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350 mag electrical problem

1

1973browning

Contributing Member
"A friend of my brother in law

"A friend of my brother in law bought a brand new 350 mag crate motor for a 25 foot Grady White From Mercruiser. He said he spent $16000 for brand new engine system and drive. I guess that was for installation also. (I hope so for that kind of money!) Anyway, he said he has had an ongoing problem at 2200-2400 rpm. Apparently, the engine breaks up at the above rpm range and as soon as you go above or below the rpm range the motor seems to level out. He had a cerified Merc. mechanic work on the boat for quite some time without a bit of luck. Mercruiser keeps telling him from day 1 that he needed a new distributor cap. (What a load of dung!) The cap,rotor,wires and plugs have all been replaced and he still has the same problem. Mercruiser also stated that it could be the ecm but refused to pay for it from day one. Is it possible that he has something wrong with his distributor or coil? I did not know what to tell him so I decided to post for him hoping one of you guys might have an answer or lead him in the right direction. He has 400 hours on the motor presently so it has been going on for quite some time. It does it when the motor is cold and when it is hot but only at those approximate rpms. I guess he has alot of money buried into the thing and I did not want to mislead him into buying parts that he does't need! I do not know enough about the 350 mag motor to help him but figured I would ask you guys in the hopes of putting him in the right direction and possibly help him solve his problems. He was also told that it could be the ecm,distributor itself,or possibly the coil but I guess he doesn't want to keep dumping money in the wrong direction. Any help would be appreciated greatly, sincerely, Tom
P.S. Merry Christmas to all and God Bless!"
 
"What he got for 16000 dollars

"What he got for 16000 dollars is screwed. I just can't imagine how anyone could have charged him that dollar amount.

Anyway i would do a vacuum test first thing. Have him or you post the result here is the links to help him.

http://www.secondchancegarage.com/public/186.cfm

http://www.users.bigpond.com/ergoff/vac1.htm

This is free to do if you have the gauge or 15 bucks any auto store will have one.

This is the first thing i would do.

It will point to a internal problem, and then we can go from there."
 
"Before anyone here can make a

"Before anyone here can make any suggestions we need more info, You say ECM but do not say EFI.

What we need to know is this a carbureted engine or a EFI engine?

Without that basic info we can not go further.

And $16000 for a complete repower, motor, transom and outdrive and labor is not that far out"
 
"Thanks chiefalen and kghost f

"Thanks chiefalen and kghost for your help. The engine is a 2003 350 Mag with EFI and ECM. It is breaking up from the mid 2s to around 2900rpm. It was bought brand new and yes they did the full repower (engine,transom,alpha one generation II). As I said before it runs perfectly at lower and higher rpms. The Merc. mechanics checked out everything but the ECM or could not find anything wrong with it and I guess Merc. was not willing to replace ECM for the guy. They kept telling him that his distributor cap was corroded. The only problem with that is that the cap and wires and plugs were replaced. This has been going on since the day he bought the engine package. I guess at this point, the guy is going to have to pay for what ever parts he needs to hopefully resolve the ongoing problem. I was not aware that the engine was 7 years old until I called him for further info. Just trying to get him a little closer to diagnosing or possibly fixing his problem with a little help from you guys. Oh, one final thing to mention: He had all injectors tested and they were all good. I told him to check out this site tonight to see if you guys had any possible direction for him. Thanks again for all your support! Tom"
 
Have him check the throttle po

Have him check the throttle position sensor w/an ohmmeter. There may be a dead spot. Also check the voltage w/the ignition on and engine not running. Use the 10 volt DC scale. It will register less than 1 volt and should not drop to "0" volts as it is turned from min. to max. throttle. The voltage change should be smooth.
 
"Before you look to A ELECTRIC

"Before you look to A ELECTRICAL PROBLEM why not rule out a mechanical problem. And for free i might add, if you have the gauge.

This problem the way i read is from new, how did the owner buy this setup with no warranty, and if there was a warranty what marina didn't follow thru finding the problem.

And lastly, just for example, wouldn't alot of mechanical problems show up with a vacuum test.

In my humble opinion before the guy spends one more nickle on parts he doesn't need he should perform one, i listed a site Charlie posted and one i found on my own.

16000 bucks jeez this is for one motor and drive?"
 
"List price ~ALL BRAND NEW MER

"List price ~ALL BRAND NEW MERC thru A Dealer

Motor (350 mag complete EFI) ~ $7000 - $9000
Outdrive, ~ $3500 - $4500
Transom Assembly ~ $2500

Labor.........who knows, but I bet it is a lot!!

So $16000 is not far off from a real cost."
 
"Hi, You might want to have a

"Hi, You might want to have a look at Page 3 of 3 of this bulletin below, What is your engine serial number???

Service Bulletin
Bulletin No. 2003-06
Page 1 of 3
V6 and V8 GMEFI with No Start, Engine Miss-Fire or Engine
Shut-off Conditions
Models Affected
Model Serial Number or Year
MCM V6 4.3L MPI 0M322781-0M645782
MCM V8 5.0L MPI 0M320415-0M642305
MCM V8 350 MPI 0M320590-0M642305
MCM V8 MX 6.2 MPI 0M320614-0M642305
MIE 350 MPI Tow Sports 0M310010-0M391684
MIE 350 MPI Inboard 0M310001-0M391684
MIE MX 6.2 MPI Inboard 0M310008-0M391737
Situation
Under certain conditions, an engine may experience one or more of the conditions listed.
Condition/Correction
No Start Condition
This usually occurs at first start-up of the day. After getting the engine started the first
time, the engine starts well for the rest of the day. The problem can occur again the next
outing. Areas with high humidity are more likely to experience this problem. Remove the
HVS distributor cap as soon as possible after experiencing the problem. Look for droplets
of moisture or condensation on the inside of the cap or on the rotor.
NOTE: Do not confuse white powder on distributor cap electrodes as moisture. Moisture
(droplets of water) has to be present under the cap.
If droplets of water are present inside the cap, order a new HVS distributor. Distributors in
Mercury Parts stock have a redesigned vent screen to allow better air circulation inside
the cap to prevent the accumulation of moisture. Engines with a higher S/N than the
ending S/N listed above have the new HVS distributor.
Description Part Number
V6 HVS Distributor 888751A02
V8 HVS Distributor 884794A02


Page 2 of 3 © 2003, 2003-06R V6 AND V8 GMEFI WITH NO START, ENGINE MISS-FIRE OR ENGINE SHUT-OFF CONDITIONS
Engine Miss-Fire Condition
First, make sure it is not spark plugs or some other part that is causing this condition. If
the other normal causes have been eliminated, then check the following as listed for each
type of engine.
1. V6 and V8 engines
a. Whenever looking at a HVS related problem make sure the rotor (under the cap)
is tight on the distributor shaft. This is a pressed fit and you should not be able to
rotate or remove the rotor/metal plate off of the shaft. If you can do either, order a
new HVS Distributor.
b. If there is corrosion or white, powder build-up on the HVS distributor cap
electrodes or on the rotor, replace the rotor and distributor cap to correct the
problem.
NOTE: Ensure no moisture (droplets of water) is present.
Description Part Number
Cap for V6 HVS Distributor 888731
Cap for V8 HVS Distributor 884792
Rotor for V6 or V8 HVS Distributor 884790
2. V8 engines only: If the performance of 1a and 1b (above) does not resolve the
problem, bring the engine to TDC (firing). Remove the HVS distributor cap and verify
that the rotor is pointing at the correct timing mark, 8, on the HVS housing between
the 2 lines shown below. If this is correct, reinstall the HVS cap. Mark a reference
line on the HVS distributor and intake manifold (so you know where the distributor
was located after you move it in the following step). Loosen the distributor's clamping
screw and rotate the distributor 1-2 degrees clockwise (viewed from top), then
retighten the distributor's clamping screw. The engine should now run well. If it does
not, then you can return it back to the original line that you marked before.
8
a
b
mc78144-1
a - Rotor tang b - "8" on housing


More then likely you will find your problem with this next test. Now if this test does fix it, Then I was told two things may fix it One is a new ecm, Or you could try to remove the shift bracket assembly off the engine and mount it to the transom of the boat if there is enough room back there with out bending the cables all up, Making it harder to shift. Mercruiser did say to me one time that the vibrations of the engine is making the ecm think the shifter switch cut out is activating and making the engine run that way at at certain times, So mercruiser just updated the ecm with a new one to fix the problem back then, But now they say move the shifter assembly because it is out or warranty. Now to be honest I do not know myself if removing the shifter assembly off the engine works because i did not try it after they told me, I was able to talk them into a new ECM for the guy a few years back, And the owner had to pay for all the labor and testing time. I'm not sure with this next fix, But if you want to look into it you could call this company that repairs ECMs and maybe they could update your ECM to try to remove the sensitivity in the shifter area of the ECM. [Long shot. But you never know till you try it]Here is the link I was thinking about. http://simonmotorsports.com/ Now if the shifter assembly does not fix this then you should try the first part of this service bulletin with checking for moisture under the distributor cap.


Page 3 of 3
CAUTION
When performing this test, the boat must be in the water. The drive unit will be difficult to
get out of gear so do the test in an area that will allow the engine to be shut off in gear
after the test so that the shift interrupt switch can be reconnected to the engine wiring
harness again before proceeding back to the dock.
3. MCM V6 and V8 MPI Alpha engines only: If you have an Alpha engine with an
engine miss-fire or if the engine shuts off above 1200 rpm, the problem may be
caused by the shift interrupt switch. To determine if the interrupt switch is the cause,
disconnect the switch from the engine wiring harness. Use a small jumper wire to go
between the A and C terminals in the engine harness connector.
BLK/PNK
B
SPLICE 100
A BC
ABC ABC
YEL/PPL
GRY
10K RESISTOR
PPL
PPL
BLK/PNK
c
a
b
mc79432-1
a - Shift interrupt switch
b - Engine harness connection
c - Jumper wire between A and C terminals
If the jumper wire corrects the engine running condition, a new ECM with the latest
calibration should be installed. Alpha engines built above the S/N below have the new
ECM 555. If an ECM is needed, order the correct one listed below.
Model Description ECM Part Number New ECM Starting Serial Number
MCM 4.3L MPI Alpha RWC or non Dry Joint
Exhaust Manifold with FWC
864270T06 0M377671
MCM 4.3L MPI Alpha Dry Joint Exhaust Manifold
with FWC
865000T02 0M378413
MCM 5.0L MPI Alpha RWC or non Dry Joint
Exhaust Manifold with FWC
864271T05 0M603858
MCM 5.0L MPI Alpha Dry Joint Exhaust Manifold
with FWC
865002T02 0M604008
MCM 350 MPI Alpha RWC or non Dry Joint
Exhaust Manifold with FWC
864272T05 0M604450
MCM 350 MPI Alpha Dry Joint Exhaust Manifold
with FWC
865004T02 0M604450
Engine Shut Off Condition Above 1200 rpm
See Step 3 under Engine Miss-Fire Condition. Replacement ECM P/N and starting engine
S/N when the ECM went into production are listed.

Good luck with it.
happy.gif
"
 
"where in the bulletin does it

"where in the bulletin does it discribe the midrange breakup the op stated was the problem?

He didn't post there was a starting problem, or did i miss that?"
 
"If the shift interuptor is se

"If the shift interuptor is seeing a intermitten activation then this could be the sympton that is being experienced. As the rpms increase any mid throttle vibration goes away and is less likely to cause this problem at higher rpm's.

So I think in summary to all the bulletin info IF I read it correctly.

1. Check for water droplets in distributor cap.

2 reset timing 2 degrees "clockwise at distributor" and see if this solves issue. If not reset to original setting.

3. bypass interuptor switch out in the water away from other boats and if this solves the problem then a updated ECM needs to be put in place to reduce sensitivity to interuptor activation. Remeber when the shift interuptor switch is bypassed you will need to shut engine of IN GEAR becuase it will be almost impossible to get to neutral.


A side note, it would appear to me that the interuptor switch is no longer a ON/OFF switch and actually sends a resistance value to the ECM ( I could be wrong) but if that is so the I would think a updated ecm makes the interuptor signal and makes it look more like a ON/OFF signal.


Many Years ago with the old roller type micro switch interuptor it had issue NOT engaging due to the weak lever of the switch and the fix was to bend it, This seems like a similar situation but in reverse, the sensitivity causes a intermitten activation of the switch.

Almost all motors have a partial throttle stumble if you will and this tends to cause a higher level of vibration at that point of rpm's. It makes some sense to me anyway that this could be the issue, especially since MERC has reconized it as such."
 
"Everything that you guys have

"Everything that you guys have posted thus far has provided a wealth of information! I do not know the serial # because I guess he has the boat at a marina all wrapped up and I know this is important and I am sure he must have the paperwork at home. I told him to look at the site last night. Hopefully, he did and he will register to become member of this site. I guess what I am saying is that it is up to him at this point to research and do the tests that you guys provided above. Can't thank you guys enough for your help! Happy Holidays, Tom"
 
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