HELP! Mercruiser 4.3L V6 Engine Problem

[liquidbimmer]

Hello All!

I just bought a 1996 Monterey 185 with a Mercruiser 4.3L V6 about two weeks ago. I took the boat on a test ride and everything was fine. I also
took it out last weekend and ran it all day with no problems. This past Wednesday I was out on the lake for about two hours, stopped the boat to swim. After starting the boat up I put into full throttle, the engine began to misfire and run rough over 2,000 rpm. I shut off the engine and tried again, same results, the boat started up fine and runs smooth below 2000 rpm. I took the boat slowly to the loading ramp and took it home. I got a mechanic to pick up my boat the other day, he took out the spark plugs and said a couple were detonated pretty bad. He did a compression test and got a low reading on one cylinder I think the lowest was a 75 and the others were like 130-150. He automatically said I needed to get my heads rebuilt without even taking off the top itself. Felt like this guy was trying to scam me, because it doesnt make sense if the compression is low in a cyclinder my engine should be running rough at low rpms and smooth at high rpm. I took the boat back and have it in storage.

Also, I had put in a full tank of gas and added some seafoam earlier that day.

Does anyone know what is wrong with my engine? Bad fuel? sticky valve?

Thanks!

Allen

 

[Bifflefan]

Re: HELP! Mercruiser 4.3L V6 Engine Problem

I think you need to re-read your post. in one part you say it runs good below 2000 and in another part you say it runs bad below 2000.
Clearify this and someone may be able to help you.

It first read I suspected a partial vapor lock. That make have caused a lean run, that can burn a hole in a piston or burn an exhaust valve. A leak down test will tell all in either case.

Have you tried it with then new plugs?

 

[liquidbimmer]

Re: HELP! Mercruiser 4.3L V6 Engine Problem

The engine starts up fine, idles fine, and run smooth under 2000 rpm.
And runs rough and misfires over 2000 rpm.

Sorry for the confusion


Thanks!

Allen

 

[liquidbimmer]

Re: HELP! Mercruiser 4.3L V6 Engine Problem

I think you need to re-read your post. in one part you say it runs good below 2000 and in another part you say it runs bad below 2000.
Clearify this and someone may be able to help you.

It first read I suspected a partial vapor lock. That make have caused a lean run, that can burn a hole in a piston or burn an exhaust valve. A leak down test will tell all in either case.

Have you tried it with then new plugs?

I asked the mechanic I took it to do to that and run it, but he refused and said he would only perform a head job on it, he stated that just changing the spark plugs and starting it would harm the engine more.

 

[boat127]

Re: HELP! Mercruiser 4.3L V6 Engine Problem

sounds fishy to me. i would find a new mechanic. get a leak down test done and proper dianosis done. how are you rmechanical abilities?

 

[Don S]

Re: HELP! Mercruiser 4.3L V6 Engine Problem

If you only have 75 psi on one cylinder, and 130 to 150 on the other 5, then it doesn't run fine. It may sound fine to an untrained ear, but it's not running fine.
I have heard this many times from customers (It runs fine at idle) only to start it and hear a dead miss at idle but they can't here it.
If the plugs show detonation, then you could easly have valve damage or even piston damage.
New plugs will not bring up the compression, and it will not cure the problem that caused the detonation to begin with. All that has to be checked out.
A leakdown test on the low cylinder will tell you if it's a valve, piston, head gasket or whatever without taking it apart.
After that is fixed, the the ignition system needs check along with the fuel system or you will detonate again.

 

[z284u2try]

Re: HELP! Mercruiser 4.3L V6 Engine Problem

Sounds like it needs fuel.
Any pics of plugs?
Have your mechanic do a leak down.
So if he didnt start in then you are not getting the correct compression numbers. Those numbers will be higher on a warm engine did he take compression then add oil to the cyl and take comp again?