1995 Mariner 225 Magnum EFI rough idle

A somewhat minor but aggravating problem I am having with this outboard. To be clear it is a 1995 Mariner 225 Magnum EFI 2 stroke. Engine runs great other than at idle. compression checks have been done and range from 100-105 on all six. Boat planes fine and is very responsive. My question is about some rough idling. When in gear and leaving no wake zones, it frequently coughs and sputters and often dies. RPMs at idle in gear are around 550-600 per dash gauge. Plugs were changed and only run quicksilver oil and non ethanol gas. Is adjusting the idle speed on this engine as simple as adjusting the throttle linkage on the hot foot? I feel if it was idling around 600-650(what I'm finding in the manuals) it would purr at idle. Other boaters tell me that a big 2 stroke just doesn't idle good and accept it. I am a type A perfectionist so I choose to investigate. I'd like to get some opinions/advice on here. Thanks in advance!

Have you checked for good spark at the end of each wire?

Spark is good, fuel is good, accelerates fine. Just seems like it is idling too low b/c when you give it a little gas it idles fine. On my carb model mercury their was a idle speed screw that was easily adjustable and was wondering if I could tweak this one a little the same way. It only needs about 100 more rpms in gear. The EFI makes me hesitant to go turning knobs until I know exactly what I am doing.

If your injectors are dirty it won't run worth a chit. Get a separate 3-6 gal. portable tank and add 3 gal. of high octane gas and a pint of SeaFoam then run the crap out of it until the tank is nearly empty. If it runs better after that then the injectors were dirty. You could pull the injectors and send then out or professional cleaning.

I was actually in the process of doing that. I did the seafoam through fuel lines and did power tune in each cyclinder. I then found an online manual for that engine (still don't know how i found it) and readjusted the throttle cam on idler arm. She smoked just as expected and blew a bunch of junk out. Pulled the plugs, cleaned em off ( they were new) and fired her up. It just sounds like it's running better, cranks at half a turn of key and idling on muffs at 1050-1100. I imagine she'll do fine now in the water and in gear. Taking to the lake in morning, may have solved the problem! Thanks for your help!

Screw the Sea Foam. That stuff is pretty much worthless for what you're experiencing. If it runs fine at all other RPMs except idle, it could be caused by a lot of different things. The EFI's have a variety of sensors which could cause rough idle if one is bad. Make sure you are adjusting the idle speed at the correct spot. A lot of people make the mistake of adjusting the engine timing, which causes bigger problems.

Oh no no no, I know not to mess with the timing and have no desire too ha! Basically what I did was realign the throttle cam with the the roller where you align the two marks and verify the small space between. A very elementary and basic adjustment which I figured was something I could start with. My first thought was the tps sensor but I don't have the special adapter to connect to my voltmeter, so I really didn't try anything too serious after learning most adjustments on that engine require electronics and special tools. I took it out yesterday and she ran fantastic. 32 degrees when I cranked her and she fired up on first turn, no choke. Idled in gear about 725rpms give or take and accelerated smoothly. Thanks for all the help!

You gotta love that SeaFoam when it works to clean the injectors. I use a 40:1 mix (1 pint SF to 5 gals. of gas) of high test gas and SeaFoam in my truck and SUV once a year to clean the injectors and combustion chambers.

I cranked her and she fired up on first turn, no choke

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A EFI does not have a choke,,,pushing in on key does nothing. As for cleaning outboard marine injectors there is no cleaner in a bottle that truly works as it wont clean the internal filter. It will clean the pintle tip but will not increase flow if filter is restricted. This is why they need to be pulled and professionally cleaned and flow tested and balanced. This and ECU problems are biggest reasons cylinder's fail on these motors. I have seen motors gain 500rpm on top end after this service..

If you want to increase the idle RPM, loosen the TPS and move it slightly. What spark Plugs are you using?

After doing some reading, I now know that the choke feature is all in the efi and is some type of enrichment feature. So if you continually turn the key on and off will it "re-enrich" it each time the key is turned on? Just curious because it seemed to have flooded it, so now I just leave the key in the on position and crank it longer. Regarding the tps sensor, I ordered the test lead that allows you to check the voltage at the sensor ( I love $75 tests) and it was off a bit @.68. Readjusted it to the manual specifications of .95. Plugs are NGK-BR8HS. I see some manuals go with the champion QL... should I be running those instead? I hear nothing but bad things about them, but can't help to wonder if the factory specified plugs need to be back in it. Will call Merc in the morning and ask about any service bulletins or spark plug recommendations.

After readjusting the tps and running it on the muffs it sounds noticebly smoother and fires right up. Haven't had a chance to take it out on the water yet, but hope to soon. After idling on muffs did notice that one spark plug is fouled, others look great. Hmm.. maybe the readjustment in tps (therefore timing) loosened something inside after the seafoam?? I'll recheck that one plug again after running and see what it looks like. Thanks again for everyone's advice, I have learned a great deal here. If I can't get everything just right I may call the mobile marine guy here ( marine service here takes foreeevverr) before spring. Tell me what ya'll think about the spark plugs.

Oh and will ask about getting injectors cleaned too.

Use the factory specified spark plug. They will need the serial number for this.

PLEASE DO NOT USE SEAFOAM IN A FUEL INJECTED ENGINE! IT WILL RUIN THE FILTER ON THE INJECTOR!!!! Instead use Mercury's Power Tune. It works much better.

Running on muffs will only tell you that the engine starts and pumps water. Performance is checked with the boat floating.

A paper clip is all that is needed to check TPS voltage, along with your meter.


Run the engine and see how it preforms. Cleaning the injectors is a larger task on your engine.

Put the champion plugs in and took to lake.. wow a completely different engine. Between the smoother acceleration and clean smooth idle, I am quite shocked. It smokes dramatically less too and seems like it's running more efficiently. Got home and pulled the plugs.. a light mist of oil and nothing else, no dark deposits or impending fouling look. I feel so confident in it now and want to thank everyone for their advice and input. I am a little pissed I payed $75 for that tps harness to simply adjust it if I could have used a home rig.. just wanted to do it right I guess.

Regarding the seafoam on injectors... I ran one treatment through it briefly, surely I didn't mess anything up? I did go and get some merc quickleen to use periodically.

Glad to hear that she is running great.


Seafoam is fine for a carbed engine. Extended use in an EFI is bad for the injector filter. It turns them to dust.

Got it, will stick to the merc quickleen from now on. Regarding the amount of smoke on these older EFIs, what is the expected or acceptable amount? When I am idling through small channels it will leave a small trail , but it isn't head turning or anything. Didn't know if there were any adjustments on the oil pump or actual Efi (utilizing the DDT) that could lean it a bit on idle. Nothing major, just curious.

There is Nothing wrong with seeing a fair amount of smoke at slow speed.