Anyone had this issue

[SeaRenity]

Thanks for looking, I have had an issue with my starboard engine that is driving me crazy.
It is a intermittent issue that has been more frequent-- Engine starts fine, than within first few minutes RPM drops to low 500 and feels like it will stall --rough idle, shakes and smell of gas in exhaust. If I raise throttle engine smooths out and runs fine. Within five minutes(almost as it warms up) it goes away and I have no issues. I fel like it is a sensor issue but I don't know where to go-- Need help
REplaced Gen 3 cool fuel module (complete unit with pumps/filters) replaced injectors, new IAC, swaped MAP sensor with other engine (no change) new plugs,wires, cap and rotor
Any Help will be appreciated
Thanks
Rich

 

[sr330daguy]

Does the engine reach proper temperature? (t stat). A defective coolant temp sensor could also be the culprit.

 

[SeaRenity]

YEs it reaches 163 degrees and levels out

 

[Espos4]

Sounds like one or more cylinders isn’t firing, allowing raw gas to be expelled through the exhaust.

There are 2 different temperature sensors. One for the gauge (1 wire sensor) and one for the ECM (2 wire sensor)
The dashboard readout isnt necessarily the same reading that the ECM sees.

Maybe a defective distributor cap?
Swap the cap from the other motor.

When you replaced the spark plugs, was there any sign of water intrusion, any rust stains on the spark plug(s)?

Next time it happens, shut it down after a few minutes of rough running and remove the spark plugs. Look for the plug that is wet with fuel.

I would consider doing a compression test as part of your diagnosis.

 

[BillK2632]

A couple of things are going on during a cold start:
Coolant temp sensor senses a cold engine.
ECM Richens the fuel mix based on what the temp sensor is saying.
Throttle Position Sensor (TPS) sets the throttle open to keep the idle up until the engine warms up.

This is all the electronic version of a choke, basically yours is not working and you using the throttle to keep the idle high until it warms up is the equivalent of keeping your foot on the gas peddle of an old car without a choke until it warms up.

I would check the connections on the temp sensor and TPS first. You could replace these items yourself, they are not expensive, but can't know for sure either one is the problem. A Mercruiser mechanic could use the scan tool to find out exactly what is going on, might be cheaper in the long run vs just replacing parts.

 

[scoflaw]

IAC opens to raise the idle when cold. TPS only opens with movement from binnacle.

 

[SeaRenity]

Bill
Thank you so much for your response, when you say temp sensor you mean the ECT sensor(engine coolant sensor that sends info to ECM)
Would this be why I smell gas??

 

[Espos4]

The throttle position sensor doesn't open or close the throttle. It is only an input sensor. It tells the ECM the position of the throttle plate.

The DTS system has an electric motor that opens and closes the throttle as commanded. This electronic throttle body does have a built in throttle position sensor (probably 2 like the automotive version has).

Mechanical throttle bodies use an IAC (Idle air control) motor to adjust the amount of intake air bypassing the throttle plate, which will raise and lower your idle speed as commanded by the ECM.

If you are mechanically inclined and up for the task, you could swap parts between the motors.
If you aren't so inclined, I would say it is time to get a qualified tech involved.

 

[Bt Doctur]

It does sound like the temp sensor is not working