2001 SCR2900 I’m stumped

[Elshu]

Hey guys,

I just picked up this boat not long ago, body/interior in great condition. Having mechanical issues though.

Basically revs in neutral to around 4600 RPM, not rough at all sounds good, start up cranks over immediately. While under load, bad vibration from 2000-2500 RPM. Hits 3000-3100 and bogs out, today I was able to push it to plain out and the rpm went up slowly to 4000 for the first time however I was windward, when I circled around I could not get it to plain back out.

Every other time I’ve gone out, 3 other times, would not advance passed 3000 RPM and vibrated at 2000-2500.

So far I’ve done the following:

Coil
Rotor
Dis Cap
Wires
Plugs

Compression tested all cylinders every one between 150-175

Lined the harmonic balancer up to ensure plugs were firing on right order and not retarded or reversed

New fuel filter, old one had about 1 teaspoon of water after I dumped it into a jar

Cleaned fire arrestor, looked good, checked the throttle body for movement also ok.

Unrelated, during winterization they left the bottom raw water line on and it cracked. I replaced the plastic housing, along with gaskets, etc...

It’s a 2001 scr2900 with 800 hrs
Thanks in advance,

Elliott

[mmwjr]

Welcome aboard!

First of never rev an unloaded engine that high in rpm as you can do damage and it tells you nothing about the engines condition!

What engine(s) do you have? Is the bottom of the boat free of marine growth? How much weight in gear and passengers are on board? Lastly are you sure it’s running on all cylinders?

[Elshu]

Originally Posted by mmwjr

Welcome aboard!

First of never rev an unloaded engine that high in rpm as you can do damage and it tells you nothing about the engines condition!

What engine(s) do you have? Is the bottom of the boat free of marine growth? How much weight in gear and passengers are on board? Lastly are you sure it’s running on all cylinders?

Prior to launch the hull was immaculate I’m on Lake Ontario in a marina clear of debris, it’s a single 7.4 mpi mercruiser, I believe it’s the l29. Its pair with a bravo 3. The times I went out were just myself, my wife and I twice and a fourth time with my two brothers and I. The water tank has about 1/4 tank, septic is empty, gas tank is full. As for all cylinders would the compression not determine that all cylinders are operating?

[jparsons121]

Compression is one part of a cylinder “operating”. Next is spark and fuel. Since you have compression, I’d pull all the plugs to get a “read” on if they are all burning well. Wet or sooty plug would indicate a possible missing cylinder. Vibration under load, with no power, lend to thinking you’re not firing on one or more cylinders.

Other thought is that you have some kind of damage to the drive if water was also left in it and it froze. My money is on the engine misfiring at this point.

[Elshu]

Originally Posted by jparsons121

Compression is one part of a cylinder “operating”. Next is spark and fuel. Since you have compression, I’d pull all the plugs to get a “read” on if they are all burning well. Wet or sooty plug would indicate a possible missing cylinder. Vibration under load, with no power, lend to thinking you’re not firing on one or more cylinders.

Other thought is that you have some kind of damage to the drive if water was also left in it and it froze. My money is on the engine misfiring at this point.

I have noticed since I installed the new plugs that they have become almost fuel fouled, but that was on all of them, what would lead to all plugs, perhaps faulty fuel regulator?

[mmwjr]

Post pictures of the plugs. If you install new ones does the performance improve? I don’t think the regulator is the issue. Has the boat acted this way since you’ve owned it?

[2004_3300_SCR]

Perhaps bad gas or issues with dirty / inefficient injectors?

[Elshu]

Looking like it’s a fuel issue, I just picked up a fuel pressure tester. According to the manual I should be at 43psi at idle, reading at 36psi. When in neutral I brought it up to 1500rpm and it went to 30psi. Far below what’s suggested. Going to start looking in that direction. There was a bulletin posted in regards to faulty pumps on these.

[Trey5684]

Not sure all the details about how mpi but, low fuel pressure fouled the plugs?

[Elshu]

Originally Posted by Trey5684

Not sure all the details about how mpi but, low fuel pressure fouled the plugs?

Agreed seems odd but can’t argue with the numbers. Perhaps the computer is over compensating for lack of fuel pressure. I’m just guessing at this point.

[2004_3300_SCR]

if the pressure gets too low the injectors won't atomize the fuel properly and just kinda pee stream into the cylinders - that could get you wet plugs. Inefficient / clogged injectors essentially do the same thing ... poor atomization.

[Elshu]

Originally Posted by 2004_3300_SCR

if the pressure gets too low the injectors won't atomize the fuel properly and just kinda pee stream into the cylinders - that could get you wet plugs. Inefficient / clogged injectors essentially do the same thing ... poor atomization.

Thanks for the clarification, that makes a lot of sense, as you could imagine this is why my initial thoughts were insufficient ignition. The pump seems to make sense based on rail pressure and every plug being identical in foulness. Ordered a new pump will update once I get it back together.

Thanks everyone for your input so far very appreciative!

[mmwjr]

The older cool fuel systems had issues with paint flakes in the system. Mercruiser denied this but many have posted about it.

[Elshu]

Well, I installed the new pump this morning, along with the cool fuel rail and regulator. Still not going above 3000 rpm and fuel pressure still 36 psi at the rail.

[mmwjr]

Originally Posted by Elshu

Well, I installed the new pump this morning, along with the cool fuel rail and regulator. Still not going above 3000 rpm and fuel pressure still 36 psi at the rail.

Did you have the ECM scanned for codes?

[Elshu]

Originally Posted by mmwjr

Did you have the ECM scanned for codes?

Ok so an update, I’ve rolled through some punches on this one. I had a mechanic finally come out today to assist with an overview, we narrowed it down to the timing is not advancing only under load. I replaced the ignition module but no luck. I’m stumped still.

[mmwjr]

What module did you replace, the sensor in the distributor or the ECM? Did the mechanic give up trying to determine why it isn’t advancing? How did he determine this anyway? Did he check for fault codes?

[Elshu]

Yes, no codes coming up. While in neutral we checked the timing to see it advance as I throttled up, at 3000 rpm we had 24 degrees BTDC but under load we were at 13 degrees at 3000 rpm. The ignition module sits under the distributor cap, I replaced that, no luck.

[mmwjr]

I wonder if the knock control module is detecting a knock and limiting advance. Have you looked at the troubleshooting guide in the Mercruiser manual #23? Page 4E-12 is the wiring diagram.

[Elshu]

We disconnected the knock sensors to rule that out. No change.