Unsteady Engine speed

[JRuszkay]

I was wondering if anyone else has experienced the engine speed varying by 1000 rpm or more when running at 6000-7500 rpm. This just started happening on my 2008 Yamaha SX210. It seems to occur during first 20 min of operation then go away. I am thinking it’s dirty spark plugs, gas fuel flow or a wire on starting to fail throttle position sensor. The engine speed will respond if I adjust the throttle position up and down, but then doesn’t hold steady once I leave it alone, until I run for 20 min or so, then the problem goes away. Wondering what is causing this issue.

 

[Gym]

I'd start with the easiest...Spark plugs. When were they changed last. You could also have dirty fuel injectors. A can of Sea Foam in your gas tank should cure that issue.

 

[Dthorpe]

If it is happening simultaneously with both engines, then it has nothing to do with the engines.

 

[JRuszkay]

Thanks for the suggestions. The spark plugs probably have 50 or more hours on them. But I am not sure the problem would go away upon running for 20 min, if it was dirty or worn plugs.

And the problem occurs on only 1 engine, so the fuel isn’t the problem.

Has anyone ever experienced a problem with the computer or throttle position sensor? I was wondering if this could be a dirty or worn TPS that is sending a varying signal to the engine computer. The throttle cables don’t connect directly to throttle body on the engine. Or is it possibly a sticky throttle body?

 

[djetok]

Thanks for the suggestions. The spark plugs probably have 50 or more hours on them. But I am not sure the problem would go away upon running for 20 min, if it was dirty or worn plugs.

And the problem occurs on only 1 engine, so the fuel isn’t the problem.

Has anyone ever experienced a problem with the computer or throttle position sensor? I was wondering if this could be a dirty or worn TPS that is sending a varying signal to the engine computer. The throttle cables don’t connect directly to throttle body on the engine. Or is it possibly a sticky throttle body?

Check your aps plug. On the side wall in the engine bay. I had what I am guessing a similar issue.

https://jetboaters.net/threads/rpm-fluctuation-then-check-engine.23298/#post-397835

 

[Gym]

@JRuszkay. Before you get too deep in the weeds on this. It's pretty common knowledge that these earlier model engines are pretty sensative to spark plugs. Is your 50 hours on your plugs all this season or over two seasons? In any event plugs quite often resolve issues such as yours. Whereas this is a transient problem I would consider a can of Seafoam in a full tank of fuel incase you have a fould injector. New plugs incase you have a fouled plug. A 1000 rpm variance is consistant with losing a cylinder.

 

[mwalker4]

I had a loose coil which caused intermittent issues very similar to your situation. Another form of plug issue.

 

[Art Traver]

My starboard engine idles fine, and seems fine at WOT. When heading into the marina at 3,000 RPM I get a 300+/- fluctuation. Looks like a new set of plugs are in order from reading all the threads. Hopefully that is the fix.

 

[srf32]

I just experienced similar on our '06 SX210 this past weekend. It was intermittent in the starboard engine - rpm would change and the tach needle would toggle up and down ~500 rpm like someone was continually flicking a switch on and off. In our case, appears to be an under voltage issue while running (see my thread about bad port stator), and I got error code 19 (voltage issue) in the YDS diagnostic page. Both engines started and ran fine otherwise on the undercharged battery. When switched to second, charged battery, the problem went away.

Not sure if related to your issue, but may be worth checking voltage and battery connections.

 

[Art Traver]

To further the conundrum we have gone out 6 times since my 8/6/20 post. No further issues have been observed. But, thinking back to July our daughter had a broken leg and when we boated we just "putzed" in the Erie Canal for a month, which is just no-wake speed. So maybe being at essentially idle speed for many hours was draining the battery resulting in the under voltage condition. I haven't hooked up the diagnostics yet, but planning on it.

 

[MidnightRider]

I just had this same issue yesterday. Out of the blue my starboard engine was fluctuating speed all over the place especially at WOT. Took it back to the dock and changed the plugs and all is good again. **I usually change the plugs every season, this last spring I did not cause our use was down last year from previous years - guess this confirms to change plugs every season regardless, these engines are sure sensitive to fouled plugs.

 

[srf32]

Just a quick update to my earlier reply. Turns out my issue was not related to voltage after all and it was just coincidence that it cleared up when switching batteries. Problem soon came back and was persistent.

So, I followed the "...when in doubt replace the plugs..." advice of the "elders" from these forums, and sure enough, problem solved .