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Author Topic: 2006 Verado 250 with erratic idle/dying at low rpm  (Read 527 times)
Pinelslandporsche
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« on: November 10, 2019, 03:34:35 pm »

I've become very frustrated with my port 2006 Verado 250. I run twin 250's on a 30' CC, both motors have 1255 hours on them, both have received Whipple flashes and both have been well maintained. The starboard motor runs perfectly, like a singer sewing machine, the port motor developed an erratic idle a year ago. When the port motor would be fired up it would come on, idle down to 560 rpm and then begin to hunt around from 400 rpm's to 630 rpm's, some times it would die idling at the dock. If it didn't die at the dock it would die when I slipped it into gear leaving the dock. It would surge or chug along and die if I didn't increase the rpm's over 850 rpm's. I lived with this for awhile hoping it would somehow cure itself.   

Determined to solve this problem I became a member here and started doing my research. I verified all of the following:

1) my pencil coil's are all new or nearly new.
2) I changed the plugs with new OEM, the old plugs showed rich fuel burn and the motor burns at least 10% more fuel than the starboard.
3) I checked all my electrical connections, cleaned them and used anti-corrosion as specified
4) I did a compression test
5) I checked the fuel pressures with a verado certified tech, all good
6) I changed the map sensor and boot as it wasn't functioning per the computer, but this didn't solve the problem.
7) I replaced the fsm and vent tube, this didn't change anything.
Cool I swapped the fuel rail to see if I had a bad injector, no change and the starboard motor continued running perfectly
9) I swapped the cam sensors, no change. I watched the timing on the tech's computer and all was perfect.
10) I had Whipple reflash the ECM's with no change at all. Starboard ran perfectly, nothing changed with the port.
11) I swapped FSM assemblies between engines. No change with either motor.
12) I removed the muffler and flooded the exhausted port with cleaner, hoping maybe an exhaust restriction was causing the engine to smother itself, but nothing changed.
13) I checked all the electrical connections, from the throttle controls back to the engine, sensors, ecm and pins, etc...
14) I checked the batteries, the battery connections, the voltage, all things electrical
15) I changed the fuel filters on both motors, I run the red filters plus racor in line filters

After all this I burned up the gas in the tank and filled with new gas including Yamaha ring-free. I ran the boat for about 4 hours continuously at varying rpm's and after 4 hours I ran the rpms up to 6300, fully trimmed in smooth water. After several minutes at WOT the port motor backed itself off about 500 or 600 rpms. I'm not sure why as it really didn't give me an alarm or a warning. I'm not sure if it was because of water temp, oil temp or a pencil coil, but I took the boat down to idle and it hunted and died. After a couple more minutes I eased it back up on plain and went back to WOT, same result with the port motor backing itself off 500 or 600 rpms. I reduced my rpm's down to around 4500 and the boat ran fine back to the dock where it died entering our canal when I pulled the throttles back to idle. Re-fired and returned to the dock. At this point I've had 2 verado certified techs look at it, I've spent a bunch of money on time and some parts and I've got nothing to show for all the effort. Can anyone think of anything I or the techs have missed? I'm concerned maybe I have a valve hanging up, just not sure if it would be intake or exhaust. I know the timing is good so it would seem to rule out the timing chain. Other than this recent issue at WOT the motor runs flawlessly over 850 rpm's. Anyone with some ideas of something I've overlooked would you please share it? I'm getting the end of my chain! LOL!!!!   
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Glen
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« Reply #1 on: November 10, 2019, 03:45:40 pm »

i know you said you replaced it but a vent tube that gets crimped or disconnected can cause a lot like you are experiencing.....

also, many here that run into problems like this can have an intermittant  short in the harness somewhere.
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Pinelslandporsche
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« Reply #2 on: November 10, 2019, 05:15:56 pm »

Glen, thank you. Yes, the vent tube was replaced and I swapped the entire FSM Assembly out with the starboard motor. Both FSM's are functioning properly on the Mercury computer. The first tech who worked on it for me suggested installing a new harness as a last resort.

I thought about going thru the harness, connection by connection, using electrical connection cleaner and some type of appropriate anti-corrosion grease/lube to see if that helped. I looked at some areas in the harness where others have reported chafing or grounding and could find any evidence of anything that would cause a problem.
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Pinelslandporsche
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« Reply #3 on: November 10, 2019, 08:34:02 pm »

Is their a chance my supercharger is causing this?
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Glen
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« Reply #4 on: November 10, 2019, 09:16:03 pm »

Itís really impossible to tell from here whatís happening without putting hands on it. You need to find a mercury tech thatís got the mercury laptop ,hook it up and see what code is giving you and telling you what is not working. This would not be the supercharger,  when a supercharger goes out you got approx 5000 RPM from the engine and nothing more.
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Pinelslandporsche
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« Reply #5 on: November 10, 2019, 11:26:08 pm »

Thanks Glen, I've had 2 certified verado techs put the laptops on it and the only code we ever got and the only alarm that ever sounded was for the faulty MAP sensor. Other than that, nothing but the erratic idle and it dying. All the pressures are within spec, all the pumps are activating at the correct times. We run it with the laptop connected and can't get a code or an alarm, yet the engine idles erractially, dies at low rpm and will chug or surge unless the revs are over 850 rpm. It's very frustrating as I've checked every single culprit you would usually expect with this motor.
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thunderstruck
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« Reply #6 on: November 11, 2019, 09:48:07 am »

A valve not closing would cause a backfire in the intake or exhaust and there would be no compression in that hole.

Do all the plugs look the same?

If the motor dropped 500 rpms all of a sudden with no alarm it is almost surely an ignition issue.  The computer cannot determine if the coil is developing voltage on the spark plug side or if the plug is actually firing so no alarms will be generated from plug side ignition issues.

Replacing the coils again or swapping with other motor is an option.

Keep watching the plugs just because they are relatively new doesn't mean they are not fouled.

If coils and plugs are good I would look at wiring harness issues (swap or replace) and/or swap ECMs.

Also go back over the list of items you have looked at very carefully and try to root out anything that was not verified by the testing you have done.

If ignition is not the issue maybe swap throttle control assembly and the boost control assembly.  (One at a time.)
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Pinelslandporsche
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« Reply #7 on: November 11, 2019, 10:06:58 am »

@thunderstruck, thanks for the input. Compression was very good in all cylinders. Only thing I didn't do was a leak down test. Thank you for reminding me about the throttle bodies, I swapped those back and forth as well, no change. On another board they suggested water getting into the air side of the supercharger affecting the the after cooler? Also water getting into cylinders 5 and 6... I'm truly confused at this point.
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thunderstruck
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« Reply #8 on: November 11, 2019, 03:12:25 pm »

Good points from the other site.  You can take the air tube off between the blower and charge air cooler and look for water or oil (the supercharger can leak oil into the intake).  Take the plugs out and get someone to roll the motor over and look for water mist pushing out the spark plug hole with a light. Water could be the charge air cooler leaking, head gasket leak.

Keep watching the plugs.  Let the motor idle.  If you get the same symptoms shut the motor off and immediately pull the plugs.  Water/oil/excess gasoline will show up on the plugs if that is the problem.  I can't imagine that the motor would run up to WOT rpm at all with a significant amount of oil/water/gasoline leaking into the motor but strange things can happen.

Also try swapping the boost control valves. ECM Swap??  Don't forget about the wiring harness, swap or replace.

A leakdown just pinpoints the cause of low compression.  No need for a leakdown if compression numbers look good.

You need to start eliminating problem causes and writing them down.  Document all your possible causes and then write down the troubleshooting that confirms or eliminates the source of the problem and move to the next troubleshooting.
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Pinelslandporsche
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« Reply #9 on: November 11, 2019, 06:28:33 pm »

Alright thank you everyone for these suggestions. Ive been thru everything so far with 2 certified techs and the harness was where we left it, nothing about the supercharger. I'll give these suggestions a shot.
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Pinelslandporsche
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« Reply #10 on: November 12, 2019, 08:30:53 am »

It's also been suggested I check the isolator boot on the front of the engine.
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Pinelslandporsche
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« Reply #11 on: November 12, 2019, 11:10:48 am »

How long does it take to swap engine wiring harnesses?
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Glen
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« Reply #12 on: November 12, 2019, 03:07:30 pm »

several hours....depends on the boat and  how it was rigged....
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