|I have a 1997 Sea Ray with the MERC alpha 6 banger that only has 150 hours on it. We just bought it this summer and have had a problem with desieling. We have taken it to the shop and they can't seem to get it fixed. (they have been very good at trying, but they are at their wits ends.) |
There is no consitancy to the problem. It happens about 50% of the time we shut down. Carb has been check and no leaks, timing is right on, plugs look good. OPerating time has no effect.
We can't seem to figure this one out. Any suggestions?
|It is a very common problem with all merc carb motors. My 4 banger does it maybe 10% of the time, our mech who is very on to it for a merc merchanic cannot fix it either.|
|Have you tried higher octane fuel?|
|seems to me I remember a post about the merc 6 cyl doing this a while back, say 6 months or better. I believe the post said there was a merc bulletin on the subject so you might try looking into that. I don't have that engine but I had a similar problem which became more pronounced after I installed a 175 degree thermostat. I resolved my issue by dropping the timing about 2 degrees, and using medium or high grade gas (like Bill J suggested). After I did these things my engine is way easier to start, better on the mileage and more consistent. What virtually eliminated my deiseling is to always let my engine idle for a short time 10-30 seconds before I turn it off. I'm guessing this helps clear out any hot spots or excess gas. hope this helps. |
|Most of the time we find the idle is simply set too high. Sometimes due to the throttle cable end being out of adjustment. This does not let the throttle arm hit the stop at idle....|
|I had that problem with low octane gas. '96 Searay. Since then, I've use at least 87 octane nad have had no problems.|
|This used to be a common problem on cars in the 70's, when most were still carburated and the smog controls hit them hard. |
The dieseling is caused by hot spots in the cylinders igniting the fuel. There are three basic ways to get rid of the problem:
1) eliminate the hot spots
2) eliminate the fuel
3) use a fuel that won't ignite as easily.
The simplest soultion is often choice 3, and it has the added benefit of solving any detonation problems, which usually go hand and hand with dieseling.
Solution 2 is usually a carburator thing: in the 70's many carburators had solenoid valves that completely shut down the fuel system when you turned the key off. Unless you want to change carburators, this is not something you can do.
Solution 1 has many aspects to it. The basic idea is that you need to eliminate whatever is getting red hot.
One way you can do this is to run a little richer fuel mixture so that the cylinder head temperature is not so high. This is not so hard to do, and if you are running a little lean anyway it can improve performance. You might improve it a little by adjusting the idle mixture, but the real fix is in the main jet(s) which will have to be replaced.
If you feel like rebuilding the engine, you can tear it apart and remove any carbon build up or rough spots on the cylinder head. Nah, I didn't think you would want to do that!
Changing the spark plugs to a "cooler" plug can often make a big difference. Spark plugs have three basic characteristics: Thread/diameter, reach, and heat range
The thread/diameter has to be right or you won't get them threaded in.
The reach has to be right. If it is too long, the tip of the plug may hit the piston and very bad things will happen. If it is too short, the spark won't be in the right area and it will miss.
The temperature range is something that you can play with. The plugs are constructed with an insulator between the center electrode and the outer case. How thick that insulator is and how far back it goes before contacting the case determins how well it conducts heat away. NOTE: it makes NO difference to the strength of the spark, just the ability to dissipate heat.
If you get too cold of a plug, then you will get deposits building up on the insulator and it will short out, or "foul". If the plug is too hot, the excessive heat can crack the insulator or even melt the tip. It can also leave the plug tipp glowing red hot, which may cause detonation and dieseling. The "heat" value of the plug is normally encoded in the number, and can vary with manufacturer. For Champion plugs, as an example, a plug number RV15YC4 has a heat value of "15". Going to a RV14YC4 would be the same plug in a sligthly cooler version.
I would start by taking a close look at the plugs. If they don't already look too cold (black deposits on the insulator) then I would try going to a cooler plug. If that doesn't work, I would simply use a higher octane gasoline.
|Replacing the thermostat helped to fix this with my 4.3L merc. Don't know why, but the problem is solved. |
I have also heard people say that they let the let the engine run at idle speed for 15+ seconds before shutting the engine off. (IE: dont shut the engine off while not in neutral, and dont shut off immediately after going into neutral, give it a few secs).
After reading the previous post, the thermostat change may help to make things run a little cooler.. I may have been switched from a 175 degree to a 160 degree thermostat, I forget...