Merc 350 Mag MPI won't start

[axman]

I have been having declining g performance with the starboard engine. (1999 Mercruiser 350 mag MPI). It began at the end of last season without being able to keep up with the port engine, or in other words bogging down. This has gotten worse and also become hard to start. I also noticed a roughidle. Yesterday it ran so horrible it wouldn't even get above 2000 rpms under load. There appeared to be an audible miss from the ignition, so I checked the wires for good connection and found one spark plug wire poorly connected and one poorly connected to the dis cap. Dis cap is also brand new. Now it won't start. Checked spark which was good. Checked for fuel pressure which had pressure, but I didn't have a gauge to check. Posted straight gas into throttle body and didn't start. Last notable point t is yesterday when it was running, the engine temp didn't get above 100 degrees. I am lost at this point.

 

[Fun Times]

Hi, Judging by your description above, it sounds like it might be time to pull, inspect & replace the spark plugs as they may possibly be fouled out due to lack of proper ignition source from bad plug wire connections and if the engine temperature isn't reaching the proper heat range of 160 degrees then the engine is adding additional fuel to the spark plugs and it's not burring away properly. You'll want to install a new 160 degree thermostat and related parts.

​Knowing the distributor cap was changed there's always a possibility that the plug wires may have been compromised not allowing the plugs to fire properly too. If you decide to install new plug wires, It's worth the extra money to stay with OEM plug wires.

To ensure the engine internals are still in good working order then you'll also want to for sure perform a engine cylinder compression test and seriously consider doing a leak down test even though it's a tough job to perform.

You'll need to still check your fuel pressure using a fuel tester gauge.

What is your engine serial number?

 

[axman]

Engine S/N is 0L092319. Thanks for those thoughts. I intended on trying to find wires today anyway. Is it possible that it's not running due to a bad wire? What about the possibility of some various engine sensors? I am not incredibly familiar with these systems, but very knowledgeable with similar GM car MPI systems. The things that I don't understand are no O2 sensor, no MAF sensor and how the computer compensates for those. I will pull pugs this morning too.

 

[Silvertip]

Hard starting cold or hot can be either fuel or ignition related or both. Don't start buying parts "hoping" you get lucky. Determine what the problem is and fix it. If you poured fuel into the intake and it still didn't light off, lack of spark is suspect. But fouled plugs can certainly cause that. A leaky fuel injector can flood a cylinder as can a leaky fuel pressure regulator.

 

[axman]

I wouldn't do that, never believed in just throwing parts at things. Either way, pulled the plugs this morning and sure enough they were heavily fouled. Threw in new plugs fired up right away, nice smooth idle. Had a mechanic on the way anyway so he ran scan on it and found a bad MAP sensor. I don't get too excited until the problem is gone for good, but looks like we may have e narrowed it down.

 

[harleyman1975]

Replace plug wires too just for the heck of it... my .02

 

[Fun Times]

Good to hear you found some easier to handle/solve problems.:eagerness:

Engine S/N is 0L092319.

Here's parts guides for your engine model if needed, http://www.iboats.com/350-MAG-MPI-AL...iew_id.1691766
https://www.perfprotech.com/mercury-...g/umodel/20267

I intended on trying to find wires today anyway.

CABLE KIT, Ignition (SPARK PLUGS & COIL) (OEM part # 84-816608Q71)
When taken care of, OEM plug wires do tend to last a long time and you'll have an easier time not fighting with the coil tower wire boot leaking spark.

You still will want to consider installing a new thermostat as well or you may just continue to slowly fuel foul out the plugs if you're really only running around 100 degrees. Items 3, 4, 5, 6, https://www.perfprotech.com/mercury-...0?umodel=20267

What about the possibility of some various engine sensors? I am not incredibly familiar with these systems, but very knowledgeable with similar GM car MPI systems. The things that I don't understand are no O2 sensor, no MAF sensor and how the computer compensates for those.

Outside of the fuel pump and fuel regulator system, one of the main controllers for the fuel system as far as EFI sensors go is the MAP sensor followed closely by the engine temp sensor for second place then a couple of other ones help fine tune it all.....Since you know the O2 MAF systems pretty well, if interested, when you get some extra free reading time, search online ( Speed Density System) as that is more of how the MAP system tends to work vs the MAF system.

 

[axman]

Thank you Fun Times. I thought all was solved but to my amazement, no... So, after installing MAP sensor and new plugs, I've seen some improvement but not 100% by any measure. Engine still has a noticeable skip/miss and clearly lagging behind the port engine. I swapped IAC's between the two today just as a diagnostic measure and no difference to either engine. I put the thermostat in since the engine was not coming up to temp, but i believe that before doing so the plugs fouled out again. I will be putting Champion plugs in today as recommended by local mechanic over NGK. Furthermore, while putting the thermostat in, I discovered the engine coolant temp sensor to have JB weld around it. Leads me to believe this MAY have something to do with why its still running rich. They are cheap enough so instead of taking a chance on the JB welded one, putting a new one in a few. I was also able to check fuel pressure today and found 21psi with key on, around 32 psi with engine running. I didn't run it at load but it seemed low to me. Which, leads me to believe it is low only because the ECM is telling the injectors to dump fuel (cold mode?) I don't suspect one bad injector as all of the plugs foul out and look the same. The only other item of concern to me would be something ignition related. Thoughts?

 

[axman]

So here is where I am at if anyone can lend anymore insight. Temp sensor, thermostat, new plugs and plug wires have made little difference. Checked vacuum to the fuel pressure regulator v and it appears sufficient. I compared fuel pressure to the port engine which has been running fine and found that the starboard engine is running higher pressure by about 6-8 psi with little to no drop at higher rpm. So, should I be looking at the fuel pressure regulator further or fuel injectors?

 

[harleyman1975]

I think that the regulator actually lowers the fuel pressure so I am going to go out on a limb and suggest if possible switching fuel pumps from port to starboard and see if the problem follows the fuel pump.

 

[axman]

Not an easy task and I believe that the fuel pressure regulator is what limits the upper end of fuel pressure because the electric pumps could probably easily push 100psi, thats why i'm thinking the regulator is bad.

 

[axman]

The regulator does lower pressure, thats why I'm thinking its bad. Those electric pumps are capable of over 100psi so thats why I'm thinking the regulator has gone bad. Has anyone ever tried to just clean one of these out?