Need some help with engine loading up

[GUY_RPM]

I am having issues with my Mercury 350 Mag motor (5.7) in my 2004 24V. Once engine is warm, if I turn off boat it will not re-fire unless I pump the throttle and when I pull throttle back to idle it will die. The smell of gas is fairly bad. Finally got it to fire and idle but it was idling very rough and the smell of fuel was bad. I was able to get it back to dock and on trailer. Here is another aspect, once I was able to get it to idle and put it in gear once I opened throttle up it took off fine and ran on top end like there was not a problem. The carb is loading up at low RPM. Any ideas?

[GUY_RPM]

Or maybe I should say throttle body is loading up. I was not getting any beeps, just not starting.

[chpthril]

MAP and coolant temp sensors are the biggest contributors to fuel ratio on that engine and most likely to have a failure. A few leaking injectors could also cause your symptom.

[GUY_RPM]

If these sensors went bad would motor still run great on top end?

[chpthril]

Yes, as a little bit rich never hurts top end, but rich doesnt make for a smooth idle

[GUY_RPM]

Are these sensors expensive, any idea on cost? If I came up with a engine scanner would there be a code for either of these sensors?

[chpthril]

May not be codes, so you would have to know how to interpret the streaming data.

[boatwakes]

Hey Guy, post your engine serial number as some of those models have an intake manifold air temp sensor (MAT) that presents the same way. I had it do the same thing on mine but not all Merc's have the sensor, hence the serial number so i can look it up.

[GUY_RPM]

My serial number is 0M393266, thanks for all your help.

[boatwakes]

Okay, you have an integrated map/mat sensor that may be responsible for your issues. Unfortunately that sensor is $280 (merc p/n 885165). But fortunately I have one on a shop engine that I'll send out to you if you want to try it. However, I think you should have someone scan the motor ECM and check for anything else it may be. It could also be a fuel pressure regulator issue, easy $30 fix if so, so again you should get the fuel pressure checked. Now it's cheaper to buy your own fuel pressure tester at an auto parts store than have a tech scan the engine, $45 for the tester, but it's up to you on both. My suggestion is to scan it to verify and test the fuel pressure to eliminate or at least narrow down where you go from there. I hate throwing parts at stuff unless it needs it but I'll help you with it as far as you want to go on your own.

[GUY_RPM]

Boatwakes that is very generous of you. I talked to my mechanic and he suggested to remove a small air filter that is inside the center intake of the throttle body. I took it out and it was very loaded up with grease and gunk. I also cleaned the flame arrester and it was quite dirty. I ran it in the driveway and it did ok but you can't simulate the engine being under a load in the driveway. I am not sure if cleaning these two things will fix the situation but it can't hurt. I am going to start with the simple stuff to rule it out. One question I have is if the MAP/MAT sensor is bad wouldn't it beep at me to let me know it has thrown a code?

[Z1Fan]

Hey guys , does this engine have a actual TBI or is it MPI ?

Sent from my HTC One_M8 using Tapatalk

[chpthril]

One question I have is if the MAP/MAT sensor is bad wouldn't it beep at me to let me know it has thrown a code?

A sensors feedback can be skewed enough to effect engine performance, but not to the point of setting a code. Ive seen it with TPS, O2, MAP CTS and AIT and others. Thats where understanding the live data stream comes in. Even with a code, that doesnt mean the sensor has failed. I have also see plenty of MAP, O2, EVAP and other codes, where the sensor was telling us the truth, but something else caused its feedback to reach the point of setting a code. Thats the difference between a technician with a scan tool and the counter guy at Advance with a code reader.

[GUY_RPM]

It has a throttle body. I replaced the throttle position sensor two years ago so I don't think that is the problem but as many of you can probably figure out I am not a mechanic. Heck I am not even a shade tree mechanic. I am not afraid of trying to fix myself but would rather take it to my marine mechanic but he can't get to it for a week so I am taking a stab at fixing some of the simple things and will test it later in the week before we load it down with people. After listening to it run without the filter inside the throttle body it opened my eyes to how much air it sucks in and there was no way it was getting as much air as it needed thru the dirty filter.

[boatwakes]

Guy, you have a throttle body but not throttle body injection (tbi), it's a multi port injected engine.