Mercury 250 EFI smoky start up and rough idle in NC

[fishnmusic]

My first post ... I have the exact same problem as "sportcraft" did in 2015. My starboard 2001 Mercury 250XL EFI serial number 0T397443 has a very smoky start up, rough idle and occasional sneezing at idle after coming off plane. Both engines run awesome at cruise and wide-open throttle. Approximately 400 hours on the trouble engine. I have read through the thread titled "first post....2001 Mercury EFI 250 rough idle issue and smoky start up" posted by 'sportcraft' on 06/27/15 multiple times and I don't see where there was a final solution. I was hoping to start there. My service manual is in route. Would appreciate a recommendation for a marine diagnostic scan tool for this engine. Can anyone help? I am a rookie forum user. Hopefully I will get better!

[EuropeanAM]

The Diacom system is a decent system for the money. Lots of bang-for-the-buck.

A good starting point would be to check the crankcase pressure check valve (for the remote oil tank), AND check the Fuel Pressure (or better yet, monitor it from start, through a run, and when returning to idle).

Also a good idea to check the other basics: Compression Test, Spark Test (including inspection of the QL77CC Spark Plugs), and fuel Quality, Delivery (and pressure) tests.

Verify that both banks of the engine are coming up to temperature properly.

[fishnmusic]

Thank you sir! I will check each of these items and respond when I have accurate info for you!

[fishnmusic]

Mr. Weed, just FYI, I replaced the stater on this engine last fall thinking it might help with the rough idle.( no noticeable improvement ) also, the excessive smoke and rough idle has been going on for maybe 10 years. Not something that started recently. I am a fairly good nuts and bolts mechanic and do all work on my engines,boat and trailer myself. I am not well versed with the electrical tech stuff. With that said; Will readings from an infrared temperature gun shot down into the temperature sensor hole (removing the temperature sensor on the right bank ) while running on the flush kit be OK for you?

[trivialpursuits]

Mr. Weed, just FYI, I replaced the stater on this engine last fall thinking it might help with the rough idle.( no noticeable improvement ) also, the excessive smoke and rough idle has been going on for maybe 10 years. Not something that started recently. I am a fairly good nuts and bolts mechanic and do all work on my engines,boat and trailer myself. I am not well versed with the electrical tech stuff. With that said; Will readings from an infrared temperature gun shot down into the temperature sensor hole (removing the temperature sensor on the right bank ) while running on the flush kit be OK for you?

I believe Don prefers to take IR temp readings on the thermostat covers on each cylinder bank -- the side *after* the thermostat.

[fishnmusic]

Thank you!

[EuropeanAM]

+1... right in the middle of the thermostat housing cover. Readings from any other locations will not be likely to be consistent with the actual coolant temps.

[fishnmusic]

Still waiting on some parts to arrive. Changing crank case pressure check valve this afternoon. Hope to fire it up soon!

[fishnmusic]

Mr. Weed, don't know if this could be useful to you for diagnostic purposes. In 2008 this vessel was struck by lightning( with me on it in The Ocean ) both computers ( on both engines ) had to be replaced. I do not believe any sensors were replaced. I recently had the ambient air temperature sensor on the throttlebody fail on the port engine so I replace the one on the starboard ( The trouble engine) also. Thanks!

[JOE54]

O M G there d been a mess to clean up if id been in it with ya

[EuropeanAM]

Not a bad idea to double-check the air temp values (and engine temp sensor values) on a Diagnostic Tool or Laptop, even though you replaced the intake air temp sensors.

Have seen a "higher than expected" incidence of failed IAT's lately... ECU's reporting 6+ digit intake temps. That doesn't compute with the MAP tables... and the "failsafe" back then for "does not compute" was to "dump lots of fuel".

See where you're at after the check valve and IAT/CTS rechecks.

[fishnmusic]

Believe it or not, I actually found two unicorns. Two brand new, still in the bag original mercury IAT's( after reading that the replacement\superseded ones were not accurate ) Before I order the Diacom system, is there any particular model you suggest? When I pulled up the Diacom system that works with my laptop, it did not mention 250 EFI as one of the engines it covered. Thanks for your patience!

[Bob G.]

Check your diagnostic cable connection on your motor and match it to one of the Rinda cables.

[EuropeanAM]

Their 94111 kit will work for your engine.

[fishnmusic]

94111 on the way. Crank case pressure check valve on trouble engine was almost completely stopped up. Surprised it was pumping oil from the remote tank. All cylinders were 114 pounds to 119 pounds on compression check. Waiting for new QL77cc Marine plugs for temperature check. Thanks!

[JOE54]

[fishnmusic]

Diacom 94111 arrived today. Of course, there's a problem with my laptop so it's going to the shop tomorrow. It's always something, isn't it. R&R next week so I will be back on the forum the following week. Never give up! Never surrender!

[JOE54]

always,,, Never give up! swhat ICONELLI says

[fishnmusic]

A big thank you to Don Weed! Thank you so much for your help and advice! The Mercury 3L service manual and the Diacom 94111 made it all fairly simple. The throttle linkage (on both engines) was set wrong. The cam roller on the trouble engine was approximately 1 inch toward WOT( did not line up with the casting Mark on throttle cam. The port engine was 1/2 inch the same) The TPS on the troubled engine would not stay set at idle. Replaced it. After repairs and adjustments, it sorta scared me when I started them for the first time. There was hardly any smoke at all! Idle is perfect! Thanks again Don!

[Bob G.]