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Engine damaged by sticky carb accelerator pump???

T

TJSepka

New member
I had my 1985(?) Mercruiser MCM 230 5.0L engine rebuilt due to freeze damage. I (finally) got the boat back the day before leaving for vacation. I launched the boat and had a very difficult time to get it to start, and it would die when shifting from reverse to forward. When I did get it to finally start, I took it out for a run and kept it at or below 3000 RPM. On the third lap around the lake, the engine slowed, stopped and then would not turn over. I seemed to be VERY hot, even though the water temperature gauge never went above 160 degrees F.

After a ruined vacation, I returned the boat to the place that did the work on my way home. The newly rebuilt engine had overheated and seized. The engine is equipped with a 4 barrel Rochester carburetor (P/N 1347-8296). The repair facility told me that the carb's accelerator pump was sticky and diluted the oil with fuel, causing the engine to overheat and seize.

Does anyone have any experience with, or believe that the engine's catastrophic failure (with no indication of overheating) was due to a sticky accelerator pump? Is this a reasonable cause of the failure?

When I started the engine in the spring it fired up just fine, as it always had, and idled normally (without roughness). I found the freeze damage when the temperature gauge wasn't coming up after a couple of minutes of starting. That's when I found it leaking water (I was running it with muffs for cooling). I don't believe the engine had a chance to get very hot, since I shut it down immediately when I saw the leaks. There was no indication of water getting into and mixing with the oil. The carb seemed to be performing just fine at that time.
 
..."The repair facility told me that the carb's accelerator pump was sticky and diluted the oil with fuel, causing the engine to overheat and seize."

Absolute nonsense! Not only would a "sticking accelerator pump" not cause fuel leakage, a rich condition would make the motor run COOLER, not hotter.

Since I have the exact same motor in my boat, I feel really bad for you. Why did it happen? I suspect a loss of oil pressure, for that's what usually causes engine seizure. It's possible you had a leaking diaphragm in the fuel pump. That would cause serious dilution of your oil (and should be detectable by smell and a higher than normal oil level).

Jeff
 
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TJSepka said:
I had my 1985(?) Mercruiser MCM 230 5.0L engine rebuilt due to freeze damage.
Define "rebuilt" for us.
Was this a complete over-haul or a partial repair, such as cylinder head work?
If complete over-haul, did the company give you instructions for a "break-in" period?
If so, was the oil level checked during the break-in period, and did you notice a high reading on the dip stick?

I (finally) got the boat back the day before leaving for vacation. I launched the boat and had a very difficult time to get it to start, and it would die when shifting from reverse to forward.
That may be an issue with the SI or SA (shift interrupt/shift assist).

When I did get it to finally start, I took it out for a run and kept it at or below 3000 RPM. On the third lap around the lake, the engine slowed, stopped and then would not turn over. I seemed to be VERY hot, even though the water temperature gauge never went above 160 degrees F.
If you made only three runs during this period, and if gasoline was entering the crankcase, it would be more likely a fuel pump diaphragm issued.

After a ruined vacation, I returned the boat to the place that did the work on my way home. The newly rebuilt engine had overheated and seized. The engine is equipped with a 4 barrel Rochester carburetor (P/N 1347-8296). The repair facility told me that the carb's accelerator pump was sticky and diluted the oil with fuel, causing the engine to overheat and seize.

The accelerator pump is only operational during a change in throttle plate position from low speed to high speed. In other words, it becomes functional ONLY when the throttle is moved from closed towards fully opened.
There is nothing that I can think of that would cause an accelerator pump (sticky or not) to dump quantities of gasoline into the cylinders.
Plus..... with the engine running, any excessive fuel would first make it's way into the cylinders of which at that time are undergoing combustion.
(the fuel would need to get past the piston rings in order to reach the oil)

Is it possible that you misunderstood what the repair facility told you?


Does anyone have any experience with, or believe that the engine's catastrophic failure (with no indication of overheating) was due to a sticky accelerator pump?
Well, first I'd like to hear what a "sticky accelerator pump" consists of.

The accelerator pump's job is to fatten up the fuel/air mixture during the transition from low speed fuel metering to high speed fuel metering.
In other words, it eliminates an otherwise "flat spot" that is typically encountered during this transition.
Once a throttle position is held, the accelerator pump becomes in-active.

Accelerator pump:
......... fuel chamber supplied by carburetor's fuel bowl.
......... piston actuated by linkage connected to primary throttle plate arm.
........ check ball valve that allows fuel into the chamber on the "up-stroke" of the piston (throttle plates closing).
........ small fuel stream into primary throttle bores ONLY during throttle plate position change (from low to high).

Is this a reasonable cause of the failure?
I'm with Chris....... NO!

When I started the engine in the spring it fired up just fine, as it always had, and idled normally (without roughness). I found the freeze damage when the temperature gauge wasn't coming up after a couple of minutes of starting. That's when I found it leaking water (I was running it with muffs for cooling). I don't believe the engine had a chance to get very hot, since I shut it down immediately when I saw the leaks. There was no indication of water getting into and mixing with the oil.
The above would not be relevant to today's issue.

The carb seemed to be performing just fine at that time.
Again..... I'm with Chris. I don't see how a Q-jet carburetor (on a running engine) could cause oil dilution without leaving some tell-tale signs.
A leaking carburetor may cause a running engine to run rich, but none-the-less, this fuel would have undergone combustion.

Keep in mind that when the engine was shut down, the fuel supply to the Q-jet would have been cut off.

If gasoline did enter the crankcase (in an amount that would cause dilution and damage), it is more likely to have come from a ruptured fuel pump diaphragm.
Click to expand...
 
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Thanks everyone for your responses!

@RicardoMarine: Great info and I appreciate your taking the time to go through my post in detail. I'll add additional info to your responses as soon as I can go through it in detail uninterrupted, hopefully in the next day or two. Your detailed response deserves that type of attention from me, and hopefully everyone might learn something from this exchange!
 
RicardoMarine said:
...... I don't see how a Q-jet carburetor (on a running engine) could cause oil dilution without leaving some tell-tale signs.
A leaking carburetor may cause a running engine to run rich, but none-the-less, this fuel would have undergone combustion.

Keep in mind that when the engine was shut down, the fuel supply to the Q-jet would have been cut off....................
Click to expand...

Many Q jets after years of service require epoxy to keep the fuel from leaking out the well plugs into the intake while the engine is off. Run your engine enough times and this will dilute the oil in the pan.

And yes it will run into the cylinder and go right past the rings.
 
Docksidemarineservices said:
Many Q jets after years of service require epoxy to keep the fuel from leaking out the well plugs into the intake while the engine is off. Run your engine enough times and this will dilute the oil in the pan.

And yes it will run into the cylinder and go right past the rings.
Click to expand...

Chris, you are correct, and I too have seen this with the Q-jets.

In post #1, the OP said "The carb seemed to be performing just fine at that time."
I am curious as to why this was not noticed during the break-in period oil level checks.

None-the-less, I don't believe that the carburetor's accelerator pump is a likely candidate.

.
 
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