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MPI 350 (1998) ECM no power

S

stringer_bell

Regular Contributor
I have straight inboard 350 MAG MPI motor paired with 72C Velvet Drive. 300hp variety.

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Came with boat, showed 1000 hrs. I went thru the motor and completely rebuilt it, top to bottom.
Installed new injectors too. Put original ECM and harnesses back on, and when I turn the key dash/gauges light up,
but I dont hear the fuel pump cycle or anything else.

I tried connecting my TechMate tool to it, and with key on tool screen lights up (DLC port has power),
but when connecting to 'MERCRUISER EFI' it fails to communicate with the ECM. :mad:

There are three fuses on the EFI harness (by the starter relay and ECM), 15/15/10 amps.

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Only fuse #16 (15amp ECM/DLC/Battery) has power to it.
Other two fuses, no juice. :confused:

Ignition System Relay and Fuel Pump Relay all ring out good in continuity test.
Main engine fuse on the starter is good as well, and passes 12v thru to the motor with key on.

For this test distributor was not plugged in, didnt think it was necessary? Also ignition coil was not plugged in.
I left those plugs off because I was working around where EFI and Engine harnesses plug into eachother and
ringing out continuity between male and female harness ends.

Any advice on how to approach this? I guess I can give power to fuel pump directly to see if it will energize?
How do I test the ECM to make sure its good? Dont want to burn anything up! All help appreciated, thanks guys :)
 

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Back of the starboard cylinder head there should be a harness connection for the power feed to relays. The two large power wires come off of the stater main lug. Cut this connector out and splice the wires together using a quality connector. This is a failure point in the harness. It will test good until it is slightly loaded.
 
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It's straight wires from starter lug to relay box, no connector on that harness run

two fat red wires go to breaker one on each lug
Orange wire goes to back of alternator
 
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My battery has 12.8v, but I only get 6.7v on the starter power wire, and same low voltage on breaker lug and relay inside ECM enclosure? What can be causing this?
 
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stringer_bell said:
My battery has 12.8v, but I only get 6.7v on the starter power wire, and same low voltage on breaker lug and relay inside ECM enclosure? What can be causing this?
Click to expand...
You have 6.7 volts at the starter and 12.8 at the battery? You have a bad connection or cable. Connections include the grounds. Check them all.
 
Engine is 1998, 350 MAG MPI serial # 0L009914

Got a fuel pump in, but cant get motor to start :confused:

Fuel pump is used, I am using it for testing purposes until I can get a new one ordered. When I turn the key on, the pump turns on for few seconds and primes the system. I removed shrader valve and installed mechanical fuel pressure gauge on there, and I get 26PSI at the fuel rail. While pump is priming is 30PSI, drops to 26PSI when pump stops.

I timed the engine to TDC @ #1 cyl, and I get spark since my timing light is flashing as engine turns over.

Batteries are new, 12.5V, they are inline and each is 750cca marine dual purpose.

While cranking oil pressure gauge goes to ~80+ PSI. Tach shows engine turning 3000-3700 RPM.

Plugs are new Autolite generic vortec small block from Advance, gapped to .40

I will check compression tomorrow to make sure its not the issue.

I am down to compression or wrong plugs. Maybe timing, I will re-check it but I did it twice and its @ #1 when pulley mark lines up with timing cover.

Any other ideas? I was gonna get AC 41-983 plugs by the way
 
I am using good fuel in a can, engine is I the shop not in the boat. Fuel pressure stays ~25 when cranking, I am not sure if the pump is running while cranking I can't hear it over the sound of the starter and motor turning over. I will check this afternoon after I do compression test.
 
stringer_bell said:
Fuel pump is used, I am using it for testing purposes until I can get a new one ordered. When I turn the key on, the pump turns on for few seconds and primes the system. I removed shrader valve and installed mechanical fuel pressure gauge on there, and I get 26PSI at the fuel rail. While pump is priming is 30PSI, drops to 26PSI when pump stops.
Click to expand...
It appears you have the correct fuel pressure @ 30 psi for your engine model.

stringer_bell said:
I get spark since my timing light is flashing as engine turns over. Batteries are new, 12.5V, they are inline and each is 750cca marine dual purpose.
Click to expand...
Nice, New batteries are good to have for testing. Does your timing light flash at all 8 plug wires while engine is cranking over?

stringer_bell said:
While cranking oil pressure gauge goes to ~80+ PSI.
Click to expand...
Oil PSI seems a bit high! Did you upgrade the oil pump or something?

stringer_bell said:
Tach shows engine turning 3000-3700 RPM.
Click to expand...
A correct setting/working tach gauge should show 300-400 RPM at cranking speed. If for some reason your tach gauge is faulty for any reason, it will keep the engine from starting. You may want to disconnect the gauge as a test. Is the engine even trying to start with an backfire or anything? or is it just a solid cranking speed? Also, have you opened up the throttle a bit to see if it helps start the engine in any way?

stringer_bell said:
Plugs are new Autolite generic vortec small block from Advance, gapped to .40.
Click to expand...
Spark plug gap should be .045 in. But this won't keep it from starting.

stringer_bell said:
Any other ideas? I was gonna get AC 41-983 plugs by the way
Click to expand...
The AC 41-983 are the incorrect plugs as well. The plug numbers you need are,
AC - MR43LTS​
NGK - BPR6EFS
Champion - RSY9C (Last resort)
Spark Plug Gap .045 in. (1.1 mm)

For your engine specifications see page Page 1B-9,
http://www.justanswer.com/uploads/ROCKETBOAT/2008-12-14_034006_24b1r1.pdf

Oh By the way, There is no oil pressure switch on the EFI engines, the fuel pump is controlled by the ECM which is looking for 12+ volt at key up to turn on the pump for 2 seconds, then back on in crank mode, then ECM reads input from other sensors at start up.

STARTING MODE​
When the ignition switch is turned to the crank position, the ECM turns ON the fuel pump relay and the fuel pump builds up pressure. The ECM then checks the Engine Coolant Temperature (ECT) sensor and Throttle Position (TP) sensor and determines the proper air/fuel ratio for starting. The ECM controls the amount of fuel delivered in the starting mode by changing how long the injectors are turned ON and OFF. This is done by pulsing the injectors for very short times.

CLEAR FLOOD MODE​
If the engine floods, it can be cleared by opening the throttle half way (50%). (Open throttle handle until resistancefrom secondary throttle [Multi-Port only] is felt.) The ECM discontinues fuel injector pulsation as long as the throttle is between 50 to 75 % and the engine RPM is below 300. If the throttle position becomes more than 75% or less than 50%, the ECM returns to the starting mode.​

​
 
Thanks for detailed list and explanation.

Checked compression 140-150 across all 8

replaced oil pump during rebuild, but don't remember with which ( meling?). Not hi perf don't think

oil pressure gauge pegs during cranking (shows 80+ since 80 is highest reading)

i will try disconnecting tach to eliminate it as cause of no start
not even attempting to start just steady cranking. Moving throttle no change

will report back
 
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Disconnected the tach, still no start

grounded oil sender while cranking, still no start

Fuel pressure was fluctuating 23-27 as I was cranking, pump was working and bumping it up

i am going to turn distributor 180 degrees tomorrow and try again
 
You need at least 40-43psi on the fuel pressure with out the engine running(key on engine off while the pump cycles). It should drop to about 38psi when you get it started.
 
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Manual #24 shows 30psi required fuel pressure, mine is an pre-2001 model 350 mag mpi?

I am also using new Magnetti Marelli 46LB injectors, they are what Mercruiser OEM's for SB mpi

0L009914 serial

i think ECM will not let engine run in case oil pressure is lost?
 
Master mechanic said:
If the the oil pressure goes to zero what cuts off the fuel pump?
Click to expand...
With this system, it will be up to the boat operator to look at their oil PSI gauge and shut the engine down or wait till the engine has seized up.:eek: It used to work like this,
POWER REDUCTION MODE (1996 AND OLDER MODELS)​
The ECM will go into power reduction mode when the following conditions are met: 1. Low oil pressure 2. Engine overheat 3. Low I/O fluid level (MCM) 4. High transmission temperature (MIE).

DISCRETE SWITCH INPUTS - POWER REDUCTION MODE (1996 AND OLDER MODELS)​
Several discrete switch inputs are utilized by the system to identify abnormal conditions that may affect engine operation. Pull-down switches are used in conjunction with the ECM to detect critical conditions to engine operation. If a switch changes state from its normal at rest position, that is normally open to closed, the ECM senses a change in voltage and responds by entering power reduction mode. This engine protection feature allows the operator full engine power up to 2800 RPM, but disables half of the fuel injectors above 2800 RPM until the engine RPM drops to 1200 RPMS. This feature allows the operator a comfortable maneuvering speed while removing the possibility of high RPM engine operation until the problem is corrected.

But then Merc realized that it was very dangerous to go from WOT down to 2800 instantly so in 1996-1/2 the removed the power reduction mode and used the discrete switchs to just sound the warning horn only.

FUEL PUMP ELECTRICAL COMPONENTS​
When the ignition switch is turned to the RUN position, the ECM will turn ON the fuel pump relay for two seconds. When the ignition switch is turned to the crank position, the ECM turns the fuel pump relay ON causing the fuel pump to start. If the ECM does not receive ignition reference pulses (engine cranking or running), it shuts Off the fuel pump relay, causing the fuel pump to stop.

CIRCUIT DESCRIPTION​
The fuel system circuit receives a supply voltage from the system relay CKT 902. The fuel system is protected by a 15 amp fuse. After the fuse, supply voltage is delivered by CKT 339 to fuel pump relay terminal “30.” The fuel pump relay is turned on by the ECM by supplying voltage to CKT 465. The fuel pump relay will remain ON as long as the engine is running or cranking and the ECM is receiving reference pulses. If no reference pulses are present, the ECM de-energizes the fuel pump relay within 2 seconds after the ignition is turned ON or the engine is stopped.​

​
 
Besides all of the above. Shoot some starting fluid into the intake and crank her over. If she fires for a few seconds or even back fires, or does nothing you will know what direction to look.
 
If i remember correctly about these,

Pull the distributor cap off and take a look see.....

if you see a lot of corrosion it may be time to replace the ign module inside.. I seem to remember this is a common problem with age of these...
 
Now that you have the ECM low voltage issue resolved, you may want to verify the ignition timing is where you thought it was...

Depending upon the ignition system used, you will have to use the "timing lead" or your scan tool...
 
Checked with tech mate and was getting 0 F as manifold air temp, is that the 4-wire sender right next to the TPS on the throttle body? Other readings were ok, CTS 81f, TPS volts 0.5, Baro volts 4.90, etc. this is key on engine off

My engine is MEFI-2
 
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