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Blocked Exhaust on Merc 5.0L MPI?

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iceman99

New member
2006 Mercruiser 5.0L MPI engine. My main problem is the engine won't go over 2900 RPM, so I can't go faster than about 10 knots and can't get on plane. No alarms, and I am not in Guardian mode.

I've looked at a bunch of possible causes. The cap and rotor look fine (they and plugs were replaced last year). The fuel filter and fuel pressure regulator were replaced. Fuel pressure at idle is ~39-40 psi (Have not checked under load yet). Ran a Diacomm scan and spark advance seems to be adjusting OK up to 2500 RPM. Most of my diagnostics have been done in my driveway since the boat is trailered and it's a production to launch it.

Someone suggested it could be a blocked exhaust. The engine (raw water cooled) doesn't overheat on the water, even at 2900 RPM. It sounds fine also, no missing, knocking etc.

Could a blocked exhaust be the problem? If so how can I test/check for that without tearing apart the engine?
 
iceman99 said:
2006 Mercruiser 5.0L MPI engine. My main problem is the engine won't go over 2900 RPM, so I can't go faster than about 10 knots and can't get on plane. No alarms, and I am not in Guardian mode.

I've looked at a bunch of possible causes. The cap and rotor look fine (they and plugs were replaced last year). The fuel filter and fuel pressure regulator were replaced. Fuel pressure at idle is ~39-40 psi (Have not checked under load yet). Ran a Diacomm scan and spark advance seems to be adjusting OK up to 2500 RPM. Most of my diagnostics have been done in my driveway since the boat is trailered and it's a production to launch it.

Someone suggested it could be a blocked exhaust. The engine (raw water cooled) doesn't overheat on the water, even at 2900 RPM. It sounds fine also, no missing, knocking etc.

Could a blocked exhaust be the problem? If so how can I test/check for that without tearing apart the engine?
Click to expand...

take the down turn elbows off the exhaust a couple of clamps to remove push the rubber boots down the wye pipe. The exhaust flappers are just below if they are intact you will see them if they are missing pull the drive and find the pieces.

check the fuel pressure under load doesnt mean much on the trailer. Doesnt take any power to rev out of the water

is it seeing the gear indicator being in fwd when you are trying to get above 2900 ?
 
Dieter said:
take the down turn elbows off the exhaust a couple of clamps to remove push the rubber boots down the wye pipe. The exhaust flappers are just below if they are intact you will see them if they are missing pull the drive and find the pieces.
Click to expand...

OK that makes sense, I'll take a look for that. Could an impeller also block the exhaust and cause these symptoms? I would think pieces of the impeller would get stuck in the manifolds though, and not make it through to the sterndrive. And that should cause overheating?
 
Dieter said:
check the fuel pressure under load doesnt mean much on the trailer. Doesnt take any power to rev out of the water

is it seeing the gear indicator being in fwd when you are trying to get above 2900 ?
Click to expand...

Understood re: fuel pressure. I'm not sure what gear indicator you are referring to, but I believe it was still in forward gear.
 
It is a serious question.... your answer eliminates an incorrect propeller (over propped) installed by a previous owner.

When did this problem start?
 
sandkicker said:
It is a serious question.... your answer eliminates an incorrect propeller (over propped) installed by a previous owner.

When did this problem start?
Click to expand...

OK fair enough, thanks. I've owned the boat for 11 years and it mostly runs fine. It's trailered so there's no issue of marine growth. The problem started about 2 months ago. As you can imagine the boat has had its share of issues over the 11 years so I have a little experience troubleshooting, but I'm not a mechanic. I've been focusing on fuel delivery and electrical system problems as these seem to be most common. Exhaust blockage was a new suggestion for me. I'd just like to rule that out if it can be done easily. I've heard that the exhaust flappers can break off, and the impeller can break apart, causing blockage in both cases. I'm just not sure if either would cause loss of power without overheating, since the seawater for cooling is pumped out with the exhaust.
 
Unless there is a water flow sensor, which I do not believe there is,

Both a broken impeller or blocked exhaust would cause an over heat condition.

There are only a couple people here who have a lot of experience with EFI. I am not one.

But if you are only reaching 2900 under load then
what comes to mind is,
No advance (should see >24* btdc with rpm over 25-2800 rpm load or no load)
Limp mode due to a fault. What that fault is could be many things.
Hopefully someone with more WFI background will chime in.
 
kghost said:
Unless there is a water flow sensor, which I do not believe there is,

Both a broken impeller or blocked exhaust would cause an over heat condition.

There are only a couple people here who have a lot of experience with EFI. I am not one.

But if you are only reaching 2900 under load then
what comes to mind is,
No advance (should see >24* btdc with rpm over 25-2800 rpm load or no load)
Limp mode due to a fault. What that fault is could be many things.
Hopefully someone with more WFI background will chime in.
Click to expand...

I checked Spark Advance using the scanner and it seems OK. It was about 28 deg at 2500 RPM. I'm sure I'm not in Guardian mode because there are no faults being reported.
 
I'd check the fuel pressure, under load, from Idle to WOT...low pressure = lean condition = lots of problems
 
Does the alarm sound when you turn the key on?

If the alarm is not sounding you will not hear the ecm screaming at you when the shift event switch goes bad.... If the ECM does not see the shift event the rev limiter is active and timing is limited.

The complete timing curve is controlled by the ECM. Timing is not adjustable..... The HVS has to be re-indexed after so many hrs due to timing chain stretch.

This is not a clogged exhaust issue.
 
Docksidemarineservices said:
Does the alarm sound when you turn the key on?

If the alarm is not sounding you will not hear the ecm screaming at you when the shift event switch goes bad.... If the ECM does not see the shift event the rev limiter is active and timing is limited.

The complete timing curve is controlled by the ECM. Timing is not adjustable..... The HVS has to be re-indexed after so many hrs due to timing chain stretch.

This is not a clogged exhaust issue.
Click to expand...

I'm very skeptical of it being blocked exhaust also. Especially since there is no sign of overheating.

If by key on alarm sound you mean the normal beep I hear when I turn the ignition switch, yes I'm hearing it. I'm not familiar with the shift event switch, but is hearing the beep enough to rule it out?
 
No! Make sure the ECM is seeing the shift. The switch is on the Starboard exhaust elbow where the shift cables mount... Plug the scan tool in, it will show Neutral and in gear status.
 
Docksidemarineservices said:
No! Make sure the ECM is seeing the shift. The switch is on the Starboard exhaust elbow where the shift cables mount... Plug the scan tool in, it will show Neutral and in gear status.
Click to expand...

Really appreciate your help. Using the Rinda Diacomm, I've only seen Gear Position reported along with Faults. It doesn't seem accessible with the realtime ECM performance data. See attached which is an old service report from my engine.

Would you happen to know how I can get access to that?

Diacom Service Report Capture 1.jpg
 
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