350 mpi fuel pressure question

[Offthepath]

I have a 2005 350 mag mpi I am trying to diagnose a hard start issue when hot. It may be vapor lock, but I'm looking at a few other items including fuel pressure. I cant find any clear indication in the service book what an acceptable pressure bleed off is.

Key on, engine off I get about 43 psi. Once the key is turned off, the pressure slowly bleeds down, loosing about 1 psi a minuite.

Is this normal?

Thanks

 

[Offthepath]

I just realized the guage had a slight leak, so less than 1psi a min leak down.

 

[alldodge]

Being a 2005 it should be a MEFI 3 but a serial number would verify

The fuel pressure looks good

Does the hard start happen after running and stopping for a hour or two and then the problem shows up? Otherwise its starts right

 

[Offthepath]

Thanks for the reply. Serial number is Ow375597.

The boat starts and idles fine cold. The issues happen after running the boat hard for 30 min to an hour and then restarting up to an hour later. When restarting, the engine has a hard time catching and you have to crank it much longer. Sometimes it will not start without giving it throttle ranging from 1/4 to fill throttle. As soon as it starts it idles fine. No other problems running.

New plugs, cap, rotor, and fuel filter. New starting battery. Changed the IAC and checked the coolant sensor range.

Thanks

 

[alldodge]

Sounds sort of like vapor lock (VL), not know much else about the motor.

Is the weather hot where your at?

Once it starts back up, does it run good, or is there any stumbling and then it starts doing better?

Being a 350 I'm assuming its a Bravo drive?

 

[Offthepath]

Yeah Bravo 3. No, once it starts it runs fine. Idles just fine after hot start too. The boat was new to us last fall and we had about 6 trips on it. Hot starting was always an issue, but it never left us stranded and would start within 10 sec or so.

First time out this spring it had a really hard time starting hot. Took several tires with some really long cranking and full throttle before it lit.

It's not hot here, but I'm leaning towards vapor lock. From what I'm reading winter fuel can really contribute to VL. When we filled the boat up at the end of the year it was November and the fuel was not ethanol free. Also were we about 4500 feet above sea level so that would contribute as well.

One question I've had but not sure it's even related.... when starting the boat cold it does not have a fast idle. It starts and just idles around 500-600 rpm. Is an MPI boat like a car that should idle fast when cold?

Thanks!

 

[alldodge]

One question I've had but not sure it's even related.... when starting the boat cold it does not have a fast idle. It starts and just idles around 500-600 rpm. Is an MPI boat like a car that should idle fast when cold?

No, the MPI will stay at about same rpm cold or hot

Since it is getting worst and there is the issue of WOT to get it to start, it may be to much fuel.
Can be caused by high fuel pressure, MAP/T allowing to much fuel, incorrect timing, TPS issue and/or several leaking injectors

The 4500 foot can also be an issue, so it needs to run leaner, but the MAP should tell the ECM to use less fuel

Pull a couple plugs are see if they are tan/cream color or if there black

 

[Fun Times]

0W375597 = MERCRUISER 350 MPI BRAVO 1.7 inch exhaust risers.
Just as a quick reference update an 2005 engine will have an 555 ECM as that started in late 02 / but really early 2003 for the majority with the 496 being the exception of starting in 2001 known as an 555 PCM.

43 psi is alright to see but you'll want to try leaving your gauge connected for awhile with no fuel leaks to see if the psi is still in range during the "event".

You changed 1 or 2 of the fuel filters inside? There would be 2 technically speaking.

Also look closely inside the gen 3 cool fuel module including the top cover around the fuel pressure regulator for any deteriorating paint particles flowing into the fuel system of the engine, injectors, etc..
https://www.perfprotech.com/mercury...ly-module-bravo/assembly/11681/80?umodel=6573

Since you can get the engine started by opening the throttle, sometimes that tends to indicate you're not getting enough air to the engine or maybe hitting the fuel cut off mode just right percentage wise of the throttle during engine cranking shutting down the fuel injectors which would be called clear flood mode to the ECM.

Check and change your spark plugs and ensure they are the correct ones either ACdelco or NGK's...Also clean your inner side of the throttle body housing, throttle plate both/all rounded sides of the plate, the hole and hoses for the IAC passage ways and the flame arrestor.

Also if you didn't, install a new clean IAC muffler item number 6 as a dirty IAC muffler could play a role in hard starts, https://www.perfprotech.com/mercury...bodymechanical/assembly/11681/100?umodel=6573

https://www.perfprotech.com/mercury-marine-parts-catalog/umodel/6573

 

[Fun Times]

One question I've had but not sure it's even related.... when starting the boat cold it does not have a fast idle. It starts and just idles around 500-600 rpm. Is an MPI boat like a car that should idle fast when cold?

Does the engine seem to flare up to around 1000 to 1300 RPM when started? as it ought to do that then drop quick to around when cold the 750+ RPM range and drop at a medium rate to 600 RPM as the engine temp climes mostly controlled by the engine temp sensor readings.

Be sure your battery is at least around 800 Cold Cranking Amps.

Winter/older fuel could play a role in starting up plus being up into 4500 feet above sea level changes things around too...So Maybe...

But again a dirty throttle/air assembly system could do this too including not allowing the RPM to fluctuate normally...Your car/truck too.

 

[Offthepath]

Thanks for the replies. When I changed the plugs last fall they all looked fine, but next time I work on the boat I will pull a few and have another look. I used AC Delco plugs.

Both filters were changed. I did not see any signs of flaking paint.

I cleaned the spark arrestor. The throttle body looked very clean so I left it alone. The muffler was not changed. I'll get a new one.

Battery was the biggest I could get, I think it had around 1000cca. One issue I noticed is the battery cables are 4awg, it seems they should be larger?? The cable from the battery shutoff to starter is 1/0awg. At some point id like to replace the 4awg with 1awg cable. Are marine grade battery cables required?

i agree I need to test fuel pressure while at the lake when the issues occure.

Regarding the idle, it idles around 500-600 rpm all the time, no matter hot or cold. It never varies or idles above that.

Thanks for all the help!


​​​

 

[Fun Times]

Depending on length of the cables you're probably okay with what you have as long as they are not internally corroded, etc..
Next time it's happening, try checking the cables for excessive heat at times throughout the boating day but that ought to be just a bit lower on the priority list if the engine seems to be turning over at about the same rate/RPM, etc. as you notice while it's cranking over and starting normal while cool...Below is what Mercuiser recommends on the battery cables.

There usually is a small RPM fluctuation between cold and warm but maybe your tachometer isn't quite picking it up but If you ever happen to get a chance to use a marine scan tool from someone or buy one, etc., you may want to connect the tool up and watch and preform a few things to see if they are within range such as double check the cold vs warm RPM ranges under the digital scale and the IAC values and maybe try the IAC test feature to see if that test works/helps your engine model any as that test seemed to vary for working or not....Also see where your engine digital temp sensor is reading at as that is what the ECM reads and it could be different vs what the gauge says at the helm. Make sure the digital temp isn't staying at a higher/hotter read all the time...< Just an idea.

Here's a couple of ideas of engines starting and running for comparisons with a good view of a laptop scan tool connected if interested to see,

MercruiserService Bulletin No. 2002-12
New Battery Cable Gauge Recommendation Information.

This new battery cable recommendation will be going into the service manuals and installation manuals.

NOTE:

Battery should be located as close to engine as possible.
Step 1. Select proper size positive (+) and negative (-) battery cables using the chart. Step 2. Add the positive and negative cable lengths together. Step 3. Divide by 2 to obtain the average cable length.

IMPORTANT: Terminals must be soldered to cable ends to ensure goodelectrical contact. Use electrical grade (resin flux) solder only. Do NOT useacid flux solder, as it may cause corrosion and a subsequent failure.

Cable Length, Cable Gauge

Up to 1.1 m (3–1/2 ft.) 25 mm2 (4)
1.1–1.8 m (3–1/2 – 6 ft.) 35 mm2 (2)
1.8–2.3 m (6 – 7–1/2 ft.) 50 mm2 (1)
2.3–2.9 m (7–1/2 – 9–1/2 ft.) 50 mm2 (0)
2.9–3.7 m (9–1/2 – 12 ft.) 70 mm2 (00)
3.7–4.6 m (12 – 15 ft.) 95 mm2 (000)
4.6–5.8 m (15 – 19 ft.) 120 mm2 (0000)


With the old battery cable recommendation, the negative (-) and the positive (+) battery cables were measured separately. Then you selected the correct cable gauge to fit the length measured for each. This was OK if they were both the same length. It did not work well if there was a battery switch installed in the positive (+) battery cable, between the engine and battery. Then, the positive (+) cable had to use a much larger gauge than the negative (-) did.

In the example below, this would have meant the shorter 91.4 cm (36 in.) length negative (-) cable would have used a 25 mm2 (4) gauge cable. The longer 579.1 cm (228 in.) positive (+) cable would have used 120 mm2 (0000) gauge.

Example:

A person measures 91.4 cm (36 in.) length of negative (-) cable between engine and battery. They measure 274.3 cm (108 in.) of positive (+) cable between the engine and the battery switch and 304.8 cm (120 in.) between the battery switch and battery. Add 91.4 cm (36 in.) + 274.3 cm (108 in.) + 304.8 cm (120 in.) = 670.5 cm (264 in.) divide by 2 = 335.3 cm (132 in.) or 3.4 m (11 ft.). So BOTH the negative (-) AND positive (+) battery cable use the 70 mm2 (00) gauge cable.By using the same gauge battery cable, 70 mm2 (00) gauge in the example above, for BOTH the negative (-) and positive (+) cable, the longer length positive (+) cable can use a smaller gauge cable than it would have if the negative (-) and positive (+) gauges were sized to their respective lengths.

New Minimum EFI and MPI Cranking Battery Requirements​

Models Affected​

All EFI (TBI) and MPI models, including all product produced prior to this service bulletin.​

NOTE: ​

All EFI and MPI products produced prior to this bulletin have to meet this newrequirement when the current battery in the boat has to be replaced.​

NOTE: ​

Carbureted engine minimum battery requirements remain the same as before.​

Situation​

Due to increasing current requirements with MerCruiser gasoline EFI or MPI engines, MerCruiser has changed the minimum battery requirement for all of these engines.​

New minimum: 750 cca, 950 mca, OR 180 Ah.​

NOTE: ​

When selecting a battery, any one of the ratings must be met. It is not necessary​

to select a battery that meets all three ratings.
https://www.perfprotech.com/blog/art...e-requirements

 

[QBhoy]

Hi
as said. Just double check those plugs. I bought AC delco spec plugs and it turned out they are platinum at best. Not the NGK iridium. It ran terribly with the ac delco. If you have put normal carb 350 plugs in it that aren’t even platinum...then it will certainly run poorly
for sure.
I would also say that my MPi and the two others like it I’m familiar with...all run a little higher rpm and fuel rich for the first few minutes...then settle down to about 650-680 rpm at idle.

 

[Offthepath]

I wanted to bump this up with some follow up info. I tested the fuel pressure with the boat running and when the hard start occured. Fuel pressure was 38psi at idle and around 43psi at WOT.

The fuel pressure did not change when the hard start occured.

I found the boat starts fine if prior to cranking, the throttle is placed at 1/8 to 1/4 and not moved until it catches. Modulating the throttle while cranking does not help and often you have to go to full throttle to get it to start after doing this.

Any ideas?

Thanks

 

[alldodge]

I'm thinking its getting to much fuel and not enough air.

This could be from
MAP not reading air pressure correctly

Fuel injectors leaking

Spark is not hot enough

ECT not accurate
160* = 450 ohms
100* = 1800 ohms
70* = 3400 ohms
40* = 7500 ohms

Low compression

 

[Offthepath]

I tested the ECT and it was within range. I'm doubtful it's injectors leaking because it hold fuel pressure for a long time after shutdown. I have not done a compression test. Can the map sensor be tested for correct operation with an ohm meter?

There seems to be conflicting statements in this thread, but I still find it very odd that a fuel injected engine does not flare up or increase RPMs briefly when started. This engine does not idle up at all, just chugs along at 500-600 rpm.

 

[alldodge]

My manual shows no way to test MAP with ohms, but does show what the voltage should be with a given pressure. I would post but unable to upload pics right now

Do agree the engine not increasing rpm on start up is unusual. Could be a ECM issue

Could call Bob at obd diagnostics and see what his take is

 

[QBhoy]

I wanted to bump this up with some follow up info. I tested the fuel pressure with the boat running and when the hard start occured. Fuel pressure was 38psi at idle and around 43psi at WOT.

The fuel pressure did not change when the hard start occured.

I found the boat starts fine if prior to cranking, the throttle is placed at 1/8 to 1/4 and not moved until it catches. Modulating the throttle while cranking does not help and often you have to go to full throttle to get it to start after doing this.

Any ideas?

Thanks

Have you looked at things like the TPS ? Just reading your last comments about throttle position to start. The sw pressure sensor and temp sensor would be a prime suspect after that perhaps.

 

[Offthepath]

Thanks for the info. What is a "SW" pressure and temp sensor?

Yesterday at the lake, same issues. Starts right away cold but any other startup is very difficult. This time after launching the boat we idled for a few minutes and the boat never even got up to temp before we turned it off. A few minutes later, same starting issue. I had a hard time getting it going using any method. Eventually switched it so it was running on both batteries and with 1/4 of steady throttle it lit.

I mentioned the battery cables being a concern earlier. I'm an leaning towards this again. Playing with it, I noticed it spins over much faster when running off both batteries. For the rest of the day after the initial hard start, I ran it on both batteries and with a 1/4 throttle it did not give us any issues. Even if the battery cables were the issue, I'm not sure why it would still need throttle to start.

Batteries are new, one group 31 950 CCA primary starting battery and one group 31 650 CCA deep cycle battery. When we got the boat last year it had undersized battery's and I suspected the lack of CCA was contributing to the starting issue.

​​​​​​​We also have finally burned off most of the fuel from last winter. When we filled the boat last it was in November so I'm sure it was a winter blend. I'm hoping a tank of ethanol free may help as well.

 

[alldodge]

When cranking, are you seeing at least 300 rpm on the tachometer?

 

[Offthepath]

I've not paid attention to that before. I'm going out Friday and will see what the cranking RPM is. If it's spinning below 300 will it not fire?

Thanks