Mercruiser 5.7L 350 MAG MPI - No Start

JrPickle

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May 11, 2020
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My dad’s boat has a Mercruiser 5.7L 350 MAG MPI Horizon MIE engine manufactured in 2006. The serial number is 0W398110. We have a no start issue with a very long back story. I’m not sure how much is relevant so I’m throwing the whole story out there. I have lurked on boating forums for years and solved many different issues that way, but the time has come to make my first post and ask for help.

The boat was running flawlessly on about 5 different outings this year up until about 2 months ago. Then things took a turn. On the day the boat died, we had used it for about 1.5 hours with at least 30 minutes of WOT, no issues. We pulled up to the dock so we could wash the boat and organize some stuff, that took maybe another 1.5 hours. My dad backed the boat off the dock, into the lake, while I dropped in the trailer so we could pull the boat out for the day. A few short minutes after he pulled the boat off the dock, it died. He cranked it a few times, but it would not start. Another person on the lake towed him in.

Fast forward a few hours later. I am diagnosing what happened to the boat. I discover the fuel pumps are not kicking on like they should when you turn the key. I verified 12v at the power plug going into the module. I decide to check the fuel relay, and some of the plastic on the pin it draws from had melted. Clearly there was an issue with fuel delivery. I pulled the cool fuel module (Gen III) off the boat and opened it up on the work bench. To my horror, there were paint flakes everywhere inside the cavity with the 2 fuel pumps. I looked it up on google and sure enough it is a known problem.

Next, I weighed all my different options but ultimately decided to fix it myself. I painstakingly removed all the paint delamination inside the module until it was all gone. Then I replaced both fuel pumps and the regulator and reassembled it. I also put in a new fuel filter in the module and a new water fuel separator on my boat. I made sure everything was filled with gas while assembling and then I primed the system to also help get air out. I also replaced the fuel module relay in case it was damaged. From, reading the above forum. I knew that the injectors being clogged could be a very real and likely probability, but I decided to wait to see if that was a problem. My logic was that, since there were no signs or symptoms of issues before the incident, maybe I got lucky and caught it before it got to the injectors.

Well, after all that work, I turned the key and the boat started. It ran pretty rough on idle for about 2-3 minutes and then cleared up. By rough, I mean RPM’s dropping to like 400 and then going back up to the usual 700 RPMs. After it cleared up, I let it idle for 20 minutes with the hose hook up and it had no issues. I took it on the lake the next day with my dad but the same thing occurred, rough idle on cold start for about 2-3 minutes and then it was good. We idled around the lake and eased into acceleration, watching all the gauges. Everything was good, we then went around on WOT for at least 40 minutes. Every 5 minutes or so we did a visual inspection of the engine compartment to make sure everything was good.

We thought the rough idle on cold start was odd but that maybe we just needed to burn through some gunk. We took it out one more time but this time in the ocean. Same thing rough idle on cold start for about 2-3 minutes. We were out all day, stopping at islands for a bit with the engine being shut off for as long as like 2 hours. Absolutely no issues all day expect for the rough idle when we dropped in at the start of the day.

Following our gunked up theory, we decided to add injector cleaner to the fuel this time. (So as a quick recap, the boat has worked on 3 occasions, once on the trailer for 20 minutes and then 2 times on the water) We dropped it in the water and tried to start it. It started but idled like horribly rough for about 30 seconds and then died. It would not start up again. We pulled it out and took it home. One thing to note here, there was now a second beep from the on-board computer. The first one was the standard beep when you turn the key halfway, the other beep occurred when cranking.

Some google searching revealed the idle air controller was a possible reason for the rough idle and maybe even the no-start. I replaced the IAC and it’s muffler and then performed the reset procedure mentioned in the service manual. The second beep that was occurring during cranking was now gone but the boat did not want to start. It would have a nice initial combustion and then no follow up.

Knowing what we knew, we decided it was time to pull the fuel rail and injectors. We were quoted $50 per injector to clean them. Instead we decided to buy new bosch injectors. I pulled the fuel rail and it was full of paint flakes. There was literally paint sticking out of one of the injectors too. After soaking and blasting the fuel rail like crazy, the paint flakes seemed to be all gone, I reinstalled it with the brand new injectors. I crossed my fingers, hoping the no start issue was gone but the boat would still not start.

I verified the fuel pumps are still engaging. I don’t have a fuel pressure gauge to hook up to the schrader valve but I poked it with a flat blade and it squirted a lot of fuel (I was wearing safety googles😊). There is definitely fuel pressure but obviously without a fuel pressure gauge, I don’t know if its within spec. I decided to table fuel for now and maybe come back to that later with a fuel pressure gauge. I am confident that with all the work I put into the fuel system, that it should be working great.

Next I tested spark on one of the sparks plugs with a cheap little spark plug tester and it only lights up like 5 or 6 times while cranking for a couple seconds. That did not seem right to me. (My only other time using this tester was on a jet ski and that lit up much more frequently and faster) So now I think my no start issue has to do with the ignition system. I tested power at the ignition coil and it was getting 12.9v with the key turned half way. I consulted the internet and discovered there are a bunch of sensors that could cause the computer to shut off the fuel system and cause a no start. Stuff like the ECT sensor, oil pressure sensor, manifold air pressure and temperature sensor, camshaft positions sensor, etc.

The only two sensors I could figure out how to sort of test was the ECT and the manifold air pressure and temperature sensor. The ECT measured 2.35k ohms which as far as I could tell was within spec. The service manual states that if you disconnect the manifold air pressure and temperature sensor, the computer will use the last known good configuration, so I unplugged it and cranked the engine. This time, the engine seemed to have sputtered for 2-3 seconds after that initial combustion. (Unless I’m just going crazy) I tried to crank again but it was the same one-off combustion no start.

At this point, I need help. There are a few more diagnostic steps I could do, like test for compression and buy a fuel pressure gauge, but I’m going a little crazy and need to take a step back for now. If anyone here can point me into some direction that would be much appreciated. If not, I will probably be calling around for a mechanic with a diagnostic computer.
 

achris

More fish than mountain goat
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May 19, 2004
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You've done really well to get as far as you have without the diagnostic software, but for your current issues, I would suggest you buy the Diacom software and have that running on a laptop. I know it's expensive, but as you seem to be capable of doing good work, I think you'll find it's worth it's weight.

And without that software (yes, many people will tell you that the techmate code reader will do 'as good' a job, but it will not) you really are shooting in the dark as far as sensors, especially the 3 wire ones that rely on a voltage divided signal, not resistance.

Chris......
 

JrPickle

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May 11, 2020
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You've done really well to get as far as you have without the diagnostic software, but for your current issues, I would suggest you buy the Diacom software and have that running on a laptop. I know it's expensive, but as you seem to be capable of doing good work, I think you'll find it's worth it's weight..

My dad and I called a Mercruiser service center; they wanted $189 for diagnostics. They were so busy that it would take them 4 weeks to even check out the boat though. We decided to buy the Diacom 94106 software kit that's compatible with our engine. It was $569 plus another ~$19 for 2-day shipping. Hopefully everything works out well and you never know, we may need the software again in the future. I will keep this thread updated with the outcome. Thanks for the suggestion Chris.
 

achris

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.... We decided to buy the Diacom 94106 software kit that's compatible with our engine....

I doubt you'll regret buying it. I haven't, and all the members of this forum who have are also very happy with it. You only have to use it a couple of times and it pays for itself.

You can also save the files and live data and post them (as attachments) and those of us with the software can look at the files as if they are live data!

Chris......
 

JrPickle

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Okay, I received the Diacom Kit today and plugged it in. The first thing I noticed was the oil pressure reading from the sensor on the computer was extremely high. It reports a steady 94.98 PSI the entire time, even while cranking. The next thing I did was look at the trouble codes, a few codes are logged for sensors I don't have so that's fine. The only other trouble codes are from my sensor testing when I had them disconnected. For kicks and giggles, I wanted to see the computer report a different value for the oil pressure sensor so I shut it off, unplugged the sensor, and turned the key to run. It did the 3-beep warning for the sensor being disconnected and the Diacom software reported 50.75 PSI. I imagine that's some sort of placeholder value. Trying to start with the sensor disconnected results in the same no-start behavior. I repeated the same process to reconnect the sensor and it was straight back to 94.98 PSI. The parts catalog states its part number 881879A6 and names it as "SENSOR (0 - 100 PSI) Oil Pressure". So, the oil pressure sensor is nearly pegged at full. Could such a high reading cause the computer to disable things resulting in a no-start? I can't find anything in the service manual about the potential issues caused by this sensor being faulty.

I ran out of daylight but can do more tests tomorrow afternoon. What else should I do? I was thinking of doing the ignition test thing in the Diacom software.
 

achris

More fish than mountain goat
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May 19, 2004
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With any sensor disconnected you should get a 'Sensor low' fault code. If the oil pressure is reading 50psi when disconnected and 95 when connect with the engine not running, you have an ECM or wiring problem. Maybe a disconnected sensor ground to that sensor. I would disconnect the oil pressure sensor and maybe the temp sensor and check continuity between the ground wires. Sensor ground is not engine block ground, so you can't just check continuity to the engine block.

Do you have the engine schematic?

Chris.........
 

QBhoy

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Sounds like that sensor is at a default extreme reading...but my amateur knowledge accumulated over the years from these mpi engines also tells me that one sensor or poor voltage supply to it or others can also lead you down a road of chasing your tale. One bad thing can have a domino effect on them all.
If there is no 5v supply to this or others, you may well get a default reading like this. Not totally sure and others that know more than I will correct or agree. Good place to start though and you’ve done so well already.
 

QBhoy

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Ok. So just realised we have left one big thing out here...have you changed that Diz cap ? As unlikely as it sounds...I’ve had that cause a whole disaster of chain reactions on a load of sensors on my mpi. Worst case it rules it out and saves a breakdown later...best case...that’s the issue. Google mpi crap cap and you’ll understand.
 

JrPickle

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Wow, thanks for the fast responses. I went outside with a flashlight to do these tests. I identified the ground wire on the sensor plugs to be the purple wire with black stripe. The reading between the oil pressure sensor ground and the water temp sensor ground was 0.2 ohms, so I have continuity. Also, I know you said the sensor ground and engine block ground aren't the same but I measured that too. It was 1.2 ohms between oil pressure sensor and engine block, and it was 1.1 ohms between water pressure sensor and engine block. As far as having schematics, I have been using the parts catalog to identify engine parts. I also have access to the service manual.
 

JrPickle

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Ok. So just realised we have left one big thing out here...have you changed that Diz cap ? As unlikely as it sounds...I’ve had that cause a whole disaster of chain reactions on a load of sensors on my mpi. Worst case it rules it out and saves a breakdown later...best case...that’s the issue. Google mpi crap cap and you’ll understand.

Excuse my ignorance, but does diz cap mean the distributor cap? The entire distributor assembly is all still original. Would I want to replace just the cap (884792) or the entire assembly(884794A02)? What kind of tests could I do to determine if it's bad?
 

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achris

More fish than mountain goat
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Diz is distributor (some funny yank slang :facepalm:). And if you don't have a problem with the cap, and at the moment it doesn't sound like you have, leave it alone!

Check your PMs. ;)

Chris.......
 

achris

More fish than mountain goat
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Messages
27,468
Ok. So just realised we have left one big thing out here...have you changed that Diz cap ? As unlikely as it sounds...I’ve had that cause a whole disaster of chain reactions on a load of sensors on my mpi. Worst case it rules it out and saves a breakdown later...best case...that’s the issue. Google mpi crap cap and you’ll understand.

I've had my 'crap cap' since new, 2006, and it's never caused me a problem. I also maintain a load of other Mercruiser MPI engines, and they haven't had a cap problem either. Just because you have had a problem, doesn't make it a 'common problem'.

Chris......
 

QBhoy

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I've had my 'crap cap' since new, 2006, and it's never caused me a problem. I also maintain a load of other Mercruiser MPI engines, and they haven't had a cap problem either. Just because you have had a problem, doesn't make it a 'common problem'.

Chris......

Chris...I almost always agree with and respect every single bit of great advice you post on here. Nothing more certain of that....but on this one...honestly...the diz cap or crap cap issue is like thee single most common problem with the short block mpi engines. Without doubt. Especially in countries that see real big temperature difference between seasons.

to the original poster...if it’s never been changed...please believe me when I say...if this isn’t your issue now...it almost certainly will be at some point soon. I’m am not the only one that’s seen this. And by a long long way. Have a quick search at mpi distributor problems. Not only on here...but just about everywhere else too. It’s undoubtedly the single biggest issue to trouble the 5.0/5.7/6.2 mpi engine. Time after time and then some !! Please believe me.
 

JrPickle

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Sorry it's been a few days. First thing, that oil pressure sensor. As I said early with my testing, the oil pressure sensor wiring tested okay. I got a new one, I plugged it into the wiring harness to test but have not bolted it on yet (I have left the old one in for now). The computer fluctuates between 0.00 and 0.23 PSI on this sensor sitting in the open air. To me that implies the wiring is good and the old sensor is faulty being stuck at 94.98 PSI. The boat still does not start. I am waiting till I get the boat running before I make the final decision if the thing needs to be replaced; though it seems like it does.

I noticed that it does not have it's good one off combustion as it did before, it's gotten worse. To ensure it is not electrical. I got a brand new battery, cleaned off all terminals on the battery, cleaned off the terminal on the main ground wire, and cleaned off all wire connections on the starter. Same issues. I hooked up my cheap little spark plug tester to one of the plugs and the spark seems really inconsistent to me. Higher levels of spark also correlate to higher engine noises. I have attached two unlisted youtube videos here and here so you can see it in action.

Then I noticed QBhoy mentioned me in another post (By the way, how do you mention people? Maybe what I did will work but the editor doesn't seem to do anything special here). I removed the distributor cap and it was very corroded, a photo is attached below. I sanded it down as best I could and put it back on but it made no difference.

What do you guys think? Do the videos look like very weak spark to you? I have verified 12v + at the ignition coil. Unfortunately, the spark plug tester I have won't fit on the main wire going from the ignition coil into the distributor cap.
 

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alldodge

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If a person has commented on a post they will get notified in their message in box
If you want to mention a person not on the thread use the (at) sign, then start typing their name, when name shows up select it. This will notify them

A QBhoy mentions the distributor cap in almost ever post with an issue, while I don't always agree, he is right with the cap causing a large amount of problems. The cap can even look brand new and still have issues which are caused internally. Anyone with one should carry a spare because they can fail without warning

In this case I would replace the cap and rotor, your spark is to erratic. If this does not fix the issue all good, now look at the crank position sensor

Question: the new injectors you bought, did they come from Merc?

Note: you can have injectors cleaned and flowed at very good marine places for about $20 each, the $50 is way out of range
 

QBhoy

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If a person has commented on a post they will get notified in their message in box
If you want to mention a person not on the thread use the (at) sign, then start typing their name, when name shows up select it. This will notify them

A QBhoy mentions the distributor cap in almost ever post with an issue, while I don't always agree, he is right with the cap causing a large amount of problems. The cap can even look brand new and still have issues which are caused internally. Anyone with one should carry a spare because they can fail without warning

In this case I would replace the cap and rotor, your spark is to erratic. If this does not fix the issue all good, now look at the crank position sensor

Question: the new injectors you bought, did they come from Merc?

Note: you can have injectors cleaned and flowed at very good marine places for about $20 each, the $50 is way out of range

You’re dead right AD. I really think the issue with these caps is further amplified in certain climates. By this I mean climates where vast temperature and humidity extremes are common place. Like here in Scotland where we often see it 15+ degrees C one day and -5 the next with the moisture levels just as wild in difference over winter. Anything inclined to create moisture variation from one day to the next and for months at a time, I think seriously affects these caps. It year after year causes issue at this time of year.
 

JrPickle

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Thanks for the response, I'm glad you mentioned the spark looks erratic. That gives me hope I am going down the right path. I ordered a new cap and rotor which will arrive on Tuesday.

The injectors were not from Mercruiser, they are bosh injectors bought on ebay with a one year warranty. I figured the $50 was way out of range, that's my local mercruiser certified dealer that is typically overpriced on a lot of things. I don't mind going new anyways since the whole paint chipping thing happened, they seemed very clogged. I still have the old injectors in the garage if I need them for whatever reason. I ran the injector test with the Diacom software and could hear each pair firing so that's a good sign, obviously the engine hasn't ran successfully with them yet.
 

alldodge

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bought on ebay with a one year warranty.

Oh crud, I was afraid of that, I'm betting right now the injectors you have while are not causing your current issue, they are probably junk.

Eaby has several places selling what they call brand new injectors. Folks that buy them have no way of testing them and therefore many (if not all) are used junk injectors which did not pass test. Injectors cannot be rebuilt, only cleaned and tested.

I can find you a place to send your old ones to for testing. I'm not in the biz, just run into this quite to often

Do understand now why the cost is $50 each, a dealer will charge that much
 

JrPickle

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Oh crud, I was afraid of that, I'm betting right now the injectors you have while are not causing your current issue, they are probably junk.

Eaby has several places selling what they call brand new injectors. Folks that buy them have no way of testing them and therefore many (if not all) are used junk injectors which did not pass test. Injectors cannot be rebuilt, only cleaned and tested.

I can find you a place to send your old ones to for testing. I'm not in the biz, just run into this quite to often

Do understand now why the cost is $50 each, a dealer will charge that much

I was afraid you would say that. After I get it running, if I run into issues with them, I will look into getting the old ones cleaned out and tested somewhere cheaper than the dealer.
 
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