2000 21V | 5.7 Mag MPI SKI - Engine Issue??

[Schnate]

Hey guys,

Thought I would offer this one up to the forum. Let me know your thoughts.

Year 2000
2100 V Limited (essentially the 21V, but with different trim and finish)

5.7 Mercruiser Mag Mpi Ski with aprx. 600 hours.

She has been a FANTASTIC boat and engine for the past 5 years, then at the end of last season, we ran into an issue.

We put in, and buzzed around the lake for 30 minutes. Then, parked and swam and ate (engine off) for 30-45 minutes. Went to start engine, and engine would turn over, but did not fire. After 4-6 attempts, the engine did started. Upon driving out of the swim spot the engine would not go above 3000 rpms, and was bogging down, coughing, and back firing all the way back to the boat ramp. (Engine died if I went too low on rpms.)

I have been able to replicate this same issue each time out after.

It seems to be tied to the engine getting hot. The issue of the engine not wanting to restart—and then bogging and dying—only happens after the engine has ran for 45 minutes to 1.5hrs and is hot.

Things I have done:

• replaced ignition coil
• replaced spark plugs
• replaced distributor cap and rotor
• replaced IAC valve
• replaced thermostat
• replaced impeller
• replaced fuel pump
• cleaned fuel lines (from filter to pump)
• replaced fuel filter
• replaced fuel lines from tank to fuel filter

I initially thought it was vapor lock, but the fuel pressure is good all around. Also, this 5.7s has the "cool fuel" system which runs the fuel through a pipe with water rushing through it to keep the fuel from getting too hot.

I also did multiple compression tests at different points, cold, hot, after the issue happened. And, each compression test was within the range. (145-160s across the board)

My hypothesis as of now:

• slightly faulty head gasket
  • When we first got the boat 5 years ago, it overheated on our 2nd time out. I later found impeller bits in the water tubing downstream of the impeller. "Thanks previous owner..." (I now replace the impeller every other season just to be safe.)
  • This overheating lasted all of 3 seconds and I pulled the boat down to idle to get back home.
  • I am wondering if this overheating issue affected the head gasket then, and it is now beginning to failing when the engine gets hot???
• Could this be some electrical thing when the engine gets hot?

The frustrating thing is the engine runs like a scalded dog for 30-45 minutes. All the power you want. Smooth. Sounds good. Then... if we turn the engine off, and attempt to restart, it bogs, coughs, and eventually dies.

OK, that is all I have... Any thoughts you may have would be much appreciated.

Thank you in advance!

[USWaker]

Perhaps the cool fuel heat exchanger has a small crack in it that allows small amounts of water into the fuel line once the engine is shut off. The fuel pressure would keep the water out when running, then once you turn it off, the water in the exchanger can leak in.


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[30Year_Drought]

Not quite 45 minutes of running, but my problem was would warm up then start lugging and surging after about 15 minutes and die at idle then wouldn’t restart. It was the coolant temp sensor. The sensor, not the sender, changes the ratio when it “thinks” the engine is cool. Easy fix to try. I replaced that and the distributor cap and rotor and it ran flawlessly all this past weekend.
Chris

[rt2439]

we had those same issues with our 2300v... we did everything you did but did not fix it... we also found what some called a "cam position sensor" that the rotor moves through under the rotor... changed that out and have had no issues since... it was corroded and would sometimes work, sometimes not...

Rick

[rt2439]

Sensor.PNG It is part number 11 in this diagram.... sensor

[boatwakes]

Post up your serial number please or compare, this issue affects serial range 0L304600-0M310009. Years ago we ran across the same issue and discovered it was the Intake Air Temp sensor (Mercruiser part #805223T), boat would run fine until shut down for a bit. It turns out, during the previous oil change, the oil had been overfilled and there was a significant amount of oil vapor returning to the intake manifold through the PCV. Unfortunately, the Intake Air Temp sensor was being coated in oil while the engine was running and when shut off and then restarted, would send incorrect air temps to the ECM and cause the poor girl to overfuel like crazy, coughing, sputtering and eventually stalling. There was only a short run of Mercruiser serial numbers (see above) that use the Intake Air Temp sensor but check the link to the diagram to see where it's located, it's item number 17.

https://www.perfprotech.com/mercury-...135?model=1467

[Schnate]

gumby "what do you mean when you say it runs fine till it gets hot? Is it overheating or normal temp hot then runs bad?"
- I mean it runs great, until the engine gets warm. (up to 160-170°) The engine is not overheating.

----

Thank you all so much for the responses! This is very helpful.

(We just returned from a trip, and I plan to dig into the boat ASAP.)

Engine Block Serial Number: 10243880

More to come here. I will keep you posted on our progress.

Thank you again.

[Schnate]

Well, I just saw this serial number on the side of the bock: L396079

(Sorry, maybe the other number is a part number.)

[Schnate]

OK guys, I may need a bit more advice here... :-/

I just replaced these items below this week and went for a lake test today, but am still having the issue of the engine running for 30-50 minutes just fine, then puttering out when i come down to idle, then not wanting to start:

• Distributor sensor
• MAP sensor
• Air intake temperature sensor
• Coolant temperature sensor
• Engine water temp gauge sender

Things I have not done yet:

• TPS sensor
• rebuild or new alternator & starter

I am back at my original idea of a head gasket issue that comes into play once the engine is warm and has been run a bit.

Any thoughts on this hypothesis?

[Schnate]

gumby I have good cylinder compression from multiple compression tests (145-160s each cylinder), but I am wondering if maybe the gasket fails only when the engine gets warm?

I would love to not open the engine up to do a head gasket job... :-D But on the other hand, I would do that job myself and enjoy seeing how the engine looks inside. But, again, not if I don't have to.

Could this be be a crack in the block or head...? That only messes up when the engine gets warm?

I am with you on the issue being related to fuel. Except I have replaced the fuel pump and fuel pressure regulator just recently, in an attempt to fix this same problem.

It is super hot outside now... Could this be a vapor lock issue? I thought the Cool Fuel system negated vapor lock? I know there is a fuel pump booster kit you can add. Any thoughts on this being related to vapor lock? The only thing I will say, is that the engine has still does this same problem any time of the year. (Even when it is cooler outside.)

The plugs have looked pretty good the past few changes. The front cylinders on each side have been more black/oil covered than the rest of the plugs. I tie this to the forward tilt of the engine, thus catching more oil at the front of the engine.

Not sure about running the boat for 50 min, then continuing to run it. When I ran it yesterday, I drove like I was pulling a wakeboarder. Stopped to pick them up, then get the slack out of the line, then get them up to speed on their run, etc. After running for 30 minutes, I ran back to the dock and it died as I was coming to idle in the no-wake zone. I bet it would continue to run if I didn't slow down...?? not sure though.

In your listing of 500hr items, you mention the alternator and starter. Could one of those be causing this issue??

Regarding fuel, I do try and use non-ethanol when I can find it as well as higher octane. I also use HEET (yellow container) in the tank every time I fill up.

[Schnate]

gumby After reading the link you shared in this thread on June 16th, I am leaning towards this issue being tied to vapor lock.

That fuel pump booster is not cheap, but I am wondering if I should give that a try. My boat has the cool fuel system with one fuel pump, which I have replaced, but maybe things are just getting too hot??

I'll let you know if I try this out, and if it solves anything.

[Schnate]

gumby I like your idea about the insulation on the fuel lines under the engine. I may also do something to the fuel rails as well..? Maybe the heat reflective wraps?

Regarding the spark plugs, I just meant during the last 2 spark plug changes. (1) when we first purchased the boat 5 years ago, and (2) when this issue cropped up 1 year ago.

Yes, AC delco spark plugs.

No, I have not ohm-ed the injectors. I just keep coming back to the fact that the engine runs so good at first. All the power in the world, any rpm. I would think the injectors would act up all the time if they were acting up. (Not just after 30-50 minutes.)

I don't think we are having any of those issues tied to a crack in the block. I am just grasping at straws at this point.

A fuel delivery problem is where my money is. I am hoping that the booster pump solves the problem. I am also planning to review the fuel tank pick up tube filter, and any over fill valve. Perhaps there is a slight vacuum on the fuel from the tank?? Doesn't hurt to look in there especially with our boat's age.

[Schnate]

gumby I guess I haven't replaced the TPS because I didn't think it would be tied to this issue. But, if you are saying it could be tied, I might as well swap it out...

Thanks.