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#10965519 - 07/09/15 08:42 AM Motor Question "Mercury 08 90HP 4 Stroke"
Waco28 Crappie Offline
Outdoorsman

Registered: 06/06/13
Posts: 210
Loc: Waco TX
Last weekend I was at the lake and my motor was runnning fine with no issues or noises. I was letting the motor idle while I used the trolling motor to throw the cast net for shad and then would use the big motor to move the boat from spot to spot. I was near a sand bar in 4 ft of water but I dont think that would have caused any issues.

All of the sudden the motor starts struggling and shaking violently, like it was miss firing or it was struggling for fuel. I killed the motor right away and checked the prop, tried to start again and it sounded rough and continued to shake before it would die after 20-30 seconds. I tried to start it and drive off to throw more gas to the engine but that didnt help. I ultimately packed it up until I could do more research on the issue.

What I have discovered so far is that supposedly while I had the motor idling I had it trimmed all the way down which normally isnt an issue but, this time I had friends in the back of the boat so it is possible that my water jet that sprays water out of the back of the boat was below the water line. Reading the manual it says this can fowl out a spark plug. I have no idea how that could fowl out a spark plug, has anyone ever heard of that happening? Anyways, I pulled one spark plug and it look relatively clean but I ordered 4 more and will change them out next week.

Now for the scary part: While I was inspecting the spark plug I noticed HALF of my damn air filter has disintegrated! After reading online I found this is an issue with Mercury's and the air filter cannot withstand the oil and gas fumes that exhaust back up through the air filter and it degrades. I hope this did not cause my problems. Some of the other forums said even though the air filter disintegrated it did not cause there engine to fail.

Any thoughts?
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#10965665 - 07/09/15 09:42 AM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
nethingthatbites Offline
TFF Celebrity

Registered: 05/07/09
Posts: 6461
new spark plugs never hurt, but make sure your jet is shooting water...could be a problem in the cooling system...clog, bad impeller, etc.

my first thought would be bad/old gas and/or issues in fuel line....how old is the gas and when was last fuel filter change?
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#10965748 - 07/09/15 10:22 AM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
Waco28 Crappie Offline
Outdoorsman

Registered: 06/06/13
Posts: 210
Loc: Waco TX
All the filters were last changed in 09/2013. The boat sat for one year before I bought it as the owner had 2 boats. He kept very good care of his boat and always had it maintenanced good. This was the first time I have taken my bought out in 4 months due to the rain and the ramps closing. I do add stabil fuel to help guard the engine. The boat had half a tank when I headed to the lake and I filled it up on the way also to try and help the engine if any of the gas was getting old. I looked online and i can change low pressure fuel filter by myself but the high pressure one it says to take to a certified tech.

Also, the water appeared to be coming out of the motor normal and I didnt see any signs of failure or trouble from that.
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#10965826 - 07/09/15 10:49 AM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
nethingthatbites Offline
TFF Celebrity

Registered: 05/07/09
Posts: 6461
Well, that just about covers everything I would know to do...hopefully one the experts around here can come in and clear it up.
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#10971684 - 07/12/15 10:21 AM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
Waco28 Crappie Offline
Outdoorsman

Registered: 06/06/13
Posts: 210
Loc: Waco TX
Well I changed out the plugs and tested it in my yard using the water suction cup things. it appeared to be fixed and had no issues idling. I took it to the lake and took off from the ramp. It ran fine for 3 minutes before it fowled out another plug and ran like [censored] from then on. I had to take it back to the ramp as the motor would then die every minute, Im taking it to a mechanic on Monday. he said it could be water in the gas. He said he has heard alot of complaints that gas stations have water in there gas due to the amount of rain we have had this year.
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#10972732 - 07/12/15 08:35 PM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
Bob Landry Online   content
Extreme Angler

Registered: 08/10/11
Posts: 2016
Loc: Austin, Tx
It could be starving for fuel. I would get a can or jar of fresh gas, stick the fuel line in it and see if it runs OK. If you still have the same problem, then you have eliminated bad fuel, a pickup problem in the tank, and/or a pinched or obstructed fuel line.
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#10976202 - 07/14/15 11:28 AM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
Waco28 Crappie Offline
Outdoorsman

Registered: 06/06/13
Posts: 210
Loc: Waco TX
just got a call from the mechanic, terrible news. Valve #2 is running at 150 lbs compression compared to the other valves of 205 and 215 compression. He said its a valve issue and he cant repair a Mercury 4 stroke and that it will cost thousands of dollars. Said most mechanics dont even bother with the repair they just replace the entire head.

I'm screwed now.
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#10976232 - 07/14/15 11:41 AM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
RangerBass21 Offline
Extreme Angler

Registered: 01/09/15
Posts: 2210
Loc: Plano
Awful young motor to already have a weak valve
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#10976557 - 07/14/15 01:59 PM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
Waco28 Crappie Offline
Outdoorsman

Registered: 06/06/13
Posts: 210
Loc: Waco TX
My thoughts exactly. Do you guys think I should even bother with the repair of this motor or should I get another motor? From doing searches online I'm seeing alot of issues from Mercury related to their 4 stroke motors in the 90-115 hp range. I'm so pissed this means that I surely wont have the motor repaired at least until February because of funding. Being on the water is the only thing that relieves my stress.
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#10977003 - 07/14/15 04:52 PM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
Bob Landry Online   content
Extreme Angler

Registered: 08/10/11
Posts: 2016
Loc: Austin, Tx
I think I would get a second opinion.
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#10978866 - 07/15/15 02:57 PM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Bob Landry]
BigDave1 Offline
Extreme Angler

Registered: 01/07/08
Posts: 1491
Loc: Allen, TX
Originally Posted By: Bob Landry
I think I would get a second opinion.


This for sure!
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#10980309 - 07/16/15 08:52 AM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
Waco28 Crappie Offline
Outdoorsman

Registered: 06/06/13
Posts: 210
Loc: Waco TX
I called Waco MarineLand and the guy over the phone said he would recommend a de-carb cleaning be done first before opening the head. He said this would cost me about $250. I looked online on how to do this and it seems real easy so I will be doing that this afternoon on my own. I never knew people did this to engines but now that I know I will do it regularly to mine maybe once every two years to remove the carbon build up.

Below are some photos I took yesterday with a bore scope. The photos that look like real [censored] are from my #2 cylinder. I dont believe a de-carb cleaning will fix my issue because the #2 cylinder has so much more [censored] in it than the other cylinders which to me suggest that oil is being pulled in past the valve when the piston moves to bottom dead center. But at least I can do the de carb cleaning myself and then tell the boat repair place to pull the head right away. Unless I decide to try and fix it myself. Check out the photos and let me know what you think.

bang



http://i1319.photobucket.com/albums/t669/Marc_Rounsavall/Boat%20Piston/1730441J_zps7zdmdgbl.jpg



http://i1319.photobucket.com/albums/t669/Marc_Rounsavall/Boat%20Piston/1731071J_zpsctjl5na7.jpg



http://i1319.photobucket.com/albums/t669/Marc_Rounsavall/Boat%20Piston/1741571J_zpscfgjowpd.jpg



http://i1319.photobucket.com/albums/t669/Marc_Rounsavall/Boat%20Piston/1741451J_zpsb6brbqvd.jpg



http://i1319.photobucket.com/albums/t669/Marc_Rounsavall/Boat%20Piston/1731071J_zpsctjl5na7.jpg
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#10980372 - 07/16/15 09:13 AM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
gary purdy Online   content
TFF Guru

Registered: 05/24/03
Posts: 10515
Loc: Littlefield Tx & Lake A. H. US...
To foul out another sparkplug that quick you have bad or stuck rings on #2. I do not believe that much oil can be pulled down the valve guides to do this but I have been wrong before. Marvel Mystery oil setting in the cylinders for 24 hours cleans them as good as anything I have tried.
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#10980958 - 07/16/15 12:10 PM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
crawdaddct Offline
Angler

Registered: 04/30/11
Posts: 428
Loc: Kingwood, TX
You can definitely tell it sat up for a year. Sounds like valve sticking, blown head gasket, or rings, as was said. Valve job is not that big of a deal, sounds like they just want to get your money, replacing the whole power head. Not sure of the setup on the merc 4 stroke. If it was me, I would pull the head and clean it good. Do a valve job and new head gasket and see what happens. Of course I do all my own repairs. Get a good manual and have at it. Its not running anyways.

Was it smoking? seems like oil would cause it to smoke before fouling. Blown head gasket would equal low pressure and oil/water fouling it out makes more sense to me. Maybe just clean it good and new head gasket. Worth a shot.


Edited by crawdaddct (07/16/15 12:20 PM)

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#10981151 - 07/16/15 01:31 PM Re: Motor Question "Mercury 08 90HP 4 Stroke" [Re: Waco28 Crappie]
Waco28 Crappie Offline
Outdoorsman

Registered: 06/06/13
Posts: 210
Loc: Waco TX
I got 4 cans of sea foam today and Im gonna let it soak on the pistons tonight and clean it tomorrow and see what happens. I'm still searching for a manual for this motor and I can find everything but this one. Once I get my hands on a manual I will try my best to fix it. Like you said it isnt running so what harm can I do. The important thing will be timing when I reassemble. I have good mechanics at work that will help on the reassemble.

Yeah, it sounded like the dealer just wanted that easy cash first. My hope is that the buildup of gunk has prevented a ring from opening so maybe when i can it will clear it out but who knows. Ill update in 2 days to see where I am.
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