Baller mechanics...340 Monsoon idle issue

[Fine409]

I've got a intermittent idle issue taking place on my 04 LXI with a 340 Monsoon engine.

 

Not seeing any issues running at speed, just idle and low speed forward gear.

 

Here's a couple videos:

 

 

(trimmed the 1st video, stumble happens ~4 mins apart)

 

 

(occurs at 30 seconds and 8 minutes)

 

In the last 10-15 hours, the boat has new plugs, wire, fuel filter and distributor. I've also checked for codes and am coming up empty there.

 

Given the quick fuel pressure drop takes place when it happens, I'm leaning towards it being a fuel pump issue but wanted to see if anyone else had a different thought.

 

Have an appt with the dealer on May 12th but would love to solve it before then.

 

Thanks in advance for taking a look!

[Bill22]

Did you replace both fuel filters?

 

My Boo has one at the engine and one under the floor.

[Fine409]

Mine just has one it seems, under the floor next to the mufflers. I traced the fuel line from there both to the engine and to the tank and I'm not seeing another one.

[Rednucleus]

Nothing more fun than an intermittent problem that's only a quick hiccup. Assume there's a fuel pump relay; maybe try changing that- quick, easy, cheap.

[DavidN]

Run some (quality) injector cleaner through it.

Cheap enough and can’t hurt.

Could be one injector hanging for one cycle.

[Bill22]

@dtm8119 I am not a mechanic so that’s all the advice I have.

[eyepeeler]

Some of the early 2000s Bus had a fuel filter hidden behind a wall in the trunk.

[Jetsetr]

Ok folks: let’s look at this a bit more analytically.

The engine needs “X” amount of fuel pressure to run correctly.

If it was a injector the main line fuel pressure would remain constant and just that one cylinder would drop.

1 it doesn’t do it all the time, so rule out fuel filters.

2 injector cleaner will never hurt, but in this case it’s not the fix...

3 it’s more than likely a electric issue/gremlin.

Fuel pump, fuel pump relays or possibly a injector (but really doubt it), bad ground on any of the above as well, intermittent positive side issue, ignition switch, ANYTHING that puts power to the pump could be the problem.

It might be a really easy fix, it might be a nightmare.

THIS is why I like carbs....

 

 

[RichardDoane]

Check the wiring/connections to the coil bracket/ module. That hiccup in the fuel pressure is a result of a loss of voltage. It corresponds to the loss of spark or “stumble”. The module controls injector pulse and spark.

[Wayne]

Can you put a volt meter at the fuel pump terminals and monitor voltage when the hiccup happens? That could rule out any electrical fluctuations causing the fuel pressure to dip.

[Fine409]

Thanks for the input everyone. I'll next check the electrical connections and monitor the fuel pump voltage and report back what I see!

[Fine409]

OK so the voltage at the pump does drop when it stumbles. I get a solid 14 volts as it is idling good and then when it stumbled it went down to 12-13 volts, then back up to 14 volts as it recovered.

 

Should I swap in a new fuel pump relay you guys think and retry?

[BraceMaker]

Relays are cheap but does the boat have two with the same part number? Usually you can swap if a boat has two and see if the fuel pump issue is resolved

[Andre]

Voltage probably went down to zero for a fraction of second.The meter just not fast enough to read it.At 12 volts,the pump would fonction normally.

Ignition switch,fuel pump relay,could also be a faulty landyard switch,something is cutting for a split second...

Follow @Jetsetr and @RichardDoane advices .

[buechsr]

Inspect the IAC.

[ReggieO]

I had a friend who was experiencing something similar and the cause was one of the rubber hoses that connect to the fuel filter was peeling back on the inside, reducing the flow of gas going through the rubber tubes. As the boats get older, the rubber hoses start to break down, especially where they connect with metal. Check it out.

 

[Conor]

Another boat in my lake was stumbling and popping. He started replacing parts. Three seasons later he had a specialist come out to the lake and it was timing all along. They couldn’t have learned that without plugging the motor in for diagnostics. He replaced a ton of parts trying to chase the problem.

Just timing. Just a thought.

[Fine409]

Just to wrap this up (I hope!) - back from the dealer and first water test went well. Ended up being the MAP sensor. Weird that it didn't throw a code.

[Jetsetr]

???

Sensor issue falls into the the electric gremlin/issue category.

It SHOULD have thrown a code. Chasing electric issues can be like herding cats...

[BraceMaker]

@Jetsetr - often they don't.

 

In these systems most of the sensors have a range of "normal" but one end is say normal for running at sea level 100 degrees high humidity idle and the other end is "normal" for a dry day in colorado at 5000 feet and 65 degrees.

 

Both are normal range that the computer expects to see but your boat has no clue that the MAP sensor is telling it that its getting too much air or too little.

 

On some of these systems like MEFI-4 you can literally unplug a sensor and the computer will toss a code and set the sensor to a default value - and if that runs better than it did with the sensor plugged in you've got your issue.

[Jetsetr]

@BraceMaker And THATS why I like a carb...

[Orlando76]

@Jetsetr but you have to pump the throttle twice before a cold crank. Too difficult.

[Jetsetr]

3 times! And it purrs like a kitten!

I get that technology changes, look at cars...they will go 300K easily now. Before if you got 100K without the body falling off/engine smoking like a old 6-71 Detroit you bragged about it.

But, sometimes simple is good.

 

This is my GF’s car (so is the Vette behind) 1475 REAR wheel horsepower, all aluminum 572, 10-71 EFI. You turn the key and it cold starts perfect. It’s a low 7 second street car...tune it with a laptop. And it’s totally drivable on the street. Not like we still have to do to start the race motors with the fuel bottle at the injector...