1996 4.3 Mercruiser idle sputter

[Bluemelj]

I have a 1996 4.3 Mercruiser that is sputtering on idle and at times during start and idle I would describe as puffing out white smoke out of the air filter on top of the edelbrock 4 barrel carborator that the previous owner added.

If I choke it and give it some gas I can not get it to sputter. At times it will start up no problem at all at other times I can only get it to start by choking it.

Little history, had a failed shutter valve and got water in the engine, only had to replace shutter valve, replace spark plugs, two oil changes, removed all water and everything else seems to be fine. The engine did not have this particular issue before it got water, is this something from that issue, it feels like a fuel/air flow problem that I would assume have to do with the carborator but I am just learning about engines through working on this boat. I have two links below one for it sputtering on idle and the other for when it is a struggle to start.

Idle sputter video: https://drive.google.com/file/d/1pgvt1iQINkep2ZEjKsVSXWFhxgKN6wIg/view?usp=sharing

start video: https://drive.google.com/file/d/13Sv0FTHdMnWypHwd5GYj2tyizb8GNr8Q/view?usp=sharing

Thanks...

 

[Rick Stephens]

By chance have you run a compression check? I can see a bad hole doing that.

 

[Bluemelj]

Rick Stephens I have not ran a compression test does it not matter that when I rev it up to 2000 to 3000 rpms it does not do the same thing. I figured with more stress the problem would get worse. thanks for the comment...

 

[Rick Stephens]

You may not notice a stuck or bent valve or a cracked piston or cylinder head at higher RPM.

 

[Bluemelj]

Rick Stephens your probably on to something, took it out for a test run today and it would not go over 2000 rpms in the water. Thought I got lucky with the water issue I had maybe not. Is not power a sign of poor compression? Thanks!’

 

[Bondo]

Rick Stephens your probably on to something, took it out for a test run today and it would not go over 2000 rpms in the water. Thought I got lucky with the water issue I had maybe not. Is not power a sign of poor compression? Thanks!’

Ayuh,........ It's a sign of something amiss,..... A compression, 'n leak-down test can point ya in the right direction,......

 

[Bluemelj]

Rick Stephens and Bondo I have watched a couple of videos on performing a compression test which looks pretty straight forward but I am not quite getting how the ignition system needs to be bypassed. Now I understand how to turn the engine over by jumping the starter slave solenoid can I use that method for the pressure test.

In addition, the link below mentions the following "Ground the ignition coil wire (that goes to the distributor cap) directly to engine ground stud to prevent a possibility of a spark from the disconnected spark plug wires."

I know where the coil is and the wire going to the distributer cap, not sure about the ground stud and not sure exactly how to ground the wire to the ground stud if that makes sense.

Link mentioned above:
https://www.perfprotech.com/blog/bo...pen,cylinder has 150 psi (1035 ... More items

Thanks as always for your input you guys are awesome...

 

[Rick Stephens]

I have always just pulled the center wire off the cap that goes to the coil. If you use a starter switch, then there will also be no power to the coil anyway since the key is off.

Label all the spark plug wires for cylinder number. Warm the engine. Pull all the plugs and coil wire. Put throttle all the way open. Test each cylinder through 5 compression strokes. Write the numbers down.

 

[Bluemelj]

I performed the compression and leak down test last night but forgot a couple of steps, put throttle all the way up and warm up the engine.

Compression test results:
1=70, 2=80, 3=90, 4=80, 5=80, 6=70

Leak down test got the following results:
1=85%, 2=85%, 3=86%, 4=90%, 5=85%, 6=85%

Compression is low, but is it consistent enough? Leak down seems good?

Opinions on results,
Thanks...

 

[louc]

Sorry those results are bad. Normal comp test results on the 4.3 are 155-165 or so. Mine had always been 160-175.
The source of the water might have been those one piece manifolds, they are known to fail due to inconsistent wall thickness because of casting problems. If you repair/replace the engine get rid of them and replace with two piece units.

 

[Rick Stephens]

If the compression gauge is accurate, then the numbers are pretty bad. A brand new motor will give 155 on a good gauge. Higher than that would require high compression pistons and usually require high octane fuel. Anything under 100 is generally considered worn out.

 

[Bluemelj]

So I don’t understand, if I have bad compression it’s been mentioned that the leak down test will show the way to the problem but that test seems good, what am I missing?

thanks again for the replies!

Jeff

 

[nola mike]

Your leakdown numbers are horrible. You are losing a ton of compression somewhere. With those numbers surprised it's running at all. Did you do a wet compression test? If you pressurize a cylinder can you tell where it's leaking (by hearing air escape)?

 

[louc]

compression test results are supposed to be high (in this case like 150-165 or so, leak down tests are supposed to be LOW...if they are high like yours you're losing a lot of compression via valves not seating or worn piston rings.

 

[Bluemelj]

So let me clarify on the leak down test I believe that I documented them wrong. We pressurized to 90 psi on all 6 cylinders, not sure why we didn’t do 100 psi.

Anyway so my numbers above wmy leak down from 90 psi which correct me if I am wrong I pasted below without the % sign are good leak down numbers. I heard zero air escaping from any part of the engine.

1=85, 2=85, 3=86, 4=90, 5=85,6=85

thanks

 

[nola mike]

Are you pressurizing a cylinder and then reading the pressure after time, or actually using a leakdown tester? Your numbers should be in %, not psi.

 

[Bluemelj]

@nola mike yes we pressurized the cylinder and then read the pressure after time. It’s not my tester but I believe it has psi on both dials.

From memory we performed the following, once we got up to 90 psi, pressure was released into the cylinder we were testing and watched the pressure change from the dial reading the cylinder hence the psi numbers which can be calculated to percentages.

First leak down test I have participated in I believe that was the process, thanks for the questions!