3 liter EFI Backfire on Startup

[Proline25]

<0T666332>

New member here but not new to outboard motors and boating.
I have a 2004 250 EFI. Bought the boat June 2020 from original owner with about 200 hrs on the motor. The boat and motor had been sitting for about 5 years. He ran the motor and had a little fresh fuel in the tank. The first thing I did was clean and scope the tank. I removed and cleaned the injectors. Cleaned cooling system and installed new thermostats and pump. Using Mercury Vessel view, the motor temp runs about 120* on plane and up to 125* max. The motor looks like new inside and out. I had an intermittent issue with the motor and finally found a bad coil was causing the issue.
Within the last month I noticed a single exhaust backfire at startup, hot or cold. I replaced the plugs and checked compression, as always twice a year. I checked the throttle adjustments and TPS. Motor starts easy and runs great at cruise, about 4100 rpm. I was told to increase the TPS a small amount and open the throttle plate a tiny bit. That is not adjustable on this engine with the single throttle body so I am at a loss as to the next move. I did notice a small amount of exhaust smoke coming from the bottom, front of the cowling after the backfire. The exhaust hose at the back of the motor to cowling is good. This engine didn't have the startup backfire for first year that I owned it. It has just recently started doing that and it does not seem normal. Anyone here have experience with that issue?

[Jasonrs]

Have you checked the reeds?

[EuropeanAM]

Check all of the bleed lines and the air handler gaskets. Any air leak will cause a lean condition in the affected cylinder (and can result in backfiring).

If you would, please take a moment to review the Announcements at the top of the Forum, and be sure to provide the Required Info (serial number) when posting.

[Proline25]

serial #ot666332. Air handler? Is that the large plenum on the front of the motor that supplies air after the throttle body? I bought new hose to replace all the bleed lines but only spent enough time to inspect the lines so far.
Reeds were good last year when I pulled the front off the engine. Would a bad reed cause only one backfire on startup and not at full load?

[EuropeanAM]

More likely that a backfire would cause a bad reed.

Air Handler is the aluminum assembly on the front of the engine, which includes the throttle plate, intake, oil pump, oil tank, reed plate and reed block assemblies.

[Proline25]

I had all those components off the engine last year and they have new gaskets and all bolts torqued. The MAP sensor is very loose so I'll be looking for a new oring for the Map sensor. I do want to correct the issue before I end up with a bad reed.
The bottom/inside of the air plenum has a thin coat of oil laying in it. I'm thinking that is normal for a 2 stroke. I'm going to pull the lower cowling and do a cold start to see if I can find why there was some exhaust smoke coming out of the front bottom of the lower cowling last time I cranked it. It did have a sneeze the last trip when slowly dropping off plane to an idle.

[EuropeanAM]

If you see exhaust up front, you definitely have a leak.

That MAP sensor is always very loose. Even with a brand-new seal.

[Proline25]

Thanks for your help with this 2 stroke. I'm sure you have worked on more than me.
After removing the lower cowling I was able to see where the smoke was coming from. The hole in the front of the intermediate housing lets me see the main driveshaft. The normal smoke is rising up through that housing and exiting through the holes by the driveshaft. I can even see the bottom of the motor and seal housing. I sprayed carb cleaner at all the possible places the gaskets and vent hoses could leak. It made no difference in the idle quality. It didn't even backfire during two cold starts.
The electronic oil pump mounted next to the air plenum is leaking oil out of the seal area and it is running down to the bottom/outside of the air plenum.

2 stroke smoke.jpg

[EuropeanAM]

That area is outside of the cowling (below the lower cowling seal). Normally don't see all that much smoke from that area, but it's certainly not impossible to be seeing it there (good idea to check tuner seals, adapter plate gasket, etc. just to make sure they're in good condition- some of these are at least partially visible with the gearcase removed).

There's also a seal/exhaust dam just aft of the water pump housing- make sure it's got a nice bead of silicone applied that will match up to the shape of it's mating surface on the midsection.

[Proline25]

My local Mercury dealer gave me some bleed hose so I replaced the lines while I had the cowling off doing some other work. None seem to have been cracked and they looked well sealed. The lower crank seal looks good by looking at the driveshaft and up into the midsection after removing the lower unit.
Another hint to finding the cause of the single exhaust backfire on startup (hot or cold) is that it does not backfire when I start the motor in the yard. It only has a single backfire when it is sitting in the water. I am thinking that the backpressure of the water covering the engines exhaust outlets may be causing the exhaust to try and exit through a blown powerhead mount gasket. I don't want to start that project but does it seem a legitimate route? Is it common to have a powerhead mount gasket to go bad? I can't really see an issue by looking up through the midsection while the lower unit is off. The engine only has about 400 hrs. on it.

[JOE54]

very common on 3.0 engines the new HP gaskets should solve your gasket issues , may need to verify all sensors and switches are operating within spec (air /head temp)

[Proline25]

Trying to order the gasket prior to removing the powerhead but all that is available is a Sierra gasket. Any other sources for the Mercury gasket or will the Sierra gasket work just as good?

[Savage]

I would only use a Mercury adapter plate gasket.....Stay away from Sierra gaskets

[Proline25]

I was hesitant to order the Sierra gasket for that area on the motor. It Seems the mercury gasket number has changed and a newer style gasket is available, but it really looks different than the original style. Part #Mercury - Gasket - 27-832933006 is what the newest Mercury manual shows for the 250 EFI motor, even though the serial number for that gasket does not go back to the older serial #OT666332 motor.

[JOE54]

832933006 p/n for latest high perf gasket for your mtr use that number for repair