Mercruiser 5.7 running on only 4 cylinders.....help!!!

pprouty72

Cadet
Joined
Jul 30, 2020
Messages
6
I have a 1995 sea ray ec 215 with the merc 5.7L. that seems to be running on only 4 cylinders. Some back story of my problem....The engine has run fine for the past 5 years. I have kept up on regular maintenance. When I started the engine Saturday morning it ran as good as always. For the next 45 minutes or so we were cruising at about 3400-3500 rpm and then decided to do some floating for about an hour. Again I started the engine and it ran fine. We got back to speed for another 20 minutes and that's when the problem began. I came off plane and noticed the engine was definitely not running like normal. It felt as if a cylinder was not firing, the rpms seemed lower than normal, and sounded like it wanted to stall. I could throttle up and it seemed to run better but still not normal. We spent a few hours at the beach we were going to and then headed home. I started the engine and it continued to run in the same manor at idle, but did not seem to have any problem getting on plane and we cruised at 3400 for the next hour no issue, but back at idle it still ran bad. Fast forward to today. I have changed the fuel/water separator. I emptied the old one and there isnt any water and hardly any debris. I also changed the spark plugs as they were due. I started the engine, no change, the problem was not fixed. I did a test at the distributor, pulling different wires off the cap to see if there was any change in engine sound. I discovered that every other wire in the firing order could be removed without any effect. Pulling any of the other wires caused it to stall. In fact, I removed the 4 "non firing" plug wires from the distributor cap at the same time and the engine ran exactly the same. The cylinders not firing are 8, 3, 5, and 2.

What would cause half the cylinders not to fire in an every other pattern on the distributor? Could the ignition interrupt switch be grounded, causing 4 cylinders to not fire (I thought it cut spark to all cylinders)? Is the ignition control module bad, or the distributor sensor?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,300
have you looked under the distributor cap at your cap and rotor?
 

pprouty72

Cadet
Joined
Jul 30, 2020
Messages
6
Yes. When I removed the 4 screws holding it down, one was not tight. There was some corrosion on the rotor and on the cap. I have not replaced them yet (cant get to the shop before they close) but I did clean up all the surfaces. Test ran and it didnt made any difference.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
Engine serial 0F593967. I believe i have the thunderbold IV ignition.

Thanks for the S/N
I'm seeing a carb motor and TB4

Thinking its a dual plane intake and one side of the carb idle circuit is clogged up
 

pprouty72

Cadet
Joined
Jul 30, 2020
Messages
6
I was thinking the same thing, except there are 2 cylinders on each side not firing. 2 and 8 on starboard, 3 and 5 on port. I would think if one circuit was clogged, all cylinders on one side would not fire.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
A dual plane intake is setup that way, every other cylinder, it doesn't do left bank, right bank
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,300
AD, that makes sense. 1, 7, 4 and 6 are on one side.......2, 8, 3, and 5 on the other side.
 

pprouty72

Cadet
Joined
Jul 30, 2020
Messages
6
I did not know that. Im mechanically proficient but certainly no expert and always learning. Is the intake set up to feed cylinder 2,3,5,8 on from one and cylinders 1,4,6,7 on the other?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
I did not know that. Im mechanically proficient but certainly no expert and always learning. Is the intake set up to feed cylinder 2,3,5,8 on from one and cylinders 1,4,6,7 on the other?

yes
 

ls169hugger

Cadet
Joined
Jul 14, 2014
Messages
7
Hi I was wondering if a carb rebuild solved the problem for you. I am having the exact same problem but with the opposite 4 cylinders not responding to pulling the plugs off the distributor.
 

finsterx8

Seaman Apprentice
Joined
Jun 4, 2019
Messages
32
I am also having a similar problem, I have rebuilt the carb and also switched with the carb on the other motor. Runs pretty much the exact same and the other motor runs fine.
I'm hoping I can check both intake planes with a vacuum gauge, hoping it is not a valve train issue.
 
Top