One engine running 4k rpms lower

[jebwm]

So I just bought a 2004 sr230 and took it out this weekend. Yesterday she ran like a champ, today...not so much. For about half the day it was fine, then things slowly got worse and worse. When I bought it I was told that one engine runs about 500 rpms lower than the other. Both engines have brand new spark plugs and I checked compression on all 8 cylinders at 170psi. Today I noticed that the one engine started running about 1k rpms lower and by the end of the day it was running 4k rpms lower. It would max out at 6k rpms and every so often shoot up to 8k for a few seconds and then back down to 6k.

I opened up both engine covers and the throttle cables looked fine and had solid even tension on both. The one visual difference that I noticed was that the engine running rough had a very light coating of oil on everything under the cover, including little specs all over the air filter. The good engine was totally clean and dry.

Any ideas what it could be? Bad seal somewhere? I'm no boat mechanic but I self-taught myself to work on my diesel engine over the years so I'm not afraid to tackle a marine engine next.

Thanks in advance!!

 

[Bruce]

I know you said the spark plugs were new but I would start with changing them. Spark plugs frequently limit the RPM of an engine.


Also if the oil was overfilled that can clog the air filter.

 

[jebwm]

I had a guy on another forum bring up the oil possibly being overfilled. I'll get another set of plugs and see if swapping them out makes a difference. Where is the best place to order new parts for yamaha jet boats? Thanks for the quick reply

 

[Julian]

Yamahasportsplaza has Yamaha parts at 25-30% off, but for plugs I just go to Advance auto.

If you think the air filter might have oil on it, try running the engine without the filter for a period (boats didn't used to have air filters because there is no road dust to suck in). If it runs fine, then get a new clean one.

Some of the older boats have had ECU issues related to fuel injectors---do you smell gas or see a sheen of gas on the water from that engine?

 

[jebwm]

Didn't notice a sheen of gas on the water, but definitely smelled a stronger gas odor than the day before when it was running fine.

As far as the air filters, I swapped them out between engines and the engine still ran poorly after that and the good engine still ran great with the oily air filter.

 

[Port0222]

Yamahasportsplaza has Yamaha parts at 25-30% off, but for plugs I just go to Advance auto.

If you think the air filter might have oil on it, try running the engine without the filter for a period (boats didn't used to have air filters because there is no road dust to suck in). If it runs fine, then get a new clean one.

Some of the older boats have had ECU issues related to fuel injectors---do you smell gas or see a sheen of gas on the water from that engine?

That was my issue- I have a 2006 and the ecu was bad, so it had a stuck injector. Constantly smelled like gas, and eventually, the oil was diluted with gas and overfilled into the air filter, which limited the RPMs.

 

[jebwm]

Yikes. I'm afraid to ask, but how much was it to replace the ECU?

 

[Port0222]

$1000 -- http://www.yamahapartshouse.com/oemparts/p/yamaha/6p6-8591a-01-00/engine-control-unit-assy

Note- that was for my boat- I am not sure it is the same part for yours.

You can also buy, or someone maybe near you with a Yamaha diagnostics cable and it can test your injectors, etc.

 

[itsdgm]

Have you checked the oil level and checked for fuel contamination?

A stuck fuel injector can lead to fuel in the oil which will blow out into the air filter. Also, contaminated oil will result in loss of lubrication and severe engine damage.

Anyway, check the oil. I'm hoping for the best.

 

[jebwm]

I'll check the oil when I get home. Does a stuck injector usually trace back to a bad ECU? I'm going to open the rear panel up later too and see if there is any water inside where the ECU is stored.

 

[Port0222]

I'll check the oil when I get home. Does a stuck injector usually trace back to a bad ECU? I'm going to open the rear panel up later too and see if there is any water inside where the ECU is stored.

Getting to the ecu is pretty involved. You have to remove the case that it is in, which requires you to remove the panel above the clean out hatch. And then you need 2 people to undo bolts. It is not a fun process. Watch for unfinished fiberglass too.

 

[itsdgm]

I'll check the oil when I get home. Does a stuck injector usually trace back to a bad ECU? I'm going to open the rear panel up later too and see if there is any water inside where the ECU is stored.

On the 04-06 boats unfortunately yes. I think it's usually the same injector too, but I can't remember which one.

 

[jebwm]

I smelled the oil just now. Honestly, both oil reservoirs and dipstick seem to have a fuel smell to me, however the engine that was running poorly DEFINITELY has a stronger fuel smell to it without question. What's the next step from here? I'm guessing I can't really do much else until I have the YDS set up? My next plan was to swap ECU's but I guess that's out for right now since it's just a pain in the ass. I was wondering why I couldn't just undo the 4 bolts. They just kept spinning and spinning and never came out.

I'm guessing I should definitely change the oil in the bad engine. Also, how do I go about checking the resistance on the injectors? Any graphics or write ups about that?

 

[jebwm]

I also pulled all the spark plugs just because it was easy and I was bored. 7 of the 8 looked normal to me (see first image) There was one plug in the bad engine that was completely black (see second image). It was the first plug closest to the front of the boat if that makes any difference. Also, How the HELL are you suppose to undo the plugs going to each injector to ohm check them? Seems impossible without removing everything else on top of them. Ugh, this is a nightmare, I bought a newer, nicer, more expensive boat because I didn't want to deal with this crap.

 

[jebwm]

Have you checked the oil level and checked for fuel contamination?

A stuck fuel injector can lead to fuel in the oil which will blow out into the air filter. Also, contaminated oil will result in loss of lubrication and severe engine damage.

Anyway, check the oil. I'm hoping for the best.

Got a hose adapter to run the engines out of water and some weird things happened. First off, I put the black spark plug in the engine that was running good, just to see if any symptoms transferred over. I ramped up the RPMs on the "bad" engine and it seemed to be fine, going up to 11k no problem (granted it's not under stress). Then I fired up the "good" engine with the black spark plug and ramped up those RPMs and it held up for a bit at 6k and jumped up to 8k. I repeated that 3 or 4 times but then it eventually cleared itself up and went up to 11k after that. So I figured GREAT!, just need a new spark plug, right? Well the issue of fuel in the oil is still a huge concern for me, and the oil reservoir of the "bad" engine definitely read way higher than the "good" engine (see photo). PLUS, after I finished all this up, I looked at the ground and off to the side, under the jet pump nozzle of the "bad" engine, it was all shimmery and full of gasoline. The other side seemed much much clearer, but to be honest it was all a giant pool of water from both nozzles after running them both for 5-7 min, it just seem to be gathered more on the side of the "bad" engine. I'm at a loss. I'm taking it to someone on Wed, but I'm still open to any advice you all might have for me. Thanks so much for your help on this.

 

[AZDANSX230HO]

My previous 2006 SX230 was extremely finicky with spark plugs, even ones that looked clean, so I always kept spares just in case. I find it hard to believe fuel getting into the oil, only way it could would be if the rings were toast, your compression test shows good. A leak down test might be in order to confirm it's in good shape or not:

https://mobiloil.com/en/article/car-maintenance/car-maintenance-archive/how-to-do-a-leakdown-test

 

[AZDANSX230HO]

If you do have a stuck injector, I would just replace it before messing with the ECU, if that fixes it I would also run several bottles of Techron through it as well to clean the existing injectors. I only use Chevron/Texaco for fuel as it already has Techron in the fuel.

 

[Port0222]

@jebwm this is exactly the same symptoms of my issue. I tried swapping Ecus, but I found it difficult to tell definitively that the ecu was the issue, so I brought it to the dealer and they confirmed ecu. I had them replace it and then do a full oil change, just to make sure all of the bad oil was out. Note that you will still have gassy oil in your muffler, so it may still smell like gas unless you take the muffler off and drain it. Gassy film going from your exhaust means an injector is for sure stuck open. -- now to determine if the issue is the ecu or injector.

Up to you if you want to try injector swaps, and see if it is an injector, but this is not the first ecu to have issues, nor would it be the last. I tried to measure the resistance of the injectors and all of them checked out. I am on my phone, so I cannot find my thread, but search for threads I started last fall ( sept) and there should be a post that showed the resistance check of the injectors.

 

[Port0222]

Here is my post -- with what sounds to be your exact issue:

https://jetboaters.net/threads/help-engine-smoking-and-lack-of-power.7417/#navigation

 

[Gym]

If you find it's a defective ECU there is a place that rebuilds them for $350. I don't have a link to the site. Maybe someone can dig it up.