Mercruiser 496 proplem
 

Hello

I had some issues with my 496 a couple of months ago with strange engine RPM that lived its own life, turned out to be the TPS sensor that was bad. Switched this and the problem whas gone.

But I still have some small issues with the engine RPM. The engine runs porely between 800-1500 rpm when I give the engine a little bit of gas (when not on plane), just cruising around.
It sounds like one cylinder is missfiring or something. But when im running the boat on plane/full gas its fine, no problems..

What can cause this to happen? Bad spark plug wirings?
I run the OEM Aldelco spark plugs.

Thanks for help where to start searching

Plug or plug wire.. Get a spark tester with the clear window where you can see the spark.

Chris

coil misfire possibly, you really need to scan the PCM and see if its throwing any codes

I bought a spark tester, gonna try that one on all the plugs.

I also gonna test to measure the cables.
What resistance should the cables have?

Okey thank you, I dont have any Scan tool, just the Mobile VesselView ”Fault codes scanner”.
Have seen codes before on the VesselView but that was the code for bad TPS.
No codes from the VesselView after I switched the tps what I could see.

Whit coil misfire, could it be different things related to this ?

Found this on another forum:

Final Diagnosis: Bad plug wire <br /><br />I know, sounds ridiculous. The 496 is a DIS (Distributorless Ignition System) engine, meaning that it has a coil for each cylinder. Four on each side of the engine, mounted on the valve cover.<br /><br />In this case, the #7 cylinder spark plug wire was leaking secondary ignition voltage at the coil tower, arcing to the coil frame. The coil frame and entire valve cover became part of the secondary ignition system. Due to a rubber gasket and appearently pretty decent paint job, the valve cover was not grounded and the ignition voltage found the easiest path to ground (intermittently) was arcing across a gap from a bare spot (chip) in the paint on the valve cover next to a mounting bolt, to a bare spot on the upper part of that mounting bolt. Just so happens that mounting bolt was directly under the ECM . The 40,000 volt discharge (about 2.5 inches away) was creating enough EMI (Electro Magnetic Interference) to momentarily shut down the ECM.<br /><br />I only found the problem because I happen to be at just the right place to actually see the spark under a wire bundle. You could not hear it due to normal engine noise. I pulled the wires away a little and I could hardly believe it, a nice fat blue spark from the valve cover to the mounting bolt. Directly under the ECM . The engine lurch happened at the exactly same times as the spark occurred. I put a jumper wire from the valve cover to a good ground. Problem went away. Took it off problem came back. Still unsure what cylinder it was, I put it on the scope. Found an intermittent misfire on #7. Visual inspection confirmed arcing from the coil tower (under the plug wire boot) to the coil frame. Changed #7 coil, wire and plug. No more problem. <br /><br />Thanks for your responses. Maybe someday this will help someone with a similar problem.

If looking for wires, FOund this option the other week, compare to the $500-$600 factory merc wires

https://sparkplugwires.com/scott-mer...plug-wire-set/

Thank you. Also found these:
https://www.marineengineparts.com/cr...-plug-wire-set

Dont know if they are as good as factory Merc wires

Those are the same as merc oe. Atleast they where up to last year when I linked those and some others here the other year. Do a search under my name and those wires.

Possibly Crankshaft Position Sensor. They can go bad without any codes. Do you have any beeping from the engine's alarm? IAC going/gone bad can cause issues at lower RPM ranges too.
Try running the engine the dark, to see if you can see plug wires arcing anywhere. Replacing spark plugs is never a bad idea. And when you do inspect plug wires and ensure they are all snapped on correctly.

Or some Moroso 8mm wires for $70. I ran a set this summer, worked good and fit right.

18-8838-1 84-881698,496-8.1 lr, distributor less ign, Ignition Parts mag star Wire Sets.Ignition Wire Set 120/140 discount high voltage ignition wire sets, s.s. coil core magstar v-6 wire sets, v-8 magstar ignition wires discounted, mercruiser h.e.i.

Old school and cheap check
1- check plug wire resistance with digital meter
2- water in spray bottle and mist wires and see if engine starts missing.

Thank you all for the advices, when im thinking of it.. I may have seen it arcing somewhere on the engine one time last year when it was darker in the engine compartment.

Definitly gonna check the spark plugs..!
What resistance should the wires have?

No beeping alarm from the engine.

Just one thing to add to this, run the blowers and double check there is no fuel leak...if the wires are bad there will be an open arc.

I wouldn't recommend that you are standing in the engine compartment, standing in the water mist, with shorts(bare skin) when you find a bad wire. I know a tech that had this happen and now he doesn't mist test, just recommends a new wire set every 2-3 years. He had to have sprayed a lot of mist expecting nothing but to this day if he gets a bit cocky, someone in the group will ask how to test a wire set on the engine - LOL

2-3 years ? Really ? All the more reason to reduce gaps to a more normal .045 on these app’s with coil and wire burning ridiculous .060” gaps.

I started with measure the spark plug cables.
With my multimeter at 20K I got these numbers, starting on port side and from the front of the engine.

Port side
1 - 2.37
2 - 3.04
3 - 3.03
4 - 2.37

Starboard side
1 - 2.38
2 - 3.06
3 - 3.01
4 - 2.35

Dont know if the resistance is good or bad.
If the resistance is good, can it still be some fault in the spark plug cables?

Or is it one of the eight ignition coils that can cause the problem?

plug wires are fine, remove and inspect spark plugs see if one is fouled

MAP sensor going out.

Im looking at the spark plugs today. See if theres someone fouled.

Okey, you think so?
Is This a common problem with the MAP sensor?

Here is some pictures of all the spark plugs in order. Starting from the back of engine.
Starboard plugs at the left, port plugs at the right.

Back of engine

8 7
6 5
4 3
2 1

Front of engine

In my eyes there is several plugs that dont look good, but the first spark plug on port side (#7 cylinder?) dont look good at all... almost completely black...

what do you guys think?

Also, when I turned the engine by hand to look at the valves opening and closing, the exhaust valve at cylinder #5 doesnt close all the way.
It first didnt move at all. But when I turned the engine a couple of turns it started to move and close (but didnt close all the way)...


https://cimg2.ibsrv.net/gimg/www.off...08152b73f.jpeg
https://cimg3.ibsrv.net/gimg/www.off...675d8e3ef.jpeg
https://cimg4.ibsrv.net/gimg/www.off...2b6e4a532.jpeg

Those plugs are toast; yes some worse than others.
How many hours on the engine?

Seeing a bit of oil burn, some rich and some lean
Have injectors cleaned and flow tested

About 480 hours on engine.
I changed the spark plugs last winter for 2020 season. So they are barely 1 year old.

Okey thank you. But the plug that is all black, feels that it is something wrong there

Could need valve seals

Two more pictures:

Port side
https://cimg7.ibsrv.net/gimg/www.off...4d62d4522.jpeg

Starboard side
https://cimg1.ibsrv.net/gimg/www.off...32f63c090.jpeg

On what spark plug/s do you see burned oil?

Not counting the oil on the threads looks could be 7 and 6, but latest pics not as much, but there is some kind of by product. Is that rust down inside 5

okey, yes it looks like rust down inside. Dont know what the wite stuff is.

Heres some nearby pictures, also nearby picture of the 3 plugs on port side, also with rust inside.

Took the boat up in september and it have been stored in my garage tent since then.

https://cimg3.ibsrv.net/gimg/www.off...81c00e5f0a.png
https://cimg7.ibsrv.net/gimg/www.off...be860a817d.png
https://cimg9.ibsrv.net/gimg/www.off...26736166eb.png

From my rookie opinion,
has injectors been serviced
leak down/compression test
Is it stock exhaust reversion issue?
for only 1 season on plugs U got deeper problems.

Have you pressure tested your exhaust? Could be a water leak and the white stuff salt.
My plugs looked like that except the white stuff when my CMI´s gave up. Also as mentioned before, check compression.

While there are some issues with the plugs, a good cleaning will have them back to where they need to be for now. The plugs are just showing how well there working with what there given to burn (so to speak). The stuff on the plugs could be stuff being added to the gas (additives and the like). Incomplete burning of fuel or things such as antifreeze or other leaking in.

There has been some good comments to have a look.

Edit: looks like I took to long to respond, others said similar

The injectors has not been serviced.
Did a leak down test last year with good results. No compression test yet.
I had some issues with my previous exhaust this year that was leaking water. (CMI exhaust).
Changed them to EMI thunder exhaust in late season this year. No problem with water after that what I know..

Something is wrong yes..

Havent pressure tested my new exhaust (EMI Thunder) but my previous exhaust (CMI exhaust) leaked as I said. Thats why I changed to the EMI and I dont think they are leaking.

Yes I will clean them up for a test later.
But doesnt it look like the #7 is missfiring or something?
Its all black.. and when I describe my problem with the missing/spitting engine at 800-1500 rpm range, doesnt it sounds like it could be a problem with the spark plugs/wiring or something..

Replace the spark plugs,if it runs ok after spark plug replacement clean your injectors

Yes a couple plugs look like they have oil on them imo. I personally would change them, not clean them.

A couple of plugs look like they have some kind of build up on them?

You had a leaking exhaust? I'm wondering if water got into the valves, valves seats, cylinder walls or rings? Compression test, leak down. I would also put a scope in the spark plug holes and try to get a look of any thing looks funky.

Is the NGK PZTR5A-15 replacement plugs for the AcDelco plugs?

Yes some build up on some of them. Looks like rust/salt or something.

My CMI exhaust leaked and then I swiched to the EMI exhaust. Could the build up on them have come from when it leaked with the CMI and then been left? I have run the boat with the EMI thunder a couple of times after so it has a few hours with the EMI and the same spark plugs..

I tired NGK plugs on my 496HO a few years back and they only lasted for 1 season ap 40 hours. My first set of Delcos had over 200 hours on them and still running strong before I changed . Perhaps I just got a bad set of NGK´s but I´ll stay with the Delco´s for now. They don´t like water though, had to change them (almost new) after my CMI-leak experience.
If you don´t buy new plugs shift the current plugs position (after cleaning them) by moving the badest ones to the cylinders where the best plugs were and vice versa. Then you probably can determine if it is the plugs that are bad or something else related to the specific cylinder(s) giving you fouled plugs. Also like someone wrote earlier in this thread, borrow one of these small cameras that you can check inside the cylinder with, the rust on the plugs can only come from water.

I will try this too thank you.

Can the rust on the plugs came from when the CMI leaked?
I mean, I changed the exhaust to EMI Thunder instead. And I have droved the boat a couple of hours with the EMI thunder exahust. Shouldnt the rust/dirt on the plugs have vanished or something?

Follow the above advice......
I would also compression check the cylinders. With the amount of rust and saltwater corrosion on those plugs, I bet your valves and seats aren't much better. Your ohm readings on plug wires are fine. You could still have a cracked boot/wire that could arc. That needs visual test to check.

I use AC 41-983 plugs on my 496, keep the .060" gap. Don't buy from amazon, ebay etc. Lotsa fake plugs out there. Rockauto sells real AC plugs.

More than likely the leaking CMI's caused this issue. Once rusted they will stay that way.

1. Compression test cylinders to get a baseline. If your plugs are rusted, likely your valves/seats are the same.
2. Replace plugs, run boat a couple hours and recheck plugs.
3. At that time/hours, have your fuel injectors serviced.

No, when I look in at the cylinders from the plug hole with a flashlight I see its rust in there..
To see if there is a cracked boot/wire it needs to be dark to check?

Is it these plugs from this site? The 41-983 is the plugs that I have.
https://www.rockauto.com/en/moreinfo...eQ73GK2A%3D%3D

Yes, I think the CMI caused this.

I will test that