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jc41762
05-05-10, 09:30 PM
Boat was running terrible at end of last season ,,,no top end ,full throttle would run about 5000RPM , Changed filters, plugs no difference .Put boat in storage for the winter after winterizing .Put seafoam in . When boat was running in the summer you would hear the motor rev to the right RPMS without touching the throttle, then it would go back to wrong RPMS. So anyway put boat in storage. last week took boat out of storage and made appt. with boat dealer . Prior to going to the appt. i took it out on the lake . As i was going down the lake at full throttle boat would only tack at 5000 RPMS . The boat suddenly rev. up to 6200 RPMs with out touching throttle and ran great . Why is the question ? Did the sea foam remove some dirt from the injectors ,
Boat is running well ,hopefully the problem is solved .Has this happened to any other owners

rookie101
05-06-10, 10:45 PM
Possibly bad gas, or a dirty injector like you mention. If it starts to act up again try draining the fuel tank and filling it with fresh fuel as well as changing the lift pump fuel filter and the fuel/water separator.

Aaron:cheers:

jc41762
05-26-10, 01:03 PM
Ok back to the lake with boat for the summer ,wish i would of taken for my 2 hour ride to the dealer ,boat again will not run up to the proper RPM .Full throttle is 5000rpm top speed 35. Boat starts well just won't run up to proper rpms .burns alot more gas in this state. Any suggestions ,bad sensor , it feels like it is telling the engine to run low performance ........Help,changed both filters and plugs no difference ,while running for about 1 hour the boat went up to the proper rpms without touching the throttle and then went back down.Thanks for the first reply Aaron

WAJetboating
05-26-10, 03:21 PM
Full throttle is 5000rpm top speed 35. Boat starts well just won't run up to proper rpms .burns alot more gas in this state. Any suggestions ,bad sensor , it feels like it is telling the engine to run low performance

How do you know it's burning a lot more gas? Do you believe that, or know it for sure?

You've done the obvious things (plugs and filters). If you're really burning too much fuel, my first thought is one or more stuck/failing fuel injectors. Is the engine smoking more than usual for a two stroke? That would be another indication.

Yours is a second-generation 240EFI. I'd find a Mercury dealer that has a DDT and have them plug it into the engine and see what the ECU thinks is going on. If it doesn't tell you something, the next thing I'd check is the throttle position sensor (TPS) because that too will really mess up engine operation. Also run the fuel flow tests suggested in the Mercury service manual (available on this site).

Beyond that, you're probably looking at taking off the induction manifold and fuel rail so you can get at the injectors. I'd send the injectors to someone like Brucato for cleaning and matching ($25 per injector, WELL worth it).

Dig into it a bit, report back, and we'll try to help.

jc41762
05-26-10, 03:50 PM
Yes smoking more then usual ,on start up , and smells rich when going arond 5 mph burns eyes .Like i said in my first thread when i put it in the lake after storage it ran bad at first then it took off and ran well all day 6000 rpms no problems now it is back to where it was . Trying to figure out why . It seems that someting clears with the throttle in one position the boat rev's up and runs great .Then it will go back to lower rpms without touching throttle.

WAJetboating
05-26-10, 04:23 PM
Yes smoking more then usual ,on start up , and smells rich when going arond 5 mph burns eyes .Like i said in my first thread when i put it in the lake after storage it ran bad at first then it took off and ran well all day 6000 rpms no problems now it is back to where it was . Trying to figure out why . It seems that someting clears with the throttle in one position the boat rev's up and runs great .Then it will go back to lower rpms without touching throttle.

Two very likely candidates: The TPS or fuel injector(s). Hopefully the TPS, it's easy to test and replace (though a bit expensive).

Get the manual (available here) and find the section on the throttle position sensor. It's just a simple variable resistor (aka potentiometer or "pot"). There is a very simple test you can perform with a multimeter that will tell you if it's failing.

When they fail, the pot no longer has a smooth, linear relationship between shaft rotation and resistance. Instead, the resistance jumps around as the shaft rotates due to wiper failure, contamination, etc. The ECU depends upon the TPS to tell it where you have set the throttle, so the ECU knows how wide the pulses to the fuel injectors should be. If the ECU is being fed bad information, it will make bad decisions, which can result in far too rich a fuel/air mixture going into the cylinders. Result: Rough operation, smoky exhaust, stinging eyes, etc. Sound familiar?

The TPS is mounted on the starboard side of the engine. It's held in place by two screws and has a connector to the engine wiring harness. Very easy to remove and replace, but you should definitely test it first because as I recall they are surprisingly expensive ($100-250 new).

If the TPS is OK, my next guess is one or more stuck fuel injectors. This will cost less in money but more in time (if you do the labor yourself). A complete Brucato job on all six injectors costs $150 ($25/injector) but you'll have to get them out and put them back, which will take a while. Not hard, just detailed. If it gets to that point I'll give you some more pointers.

You need a manual. I believe it's available as part of the "premium" membership here. Find out, get one, and test your TPS. If that's the problem you can be back up and running pretty fast.

Waiting to hear back from you....

jc41762
05-26-10, 10:40 PM
Thanks for all your help will try to test the tps and will let you know , will gladly
spend the $200.oo. One question will the tps fail and then work . Or does it fail and stay bad.

jc41762
06-02-10, 08:46 PM
OK TPS checks out well within range and is smooth on pulse for throttle up and Throttle down ,,,,ran boat best i can do is 35mph and rpm at 5300 wide open not alot of diffrence between half throttle to full throttle.What do i check next

WAJetboating
06-02-10, 08:58 PM
...then it sounds like a fuel problem.

How do your plugs look? Take a couple out and see what they look like. Any chunky stuff building up on them? You might need to decarbon the engine.

Make sure the wires going to the high pressure fuel pump on the VST are clean and snug. If the HPFP is getting intermittent power, that would cause a low pressure situation in the fuel rail that could keep the engine from opening up.

I'd be tempted to open up the VST and check the two internal filters within, but let's see what the plugs look like first. I know they're new, but if you have a carbon problem you might already be seeing buildup on the new ones.

What did the old plugs look like? They were exposed to the internal environment for a longer period of time.

jc41762
06-02-10, 09:25 PM
OK will check plugs , are there screens in the fuel injectors and what is the VST ? I will be pulling the boat from the lake and working on it at home . Easier then being on the lake . What is the black box after the small silver filter on the fuel line ? I will report back after i check the plugs ,thank you for your quick reply.

IDJetboating
06-03-10, 11:18 AM
OK will check plugs

Check the old ones too, if you have them.


are there screens in the fuel injectors

Yes, and if you get around to pulling the injectors you should send them in for cleaning and matching. I recommend Brucato for that. They will also replace the injector screens (filters) with new ones. But let's not go there quite yet - that's a reasonably involved job.


what is the VST ?

Vapor Separator Tank. All fuel injection systems have one, or something that does the same job. On your engine it's an oddly shaped silver colored tank on the port side of the engine. It's shaped like that because this is an outboard engine, and has to fit inside that big black cowling Mercury puts on their outboards. The VST contains most of the fuel injection system including the high pressure fuel pump (and its filter), the fuel pressure regulator (and its filter), etc. Within the VST is also where oil gets mixed into the fuel.

One of the next steps may be to pull the VST. That's not terribly difficult, and will give you access to the two filters mentioned above.


What is the black box after the small silver filter on the fuel line?

The electric primer fuel pump. When you turn on the key, you'll hear a ticking sound. That's the electric fuel pump charging the low pressure side of the fuel system. It turns off after about 30 seconds. Once the engine is running, fuel is drawn out of the tank by the pulse pump which is mounted on the side of the engine. The sequence is fuel tank, to inline filter, to electric pump, to pulse pump, to fuel/water separating filter, to VST, to fuel rail, to injectors.

Report back...

jc41762
06-03-10, 06:43 PM
WILL do thanks again

jc41762
06-09-10, 05:54 PM
Pulled all six plugs they look ok no carbon build up , not real clean burning looks like alot of oil but might be normal for oil injection .

WAJetboating
06-17-10, 06:09 PM
Should i pull the filters on the VST and how do i check to see if the filters are plugged ,thanks

As noted above, the VST is the oddly shaped silver tank on the port side of the engine. It has screws around its top and a brass plug at the bottom of the tank.

Unscrew the brass plug and drain the contents of the VST. Then remove the top screws so you can separate the top from the tank.

Now you will have exposed the high pressure fuel pump and its filter (along with the float, etc.). The filter is on the bottom of the pump and can be (quoting the manual) "pried out of pump and cleaned".

Reassemble the VST tank and top, being careful with the gasket.

The high pressure regulator sits on top of the VST, held on by two screws. Remove those and remove the regulator. Its fuel filter will become visible. Clean as necessary. Reassemble. Again, be careful with the gaskets.

When you restart the engine, turn the key to "On" (but not "Start") and let the electric pump run for 30 seconds. Turn the key "Off" and repeat. This will give the electric pump a chance to refill the VST. Otherwise you might have a rough/sputtering start that could worry you {grin}.

Report back!

jc41762
06-20-10, 07:28 PM
OK it appears that it would be easier to remove the whole VST by removing three rubber mounted bolts holding the unit to the motor an then separate the vst.Do you agree ,not alot of working room to try and match the two halves back together with the gasket. Have you done this ?

jc41762
06-25-10, 04:36 PM
Ok found a boat dealer with a DDT ,listen to this one ,brought boat in on 06/ 24/10 in the A.M. on 06/24/10 P.M. the DDT tested the injectors ,fuel flow ,timing ,Oil injection system and all sensors ,now while testing the TPS if the mechanic wiggled the TPS arm the RPM's Jumped 700rpms . Which is almost exactly what the boat is running low from. Boat is running at 5000rpms at WOT and should be 6200 at WOT. So it appears to be the TPS ,the mechanic would move the TPS lever side to side not front to back and the RPMs changed.. 700RPM's ,explains why the boat would change RPMs without touching throttle. If i would of known to do this while the boat is at idle it would be easy for me to change the TPS . So if any member experience the same problem this is how to check it . Mechanic is going to run the boat down the lake and have another person move the tps side to side to make sure this the problem but sounds like this is it ....will update

WAJetboating
06-25-10, 05:34 PM
while testing the TPS if the mechanic wiggled the TPS arm the RPM's Jumped 700rpms... the mechanic would move the TPS lever side to side not front to back and the RPMs changed

Well, I did say the TPS was a likely candidate earlier in this thread! {grin}

The TPS is just a variable resistor (aka potentiometer). It has a semicircular resistive element with an arm that "wipes" around on that element as it is rotated. I don't know how you tested it, but since it's a mechanical part I would have wiggled it quite a bit. Did you physically remove it when testing it, or just run the throttle through its range with it still mounted on the induction block?

Glad to hear you've probably found the problem. A new TPS, while not exactly cheap, is a nice clean solution.

Report back!

WAJetboating
06-25-10, 05:37 PM
By the way, if you do end up replacing the TPS, I'd be interested in getting the failed one. There might be a way to repair them but I'd want to experiment on a broken one.

jc41762
06-25-10, 06:08 PM
we tested it with a multi meter on the boat with the tester on i moved the throttle up and down with a smooth reading ,but if i would of moved the tps lever side to side it would of gave a higher reading. Yes you were right on with your TPS theory,and for your reward and help i will send the old TPS to you .

WAJetboating
06-25-10, 06:34 PM
Yes you were right on with your TPS theory,and for your reward and help i will send the old TPS to you .

Thanks, I'll see what I can learn and share it with everyone. The more knowledge, the more broadly disseminated, the better!


we tested it with a multi meter on the boat with the tester on i moved the throttle up and down with a smooth reading ,but if i would of moved the tps lever side to side it would of gave a higher reading.


Since there is no lever exposed while it's on the induction manifold, I presume the mechanic must have removed it for the test?

When you say "side to side", can you be more precise? While on the induction manifold, the shaft of the TPS is rotated radially. Are you saying you get the changing readings by moving the shaft axially (i.e. along its longitudinal axis)? If so, that would mean it's changing the pressure applied by the wiping arm against the resistive element.

Just trying to better understand how your test changed from "looks OK" to "ah ha, there's the problem!".

jc41762
06-25-10, 06:47 PM
Yes axially it changed , the motor reved up 700rpms. I hope this is the problem.

jc41762
06-25-10, 06:50 PM
Yes axially it changed , the motor reved up 700rpms. I hope this is the problem. When i tested it i just moved the throttle slowly up and back down and received a smooth reading.

WAJetboating
06-25-10, 07:05 PM
Yes axially it changed , the motor reved up 700rpms. I hope this is the problem. When i tested it i just moved the throttle slowly up and back down and received a smooth reading.

You got a smooth reading, but it must have been artificially "low" to have limited the WOT RPM's.

Interesting note about the TPS: The ECU uses its output to **cut** injector pulse width. Therefore, it doesn't play a role at WOT. The engine will run properly at WOT even if the TPS is disconnected. It will be very hard to start, though, because at lower throttle settings the injectors will still be pulsed as if WOT, meaning the engine will be receiving an ultra-rich fuel/air mix.

Since your WOT RPM's were low, that indicates the TPS was emitting an artificially low reading - most likely due to increased wiper resistance. That's a common failure mode for potentiometers. Might be due to resistive element or wiper deterioration, contamination of either/both, or a mechanical failure that prevents proper pressure of wiper against element. Hopefully dissection of your failed one will make us smarter!

jc41762
06-25-10, 07:36 PM
Great information, this would give me a rich smell at idle ,like in my earlier posting burning eyes at slow speed, alot of fuel consumption ,now the question is would a normal TPS raise the rpms at idle if you moved it axially?

WAJetboating
06-28-10, 12:27 PM
Great information, this would give me a rich smell at idle ,like in my earlier posting burning eyes at slow speed, alot of fuel consumption ,now the question is would a normal TPS raise the rpms at idle if you moved it axially?

It shouldn't, because a normal potentiometer shouldn't have axial play nor be sensitive to pressure along that axis - unless it was worn/damaged.