Logo
FULL SITE MENU

07 350 Mag MPI bad miss/stumble 2k-4k RPM's only under load

B

BigGib

Member
Hi, I have a 2007 Four Winns 240 with a Merc 350 Mag MPI and Bravo 3. It has a pretty bad stumble or miss, only under load, from around 2K to 4K RPM's. It runs great at idle and seems to run fine at WOT.

Serial Number: 0W664460

Here's what I've tried with no luck:

All New Parts:
Plugs and wires
Coil
Fuel filters
IAC and filter
Shift Interrupter switch
Cap and rotor

Fuel pressure at idle is around 36psi, holds steady at 40psi under throttle (even when it's missing)

Put on 2 different scanners with no relevant codes showing up.

Pulled the cool fuel module apart and replaced the rubber bushing on one of the pumps because it was ripped but this didn't fix anything

Plugs look fine when I check them

I'm running out of what to try. May clean the injectors next, but worried it may be the distributor or ECM or a timing chain issue? It's just odd it runs so good at idle and WOT.
 
Also did the injector test on the computer with no issues, but haven't flow tested them. Also cleaned the FPR but haven't replaced.
 
Have you tried disconnecting the in-gear switch on the shift plate?

Your fuel pressure gauge is not accurate then things could be ok, but if accurate pressure should be closer to 43psi
 
AllDodge said:
Have you tried disconnecting the in-gear switch on the shift plate?

Your fuel pressure gauge is not accurate then things could be ok, but if accurate pressure should be closer to 43psi
Click to expand...
Yeah it was right around 40,but held steady.

I haven't tried disconnecting the switch,but I did replace it already.
 
I also tried to get gas today, and you could see the gas in the fill tube like it was over full, but Last time I filled up last season and I've put probably 10 hours on it this year. It kept kicking off and spitting, not sure if it's a clogged vent?

Also not sure if it would run so well at WOT if the vent issue was causing the stumble/miss
 
You may want to consider changing the relays...I'm not a fan of throwing parts at a problem; but I've seen many relays that were over a decade old cause intermittent, non-repeatable issues...just one opinion....
 
BigGib said:
I haven't tried disconnecting the switch,but I did replace it already.
Click to expand...
You said you replaced the shift interruptor, meaning you have an Alpha drive. The Alpha drive will have 2 switches, interruptor and in-gear. If there was only one switch then you have a Bravo drive

The in-gear switch tells the the PCM that the motor is in gear or in neutral. If PCM thinks it's in neutral then it will limit rpm. If something is loose, has some slop the switch might not be working correctly. When throttle goes to WOT it may be in correct spot
 
AllDodge said:
You said you replaced the shift interruptor, meaning you have an Alpha drive. The Alpha drive will have 2 switches, interruptor and in-gear. If there was only one switch then you have a Bravo drive

The in-gear switch tells the the PCM that the motor is in gear or in neutral. If PCM thinks it's in neutral then it will limit rpm. If something is loose, has some slop the switch might not be working correctly. When throttle goes to WOT it may be in correct spot
Click to expand...
Yeah I just have the one switch back on the throttle plate that I replaced, it's a Bravo 3
 
AllDodge said:
OK you replaced the in-gear switch
Click to expand...
1000013024.png


Yeah, number 18 on this diagram.

I've talk to a couple people online that think it could be timing chain stretch throwing off the timing,but I would think it would run bad at all RPMs then?

It only has 260 hours
 
Correct
Don't see it being the timing chain. Has something to do with ignition and that's why I say disconnect the in gear switch
 
AllDodge said:
Correct
Don't see it being the timing chain. Has something to do with ignition and that's why I say disconnect the in gear switch
Click to expand...
I will try that next! I have to get the fuel/vent line figured out first, I'm low on gas and can't currently get any in the boat.
 
I unplugged the in gear switch,no improvement. Also tried removing the vent hose from the tank and no improvement. Also replaced the anti siphon valve with a straight fitting.
 
Need to have the motor scanned, and needs to be done with either Rinda Diacom software or Merc Dealer G3. Need to see a recording of what is happening.

If it's a fuel issue than I would look at having the injectors cleaned and flow tested, but we're just working in the dark without a scanner
 
I tried swapping out the cap and rotor and cam sensor today,and I noticed a lot of rotational slop in the distributor. I believe this should be springing back with some tension for the mechanical advance right?

Below 2k wouldn't it be at base timing and then above 4k it would be fully advanced because the weights would be set? Here is a video of the slop

 
BigGib said:
I tried swapping out the cap and rotor and cam sensor today,and I noticed a lot of rotational slop in the distributor. I believe this should be springing back with some tension for the mechanical advance right?

Below 2k wouldn't it be at base timing and then above 4k it would be fully advanced because the weights would be set? Here is a video of the slop
Click to expand...
There are no springs in the dizzy, the crank sensor does all the advance. The slop is normal gear to gear slop

Timing can not be adjusted once the dizzy has been installed correctly. Most motors get full advance above 1800 rpm

I don't see anything abnormal in the screen shots other than coolant temp being empty or 32*
 
AllDodge said:
There are no springs in the dizzy, the crank sensor does all the advance. The slop is normal gear to gear slop

Timing can not be adjusted once the dizzy has been installed correctly. Most motors get full advance above 1800 rpm

I don't see anything abnormal in the screen shots other than coolant temp being empty or 32*
Click to expand...
I'm curious why when I rotated the distributor a couple of degrees, it caused it to miss fire and pop so bad it wouldn't run above idle?
 
AllDodge said:
There are no springs in the dizzy, the crank sensor does all the advance. The slop is normal gear to gear slop

Timing can not be adjusted once the dizzy has been installed correctly. Most motors get full advance above 1800 rpm

I don't see anything abnormal in the screen shots other than coolant temp being empty or 32*
Click to expand...
I'm curious why when I rotated the distributor a couple of degrees, it caused it to miss fire and pop so bad it wouldn't run above idle?
 
I'm assuming it's throwing the phasing off?

My coil wire plug is so tight on the cap against a bracket, I'm wondering if it may be off one tooth? Maybe the cam offset is also off?
 
Since it's been moved already,I wonder if it's just in the wrong position. Is the only way to verify this checking that the cam offset/retard is at 0 degrees, +/- 2?

I don't currently have a scanner that is showing cam offset.
 
You know your close because the motor will run.
Rotate crank so your close to TDC by watching the distributor.

Use a stop bolt in No 1 spark plug hole and keep rotating it by hand until piston hits bolt and mark damper
Reverse rotation until piston hits bolt again and mark damper

Measure between both marks, that is TDC. Now install dizzy
 
AllDodge said:
You know your close because the motor will run.
Rotate crank so your close to TDC by watching the distributor.

Use a stop bolt in No 1 spark plug hole and keep rotating it by hand until piston hits bolt and mark damper
Reverse rotation until piston hits bolt again and mark damper

Measure between both marks, that is TDC. Now install dizzy
Click to expand...
Thanks, I will try this next!

It runs great at WOT and below 2k,so if this is the problem is can't be far off. It's definitely getting worse though.

I also have a noticable surge at idle now where the RPMs will raise like it's catching itself and then will settle back down. Not sure if that's related?
 
I would pull the injectors and send them to smittysengtrans.com there in Michigan 989 213-7361
He is on Offshore Only and does my work when needed.
 
This may not be your problem but your symptoms sound amazingly familiar to me.
I re-powered with a 6.2 Mag MPI SBC engine in 2007. It was set up for a Bravo drive but I kept the original Alpha One Gen 2 drive that was in the boat, with a swap to 1.47 upper gears.

From day 1 I noted intermittent missing in the 2200 to 3000 rpm range. Not only that, the engine suddenly just went stone dead at about 50 hours. The local boat shop replaced the rotor and the crab style dist cap which solved the problem.
Then the same thing happened again about 40 hours later. I talked with the folks at MerCruiser technical and they said the cap had already been re-designed at least twice to try and cure the failure problem.
To no avail: After the third Merc cap failure I replaced it with one we matched up at the local NAPA store.
That was nearly 100 hours ago and its still running strong.

Now the part that will interest you: It also completely eliminated that annoying miss. It has run smooth at all rpms ever since.
So you might want to replace your dist cap and rotor and see if it helps.

Rod
 
AllDodge said:
I would pull the injectors and send them to smittysengtrans.com there in Michigan 989 213-7361
He is on Offshore Only and does my work when needed.
Click to expand...
I appreciate the help! Going to check the indexing on the distributor probably tomorrow and then I have a local guy who is going to flow test and clean the injectors.
 
rod_stewart said:
This may not be your problem but your symptoms sound amazingly familiar to me.
I re-powered with a 6.2 Mag MPI SBC engine in 2007. It was set up for a Bravo drive but I kept the original Alpha One Gen 2 drive that was in the boat, with a swap to 1.47 upper gears.

From day 1 I noted intermittent missing in the 2200 to 3000 rpm range. Not only that, the engine suddenly just went stone dead at about 50 hours. The local boat shop replaced the rotor and the crab style dist cap which solved the problem.
Then the same thing happened again about 40 hours later. I talked with the folks at MerCruiser technical and they said the cap had already been re-designed at least twice to try and cure the failure problem.
To no avail: After the third Merc cap failure I replaced it with one we matched up at the local NAPA store.
That was nearly 100 hours ago and its still running strong.

Now the part that will interest you: It also completely eliminated that annoying miss. It has run smooth at all rpms ever since.
So you might want to replace your dist cap and rotor and see if it helps.

Rod
Click to expand...
I appreciate the info! What's odd is I've tried 2 new caps and it just keeps getting worse,no improvement at all. The threads are stripped on my distributor screw holes so I have one of those plates under it that holds the cap on tight as well.
 
Back
Top