RockandRoll24

Something just dawned on me. Hoping you all can give some input. When we were reinstalling the motor, the module on the back of the intake plenum got hung up on the fire extinguisher mounted on the transom. It took a good deal of weight and cracked a bit. Not enough to come off of the plenum, but enough to realize something had snapped. We looked it over, and aside form the cracked mounting post, didn't see anything else obvious. looking at pictures, I believe it was the ignition module. Could stalling be a symptom of a cracked up ignition module?

SB

iTrader: (1)

Ignition module is inside distributor, under the cap.

#19 is the map sensor. Ooops ! Test it, but it is probably fubared.

camptappakeg69

Is it the map sensor that broke on the left side of the plenum, that could probably cause some issues with it not running properly. In the service manual, there is a lot of trouble shooting flow charts that will help.

BUP

iTrader: (9)

you need to scan this engine and also spark test this engine. If the map sensor is crack most likely the engine is not going to run well period but whatever is cracked I would replace to see if your problem goes away. The scan also can show if in fact your MAP is working correctly by the readings thru out rpms.

Also this app has a spark control module that can be going bad on you., Heat and or voltage spikes can knock them out but after cool down will work fine again. This is NOT under the distributor cap but mounted on its own bracket externally.

I would also test with a fuel pressure gauge as the VST components can be problematic alot of the times.

you really need to figure out if this is ignition or fuel related or sensor related or all 3 causing the issue(s).

RockandRoll24

It is indeed the MAP sensor. The body appears fine, its just one of the retaining "ears" with the small bolt through that has a piece missing. I will get one ordered for piece of mind. I borrowed a friends fuel pressure gauge, but unfortunately it had the wrong coupler size. He's grabbing the correct one tomorrow. Will test tomorrow eve.

I spoke with my local merc shop manager today. I trust his opinion and have relied on him for advice many times in the past. He's thinking that the VST pump is not working correctly (or at all). Said it sounds like I'm just using the fuel in that tank to run for 30 or so seconds. If the VST pump isn't working, fuel pressure can't keep up and the tank runs dry - engine instantly dies. I'll know more about this theory after I get a gauge on it tomorrow...

SB

iTrader: (1)

You can test MAP sensor easily instead of just replacing it for the hell of it.

RockandRoll24

I think I got it figured out! Still need a bit of input from you guys though. Heres the deal...

Put a fuel pressure gauge on it. I'm seeing about 35 psi at idle and as soon as I brought RPM's up to around 3500, pressure would drop to nothing (motor RPM falls off) and slowly rebound. This was on the trailer. In the water, she just died.

I started looking closer at the fuel system. I realized someone, at some point, added a booster fuel pump before the mechanical pump. They teed the wiring into the VST pump wiring. When re-installing the motor, we didn't catch this and inadvertantly plugged the booster pumps connectors to each other (oops). So...hooked the booster pump up to the VST pump wiring and pressure stayed up again. However, when running the motor out, fuel pressure slowly dropped to 28 psi and stayed there. I believe spec calls for pressure to stay at 37 or above. Is it possible that the VST pump is failing and not able to keep pressure up where it needs to be? This obviously was an issue before I bought the boat. The PO must have added the booster to try and overcome the weak pressure. Is 28 psi an issue? What changes can I make to boost pressure? Replace VST pump?

Thanks for all the input guys!!

RockandRoll24

Anyone got any ideas on the low fuel pressure? I'm going to replace the vacuum line tonight. Its old and very brittle. I dont think its even secured at the nipples. Not sure if that could be the culprit though...

SB

iTrader: (1)

The mechanical pump pulls fuel better than an electric, so not sure why electric was used unless maybe the mechanical was going bad and they did this in case ? Or maybe some update kit for fuel vapor locking issues ?

What kind of pump is this ?

Have a pciture of it and how it's mounted ?

SB

iTrader: (1)

Yes replace it.

However, if good vacuum, the fuel psi will be less with more vacuum. The fuel psi raises when vacuum is reduced.