2003 5.0L MPI - Loss of RPMs

EricP-NH

Cadet
Joined
Aug 20, 2016
Messages
25
Hi,

I can't seem to get above about 3600 RPMs. I have a 2003 Monterey Montura 200LS with a Mercruiser 5.0L MPI engine, Aplha One Gen II. I've replaced the fuel/water filter. When I looked at the parts catalog online there's a second filter shown, which looks like an automotive style filter but I can't find it. I have a new prop as well, was going to get one anyway so I ruled out a problem with the current prop.

I am trying to see what else I need to do, but in the mean time I have a question. I replaced a failed IAC about a week before this RPM loss started with an aftermarket IAC. I understand the IAC has to do with idle, but could it also affect top end RPMs? Just curious if I caused the issue by using a non mercury part. This is my first boat, done plenty with jetskis, but this is a whole new world for me.

I am going to pull the plugs tomorrow and check them out as well as the distributor cap and rotor.

I can jump right to 5000 RPMs in neutral. If I try to start quickly I get to about 2800 RPMs and she climbs slowly on plane, then once on plane I can slowly get to 3600 RPMs and if I mess with trim, I can squeeze a couple hundred more depending on water conditions.

Hoping some folks here will have some thoughts as well.

Thank You,
-Eric
 
Last edited:

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
Howdy Eric

The IAC will not have much effect with performance above 1500 rpm. It can cause a lean condition and some loss in power but not that much. If the IAC was messing up you would have trouble with idle.

If you could give us your motor serial number we could have an easier time looking things up.

If your talking about item 27 in the link below, then this is only used if the boost pump (item 14) was installed.
http://www.mercruiserparts.com/bam/subassembly/31848/7585/70

Is this a new to you boat?
Has the boat always done this or is this a new problem?
 

Rick Stephens

Admiral
Joined
Aug 13, 2013
Messages
6,118
AD's question: has it ever run well for you, is key. Putting on a new prop, if you don't know whether the first prop ever ran correctly, can all by itself cause the symptoms you describe. Depending on your answer on the history, would determine what types of things are likely to benefit, like doing a compression check or checking timing.
 

EricP-NH

Cadet
Joined
Aug 20, 2016
Messages
25
AllDodge, yes I am talking about item #27, I don't think I have the boost pump so that solves that mystery, thank you.

Yes, this boat is new to me, I bought it 3 months ago.

Problem is new, boat ran great after I purchased it. No issues until the IAC failed, which I understand can be a coincidence. I replaced the IAC and it ran fine after that as well, no issues.

Then a couple weeks after the IAC replacement we were out cruising, I would say at about 3600 RPMs. Open water, no obstacles, I just sort of felt a drop in RPMs, it was subtle, but I do recall feeling a drop. We had small children in the boat so I didn't try to increase speed, we just finished the ride without issue. Next time out is when I tested things and realized I couldn't get above 3400 RPMs so I started to look for reasons. The prop that came with it is fine, and also worked fine all along until this event. I only mention the new prop to rule out a prop issue. There is no real difference between the two props in terms of this problem.

I should also mention, there is no sputtering or rough running. I hit the ~3600 RPM mark and throttle is about 1/2-2/3 forward. If I push it all the way down there is no change, I hear no difference in the engine, she still runs smooth, handles fine, etc... It's like that's it, no more throttle. If it was being starved for fuel wouldn't I feel surges or something? I don't it just stays there and keeps running smoothly.

Engine serial# OM368430
Outdrive serial# OM459160

Thanks,
-Eric
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
I think someone might have taken the boost pump and filter off your motor. Your serial number is listed as having it (0M336102 and up). You just never know what someone did prior to selling it. Hope they didn't remove yours and swap with another boat having issues.

Your motor should run without it, but if the high pressure pump is having trouble, or if there is a restriction (anti-siphon valve, line or filter) the pump may not be able to over come.

Reving the motor in neutral (no load) doesn't tell you anything.

Check your fuel filter contents and if it has not been replaced do that also. Next thing your going to need to find out if the computer shows any codes. Since you have a 555 ECM it will need a scanner to determine. Would also check fuel pressure at the rail, should be 43 psi at 1800 rpm
 

EricP-NH

Cadet
Joined
Aug 20, 2016
Messages
25
I replaced the fuel/water filter as part of my trouble shooting, forgot to mention that. It makes sense what you're saying about someone possibly removing the boost pump based on the parts diagrams, but I wasn't sure.

Where is the anti-siphon valve? I disconnected the fuel line where it goes into the filter assembly, there's a barbed brass fitting, I thought that may be it, but it is clean through.

I will be ordering a code reader and will check to see if there are any codes once it comes in.

Can fuel pressure be tested on the trailer using muffs to run the motor, or do you need it in the water under load? What does "at the rail" mean? I've never tested fuel pressure, but will if i need to :)

Thanks,
-Eric
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
The anti-siphon valve is the fitting the fuel line connect to on the gas tank. It's a hose barbed fitting and has a spring loaded check ball inside.

http://www.iboats.com/Moeller-Anti-S...view_id.392106

There is a shrader valve on the fuel rail which feeds the injectors. Note item 18 in the link below

http://www.mercruiserparts.com/bam/s.../31848/7585/90

Should be able to rent or obtain one from an auto parts store. It needs to be able to read up to at least 60 psi, but 100 is more common.

Yes pressure can be read on the trailer with muffs, but can also be read just turning key ON without starting. Need to cycle ON/OFF a few times and watch how far it goes up and how long it takes to start dropping
 
Last edited:

EricP-NH

Cadet
Joined
Aug 20, 2016
Messages
25
Thank you for the information. I will check into fuel pressure and also the anti-siphon valve. Not sure which size thread I have so will have to get into it first.

-Eric
 

harleyman1975

Ensign
Joined
May 12, 2003
Messages
959
Another possibility is dirty or clogged fuel injectors, bad plug wires, dist cap and rotor could also be the cause of this.
 

dingdongs

Chief Petty Officer
Joined
May 29, 2009
Messages
649
I would change the plugs and make sure the leads are onnthe correct plug as sometimes they get crossed by accident.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,464
and also the anti-siphon valve. Not sure which size thread I have so will have to get into it first.

Ayuh,.... Welcome Aboard,..... The anti-siphon valve is the hose barb comin' outa the boat's fuel tank,....

Usually aluminum, but could be brass,....
Usually 1/4" pipe, but could be 3/8" pipe,....

Another Test, is to plumb a remote fuel tank fulla fresh clean gasoline, Right to yer fuel filter, 'n run it,....
That eliminates the boat's side of the fuel system,....
 

EricP-NH

Cadet
Joined
Aug 20, 2016
Messages
25
All great ideas, and things I am planning to do. I did speak with Monterey they advised that they typically don't use the booster pump because the engine is very close to the fuel tank. So it's not surprising my serial number may be in the range of those with the booster pump, but I don't have one. He also suggested checking the throttle cable because when I am as high as I can go, it never acts like it is starved for fuel, it just doesn't go any higher no matter how much I mess with the throttle, so another item to check :)
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
I did speak with Monterey they advised that they typically don't use the booster pump because the engine is very close to the fuel tank

I will say this one gave me a good chuckle. Monterey doesn't build motors, they just install them in their boats. If they removed a booster pump that Merc installed they would void the warranty. The booster pump is installed next to the fuel filter, its not mounted elsewhere on the boat.

If this was a carb motor and it was running low on fuel it would miss, the MPI just doesn't go any faster. It does lean out a bit but not like a carb.
 

EricP-NH

Cadet
Joined
Aug 20, 2016
Messages
25
I will say this one gave me a good chuckle. Monterey doesn't build motors, they just install them in their boats. If they removed a booster pump that Merc installed they would void the warranty. The booster pump is installed next to the fuel filter, its not mounted elsewhere on the boat.

Well, I take what they say at face value. The logic makes sense, and they may order the engines without the booster pumps. There's no evidence there was ever one there. They have had Merc on site to approve their installation processes.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
I'm not bad mouthing them, I look at it as they hear booster pump and see in their mind another pump mounted away from the motor, not on the motor. You probably wouldn't notice if it was removed because it fastens using the same two bolts the hold the fuel filter on. The pump wiring Tee's into the high pressure cool fuel. Remove it and it was like it was never there.

Either way its no issue right now with your problem, I would like to know what your fuel pressure is
 

EricP-NH

Cadet
Joined
Aug 20, 2016
Messages
25
I'll be able to check the pressure Saturday and reply back. I'll also check the throttle cable per their suggestion.

BTW, I didn't mean to imply you were bad mouthing, apologies if it came across that way.

Thanks,
-Eric
 

EricP-NH

Cadet
Joined
Aug 20, 2016
Messages
25
Another thing came to mind. Inside the throttle body there is a little white mesh filter, I believe called the IAC Muffler. What does it do? I had replaced it when I replaced the IAC and the other day I took it out to check it and it was filthy. I washed it with soap and water and will put it back in this weekend. I'm just curious what it does and could it have an affect in my case?

Thanks,
-Eric
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
No problem-o
Yes it can cause some issue. When the IAC opens the air is drawn thru the muffler. So it should be clean and the port leading past should also be clean.

Note: don't use carb cleaner or harsh chemicals on the IAC or muffler area
 

EricP-NH

Cadet
Joined
Aug 20, 2016
Messages
25
Checked the fuel pressure at the rail. ~45 PSI, when I turn the key off it drops to 40 and stays there, so I don't believe it's a fuel pressure issue. Throttle linkage moves fine all they way through, I haven't tested that under way, just on the trailer, but there doesn't seem to be a problem there either.

I plan to get at the anti siphon valve next weekend.

-Eric
 

EricP-NH

Cadet
Joined
Aug 20, 2016
Messages
25
I have ruled out the anti-siphon valve. Got out today and was able to get to 4000 RPMs, but still not quite there. Going to try distributor cap and rotor.
 
Top