2001 Mercruiser350 MAG Horizon Low RPM @ WOT

neptune3277

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I have a 2001 Mercruiser 350 Mag Horizon engine that received a valve job last summer and was running okay, but now it does not turn over past 3200 rpms. The mechanics at my marina took a look at it and seemed to be baffled as to the cause. The scan showed no fault codes and the timing was checked. The engine runs smooth but right at half throttle it gets stuck at 3200 and goes no further at full throttle. The boat gets on plane but I think it is at the expense of the other engine. From what I have read, a number of issues, the engine guardian system will retard the speed if a problem is sensed to avoid engine damage, to a bad injector, electrical etc. I have replaced the coil, spark plugs, wires, cap & rotor.
 

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Re: 2001 Mercruiser350 MAG Horizon Low RPM @ WOT

When the mechanics at the marina took a look at the engine, do you recall what they tested?

Did they test the fuel pressure?

Being it's a 350 MAG Horizon, Is this a inboard engine or a stern drive set up?

Do you have the engine serial number handy?

P.S, Just so you're aware, you accidentally posted you're question in the wrong forum section, This is the Mercury outboard forum, You're thread should be moved down a few pages out on the home page to the Mercruiser forum shortly.:)
 
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alldodge

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Re: 2001 Mercruiser350 MAG Horizon Low RPM @ WOT

Maybe th knock sensor, and check if someone didn't place the wire coming from the knock sensor close to plug wires.

Guessing this is an EFI engine
 

neptune3277

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Re: 2001 Mercruiser350 MAG Horizon Low RPM @ WOT

When the mechanics at the marina took a look at the engine, do you recall what they tested?

Did they test the fuel pressure?

Being it's a 350 MAG Horizon, Is this a inboard engine or a stern drive set up?

Do you have the engine serial number handy?

P.S, Just so you're aware, you accidentally posted you're question in the wrong forum section, This is the Mercury outboard forum, You're thread should be moved down a few pages out on the home page to the Mercruiser forum shortly.:)

As far as I know the put a meter scan that showed no fault codes and checked the timing. No they did not check the fuel pressure. This is a EFI MIE v-drive inboard set up. Serial no. is OL675956
 

neptune3277

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Re: 2001 Mercruiser350 MAG Horizon Low RPM @ WOT

Maybe th knock sensor, and check if someone didn't place the wire coming from the knock sensor close to plug wires.

Guessing this is an EFI engine

Yes it is a MIE v-drive inboard.
 

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Re: 2001 Mercruiser350 MAG Horizon Low RPM @ WOT

I have a 2001 Mercruiser 350 Mag Horizon engine. From what I have read, a number of issues, the engine guardian system will retard the speed if a problem is sensed to avoid engine damage, to a bad injector, electrical etc.
You may want to be careful with what your reading as it may get confusing between all the different engine models out there now.

You're engine has the MEFI 3 ECM and it does not utilize the guardian system reducing engine power.

As far as I know the put a meter scan that showed no fault codes and checked the timing. No they did not check the fuel pressure. Serial no. is OL675956

The engine received a valve job last summer and was running okay.
Thanks for the serial number, it helps a lot.

Fuel pressure is important to know, Also you may want to do another cylinder compression test to ensure everything is still working correctly.

Do you know if each engine has its own fuel pickup tube coming from the fuel tank or does one pickup tube feed both engines? Reason I'm asking is because maybe you have one bad fuel anti syphon valve at the fuel tank restricting fuel flow.

Are you sure both props are in good condition?

Is the bottom of the boat clean?

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.
 

neptune3277

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Re: 2001 Mercruiser350 MAG Horizon Low RPM @ WOT

You may want to be careful with what your reading as it may get confusing between all the different engine models out there now.

You're engine has the MEFI 3 ECM and it does not utilize the guardian system reducing engine power.

Thanks for the serial number, it helps a lot.

Fuel pressure is important to know, Also you may want to do another cylinder compression test to ensure everything is still working correctly.

Do you know if each engine has its own fuel pickup tube coming from the fuel tank or does one pickup tube feed both engines? Reason I'm asking is because maybe you have one bad fuel anti syphon valve at the fuel tank restricting fuel flow.

Are you sure both props are in good condition?

Is the bottom of the boat clean?

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.

Since this is a MEFI 3 ECM engine, the other suggestion about a bad knock sensor would not have an effect? Thank you for the other possible causes, they make sense, it gives me items to further explore. Looking at the owners manual schematic , it appears that the fuel tanks may be independent to each engine. Perhaps I will start by replacing that anti siphon valve.
 

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Re: 2001 Mercruiser350 MAG Horizon Low RPM @ WOT

Since this is a MEFI 3 ECM engine, the other suggestion about a bad knock sensor would not have an effect?
AllDodge's suggestion of checking into the knock sensor certainly has merit and worth looking into.

Your MEFI 3 does utilize the knock sensor and the knock sensor "module" is located internal to the ECM.


Extra Verification in writing from the service manual for you since this is the internet.:D
Caution,
The correct Knock Sensor and KS module for an engine must be used or a spark knock may not be detected causing sever engine damage.

KNOCK SENSOR CIRCUITRY (MEFI-3 MODELS ONLY)
The MEFI-3 ECM is used with the knock sensor to control spark knock. The KS module circuitry
is within the MEFI-3 ECM. When spark knock is present, a small AC voltage signal
is sent from the knock sensor to the ECM through pin connector J1-30. (If the engine has
a second KS, its voltage signal goes through pin connector J1-14). An AC voltage monitor
inside the ECM will detect the spark knock and start retarding spark timing. A Code will be
set only if the ECM does not see any activity on the KS signal circuit(s).

If interested, Here's your service manual,
http://www.maxumownersclub.com/forum/site_files/manuals/mercruiser/24.pdf

And here is your parts catalog,
http://www.mercruiserparts.com/selectDocs.asp?doc_nbr=804703+99
 

neptune3277

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Re: 2001 Mercruiser350 MAG Horizon Low RPM @ WOT

Yes it is a MIE v-drive inboard.

Thank you for the suggestion. I just have a question though, if it is bad knock sensor would'nt it show up as a fault code. Please verify, the knock sensor is usually located in the intake manifold and retards the timing if it detects pre-ignition or a mis-firing plug?
 

alldodge

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Re: 2001 Mercruiser350 MAG Horizon Low RPM @ WOT

Thank you for the suggestion. I just have a question though, if it is bad knock sensor would'nt it show up as a fault code. Please verify, the knock sensor is usually located in the intake manifold and retards the timing if it detects pre-ignition or a mis-firing plug?

If the knock sensor picks up a knock and retards timimg it should through a code, but not always. The ECM's are getting better at picking things up but the last guy with this problem it never showed up. He tiewraped his knock sensor wire with his plug wires and went weeks trying to find it. It looked real good but caused him a problem.

The knock module is on the manifold, the knock sensor is on the side of the block below the exhaust manifold
 
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