Gas in Exhaust

[hinzee623]

Hi All,
Yesterday noticed an abnormal amount of gas in my starboard engine exhaust. At first, I thought I might have flooded the engine, so waited to see if it would eventually clear up. It didn't. My gut reaction is that the CARBUREATOR(??) needs to be replaced.

Important to note: Last time I took the boat out, right after we got back to the dock. I had a brain fart, and when I tried to bump the engine into gear, I mistakenly bumped the throttle into gear, which caused the engine to rev at High RPM for a split second (Luckily I was in neutral) I am thinking that might have been the cause, my issue with the fuel in the exhaust.

Anyone have experienced gas in the exhaust? What was the out come? Is the fuel injector the most common cause?

Thanks in advanced!

[1986 36' 3607 Aft Cabin, Twin MIE340 merc cruisers]

[buster53]

First of all, I’m not a mechanic and my engine knowledge is limited. That being said….
Unless the engines in that boat were replaced or upgraded, that era engine was carborated, not fuel injected.
Secondly, on my Crusader 8.1’s, I have an injector for each cylinder (8), not just 1 for the whole engine.

[hinzee623]

Source of the post First of all, I’m not a mechanic and my engine knowledge is limited. That being said….
Unless the engines in that boat were replaced or upgraded, that era engine was carborated, not fuel injected.
Secondly, on my Crusader 8.1’s, I have an injector for each cylinder (8), not just 1 for the whole engine.

My bad, you are correct, I doubled checked. It is carborated.

[buster53]

You may be due for a carborator rebuild.

[RobGunn]

I have a 1990 3607 Aft Cabin with Twin 454ci Mercruiser engines with carburation. 2 seasons ago I started to have the same issue that you are describing. I first noticed the gas smell when I came back to slip after a boat trip. Seemed to get worse at the end of each subsequent trip. I also jumped to the early conclusion that it was a carburetor issue so I pulled the car and had it rebuilt. I put the re-built carb on the engine and found that the problem was still a issue. That's when I noticed that my engine oil was showing over-fill. What the hell...How can a engine make oil? And I was seeing oil pressure dropping. I drained all the oil and filter and replaced with new. When I drained the oil it was found to be very diluted and full of gas. OK now I know where the gas smell is coming from and why I got pressure drop. I installed new oil and filter and made a couple short trips in the boat and got the same results as before, gas smell, over-filled oil and lower oil pressure. It this point I was at a brick wall in my head, how is the raw gas getting into the oil pan? Had a big meeting of the minds at the Tiki-hut at the marina and got a number of "old timers" on the case. What I found out is the mechanical fuel pump can go bad and pump raw fuel into the oil pan. Run your engine and take a look at the small clear fuel return line running from the mechanical pump to the bottom of the carb, you should se NO movement of fuel. If you see movement (bubbles) then you have a bad fuel pump.
I replaced the mechanical fuel pump, changed oil and filter once again and took the boat for another test run. The issue is resolved. Been running great ever since.

[hinzee623]

Source of the post I have a 1990 3607 Aft Cabin with Twin 454ci Mercruiser engines with carburation. 2 seasons ago I started to have the same issue that you are describing. I first noticed the gas smell when I came back to slip after a boat trip. Seemed to get worse at the end of each subsequent trip. I also jumped to the early conclusion that it was a carburetor issue so I pulled the car and had it rebuilt. I put the re-built carb on the engine and found that the problem was still a issue. That's when I noticed that my engine oil was showing over-fill. What the hell...How can a engine make oil? And I was seeing oil pressure dropping. I drained all the oil and filter and replaced with new. When I drained the oil it was found to be very diluted and full of gas. OK now I know where the gas smell is coming from and why I got pressure drop. I installed new oil and filter and made a couple short trips in the boat and got the same results as before, gas smell, over-filled oil and lower oil pressure. It this point I was at a brick wall in my head, how is the raw gas getting into the oil pan? Had a big meeting of the minds at the Tiki-hut at the marina and got a number of "old timers" on the case. What I found out is the mechanical fuel pump can go bad and pump raw fuel into the oil pan. Run your engine and take a look at the small clear fuel return line running from the mechanical pump to the bottom of the carb, you should se NO movement of fuel. If you see movement (bubbles) then you have a bad fuel pump.
I replaced the mechanical fuel pump, changed oil and filter once again and took the boat for another test run. The issue is resolved. Been running great ever since.

Appreciate the detailed response. Glad it worked out for you! I will definitely look at it from this angle.

[km1125]

Definitely follow RobGunn's suggestion. Doesn't' cost anything to check and is very easy to do.

Next up would be the choke mechanism. When the engine is warmed up, remove the flame suppressor (on top of the carb) and see if the choke is fully opening. Yours could be either a mechanical thermostat-like mechanism, or an electrical thermostat mechanism. If the choke is not fully opening, you need to troubleshoot that because it will cause the engine to run rich.

How does the engine run overall?? Any rough idle issues? That could indicate a spark issue, causing one (or more) cylinders to put unburnt gas into the exhaust.

[hinzee623]

RobGunn - As a follow up question, prior to discovering this issue. Did you notice that particular engine was a little more difficult to start up cold? Like you have to give it a few pumps of the throttle for it to turn over?

For me, my port engine turns over relatively easy, however my starboard (the one in question) it always takes a few tries, and more or less i need to move the throttle as i am turning the key in order for it to catch. So I am now thinking, maybe my fuel pump has been on its way out (unbeknownst to me, until now). I will still test your theory.

[hinzee623]

Source of the post Definitely follow RobGunn's suggestion. Doesn't' cost anything to check and is very easy to do.

Next up would be the choke mechanism. When the engine is warmed up, remove the flame suppressor (on top of the carb) and see if the choke is fully opening. Yours could be either a mechanical thermostat-like mechanism, or an electrical thermostat mechanism. If the choke is not fully opening, you need to troubleshoot that because it will cause the engine to run rich.

How does the engine run overall?? Any rough idle issues? That could indicate a spark issue, causing one (or more) cylinders to put unburnt gas into the exhaust.

Overall has been running fine. Other than the quirky start ups. I keep a detailed log of my fuel consumption. and for two seasons now, my burn rate has been very consistent.

[Viper]

When was the last time it had an ignition tune up? Pull the cap and take a look at the contacts inside. If they're corroded, you might not be firing all cylinders and sending raw fuel out the exhaust.