454 MPI smells like unburned fuel when starting

S

SK

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My 2001 7.4 MPI merc has started smelling like unburned gasoline each time I start it. It is coming from the exhaust, not from the engine compartment. The boat has 800 hours on it, but I have kept it in great condition with maintenance and repair. I was pulling the kids on a tube when I first noticed the smell each time I would start the engine after switching riders. Then I noticed it every time thereafter, hot or cold engine.

My first thought is an injector leaking fuel from the rail that still has pressure after shut off. I will pull the plugs next weekend before we take the boat out and see if I have a wet cylinder or a fouled plug. I will also bring a fuel pressure gauge and check if the rail can hold pressure for an extended time.

I noticed another strange thing while pulling the kids on the tube. We ran for about an hour and then I turned off the boat to wrap up the lines and secure the tube before heading in. The boat was off for maybe 5 minutes. When restarting (smelled the gas again) I also notice my temperature gauge was at 100 instead of the usual 175 that it stays at when full warmed up. I had to drive the boat about 3-5 minutes to get the temp back up to 175. It was like we had just left port but we had been running for a hour. ????? It went back to 175 and stayed the rest of the time. So maybe the temperature sensor is whack or thermostat and the computer is purposely dumping the gas into the engine thinking it's cold.

Any thoughts on how to diagnose this issue?
 
i think both of your suppositions are valid. pull the dip stick and smell it to see if any gas has gotten into the oil. i don't think so but it's a precaution..it'll dilute the oil.
 
I just changed the oil 3 weeks ago before I noticed this, and currently it is still nice honey color and is not getting any fuller on the dipstick. That is something I always check before I leave the dock each outing. I'm a little anal that way some have told me.
 
If you smell it, you should be seeing a sheen in the idle exhaust as well.

Good plan with the fuel pressure tester to help check for leaky injector after shutdown, and checking plugs for wetness.

You might try an inductive timing light on sp. plug cables to check for strong strobe on each, but note you could see that even if the plug boot has fallen off off and plug clip conductor is shorting to ground.

I haven't tried it, but use of a noid light on the injectors will indicate if each is getting good signal, but since it appears to be a case of unwanted gas rather than no gas delivered lack of signal is relatively very unlikely compared to debris or other reason an injector is held open, or insufficient compression or properly timed spark at a cylinder. .

How do you observe engine temps? Through SmartCraft or similar with input from the ECU/ECM? Or conventional analog type gauge?

If from the ECU/ECM, I'm wondering as a long shot if the engine ECT sensor could be erratic and affecting both ECU computed fuel mix response and dash temp reading? An analog dash gauge would just use a separate engine coolant temp sensor, I believe.
A Merc scan device should reveal if that ECT or other sensor like MAP was erroneously causing the ECU to signal all injectors to stay open longer than they should , for over-rich condition.

Last quick thought- If it happens to be a Faria analog temp gauge and temp again shows lower than expected , try giving it a few "Faria Raps" to see if it bounces back up .
 
You assume correctly Sandy, it is a Faria temp gauge. I have already replaced the tach after I played the "thumping" game for two years. This was the first time I notice the temp was below normal and I did whack it with no change. These could be unrelated problems, but maybe not.

I don't know if this engine uses two coolant temp sensors (one for gauge and one for computer). I know my Pontiac does. There is one sensor in the thermostat cover housing, I know. I have the Merc factory service manual for this engine but it is with the boat and I can't look it up right now.

Getting a Merc mechanic to scan it will cost some bucks and be a major hassle down at the lake, so I will do that only if I come up with dead ends on all the things I can diagnose myself.
 
This is for a Merc 7.4 MPI up to yr 2000 (they just don't show anything past that) but as suspected the parts diagram shows 2 separate temp sensors [ECT part #11, & Water Temperature Sender (gauge) , #21. ]

http://www.marinepartsexpress.com/merc_sch_fs.html

So the ECT has nothing to do with the gauge anomaly.

Hope you find something obvious and relatively simple to fix.
 
"A Merc scan device should reveal if that ECT or other sensor like MAP was erroneously causing the ECU to signal all injectors to stay open longer than they should , for over-rich condition"

Exactly, why shot gun it when the engine diagnosis itself?
 
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