3.0 High RPM Misfire

sgtnoah

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Last winter I put a new long block in my boat, the old motor broke a ring and did significant damage to a cylinder. Other than this damage, the boat ran great, was able to pull full nameplate RPM (with current prop, of course), and otherwise had no operational issues.

Fast forward to the present time. Long block replaced, transferring all associated parts from old motor. Manifold and riser looked wonderful, low hours and fresh water use only. Working my way through the 10 hour break-in cycle that the vendor recommended, everything seemed fine until the stage where higher rpms were allowed. There was a severe misfire about 3500, sounded like a coffee can full of bolts rattling. I did the following:

- Rechecked the timing and advance curve, 1 degree after TDC as stated by engine manufacturer (new from GM Marine). Also checked the advance at higher RPMs against the curve in the book. All good.
- Fresh fuel in tank, 91 octane
- New fuel filters
- New cap and rotor
- Rebuilt the carb, being sure to run a soft copper wire through all passages. Main jet and power valve should handle high rpms, ensure they were cleaned and clear
- Built a test hose with fuel pressure gauge and clear section to test for bubbles. Fuel pressure was low, bubbles at high RPM. Replaced pump. Rechecked, no bubbles, 5-6 psi.
- Now I can get to almost 4000 before misfire happens, sounds horrible above that though.
- Plugs gapped correctly and look fine.
- Engine serial number 0W042772, has electronic distributor.

I don't want to start throwing parts at it, but am at a bit of a loss here.

Any advice would be greatly appreciated!

Thanks,
Pete
 
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alldodge

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First are you putting the motor in base timing mode when timing is adjusted?
According to your serial number timing should be set to 2 ATDC

1? BTDC8 / 1? ATDC9 / 2? ATDC10 10Serial number break: 0L341000 and above.
 

sgtnoah

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Thanks for the reply! I jumped the two wires together to put it in base timing mode, it began to run really rough as it should. The long block that I put in is brand new, not a reman, and had a tag on it to set the timing to "-1 Degree BTDC" -- this was a bit confusing at first, but I interpreted it to mean, 1 Degree ATDC.

To double check the timing, I used the chart in the service manual that tells you the timing vs. rpm. My timing light has the advance function, so I was able to verify that the timing was where it was supposed to be through the rpm range (when not in base timing mode).
 

alldodge

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In that case I would suggest contacting the engine builder and ask if they can advise.

Otherwise I'm going to lean toward it's running lean. (Had the issue at 3500, changed the pump out and it has made it to 4000)
Lean can be caused by the float level set to low, jet size and/or restriction in the carb.
 

sgtnoah

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I made a few changes and then took it out to the river for a test today:

- Took the car apart again just to see if I missed anything. Ran copper wires and compressed air through all high speed passages -- main jets, power jets, and venturi assembly. All were clean and clear. Replaced needle and seat and set float height as per instructions that came with kit.

- Called distributer and requested more info on timing "-1 BTDC", got some conflicting info from them. Ended up leaving it at 1 degree after. This should be the more conservative setting from a pre-ignition standpoint, correct?

- Tested coil as per the book and ordered a new set of plug wire (just because???)

- Checked total advance on the water, it's 23 degrees at around 3000 rpm and gets no higher.

Still getting a noise at 4000 rpm and power tapers off sharply. With the previous motor it would pull this same prop to 4600.

I'm stumped.

Thanks in advance for any thoughts or advice.

-- Pete
 

alldodge

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Sounds are had the hear with words, can you record and post?
 

sgtnoah

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I didn't have much luck with the recording, between the wind and normal mechanical noises, the problem sound didn't come through. I've tried a few more things since last posting. Here is a recap:

2005 Bayliner 175, Mercruiser 3.0

Issue: High RPM Miss. Idles fine, pulls strong and runs well till 3800 rpm. Will start to miss/ping above that point.

The backstory: The boat sat for many years without being fogged first. The rings on two pistons scored the cylinder walls upon getting it going again. The compression was good, so I ran it this way for two seasons. It ran well and pulled hard up to 4500 rpm with a 20 degree, four blade prop (the currently installed prop). The oil consumption was excessive, so I put a brand new long block in it over the winter. It started up and ran great after the replacement - I followed the break in instructions and it's run great -- just won't go over 3800 rpm. It sounds like either a miss or pinging / detonation. Of course I did not run it that

way, as soon I the noise started, I pulled the throttle back.

What I've Done To Try To Fix It:
- Set timing to 1 ATDC, 23 degrees total advance.
- Cleaned and put a kit in the carb (it was not dirty - used fuel treatment last winter)
- Changed fuel filters
- Changed anti-syphon valve
- Made a test hose with a fuel pressure gauge and clear section to check for air bubbles
- Initially the fuel pressure would drop to zero at higher rpms and show some air bubbles
- Replaced the fuel pump and now no bubbles and 4-5 psi at all rpms
- New Rotor, Cap, and Spark Plug Wires
- Replaced ignition coil
- Changed to a solid needle in the carb, set float height as per the manual
- Cleaned Spark Arrestor
- Took exhaust elbow off to make sure that the flapper valves were intact and moved freely
- Enlarged main jets from 1.55mm to roughly 1.60mm to see if a bit more fuel would help (no change)

This issue is really driving me crazy, and guidance would be much appreciated!

-- Pete
 

Bob79

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Maybe try what All Dodge said about your engine serial number and timing being 2atdc.

He also advised me on this a couple of weeks ago and once i done it all the noise at high RPM on my engine were gone.
 

sgtnoah

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Thank you both for the input. Making some progress, but no cure yet. Adjusted lifters and checked timing again.

I'm 95% certain that it's a fuel issue. Plugs are way too white, and a very small amount of the electrode is eroded. Why is it running lean? Here are the measurements and tests that I have done regarding this:
- Fuel filters changed
- Fresh gas
- New check valve.
- 5 psi fuel pressure
- Carb apart numerous times:
Needle and seat replaced
Float level set as per manual
Jets cleaned
Power jet and horizontal passages clean
Blew air through all holes and made sure they were clean

The old motor was a 2005 and ran great (carb wise anyway), the new long block is current production. Could something be changed that would make the carb not correctly calibrated for the new block? I see that the new 3.0 engines have a different carb -- is this my issue? New long block with old style manifolds / carb?

I'm at a loss at this point - please help!

Thanks in advance,
Pete
 

alldodge

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way too white, and a very small amount of the electrode is eroded
Why is it running lean?

Your cleaning and adjusting the carb, but its still no supplying enough fuel. I would get another carb

The production line on old carbs stopped so they started building other carbs. Do see the newer style carb being any better then a properly tuned original
 

sgtnoah

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Thank you for the quick reply, I really appreciate it.

I looked up the last serial number of the Mercruiser 3.0 and came up with a 3310-8M0084193 part number. Would this carb bolt up and work?

Thanks,
Pete
 

alldodge

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That part number is for the current TKS carb, which is also listed in the serial number series which yours is part of "0W042772"
http://www.mercruiserparts.com/3-0l-gm-181-i-l4-0l097000-thru-0w30199

I don't remember seeing that you had a TKS type carb

The manifold for your motor is the same manifold which is used on the upper serial number 1A020544 and up, and with that it does appear that it should fix.

Now if you don't have the TKS carb but have the Merc type then the issue is yours is not wired for it. Can ask Fun Times to see if I'm missing something
 

Fun Times

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From new your engine has the Mercarb, the TKS carb you found after your serial number started in early 06. For you to install the TKS carb, you'd have to order the TKS kit Merc # 865959A01 which wouldn't necessarily resolve your problem unless you have a severer unseen problem with the carb you have now. While the intake manifold did change part numbers mostly due to the exhaust endcap design, the number change happened before your original engine was made.. So the newer one you have now would be the same even though they switched carb designs in 06+.

The only difference between the two carbs that you have to be more aware of when ordering a full repair carb or engine kit is the base gasket design as they are different but both come in the gasket kits sold. The TKS base gasket (27-866027) is more of a open design vs the older Mercarb (27-807982 1). ​

You're fuel pressure is a little low at 4 - 5 psi as the service manual has it at 6 - 8 psi @1000 RPM, http://www.boatinfo.no/lib/mercruiser/manuals/mercruiser26.html#/20

Do you recall the thickness of the fuel pump mounting gasket changing at all?

Is the cylinder head new with the long block or original and swapped over? If original, was it rebuilt at all or all looked good? Possibly tuliped or misadjusted valves.

If the sound is pretty loud, the valve Valvetrain would be suspected.

What spark plug part numbers are you running and have you installed/tried new spark plug wires?
 

sgtnoah

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Thank you both for the fantastic replies!

Answers to your questions:
- Running a thick (roughly 1/4") pump gasket that appears to be made of cork or something like it.
- Cylinder head came with long block. I re-adjusted the lifters yesterday, they were less than the 3/4 turn suggested, though not by a large amount.
- Replaced the plugs and wires, used the MR43LTS plugs.
- I'll be out on the river today with the fuel pressure gauge attached to see if anything has changed and to see if the valve adjustment helped.
- The fuel system is has three filters, could this be an issue? They go: Tank -> Anti-siphon valve -> Racor Water Separating Filter -> Filter in the fuel pump -> Carb Inlet filter. All filters have been replaced.
- The plugs are just screaming out lean... There is a bunch of info on the net for "too rich", but not a lot for "too lean"

Thank you in advance for the help, I am thankful for the expertise!

-- Pete
 

alldodge

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I would say yes, that is to much restriction in the inlet side. I would remove the filter in the pump and try it, the racor is all you need.

Is the racor filter mounted even with the top of the fuel tank or maybe a bit lower? The issue is you don't want it much higher then the tank, the pump will have trouble lifting the fuel
 

sgtnoah

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Just got back from the river and have a bit of an update here:

As suggested, I removed the fuel pump filter. I also re-installed my fuel pressure gauge / clear section of line tool to see how the pressure was and if any air bubbles were leaking in somewhere. No bubbles at anytime. Fuel pressure 6 psi at idle tapering off to 3 psi at 3900 rpm, where the missing starts. This is what's baffling me.

When I put the long block in, I put a new Mercruiser OEM fuel pump on it. It the pressure would drop off quickly above idle, down to zero at 1500 rpm. I disconnected the line from the carb and cranked the starter with the line in a bucket, it gushed tons of fuel. After removing the pump, the wear mark from the cam lobe was in the correct place.

That pump got warrantied and I put in a new OEM pump. Now we are good to 3900 rpm before it tapers off.

It's almost like the pump is "floating" at high rpm.

The lack of air bubbles and strong flow into a bucket makes me think that it's not a restriction or an air leak... But what could it be?

Should be seeing 5-6 psi at high rpm if I'm not mistaken?

Thanks in advance for your thoughts,
Pete
 

Fun Times

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Do you still have the original fuel pump? Did it work before installing another one? If so, you may want to try the old one.
Also using an portable fuel tank with an outboard primer bulb, try bypassing the new water separating fuel filter base assembly to see if it helps keep the fuel pressure up when adding extra pressure by hand at all RPM's.
 

alldodge

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I put a new Mercruiser OEM fuel pump on it. It the pressure would drop off quickly above idle, down to zero at 1500 rpm

Pressure is created by a pump pushing against a restriction. So your pump drops quickly after coming off idle. This is where the main jets start to open. Which also leans toward sucking air but clear line says that isn't happening.

Issues:
Pump looses pressure quickly which would lead to flooding
Plugs are white and eroding which leads to lean condition

These are two conflicting issues

The issue which is unknown is the cam lobe, are we getting a full stroke for the pump?

How to test:
Disconnect power to coil.
Remove fuel pump
Measure the distance the rod moves as the motor is rotated
 

sgtnoah

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Unfortunately I don't have the old pump. When I put the long block in over the winter, I tossed the old pump, thinking that after 10 years the diaphragm might be questionable. It did work great though and the previous block pulled hard all the way to 4600.

The pump on this engine is the lever arm type: 8M0073435 so I'm not sure how to measure the stroke, I can say that the wear area from the cam is in the right place on the lever.

Just to be sure that there are no restrictions, I hooked up a facet electric pump to where the mechanical pump connects, put the other end in a bucket and let it run. Fuel gushed out at a high rate, so pretty sure there are no obstructions.

I suppose the next step is to either add a electric marine lift pump in-line, replace the mechanical pump again (on the second O.E.M. one now... could two new pumps really be bad?), or completely replace the mechanical pump with an electric and block it off with a plate on the block.

I do not have an outboard tank with a primer, so probably would go the lift pump route for further testing.

Am I missing something here?

Thanks,
Pete
 
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