1998 Merc 7.4 MPI won't start

[SDSeville]

Hi Everyone,
I have a Merc 7.4 MPI (300 hours on it) in my 1998 Maxum 2700 SCR. I have had the boat about 16 months and keep it in a slip. I have only put about 15 hours on it in the 16 months. When not in use, I have started it every month and ran until it was at operating temp. The starter has always turned a little slow but it always started in a few seconds.

last week, it just turned (slow as usual) and did not start. I thought it might be the battery, so I replaced it and no difference. This is my first fuel injected engine, so I am not sure where to start on this one. BTW, I have 1/4 tank of untreated (I know, dumb...) year old gas in it.

Any thoughts on where to start troubleshooting?

Thanks in advance for the help.

 

[tpenfield]

Check the basics, if you have not done so already.

Kill switch ? exercise the switch a bit

Circuit breakers for the ignition - exercise those too

I assume the engine cranks slowly in the new battery . . . just does not start up (?)

If it comes to this . . . remove the spark plugs and see if you have any water or dampness in the cylinders.

While the spark plugs are out, crank the engine with the starter. Position one of the spark plug wires where you can check for the presence of a spark.

The MPI enignes set a cranking injector pulse that is about 6X the normal idling pulse. They also use the distributor module for spark timing during the cranking, until the ECM takes over once the engine starts up.

Your fuel is becoming an issue, but I am not sure it is the show stopper at this point.

If you have spark and suspect a fuel issue (like clogged injectors) you can check the fuel pressure at the valve on the fuel rail.

See what you can determine and report back.

 

[QBhoy]

Diz cap and rotor.

 

[Scott Danforth]

motor needs to spin well over 200 RPM to fire. so if it is turning slow, even with your new starter, check your battery cable connections at each end.

every connection needs to be clean enough to eat off of and should be shiny. I suggest 3M scotchbright or a wire brush in a drill. pull each connection, clean the terminal, clean the cable lug. if the cable is stiff or shows any signs of green corrosion on the cable itself under the insullation......buy new cable.

 

[SDSeville]

Thanks everyone! I will run down to the harbor on Monday and work through these lists.

I will let you know how it goes.

 

[SDSeville]

Checked battery connections and added new gas and stabilizer yesterday. No change. I tried to change cap and rotor, but stripped the cap bolts with a phillips and didn't have the correct socket with me.

Now that I am back home (where most of my tools are), it looks like both 5.5mm and 7/32 sockets fit pretty well. Does anyone know which is the correct size for the cap? I would really hate to round the bolt if I am off just a little.

Thanks

 

[SDSeville]

Can I use PB Blaster to get the distributor cap bolts off or is it too flammable to be near all that spark stuff?

 

[SDSeville]

OK, so cap and rotor were badly corroded, but still didn't start after replacement. Next step will likely be the starter, but it is crammed way in the back on the bottom starboard corner. There was barely enough room for me to squeeze back there to look at it, so replacing it is going to be real fun.

Have any of you replaced the starter on a 7.4? Any suggestions...besides having someone around to pull me out when I get stuck back there?

 

[scoflaw]

Not sure it was you, but didn't you post a you tube video of the slow cranking ? If that was you, that's not too slow to start. I'd be looking elsewhere for the no start issue

 

[SDSeville]

Not sure it was you, but didn't you post a you tube video of the slow cranking ? If that was you, that's not too slow to start. I'd be looking elsewhere for the no start issue

Wasn't me, but I will take a video of it this evening and post it.

I think I found the video you were talking about -- http://forums.iboats.com/forum/engi...drives/10500507-merc-502-slow-crank-or-normal

Mine cranks much slower than that one.

 

[alldodge]

Have replaced and easiest way is remove the exhaust manifold. A new exhaust gasket is not that much. Get some 3/8-16 thread all rod at the hardware store to place on the two outside ends. Remove those bolts, install threaded rod and then remove the rest of the bolts. Need it more for going back on then off, but off is also good.

Before you do the starter, look good at the terminals and see if they are also corroded. Would also suggest get some jumper cables and connect to starter post then to battery positive. If it still cranks slow, pull it and replaces with permanent magnet type

 

[tpenfield]

. . . Next step will likely be the starter, but it is crammed way in the back on the bottom starboard corner. There was barely enough room for me to squeeze back there to look at it, so replacing it is going to be real fun.

Have any of you replaced the starter on a 7.4? Any suggestions...besides having someone around to pull me out when I get stuck back there?

Me . . . Me . . I changed the starter on a 7.4. It was wicked fun, only took 4 hours :facepalm:

I'm with scoflaw on this . . . may not be the starter, since it is an MPI. It could be a whole bunch of other stuff that a computer will have to analyze.

 

[SDSeville]

Me . . . Me . . I changed the starter on a 7.4. It was wicked fun, only took 4 hours :facepalm:

I'm with scoflaw on this . . . may not be the starter, since it is an MPI. It could be a whole bunch of other stuff that a computer will have to analyze.

Oh great. If it took you 4 hours, it will take me all weekend. I am going check the starter terminals (neglected to check those earlier) and try the jumper cable trick.

 

[SDSeville]

OK, so I was finally able to stand on my head long enough to get a good look at the starter connections and....
So looks fine right?




I didn't have my tools with me, so I will have to go down there in the next couple days and clean it up. I need to break down and buy an entire set of tools to keep in the locker at the slip.

 

[alldodge]

Suggest either replacing the cable end (will need a big crimper) or the cable. The corrosion has gotten under the insulation jacket

 

[SDSeville]

So I finally got down to the harbor yesterday to clean everything up. I tried to start it before I got to work and the starter had gone from turning very slowly to just a loud click and no turning at all. I cleaned everything up and replaced the positive battery cable and connectors that attached to the starter. I was literally on my head in the back of the bilge off and on for more than an hour. When I was done...just the same loud click/clunk. Both batteries, an 8D deep cycle and a brand new 4D starting battery, were fully charged (both around 12.5 on the meter).

I noticed the negative battery cable looked pretty corroded too, but it is in an even harder to reach area than the starter. I guess my next step will be to replace the negative cable (going to be a major PITA) and the starter. If that doesn't work, it will be time to call the marina and have them send a guy out to my slip to get this thing running. I am sure that will not be cheap.

 

[alldodge]

Have you tried the jumper cables yet? They can also be used both for a ground connection

 

[SDSeville]

Have you tried the jumper cables yet? They can also be used both for a ground connection

No, I have not. I was so sure that the horribly corroded positive cable and connections were the problem that I bypassed that. Thanks for reminding me. I will try that tonight. So do I go positive cable from positive on battery to the starter and negative from battery to a good grounding spot somewhere on the engine?

 

[alldodge]

Since the starter has a new cable between bat and starter, I would connect both cables to the Negative terminal on the battery. The other ends connect to two different locations on the block as a double ground. If it still won't crank, jump across the two larger terminals on the slave solenoid

 

[SDSeville]

If it still won't crank, jump across the two larger terminals on the slave solenoid

OK, thanks. Do you mean the positive jumper cable clamped to both terminals on the slave solenoid at once?