502 MPI - Boat won?t run on high RPMs (3000-3400 RPM maximum)

[ojitsu]

Engine: Mercuiser MAG 502 MP1 8,2L V8

Problem: Boat won?t run on high RPMs (3000-3400 RPM maximum)

To begin with I haven?t had any problems to start the engine (cold nor hot). But when the boat has planned out I can?t accelerate over 3400 rpm on full throttle. Before this issue occurred I usually ran steady on approx. 3500-4200 rpm. When boating steady on 3400 rpm, for a couple of minutes, it now automatically runs down to 3000-3100 rpm. On 3000 rpm no problems.
Local mechanics has informed me that the fuel pump doesn?t receive enough fuel so they replaced the fuel pump, fuel tank hose, fuel filter, the nonreturn valve on top of the fuel tank was removed and STP injection cleaner is used in fuel tank. They also replaced the dist. cap and spark plugs. Same issue remains, even when trying a separate/portable fuel tank the engine shows the same symptom.

I haven?t yet done a diagnostic on the engine but before I spend some more $ on a mechanic I would appreciate your comments.

I?m out of ideas! Anyone care to help?

/OC

 

[Fun Times]

Re: 502 MPI - Boat won?t run on high RPMs (3000-3400 RPM maximum)

Anyone care to help?

Sure would like to try, do you by chance have the engine serial number handy? Engine/boat year? Boat Model?

Do you know what the fuel pressure is at those RPM's?

Is the exhaust open with thru transom exhaust?

Have you tried running a separate 5 gal fuel tank at high RPM?

Engine compression test may be in order as well.

Here is a list of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test
10. Carburetor defective, or wrong type.
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully.

 

[ojitsu]

Re: 502 MPI - Boat won?t run on high RPMs (3000-3400 RPM maximum)

Thanks for the tip!

Engine Serial Number: 0L080129 (2003)
Boat: Rinker Captiva 236 Cuddy Sport (1991)

Unfortunatly I don't have the numbers of the fuel pressure. I have tried with exhaust both thrugh transom and prop, no difference noticed. I ran with a separate fuel tank for a couple of minutes but no change noticed.

 

[Fun Times]

Re: 502 MPI - Boat won?t run on high RPMs (3000-3400 RPM maximum)

Thanks for the tip!

Engine Serial Number: 0L080129 (2003)
Boat: Rinker Captiva 236 Cuddy Sport (1991)

Unfortunatly I don't have the numbers of the fuel pressure. I have tried with exhaust both thrugh transom and prop, no difference noticed. I ran with a separate fuel tank for a couple of minutes but no change noticed.

You're welcome!

One additional thought would be to ensure that your ignition timing is advancing correctly by both using the scan tool and double checking it using a timing light.

Here is the parts catalog for this engine model if needed,
http://www.mercruiserparts.com/selectDocs.asp?doc_nbr=824418+98

Keep us updated.

 

[ojitsu]

Re: 502 MPI - Boat won?t run on high RPMs (3000-3400 RPM maximum)

A few months has passed and its time for boating soon. Still I havent found the problem. Injectors has been checked and computer diagnostic shows no error codes. Ignition timing is next I guess. Any more tips is much appreciated!

 

[Walt T]

Re: 502 MPI - Boat won?t run on high RPMs (3000-3400 RPM maximum)

Go through the list Funny Times posted. It sound to me that you've never achieved the correct rpms. What prop and what gear ratio do you have? It should scream with that boat.

 

[alldodge]

Re: 502 MPI - Boat won?t run on high RPMs (3000-3400 RPM maximum)

A few months has passed and its time for boating soon. Still I havent found the problem. Injectors has been checked and computer diagnostic shows no error codes. Ignition timing is next I guess. Any more tips is much appreciated!

Find your knock sensor, it's the exhaust manifold and to the rear of the starter. You should have only one but there may also be one on the port side. Next time you have the problem pull the wire off you knock sensor. If your rpms comes back you either have a knock or your sensor has gone bad. If it does speed back up, put the wire back on before and find out which it is. If your engine has a knock it will destroy it self if left unattended.

If your a person you is a neat freak type, make sure you have not tie-wrapped the knock sensor wire with the spark plug wires.

 

[Walt T]

Re: 502 MPI - Boat won?t run on high RPMs (3000-3400 RPM maximum)

Hah! Yea I've seen the old tie the knock sensor wire to the spark plug wire trick. Knock sensor definitely a possibility.

 

[ojitsu]

Re: 502 MPI - Boat won?t run on high RPMs (3000-3400 RPM maximum)

Thanks guys for the tip! I will check the knock sensors when the boat is in the water. I will also check the spark plugs wires! While searching the forum for answers and tips I come across a similar case where the rotor in the dist. cap was the problem. Can that also be the problem here? To be continue.

 

[Fun Times]

Re: 502 MPI - Boat won?t run on high RPMs (3000-3400 RPM maximum)

the rotor in the dist. cap was the problem. Can that also be the problem here? To.

Unfortunately just about everything should suspected until proven otherwise. Keep us updated, Good luck.

 

[ojitsu]

SOLUTION FOUND!
Short version: Ignition module (read more in the long version) and spark plug wire(s) was caput!

Long version: A lot of beer, headache, thousands of dollars on spare parts, lost summer days and a lot of ideas from certified mechanics have led to this day. This morning my father called me and said that my boat now runs approx. 70mph at. 4800rpm again.

As I have written in previous post mechanics could not find the fuel problem so they replaced the fuel pump, fuel filter, fuel tank hose, the nonreturn valve was removed and injection cleaner was used. They tried a portable fuel tank as well. No solution found. Second attempt was to replace the distribution cap and spark plugs. No solution found there either. At the third attempt they run a diagnostic without error codes. This was last season. This winter the injectors were pressure tested with good result and the rotor was changed. A couple of weeks ago the boat was in the water again with the same symptoms (read my first post).

Now a couple of days ago, with less hair on my head, I went to a local performance store for cars I never visited before to buy new spark plug wires. I explained to the clerk about my problem and he gave me One tip; check the Ignition module! He explained that the ignition module has a round small black label on it that turns yellow if it’s gone bad. I later checked the ignition module, which is mounted with two screws under the distribution cap and rotor, and found a yellow dot on it (!). I bought a new module and installed it. It ran much better, approx. 4100rpm. I then changed the spark plugs wires too, and it hasn’t run better!

I’m not sure but perhaps one of the spark plugs wires made the ignition module bad (?!) Anyhow, I hope this tip can help others in the future.

Oh! I found out that the clerk who gave me the tip about the ignition module is an 8 times European Pro Stock Champion.

 

[alldodge]

Sounds good, glad your running and thanks for following up

 

[Fun Times]

Any chance of getting a photo of this yellow dot?