Help! Where do I start? 115hp four stroke rough running and back fire

[TD121]

I have a 2008 Merc. 115 fourstroke, SN 1B693746. I purchased the motor last winter and was able to hear it start and idle, but could not test it (lakes were a little stiff here in WI...).

The fuel tank was nearly empty. I added one can of Iso-Heat (for 10gal) and added 15 gallons of premium no ethanol fuel.

I got the boat out last weekend. I turned on the key, waited for the fuel pump to shut off, turned the key to start and before I could count to three she was running and sounded nice and smooth. I idled out from the landing to deep water and thought I might be hearing a little miss-fire. I throttled the motor up and while it took a little more than expected to get on plane it ran fine. Suddenly the motor started surging and she lost power and died. I waited a couple of seconds, turned on the key, again waiting for the fuel pump to stop, and restarted. It started right away, but ran really rough and would not throttle up. Then quite suddenly it took off and ran great. After a lap around the lake, I dropped off plane idled around a bay, turned the boat back toward the landing and throttled up only to have it backfire and start running really rough. I struggled with the back and fourth behavior all the way back to the landing. It run great, then very poorly, then great.

Back in the garage, I pulled the plugs, all four were rather white. I then pulled the lift pump fuel filter (low pressure?) and tried to capture the fuel to check for water. Dumped too much on the floor to know. I then pulled the high pressure fuel filter and it was nearly empty.

I am not sure where to look next. I am new to fourstroke outboards, but have worked on 2 stroke and older cars all my life. I currently have new plugs, fuel filters, fuel line and a water separating fuel filter on order, but I am not confident that water was the issue.

Motor only beeped for a short period when it was in neutral and I was trying to reve it up. Highest RPM I could obtain in this state was 3000 and that was a stretch. I could get 5000-6000 RPM while it was running well and on plane.

Help!

[EuropeanAM]

Don't rev the engine in neutral as it does not have any load on it, and you can easily cause extensive damage to the engine.

What is the MAINTENANCE history on this engine?

Items to consider include (but aren't necessarily limited to) Oil & Filter, Gear Lube, Water Pump, Fuel Lines from Tank to Engine, Primer Bulb, Fuel Filters, and Spark Plugs. Several of these could be contributors (and note that "white" spark plugs can be deceiving on these 4-strokes... I often will not TOUCH a performance problem until I've tried plugs and proven the problem persists.

[TD121]

I wish I had more data on the maintenance. I have a filed that indicates it has been serviced 2x per year by the dealer that sold it, but the service reports are vague.

Oil is clean, looks like new. Filter also looks like new, but so does the whole motor...
Records show gear lube was changed last fall.
The indicator stream from the water pump was large and steady, but seemed to be weaker than my 90ELPTO (but then I replaced that impeller every other year)
No primer bulb.
Fuel filters - as I noted above, one of them was empty when I pulled it off. I could easily blow through either one.

I have new plugs and filters on order. (Sure will be glad when I can get back to my local dealer...)

I pulled the floor up to access the fuel line. I should also be recieving a water separating fuel filter this week and need a longer fuel line to reach the installation location. I found the fuel line to look good (externally). So good I suspect it has been replaced, particularly with the hardware store variety of hose clamp used. The fuel line has a barbed bulkhead fitting where it passes thru a compartment wall to below the deck. When I separated the tank side fuel line from the fitting I cannot blow back into the tank. Is there a check valve in the tank, or might the pick up be blocked?

I don't want to change to many things at once, but also don't want to take the floor up twice...

[EuropeanAM]

Run line (3/8") all the way to the engine. Avoid ANY 90 degree fittings, the separator should be a high-quality (recommend the Racor kit offered through Merc with Water in Fuel for best results).

You should not be able to blow backwards through the line- standards require an anti-siphon device.

A good idea would be to try a portable tank, line connected DIRECTLY to the engine. If there is a primer bulb on that line, DO NOT use it.

Run the engine and see if the problem persists AFTER 10-15 minutes of running. Allow 10-15 minutes as you may possibly have some air in the system.

[TD121]

Thanks! I'll get it put back together with the new filters and plugs and try that first. Then I'll go to an alternate tank.

Any idea why the high pressure filter would be nearly empty of fuel?

[EuropeanAM]

Starving for fuel (or a fuel restriction) would be the most common.

[TD121]

OK. Vessel View came in Thursday night. I installed it and the only codes it reports is due to turning the key on with the ignition coils unplugged....

Motor reports 90.4 hours. Maintenance is telling me plugs and alternator belt is due in 10 hours.
I completed installing new fuel line from the tank to the new water separating filter. Reused existing line from filter up to bulkhead fitting (confirmed I could see through it when held up to the light. Gas is easily drawn up to the point (although I learned I need a new siphon pump, and got a reminder as to how bad gas tastes ). With the two filters removed from under the cowl, I can easily blow through the bulkhead fitting up to where the first filter connects. (When this filter is installed, I can no longer blow through.)

Both under the cowl filters are now new and (4) new plugs have been installed.

Took her out this afternoon and she runs better. No issues at idle and she will run up to WOT and cruise for a short period, then start to stutter and loose RPM. Throttle down to 4800rpm or less and she settles down and run fine. If you go from WOT and stuttering directly to idle, you have to wait a few moments before you can throttle up. No Codes are reported on the Vessel View.

Water temp is 120 degrees. Oil pressure varies from 50 (idle) up to 80 (WOT).

One thing I realized is I was mislabeling the fuel filters in my previous posts. The filter AFTER the high pressure fuel pump (between the fuel module and the fuel rail) is always full of fuel. It is the filter that leads up to the fuel model is empty when I open it up.

Any ideas before I start looking for a dealer that is open (probably won't find one for another month...)?

[R2Tracker]

Have you checked the fuel pickup inside the tank for debris or plugged screen?
Have you tried the portable tank as Don suggested? This can be a fantastic troubleshooting measure.

[TD121]

I had no issues drawing fuel up from the tank, but maybe the flowrate is too low...

I do not have a portable tank. The best I could do is a 5 gallon can and some 3/8" line. Next several days here are supposed to be very rainy, but I'll give this a try.

[EuropeanAM]

Have the plugs and belt EVER been changed on this engine?

[TD121]

I have now way of knowing about the plugs except they looked good. Given they way they are set down inside the head with the long boots and coils...
The alternator belts looks way to good to be 12 years old.
I have receipts for "annual maintenance and winterizing" for the last 8 years. They are not detailed service tickets, so I don't know what they actually did.
Everything is so clean, it is hard to believe they were never changed, but looks can be deceiving.

I did get a chance to run a "portable" tank on the motor tonight. I used a 3 gallon gas can with a 3/8 in line weighted with (2) 1/2" nuts on the end to keep it in the tank and ran the line directly to the first fuel filter inside the cowl. No change from yesterday.

[EuropeanAM]

Try a fresh set of spark plugs (properly gapped) before going any further.

[TD121]

Plugs and fuel filters were changed prior to last Sunday and Monday nights tests. I dropped the boat at my nearest authorized Merc shop. We'll see what they come up with.

I'm a newbie to four stroke outboards, but it seems like the motor is starving for fuel. Replacing the fuel filters (and plugs) resolved the low end misfires and backfires (I had also added a water separating fuel filter), but the top end still has issues. The run time on plane before it starts to struggle is quite predictable and when keeping the boat below 4800 rpm it never acts up. No codes are reported by vessel view. My guess is the FSM is not supplying sufficient fuel for the higher RPMs, so I am predicting it will be a bad float switch, partially blocked lift pump inlet screen, or a bad lift pump.

Now I'll wait for the experts to tell me what is really wrong... I'll share what I find out.

[EuropeanAM]

If problems continue- it may be necessary to put a vacuum gauge in line (near the mechanical pump) and check to make sure that pump is capable of pulling adequate suction.

This also allows you to verify that there no substantial RESTRICTION in the supply system. If your comments about the tank, 3/8" line and nuts included NEW, totally separate 3/8" line that has never been installed in this application- it's unlikely that a restriction is present OUTSIDE the cowling.

[TD121]

The line was brand new, never used. (The nuts were slipped over the outside of the line to act a weight to keep the line in the tank)

What would be an acceptable range for vacuum? I had not considered checking this, but it makes complete sense - I guess I am at the mercy of the shop's schedule now anyway...

I do thank you for your advice!

[EuropeanAM]

Fuel inlet line vacuum should not exceed 2.5" hg Mercury for open-vented tanks.

3.0" hg for Pressurized systems (that must have a fuel demand valve).

Those specs are at any engine RPM (including sustained WOT operation).

With the line going into the engine PLUGGED, the pump should be able to pull at least 4" hg (most healthy ones will do considerably better). Keep in mind that this must be monitored long enough for the FSM level to drop, float to drop which turns the pumps on. These tests are BEST done with the Computer Diagnostic System attached... which allows the tech to monitor float position and pump "on" time.

[TD121]

Heard back from the dealer. They had it "running on a hose" - I am assuming water hose for cooling - and found no issues. The computer did indicate overheat conditions with which the guardian system limited the RPM's. I was planning on replacing the water pump impeller when I got the boat back as the indicator stream was steady, but "weaker" than my older 90hp ELPTO.

The brief time this was on Vessel View it showed a max. temp. of 120 degrees. What is too hot? I also noted that Vessel View did not report any water pressure. Does this motor have a water pressure sensor?

[SilverFox579]

Possible your engine is not equipped with a digital water pressure sender, Don will know.

[TD121]

Picked the boat up tonight after work. Reviewing the print out given to me by the shop, overheating is not likely the source. The overheat condition occurred prior to my owning the boat, not within the 2 hours of run time I have on it. I'll still change the water pump and test it, but I don't have high hopes.

[TD121]

Didn't even bother to take it to the lake. Replaced the water pump, learning that you should drain the oil on a four stroke before pulling the gearcase, then did a complete oil change.

Put the motor on a water hose and started it up. Started fine, ran a 1-2 minutes, then I noticed the idle became rough, a few moments later it started to stumble and died. It would not restart. It would fire will cranking, but would not run on its own. I tucked a rag under the high pressure "bleeder" - looks like an air valve on a tire, and let some fuel out. While it was under pressure, it seemed to have a lot of air in with it. I turned the key on, waited for the pumps to stop, turned the key off and repeated the fuel bleed. I did this 3-4 times until the flow of fuel seemed to have less air. Motor started up, stumbled a little, then sat and idled nicely for a couple of minutes before repeating the previous symptoms.