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Mercury/Tohatsu MFS 30 EFI 4 Stroke weird no start condition

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LeoRie

New member
Hi guys,

I have been reading this forum for quite a while now, and have always loved all the tips and knowledge you shared. Thank you!

This beeing said, here goes my first post :)

I wrench quite a lot on outboards and love the challenge, so I bought a Tohatsu/Mercury 30 hp 4 stroke, which the previous owner said just needed servicing. Turned out there is something wrong with the fuel injection/ECM.

The symptoms:

-Typically starts fine, idles great
-P.o. said it runs fine under load, could'nt test that myself so far
-After sitting a couple of minutes it sometimes (maybe 1 out of 20 times) does'nt start back up
-If I unplug the high pressure pump it starts right up with the pump diconnected
-If I then plug the HP Pump back in it continues to run fine


The P.o told me, that the motor once had a fault with the fuel pump driver, so he powered the HP pump externally. He said this eliminated the problem.
The HP pump has a connector with two connections. One is connected to an earth, and the other is connected to the starter relay. The former ground cable is just cut and isolated. When the motor is running it reads 0.2V (strange, should have continuity I think?).


Here are my findings:

-Fuel pressure is adequate (48 PSI). As soon as you crank the engine it builds in a matter of seconds. Also in the no start condition
-Spark is adequate, even in the no start condition
-Injector resistance is within spec.
-Injector pulses are abesent in a no Start condition (tested with noid light). When the motor runs, all three injectors fire. As soon as I unplug the HP pump, the pulses work again.
-If I connect my voltmeter to the injector connector, I read 7.2V in a no start, and about 16V in a run scenario.

So, the problem is, that the injectors don't get enough Voltage in a no start condition. From what I saw in the Service manual, the HP pump and the injectors get their power from the same ECM outlet. I wonder if I could fix the Problem, by just hotwiring the Injectors (the power side).
But it still puzzles me, that it will restart with the HP pump diconnected.

I'm sure there are many people on this board that know a lot more than me about troubleshooting this system, please feel free to share your knowledge.
I appreciate all the help I can get :)

Thanks in advance!
 
Hi,

so here is a little Update. Yesterday I probed the number one ECM pin with my multimeter while cranking. I get 6.8V in a no start condition, same thing I read on the injector connector. So I unplugged the HP pump and it jumped up to 12.8V immediately. Motor started.

I guess that rules out the wiring. I wonder what would cause the ECM to put out less Voltage? The HP pump has no direct connection to the ECM, so how does the fuel pressure affect the injector Voltage?

Any input is appreciated!
 
Hello guys,

I was back on it yesterday and I found something. I disconnected the ECM plug and put one lead of my multimeter on the number one pin, which supplies constant power to all sensors and the HP pump. The other lead was put on the power side of the HP pump. I expected to see nothing, as I thought the pump was rewired completely new and not connected to the ECM. Well, I got continuity, so the only thing that has been changed by the previous mechanic is the ground of the HP pump.

What can we conclude then? I can now see, how the voltage in the power circuit can be affected by the HP pump. It is not pressure related, but I'm thinking something is up with the pump. The devices (sensors, Injectors, pump etc.) are put in parallel, so if one device is shorting the circuit will suffer voltage drop.

Please feel free to comment, this is just my hypothesis for now.

What would be the best way to diagnose an intermittend HP pump problem? What could be the cause? Dirty VST?
 
Welcome, Leo. Perhaps the pump is drawing too much current at times. I'm not familiar with this motor, but that's worth testing. Is there specs in your manual for pump resistance and/or amperage draw?
 
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Thanks for the welcome timguy!
I will have a look in the manual for sure. For now I'm waiting on a clamp meter, to measure how much the pump draws. In theory I should see a higher reading when the motor doesn't run, and a lower reading when it is running, correct?
In general, what would a typical draw from a pump be?
 
Right. Can't speak for amps for sure at 12 volts, but certainly less than 3 or 4 amps, I would guess. An automotive size high pressure can run up to 5 amps @ 12 volts......that produces about 50 psi.....a 5 amp pump will use maybe a 5/16" hose and deliver about 100 liters per hour.
 
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Little Update:

Got my clamp meter hooked up to the fuel pump, which showed about 4.3Amps with the motor running. Amperage at the output side of the ECM was 4.8Amps.
The difference makes sense to me, about 0.5 Amps for the injectors and sensors.

Here comes the big surprise:

The amperage at the pump is lower, when the motor doesn't start. It's around 2.5A. The amperage at the ECM is a bit higher, like 2.8A.

I guess that proves the theory wrong. Will have to think about it for a while, but for now there is really nothing I could think of, that would cause this issue.

Do you guys have any idea what to check next? Thanks alot in advance!
 
Will the pump run if you just cycle the key on/off, without cranking? If so, what does it draw? You can probably guess where I'm going.
 
I checked today, interestingly enough the pump doesn't cycle when I turn the ignition on. Fuel pressure also doesn't build. As soon as I crank it over the fuel pressure builds up to 45 PSI immediately. It seems like the pump doesn't really care if it draws 2.5 or 4.5 Amps, the fuel pressure is always adequate.

Timguy, thanks alot for troubleshooting this motor together with me, really appreciate it! Can you explain where you're going with the pump cycling question?
 
So when the pump is drawing 2.5, it will not start, eh? That means your likely using more power for cranking, (at times.....sometimes). So you have fuel pressure regardless, right? So.....possibly low power available for spark....so weak spark. I wonder if the starter is drawing too many amps. Would it be possible it's seen alot of cranking over the years and the brushes are worn down?.....Happy to help, wonder where the other techs are here? Some are way better than me.
 
Yes, that is right. Well, that is an interesting idea. I will try to pull start it, when it doesn't want to run. See if that works.
On the other hand I the Motor can be started without a battery, so the power supply of the pump, ignition coils and injectors should be independant of the starter.
But this motor is crazy in many ways, so I wouldn't be suprised :).
Do you think it could be an ECM Problem? Or would that make the motor have different symptoms?
 
Yes it could have ECM problems, certainly. But.....we must explore all simpler solutions first, right?........I don't think a new ECM is too cheap. So it will pull start without 12 volts?....that seems nearly impossible for a fuel injection. Can a stator turning that slow, produce power for everything? FYI, fully charged battery is 12.65 v. and likely at least 650 amps
 
Big Update!

I tried to pull-start the motor when it wouldn't start by cranking it. It started right up after 2 pulls.
So I investigated further into the starting system. I found the starter cable to be loose on the starter relay.
I tightened it up and started the Motor about 50! times with no issues. Will try it 100 times more in the next days, before
I call it fixed, but I'm positive. Who would have thought of this? The starter was cranking the engine fine, while having a bad connection.
Never have I seen this.

About pull starting it with no battery, I know it sounds impossible considering the requirements for the fuel pump, but it does work (at least on this engine).
I tried myself.

Well, if this was the fix, I guess cold beverages are on me timguy :eek:
 
Ha....sweet. I'll share a Hamm's with ya, my friend. YOU the guy with the strong arm. Good thinker too.
 
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I have 2016 tohatsu 30hp 4stroke, and couldn't get any spark, even tried changing out the stator, though it might be the timing, or something, grounds were checked, pulsar coil giving out 0.1 on the right and the left 0.2, any good info?
 
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