[jad050]

Bought my first boat ever.... 1996 Mariah Shabah 180 w/ 4.3LX.... One previous owner, looks like he kept it up pretty well... Except for some cracking in rear seat base upolstery, it's perfect...

Last ownerd did tell me it would occasionally diesel when shut off... It didn't the first few times I cruised around on lake. Started pulling folks on tubes and wakeboards, and sure enough, the engine won't shut down at times... What I have noticed when this occurs is the idle is 1200+ and won't go any lower... When idle is a more typical idle, say ~700 is turns right off...

In the years before my hair turned grey, and cars had carburators, I think I could've have fixed this in in my sleep... Looking at a carb now is like looking at an artifact at the history museum. Carburator looks familar and I almost remember how they work.. So far the only thing I did is spray the linkages with carb cleaner/lube hoping for the simple solution...

Any insight to what could be happening? What could be causing my idle to sometimes run higher?... Could I be erroneously tying the idel problem t the dieselproblem? I believe the problem is only occuring when I have been pulling somebody. The temp stays constant (can't remember what gauge said)... Any recommended sites to get good info on mercruiser, there seem to be a lot out there, but like everything, I'm guessing some good/bad/excellent...

JD
1996 Shabah 180

[bpfirrman]

I also had this happen when I was pulling someone but it was operator error in my case. When I would pull up to the person in the water to pick them up I would shut it off too soon, needed to put it in neutral and let the rpm go down first. I was so concerned with the person in the waters safety I was jumping the gun when on turning the key off. From then I put it in neutral a little further out from the person in the water and coasted to them, then shut if off. That took care of it. Hopefully this helps you.

[jad050]

mmm, too quick shutdown might be my problem also... I haven't paid any attention to when/if the idle lowers on it's own... doing exactly what you stated... pull someone, swing aroung to pick them up, shift to neutral and turn off key... How long does it take to simmer down?

Be real easy is this is just a simple 'newbie' user error...

Thanks

JD

[2006sx18]

this was happening to me as well and I did a couple things - kept blower on and added mid grade gas instead of regular unleaded. Waiting a couple seconds before shutting off the key would probably work as well.

[All In]

Welcome to the club. I don't think that you will have to go anywhere else to find the answers that you need for your new boat. This site is great. In answer to your question about the diesel effect that you are having, you will notice that on the lake they only sell midgrade fuel, that is because most boats require this. I have a 4.3 shabah and I have found that higher grade fuel makes the boat run much better. I put the highest octane that I can in and also add an octane booster. I am constantly pulling someone on the lake, either a tuber or skier and this seems to work well for me. Also one other note, when you are coming about to pick up the person in the water, you have more control over the boat if the engine is on and in the idle mode rather than off. I agree having someone in the water can be a hazard but you don't want to add to it by not having good control over your own boat. I have a 4 and 7 year old and the last thing that I would want is to become what I am trying to protect them from. They love the tube. I think they would sleep on it if I let them.

[paulbroeder]

Hey man, Ive had two 4.3lx enigines, one in a 1900z and now in a z210talari. They both did the same thing until i got a spray can of "SEA FOAM" lube and penetrating oil. I cranked the motor and brought it up to operating temp and then flooded the intake through the carb until the motor almost died. I shut it off and let it sit for 10 min. abd then cranked motor and blew all that crud out. I then repeated it one more time. Now, when I go to kill the engine , I bring it back to idle and the rpms come down to about 650 or 700, then I kill the engine and no problems. What is happening is you are getting alot of carbon build up on your intake valves causing you valves to not seat properly, thus keeping you idle up and not shutting off the flow of fuel into your cylenders. Give it a shot and you will be pleased. If the problem persits, switch to super unleaded gasoline. I had to do that on my 1900Z.
GOOD LUCK

[Threejakes]

I had the same problem, I just use premium fuel, and the dieseling stopped. I think I will try that seafoam, or the GM stuf made for this problem.

[jad050]

I switch to premium over the weekend + added actance booster to currnet fuel... The problem is much better.... Also picked up can of seafoam, going to give it a try before next weekend....

Reminds me of a mechanic I knew in the 70's, would stream a cup of water @ fast idle into to carburator to clear out carbon.... Car could cough, sputter, smoke, the run smooth as a top. He claime dthe valves would shine afterwards... The "good old days", when life was simple..

Thanks for all the advice so far....

[dgtek]

I was looking for answers and found them! Thanks a bunch...

[SEMIJim]

Some people swear by Marvel Mystery Oil (MMO). I don't know the amount you put in, but you put it in your gas and it allegedly cleans up valve seats. I don't know how effective or necessary such measures are, these days. Back in the late 60's, early 70's, Oldsmobile pioneered positive valve rotators. I can't imagine modern (large?) engines haven't been using them for the last couple of decades.

Random thoughts...

• The reason higher-octane fuels sometimes alleviate dieseling if they have somewhat higher ignition and vapourization temperature points.
• One cause of dieseling is too-lean a fuel mixture.
• You want to avoid running an engine at high speed, under high load, then quickly shutting it down. Let it down gently--give it a bit of cool-down running at lower speed and load, before shutting it down.

Jim