2020 R5 bucks like a mule

[Luv2HuntAR]

After starting the engine, it bucked like a mule, shuttered, then purred like a kitten - all in a 5 second period. My issue happened twice over the weekend. Once after watching an incredible sunset at Lake Ouachita then again before running up my lift. Each time, I had been running at 1200 - 900 RPMs for an extended time just before shutting down. Other than the above, I haven’t noticed any other vibrations or performance issues. Is this issue indicative of a bad U-Joint?

[billk2632]

I can certainly tell you it's nothing to do with U-Joints or anything like that. This happened on a hot start vs a cold start in the morning? My R5 also with VP is older, but I have had it stumble like that on a hot start - I think on mine it was the start of vapor lock. That's when fuel in the fuel lines starts to vaporize due to the heat soak of the hot engine in an enclosed engine room - it can cause a no start condition because the fuel pump cannot pump vaporized fuel -- once cool, things work normal. I always turn my key on and let the engine do it's initial checks and the gauges come alive. Also allows the fuel pump to build pressure before you start it. This is somewhat of a guess on my part and it could be a bigger problem related to the various sensors that determine how to set the fuel trim based on the start condition - ie engine hot, cold etc. If it is only happening occasionally and in hot conditions, I would not worry about it.

[Luv2HuntAR]

They were “hot” starts at the end of the day. I usually wait a bit after turning on the ignition before engaging the starter …. Usually a few seconds after the “beep”. When I say she bucked like a mule, the starboard side of the stern raised 3”-4” first and then it moved to the port side; all of this transmitted through the entire length of the boat, and then it was over. The last time it happened we had been out all day, and she sat in the sun on a hot day for over an hour before I ran her up the lift. No response from my dealer yet.

[Krob1114]

That picture is what my wife is always looking for. Thanks.

[jhnmdahl]

HuntAR, there's a good chance it's storing an ECU code that could help identify what's going wrong. May be worth swinging by the dealer with the boat on a trailer sometime to have them hook it up and see what's up. Beyond that, vapor lock is possible, but modern marine engines tend to run cool enough to mitigate most of that. Perhaps keep an eye on engine temps next time you're out and about, and see if you see anything unusual.

[Luv2HuntAR]

jhnmdahl ... Thanks for the comment. I did not think about it throwing a code because the engine started each time; I will ask Dealer to check it out. Dealer did tell me flatly that there are no vapor lock issues on Cobalts; if there was, it would not start. Dealer threw out new spark plugs. She runs great at idle and while underway .... Hot or Cold, she starts every time I turn the key. Vapor Lock? Spark Plugs? Doing the math here makes me think I might have some bad gasoline. All three? May be an expensive rabbit hole for me. Except for a little bucking a couple of times, she runs like a champion.

[bruceb58]

You need to get it scanned. My boat had an occasional miss after I bought it. I asked the previous owner if he had the issue and he of course said no. Before I bought my scanner, I replaced the 4 coils(Mercruiser) and it fixed the issue. When I later bought a scanner, I found a lot of history of misses before I bought the boat. Moral of the story...either have it scanned or buy a scanner and of course...never believe a previous owner.

[billk2632]

Agree probably should get it scanned before you throw any parts at it. I feel it's fuel related, as if it's sensing a cold engine when it is hot or something like that - but that's really not much more than a guess.