Merc 250EFI low power and misfire on garden hose.

[chevy355mark]

Same problem. Fires right up, a sneeze every once in a while, lacks power planing but runs just fine. I’m gonna check the plugs again if there’s not any signs of a lean cylinder I’m gonna live with it.

These outboards are matching years but nothing looks of the same age on the 2 of them and none of the hardware matches. I’m assuming one has a newer Powerhead, they may not have matching port layouts or my older Powerhead may have carboned shut exhaust ports. I don’t know but it doesn’t seem like I’ll be able to fix this. The props may not have matching vent plugs by the time this is over lol

[big tom]

Chevy, I have the same motors as you. There on a 31 fountain. I got too deal on the boat because the compression was lower (90 psi-105 psi) on both motors. After @100 hours of running I started having pretty close to same problems your having. No start/sneeze at cold start sometimes, lost power getting on plane and dropped 600 to 800 rpms on top. But always run smooth. Tried a number of things, but didn't help. I run BBlades props with the 7/8 vent holes with 2 plugs pulled at first. I later had to pull the other two out of the trailing motor to keep up coming up on plane. That's a pain in the azz. The slow motor spins up faster than the good motor out of the hole so than you back that one a little, then you hit plane and the vent holes become useless and it drops rpm again. To much throttle work just to get on plane.
I had a reputable motor builder pull both powerheads before I put it in winter storage. So far I'm hearing I'm very lucky. They were on the edge. The slow one had started soring around the ports in one cylinder , One bank was running lean. He also found the t-stats where stuck open on that motor so it was running cold. We boat lake Michigan and in spring we start when water in the 50 degree's,might have been a bad start to the year if I had let them go another year. I run snowmobiles in the winter and have done some builds and have a buddy who builds mod sled engines look down my cylinders with his Borescope a few times on my sleds to check for lean burn.. Might be a tool to think about owning?.

[chevy355mark]

90-105 psi is by the book spec on these motors. My lower and higher cylinders are a dead match between the two which means it’s just a tolerance or port hieght difference common amongst all them. As it’s been mentioned before the left bank is always lower, some guys cut that head more then the other to even them out. I don’t have any problems with rpm at top end or after planing at all, it’s just the getting up on plane


i can take a look with the bore scope, not a bad idea. Normally it’s very tough to see anything worthwhile with one but what else can I do at this point.

[Merc 2.5]

Pull the head but I can usually see alot of the cyl with just a bend o lite

[chevy355mark]

not to offend anyone as the advice so far has been very good but im sorry, pulling a cylinder head on a running saltwater engine for no real reason doesn't seem wise. Bolts will break off, will turn into an enormous project. I just broke 2 off doing the thermostats and i welded/built some special tooling to tap and helicoil the block all the way through and past the cylinder heads then add studs. this was done on the good engine.

I pulled the lift pump apart and pumped some fuel from the hand primer to verify no blockages and fuel quality seems ok, no water but to be honest it could be better. lift pump checks out ok, had one gasket that was a bit out of place but it didn't leak and it probably was no problem. on to the bore scope and i might think about doing something with my new additional fuel tank, it seems to have some junk in it from the installation process, might need to run it through a filter and back into the tank with an electric pump for a few hours.

[chevy355mark]

for anyone with a similar problem it was in fact a bad tps sensor. Now that I have a real scan tool for these, i ran the boat with the techmate hooked up and found the tps signal was all over the place on the lazy engine. not something that a multimeter could catch on the trailer. swapped them from engine to engine and the problem follows.