350 mag mpi problem

[Fixit]

engine is doing some weird stuff,

starts, idles and runs very nicely.

problem occurs after the engine is hot, and is brought back to idle. after 10-15 mins at idle ill notice a 50 rpm drop to 500rpm, then it will nearly die, 350 rpm. If i give it gas it will backfire into the intake manifold and then die, but if i pump the gas i can get the rpm to climb up to 2-3000 (still coughing and popping). if i do this long enough, it suddenly clears up and seems perfect. 15-20 mins later it will all repeat.
seems that engine temp climbs up from 161 to 163 when probem happends, and after reving it up and problem stops temp is back down to 159-161

its an 08 mercruiser seacore 350 mag MPI bravo engine

no codes (ok trim indicator error and fueltank level sender are both nfg but shoudnt be related)
water pressure is 2psi at idle,
fuel pressure is steady at 42.8
good oil pressure
temps stay from 159 to 163f

ive had it hooked up to the computer and everything seemed great, but it wouldnt do this hiccup for me when mechanic was in boat

 

[baddogg]

Just out of curiosity, when was the last time you changed your risers and/or risers? I had a similar problem that eventually led to an engine failure. Risers and manifolds were obstructed due to corrosion.

 

[daddystoy]

Just throwin it out there, Good clean fresh gas with new fuel/water separator?

 

[Sir Reel]

I had an idle problem with my 4.3L MPI. It turned out to be the idle air control valve. Sounds like you might have a similar problem. Is a real easy fix or something to try anyway.

http://forum.chaparralboats.com/index.php?showtopic=15216

 

[Fixit]

Risers are of dryjoint variety and run cucumber cool all day long

Fuel should be fine, I've burned 500l through her this year with a new water seperating filter yearly, fuel pressure never drops off so that Probably isn't the issue

I've actually ordered a iac to put in, so I'll see how that goes

 

[Island Fish Lifter]

agree with baddog. Sounds like riser/ manifold to me.

 

[Fixit]

how do you figure its risers or manifolds?

manifolds are freshwater cooled,and ive never had issues with them overheating or running hot

 

[claymcpher]

If it has a distributor cap pull the cap and check/wipe it out of moisture and check it for cracks as well

 

[Fixit]

Cap rotor and plugs have 2 hrs on them

 

[Califbill]

Volvo had fuel pressure regulator problem. Leaking past the regulator. There is a bulletin on it. Saw this problem on another site, where flooded when hot.

 

[TenMile]

Could be the IAC but the boat would generally throw a code. If ordering an IAC you can get the black Merc one for $150 or a silver Ford Windstar variant for $30 (same part).

This sounds like it could be dirty injectors or a wet fuel filter too. I have had great success with my last boat 350mag MPI doing the Seafoam shock treatment. Disconnected the fuel line from the tank and ran it into a 1 gallon portable tank full of gas and an entire can of Seafoam. Start a warm engine and run up the engine to 2000 RPMs. When treatment works its way through the engine will sputter and stumble. Shut down and let sit for 30 mins. Start it up and the engine will initially smoke like hell (crud burning off). I would repeat this process at least once more. Replace your old fuel filter, fill with the gas and Seafoam mix and take the boat for a hard run.

Last thing to check might be the MAF sensor too but again would usually throw a code.

The temps and pressures you posted all look correct to me. Dont think this has anything to do with cooling. Sounds like fuel or air.

 

[runamuk]

did the problem ever get solved ? If so what was the issue?

 

[Fixit]

ended up changing a raw water pressure sensor, it was reading 0psi at idle, with engine running cool. new sensor reads 3.5-4psi

also changed idle air control valve. havent been able to replicate the issue again yet. hopeing ive got it solved

 

[TenMile]

If it does it again try cleaning the injectors like I mentioned.

with the IAC its a common part that fails. With the computer hooked up your mechanic can watch the duty cycle of the IAC. If faulty it will slowly fall below 50%.