Fuel injected 350 MAG MPI backfire. any suggestions?

[3ball]

My boat is backfiring. It runs perfect at idle & runs perfect at WOT. Backfires pretty much anywhere in between (especially when I pull back on the throttle a bit). Things I have done so far:

1. Replaced plugs
2. Cleaned Flame arrestor
3. drained tank & refilled with fresh gas
4. changed fuel filter (Twice)
5. added Lucas injector cleaner

Most of the posts I have found are about backfire on a carbed engine or backfire at start or WOT. Mine backfires only under load and at slow to medium speeds.

Anyone have a similar problem or suggestions for what I might do next?

My nearest lake is just under 3 hours away so it makes it very difficult to run back & forth for a water test. I would like to do as much as possible between lake trips.

Engine is a 2002 Mercruiser 350MAG MPI

 

[NHGuy]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

Go to the Mercruiser shop manual download here, http://www.4shared.com/file/119353633/70c4dde5/Mercruiser_Service_Manual_31.html
and check out the factory diagnostics process. There could be some known problems that are part of the FAQ's.

You may be s... out of luck though. I believe that engine uses an electronic controller which probably uses a scan tool to read trouble codes.

If you get to the point of no return maybe you could find someone to pull codes and allow you to check the diagnostics based on that.

I work in the auto repair business (not a tech, service sales-I only play a mechanic in fantasyland) , your complaint sounds like a sensor issue. Maybe crank position sensor. Now don't go buy one and put it in! If I knew whether your engine has emissions controls I'd have a remark about EGR- exhaust gas recirculation- being a possibility.
Some engines override the emissions controls at wide open throttle. Might be a contributing factor.
Good luck, I hope you don't have to spend too much to make it right.
You might have to see a tech if it gets too complicated.

 

[JustJason]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

Just to confirm your terminology, is it backfiring out of the exhaust and not popping in the intake?

If you don't have a scan tool there's not going to be much that you can do on your end. It's not codes you want per se, but you need to read live data when the motor is running.

 

[3ball]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

Jason, it must be popping in the intake because I hear it from the engine when I remove the doghouse.

 

[JustJason]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

popping in the intake usually indicates a lean condition, incorrect timing, poor quality fuel (water in fuel), or a bad intake valve/valves.

If your going to self diagnose, you can start with a compression test, running the motor and with foaming oil looking for an intake leak, and hook up a temporary outboard style fuel tank with known fresh gas. Your going to need the scan tool to check the sensors.

 

[3ball]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

When we swapped the spark plugs they were wet and were covered in black soot. Both of those are symptoms of running lean right? they were also gapped incorrectly if that matters. (that has been fixed.. back to .060)

When we bought the boat it had a full tank of bad gas. It smelled funny & was darker then normal. We drained the ENTIRE tank added a few gallons of fresh gas & drained that as well. We then topped it off with fresh fuel, changed the filter twice and have burned through about half a tank since then.

We also noticed a smell of oil in the gas.

I do have the manual by the way (Mercruiser #31). I have been looking over it as well as past posts here.

 

[3ball]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

Should I still run the boat on an external fuel tank even though I have replaced the gas.

 

[JustJason]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

When we bought the boat it had a full tank of bad gas. It smelled funny & was darker then normal. We drained the ENTIRE tank added a few gallons of fresh gas & drained that as well. We then topped it off with fresh fuel, changed the filter twice and have burned through about half a tank since then.

Depending on how long the boat was sitting with bad, untreated gasoling, you may have intermitent sticking injectors.

 

[3ball]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

That's kind of what I have been thinking all along. The boat was a Repo so there's no telling how many seasons it sat un-used. Will the injector cleaner help?

Can I pull the injectors? & if I do will the problem be visible.

Also one of the fuel rails is slightly bent. It dosent appear to be cracked or leaking. I will post a picture of it if anyone thinks that is an issue.

 

[T-Bone Coug]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

GWet and black plugs indicate running rich. Also, a backfire when decelerating means unburned fuel is igniting in the exhaust (in cars anyway). Check fuel pressure regulator also.

 

[3ball]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

So I have the boat scheduled to get a diagnostic done but they cant get me in for 3 or 4 weeks. I want to do as much as I can before then because

1. I have the time & I might (& hope to) figure it out
2. I feel I need to have a good understanding of my engine

Since my last post I have;

• Checked the vent
• Checked anti siphon and
• Checked Fuel line to filter (all OK)

Next I am going to do compression test. However, I cant do that until next week. In the meantime I will check the fuel pressure regulator & possibly the lines on the other side of the water fuel separator.

It looks like there is a filter near the regulator. Are there any other filters I should check. Also, are there any tricks or pointers to getting to the regulator & what should I look for? Will it be gummed up or clogged?

 

[rad1026]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

Hey 3ball, where are you in NM? I'm in Albuquerque. My boat is carbureted, so I'm not much help on the Fuel injection but I have a good set of tools and spend a lot of time at the butte. If I can be of help let me know.

 

[z284u2try]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

I would do a leak down over compression test.
pic of a plug?

 

[rad1026]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

Is it a standard ODBII port just like a car? I have 1996-2006 scan software and ODBII plug on my laptop, would that work?

 

[JustJason]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

Is it a standard ODBII port just like a car? I have 1996-2006 scan software and ODBII plug on my laptop, would that work?

No, OBD is an automotive protocol. Boats use what the call the "MEFI" protocol. 2 different languages.

 

[rodbolt]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

test the rail pressure, if its ok most likly the injector screens have clogged. they come out just like a car,in fact the are an AC delco part.
I send mine to flagship marine in FL for cleaning,testing and screen replacements but any injector shop can do it.

 

[Rat Capri]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

No, OBD is an automotive protocol. Boats use what the call the "MEFI" protocol. 2 different languages.

OBD is self-diagnostic and reporting capability,not a protocol, neither is MEFI,it's a term used by GM for marine based ECM's...for GM based products the protocol will be SAE J1850 VPW, just like any other GM product.

 

[3ball]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

Here is a picture of the spark plugs. The 4 on the bottom are the ones that were in the boat. The one at the top is one of the new ones. Old were gapped to .035 & new plugs are gapped to .060. Hard to tell whats going on with the new plug due to the black thread.

I checked the order of the wires & they seem to be correct. If they cross over each other can that cause a misfire or crossfire?

Hey Rad1026, we were just at the Butte on Monday (beautiful day other than the boat running like crap)

z284y2try, I will try a leakdown test as well

 

[JustJason]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

bad compression doesn't fit your symptoms except for maybe a leaking intake valve.
I would start with an injector balace test.
You already know that the engine has had bad fuel at least once, why not keep going in that direction.

 

[3ball]

Re: Fuel injected 350 MAG MPI backfire. any suggestions?

Jason, What is involved in an injector balance test? do you just pull the power to each and look for RPM drop or do you need a special gauge?

Rodbolt, what do i need to test the rail pressure? I assume I do that from the valve on the bridge between the two fuel rails. Also what PSI should it be at? & If that pressure is good.... can I assume my fuel pressure regulator is OK?

The injectors are out right now. Has anyone had any luck soaking & flushing with injector cleaner?