 1-2 feet; never happened yet in low wave conditions). Neither factor seems to cause the problem consistently.

 

Randy

 

TikiHut2

This comes up so often it almost deserves a "sticky/ top post" with common solutions somewhere.

212rowboat

what year is your rig?

this seems to be a popular issue.. well, not popular.. infamous, maybe.. anyway:

after reading a lot of folks descriptions of similar issues, I'm starting to believe what y'all are pretty much all experiencing is an ignition problem with the ignition module.. it is solid state, and reads the coil counts in effort to provide the right spark.. I'm willing to bet it's on it's way out, as it is showing characteristics of a solid state item breaking down... you know, on again off again without real good explanation or apparent cause.. if that thing is losing the ability to count RPMS, (CKPS), and know when to fire the cylinders (CPS) and shield itself from inference, well then there is your huckleberry.... it will cruise along at the last place it can positively identify the mapping of those two....

if it isn't the module, then I'd be looking at the crank position sensor (CKPS) or the camshaft position sensor (CPS), starting (and likely ending) with the wiring.

randys58

The boat is a 98 but the engine was replaced in 07 (new motor, not rebuilt).

212rowboat

was it replaced with a duplicate engine?

a 98 uses the thunderbolt ignition module I'm holding suspect... if it is a newer electronic fuel injection engine with eight fuel injectors (not throttle body injected), then I am flat wrong, and your issue may be in the PCM, or flat out somewhere else..

randys58

Regarding the engine; it's a 2007 Mercury 5.0 MPI but I'm not certain if it's fuel injection or not.

TikiHut2 posted this is a common problem and should have a 'sticky/top post' designation but I've been unable to find other posts about this. Can someone direct me to other comments relevant to this issue?

 

Thanks,

212rowboat

MPI= Multi Port Injection... it's fuel injected, and relies on a computer to collect data and provide information for it to run- such as when to and time of fire, amount of injection, ect...

check the connections at the PCM- pull the harness off, and reinstall it..

you're in luck that it is a FI/PCM engine, as it can be scanned for specific faults... I'm willing to bet one of your fuel delivery sensors is outbound.. take your pick: throttle position sensor (TPS), or manifold absolute pressure (MAP) sensor... it could also be a vacuum leak, evidenced more by the fact it does it on your outbound leg... an intake manifold leak, where it marries the heads and block, can leak a bit, and then seal itself when the engine warms.. they are hard to find...

I'll tell you something else, too- that I would likely do just because: distributor cap... I've seen them be cracked and have water infiltrate, and cause all kinds of issues from misfires to limiting ability to achieve RPM, just to dry themselves out under the heat of running, and work fine... and it's a devil to diagnose because usually by the time a mechanic gets it, it's already dried.... the only real indication for that is difficulty starting it up... are you having that issue at all?

TikiHut2

Search the archives in the box at the top of the page for a few key words like rpm or whatever. Sure seems like we've seen a few of these threads that have "limited RPM". I haven't experienced it but have read through the posts and it seems to be a variety of things from fuel filters to protection faults that limit engine damage from overheat scenarios. Drew has quite a few ideas to check too.

When it comes to the tricky fix everybody wishes they had a reliable OCD marine mech next door that lived to solve mysteries.

Good luck. Hope you find your gremlin, Mike

212rowboat

for what it's worth, and basically because I'm awaiting emails/calls at my desk, and have exhausted all my 'candy crush' life, I've been thinking about this issue.. :-D

first up, get her scanned.. there may be a code present, and there may not be a code present (but one pending)... I'm thinking you'll need a real time scanner, and somewhat duplicate conditions to recreate your issue for a mechanic.. but here is what I'm thinking, for whatever value this has:

Q*: do you bring the engine to full operating temperature before dropping her in gear?
Q**: have you ever experienced this thing on the return leg or after the engine has been running within a few hours previous?
Q***: have you ever heard a beep at the helm from the warning system?

* if not, I'd look toward a gummed up IAC which isn't allowing the PCM to realize the mapping of the needle, and therefor limiting power- but warming up and releasing gunk before a hard code it tripped... check your Positive Crank Ventilation (PCV) valve to see if it is functional (it should rattle in your hand when you shake it).. a bit of oil due to blow-by or a stuck PCV landing on the IAC can wreak havoc on it..

** if not, check your fuel tank vent for obstruction... I'm thinking vapor lock and limited flow, here..

*** I've not experienced it, ever, but it stands to reason that a pending code, while it is happening and before the ECM/PCM corrects itself, may in fact beep...

randys58

In the end I'll have to take it to the mechanic but would like to be able to check scan codes on my own. Are there relatively inexpensive scan tools available? And where could I purchase one?

212rowboat

you can google for them... they are pricey, no matter the flavor..  cheapest way out is a cable, for around $50, which converts the merc scan port (specific pin out) and software for your windows OS computer for around $250..

when you turn on the key, do you have an alarm? - I'm asking you this to make sure your alarm/beeping thing is working..

**check your trim sensors... they may be sticky.. run the trim to the top (trailer) and to the bottom several times after you start the engines... see if it persists..

my reasoning, in bold, below; shamelessly lifted from the efforts of another poster in another forum:

Warning system Faults, SC1000 Yes/No, Audio Alarm, Available Power %, Description.

(1) Cam Sensor Yes 2 Bp/min 90% Open or short, engine must be cranking to set this fault code.
(2) ECT CKT HI Yes 2 Bp/min 90% Open
(3) ECT CKT LO Yes 2 Bp/min 90% Short
(4) ECT Coolant Overheat Yes Constant 6-100 % Engine guardian overheat condition
(5) EST 1-8 Open Yes 2 Bp/min NA Coil harness wire open
(6) EST 1-8 Short Yes 2 Bp/min NA Coil harness wire short
(7) Fuel Injector 1-8 Open Yes 2 Bp/min NA Fuel injector wire open.
(8) Fuel Injector 1-8 Short Yes 2 Bp/min NA Fuel injector wire short
(9) IAC Output Yes 2 Bp/min 90% Only with rpm
(10) Knock Sensor 1 Yes 2 Bp/min 90% Alarm sounds for 20 seconds in NEUTRAL and indefinitely in gear.
(11) Knock Sensor 2 Yes 2 Bp/min 90% Alarm sounds for 20 seconds in NEUTRAL and indefinitely in gear.
(12) Low Drive Lube Strategy Yes Steady Bp 0-100% Low oil in sterndrive.
(13) Low Oil Pressure Strategy Yes Constant 0-100% Low oil pressure strategy.
(14) MAP Sensor 1 Input High No 2 Bp/min 90% Short, no visual on SC1000.
(15) MAP Sensor 1 Input Low No 2 Bp/min 90% Open, no visual on SC1000.
(16) MAT Sensor Yes 2 Bp/min 90% Open or short in MAT circuit.
(17) Oil PSI CKT Hi Yes 2 Bp/min 90% Short, defaults to 51.7 psi.
(18) Oil PSI CKT Lo Yes 2 Bp/min 90% Open, zero oil pressure.
(19) Overspeed Yes Constant RPM Limit Engine over rpm limit
(20) Port EMCT CKT Hi Yes 2 Bp/min 90% Open, defaults to 32 degrees F.
(21) Port EMCT CKT Lo Yes 2 Bp/min 90% Short, defaults to 32 degrees F.
(22) Port EMCT CKT Overheat Yes Constant 6-100% Overheat condition, 212
degreesF(100 degrees C) limit.
(23) Sea Pump PSI Lo Yes Constant 6-100% Low water pressure strategy, defaults to 43.4 psi.
(24) Sea Pump CKT Hi Yes 2 Bp/min 90% Open.
(25)Sea Pump CKT Lo Yes 2 Bp/min 90% Short.
(26) STB EMCT CKT Hi Yes 2 Bp/min 90% Open, defaults to 32 degrees F.
(27) STB EMCT CKT Lo Yes 2 Bp/min 90% Short, defaults to 32 degrees F.
(28) STB EMCT CKT Overheat Yes Constant 6-100% Overheat condition, 212 degrees (100 degrees C) limit.
(29) Steer CKT Hi Yes No No Open and short.
(30) TPS1 CKT Hi Yes 2 Bp/min 90% Short, signal to 5v+, engine will not start. Refer to data monitor screen.
(31) TPS1 CKT Lo Yes 2 Bp/min 90% Open.
(32) TPS 1 Range Hi Yes 2 Bp/min 90% Above 4.8v, 994 counts.
(33) TPS 1 Range Lo Yes 2 Bp/min 90% Below 0.5v, 35 counts.
(34) Trim CKT Hi Yes No No Short, high range, visual warning on SC1000 only.
(35) Trim CKT Lo Yes No No Open, low range, visual warning on SC1000 only.
(36) 5 VDC PWR Low Yes 2 Bp/min varies Short any 5v+ to ground.

frodo13056

My guess is that the Mercruiser Engine Guardian is kicking in. Engine Guardian is supposed to protect your engine from various issues that could destroy your engine - things like low oil pressure, low water pressure, etc. as pointed out by DrewActual's post above. When Engine Guardian kicks in, it limits your engine RPM and allows you to "limp" home. You mention that you keep your boat in a shallow location - so my guess is that your seawater pickup may be getting clogged with sand or mud, resulting in low water pressure from the seawater pickup pump (condition 23, 25) in Drew's list. The alpha drives have the seawater pump located in the lower unit and the bravo drives have a engine mounted seawater pump so there is a difference and I'm not sure where the seawater pump pressure switch would be located. Anyway, it might be a good idea to replace the seawater pump impeller. You keep the boat in shallow water which means that you are much more susceptible to sucking in sand, mud into the impeller which will score the impeller blades and the impeller housing. The engine may not be running hot (yet) but if the impeller and housing are scored or damaged, you will have less water flow (less pressure) until you get the RPM's up - and you also run the risk of complete impeller failure and you know what that means.....engine overheat.