8.1 Mercruiser Problems

Discussion in 'Gas Engines/Drives/Transmissions/Props' started by sejensen737, Jun 21, 2021.

  1. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    2006 340 8.1 Horizon 300hrs port engine

    Last year it became hard to start. It would idle rough and stop. Required restart 3 or 4 times until it would continue to idle. Any application of throttle would cause it to stop running. If it would continue to idle rough and warm up it would accept throttle inputs and accelerate and run fine. Once at this point leaving the dock and going out in the lake resulted in flawless performance. Shutting down at anchor in the lake with a restart a few hours later the engine starts and idles normally.
    Checked IAC filter. Tested IAC valve. Filter a little dirty and IAC valve checked OK. Replaced filter and IAC valve anyway. No improvement.
    Launched boat a few weeks ago. Replaced fuel filters in Cool fuel Module before launch. Both were a little dirty but not excessivley so. None of the "loose paint" problem evident. Same symptoms on first start up of the year. Took it for a run and it ran well. Back to dock and tried to start a few days later. Ran for 10 seconds first attempt. Ran for 5 seconds second attempt. Would not start after that and exhibited what I can only describe as "starter kickback" on next attempt. Decided not to push further until examination. Was afraid of possible hydrolock. Pulled all plugs , which by the way only have about 30 hours on them. None were wet with water. No water in oil. Removed left exhaust manifold to inspect. Gasket was not compromised. No sign of internal water damage but the outer manifold was quite rusty , which is why I removed that one rather than the easier right side. Pressure tested it and it appeared not to leak. Borescoped exhaust ports and all 8 cylinders and no abnormalities noted. Turned engine over with plugs out and manifold off and no abnormalities noted.
    So ...a ton of work and still no obvious faults. I'm thinking now , possibly bad injectors?
    Any ideas?
     
    Last edited: Jun 21, 2021
  2. Espos4

    Espos4 Well-Known Member

    Jan 1, 2017
    Long Island NY
    2007 240 Sundeck
    350 MAG Bravo 3 W/DTS
    Injectors could be causing the issues. Also note that the paint issue with the CF3 modules happens downstream of the filters. Did you remove and inspect the fuel pressure regulator on the CF3 module? That’s where you’d see the paint chips/flakes collecting.

    A fuel pressure test at various rpm under load would be some good data to collect too.
     
  3. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    Espos4

    Thanks. I will check the regulator. Will buy a fuel pressure gauge and check system pressure. Pump is running and producing pressure as I could hear it and smell gas when cranking with plugs out.
     
  4. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    Update...removed fuel regulator. No sign of contamination. No sign of fuel in vacuum line. Switched regulator from starboard engine just in case. Reassembld port manifold , replaced temperature sender , replaced a plug with cracked insulator.
    Moment of truth...started and idled well for 30 seconds , I'm thinking halleluah , then it falters ,picks up for 10 seconds and dies. Restarts result in running for a few seconds and dying. Started starboard engine and it runs fine with switched fuel regulator.
    Frustrating. I'm getting ready to call in the pro's but if I do it will be the first time I've had to since owning the boat...
     
  5. Mig22

    Mig22 Member

    85
    Apr 24, 2016
    Enjoying life on the water
    2000 - 340
    2001 - 340
    Current 2001 - 380
    8.1 S Horizons
    Did you get a fuel pressure gauge on it? With it cutting out sounds like it may be getting starved for fuel. If I recall those motors have a low pressure fuel pump which feeds the fuel filter then down the line it flows into a high pressure fuel pump which feeds the fuel rail/injectors. (On the fuel rail there will be a valve you can hook the pressure gauge to see the pressure typically the pressure should be in the 40-45lb range). Another item to switch between engines is the cam shaft position sensor it tells the ECM the position of the cam shaft, then the ECM tells the injectors to squirt and ignition to fire along with the crankshaft position sensor that does similar. Curious to if have you hooked up a diagnostic tool to check for any codes or history?
     
    Chris-380 likes this.
  6. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    Thanks Mig22. I have to buy a fuel pressure gauge and test. Your understanding of the system is correct. Low pressure to high pressure.
    I have seen others in the forum talk about the cam sensor and crank sensor. Sounds like another road to explore.
    The service shop here has a diagnostic so at this point I'm thinking that I should wave the white flag and have them come in and scan it.
     
  7. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    Update

    Got tired of waiting for my turn for a mechanic. Was told 4 weeks ago it would be 2 weeks. Anyway...

    Bought fuel pressure gauge. Put it on bad engine and checked fuel pressure via recommended Mercruiser method. Good pressure , 42psi but bled down immediately to 10 psi. Hmmm...not right I'm thinking so I check fuel pressure on good engine and 42psi and holds pressure.

    So , could be a number of things but does anyone out there have experience with this particular symptom and how was it rectified?
     
    Chris-380 likes this.
  8. Mig22

    Mig22 Member

    85
    Apr 24, 2016
    Enjoying life on the water
    2000 - 340
    2001 - 340
    Current 2001 - 380
    8.1 S Horizons
    Your correct it should hold pressure and not bleed down. From what you described the “high pressure fuel pump” is failing, replace it. Also verify the pressure after replacing I recently had a new one that was bad out of the box.
     
  9. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    Hi Mig

    The pressure test was done without engine running (just energising fuel pump) so I don't understand how it would be the high pressure fuel pump. It's bleeding down somewhere in the system and my first suspicion was fuel injector(s). Removed them yesterday and all look visually OK. Did impedence test and all at 12.5 ohms. Am going to rig up a little test rig and blow carb cleaner through them today. I'm not too hopeful any of this is going to solve my problem. I am having trouble determining 100% if there is a check valved somewhere in the system. Most fuel injection systems have them in the fuel pump and I would suspect a stuck valve.
     
  10. paulswagelock

    paulswagelock Well-Known Member

    Oct 25, 2010
    pa
    2018 SDX 270 OB 300 Verado
    Verado 300
    Clean the injectors with your home made rig. 8.1’s have a hard time with them. Dock neighbor is on his 3rd set in 12 years on his. He gets either clogged or stuck open ones. This year it is a clogged one, cylinder 2.
    Will be interesting to see if you have a stuck open one.
    Btw- if you search my posts I have detailed instructions on pulling injectors on an 8.1.
     
    Jimmy Buoy and Chris-380 like this.
  11. Nagaj54

    Nagaj54 Member

    34
    Feb 28, 2021
    Chesterfield Twp
    38' Searay Sundancer
    2006
    Mercury 496 C.I.D. 8.1-liter H.O. 420 H.p
    Mercury 496 C.I.D. 8.1-liter H.O. 420 H.p
    I am having issue with starboard engine.
    Started this season at max 3400RPM
    Now it maxed at 2500 RPM
    Did plugs and test wires.
    Fuel rail reads 40 Psi even under load.
    Can only be injectors now?
    Correct?
     
  12. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    paul

    Yes , I read your injector post and it was very helpful. I am finding some anomolies as I proceed and one of them is the black plastic cap on the bottom of #7 injector was missing. I'm not sure what it does or how important it is but I borescoped that port and there is no trace of it...probably eaten up and spit out.
    I cleaned all injectors with my half assed rig and none were stuck and spray patterns appeared reasonable.
     
  13. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    Update

    Put everything back together and it started but ran like crap. Idling initially at 580 rpm and would stall if throttle applied but it did keep running which is an improvement (sort of)
    Once warmed up a little , will accept throttle and ran it up to 2000 rpm but running roughly. I ordered a new injector to replace the one with the missing cap but I thought I would try an engine run with the defective injector. I find it hard to believe the engine would run so rough because of that injector.
    The faulty injector was cylinder #2. You can see the o-ring stuck in the port. It was pretty scabby in that area. Cleaned it all up. Bottom of fuel rail was scabby too but no perforation. Cleaned it up and painted.
    So it appears I'm not that much further ahead...
    By the way , I've been puzzled by the corrosion on the fuel rail and as previously mentioned , on the far starboard exhaust manifold. I have not been able to find any source of water spray in that area but I did notice that my CFM's have threaded nipples for the water coolant. I'm thinking that the CFM's may have been replaced by the previous owner due to the well known lack of integrity of the original design with the plastic fittings.
     

    Attached Files:

    Last edited: Jul 18, 2021
  14. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    Update

    Today I tested the fuel pressure while engine was running. Not easy to do when engine does not want to run.
    Anyway , got it started. Fuel pressure 42psi when pump energised and dropped to 35 psi while running. I think this is normal because I performed the same test on the good starboard engine and the results were the same.
    I was hoping the pressure would drop off and confirm a problem with the high pressure fuel pump. No such luck.
     
  15. Nagaj54

    Nagaj54 Member

    34
    Feb 28, 2021
    Chesterfield Twp
    38' Searay Sundancer
    2006
    Mercury 496 C.I.D. 8.1-liter H.O. 420 H.p
    Mercury 496 C.I.D. 8.1-liter H.O. 420 H.p
    ok, the boat after injectors were cleaned today ran Amazing.
    2500 RPM to over 4500 and humming like a bird.
    So, I am learning so much about these 8.1 HO motors.

    The injectors were so badly clogged they were causing much more noise and vibration. Also noise from the V drives it sounded like.

    Boat ran like new after I installed clean injectors. 4 out or the 8 were clogged.

    Now onto impellers

    cheers
     
    paulswagelock and Richey rich like this.
  16. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    Naga

    glad to see your problem solved

    good luck with the impellers. They are a B****
     
  17. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    Update

    Ok guys. Solution!

    I finally caved in and switched the cool fuel units. I was avoiding this like the plague due to complications that might arise especially due to the water inlet and outlet fittings. Turns out I had the barbed nipple fittings which are much easier to deal with and wont crumble in your hand as the plastic ones would.
    Lots of grunting and swearing but they actually came off without too much drama. I removed the complete muffler system on the port side to gain access. The job would be VERY difficult without doing so.
    Once the starboard engine CFM was mounted on the port I did a static fuel pressure test. Pressure held in the rail. Started engine. It ran to 750 rpm and then down to 580. Then hunted for a minute and settled down at 650. Then it ran up to 1200 and settled down to 650 and stayed there. I can only assume that possibly air was being purged from system while it refilled with fuel because all is well now. No more rough running and runs smoothly through RPM range.
    I disassembled the suspect cool fuel unit and it was absolutely pristine inside. No paint chips , gunk or corrosion. I jumped the low and high pressure pumps and they ran fine. I blew air through the high pressure pump from the inlet and the outlet to perhaps reseat a stuck return valve. At this point I am highly suspect that that was why the fuel rail was not holding pressure. I put it all back together and will mount it and try it on the starboard engine later today. Might work but if not I'm in for at least a high pressure fuel pump or possibly a new CFM.
    Will let you know how it works out...
     
  18. paulswagelock

    paulswagelock Well-Known Member

    Oct 25, 2010
    pa
    2018 SDX 270 OB 300 Verado
    Verado 300
    Great or hear you isolated the issue. Interested to hear what happens on reinstallation. I had to replace my CF module on my 8.1, it died completely. Dock neighbor had to replace his on 8.1 as well, ran erratically.
     
  19. sejensen737

    sejensen737 Member

    55
    Apr 3, 2010
    Sarnia
    2006 340
    8.1 Mercs
    Update

    Installed reassembled suspect CFM. Fuel pump runs for 2-3 seconds and then shuts off. No pressure in fuel rail. Cycled pump numerous times trying to build pressure but the pump will not prime. Took filter unit off and filled with gasoline. Still won't prime. Attached fuel line to CFM inlet and put other end in jerry can , still no prime. Removed fuel line from pump pressure side , still won't prime. Anyone know of a method of priming these CFM's?

    Looks like I'm going down the road of a new CFM. What bothers me is that this suspect unit would pressurise and pump fuel before I took it apart. It just would not hold pressure.
    Now it won't even pump which makes little sense to me.

    Another interesting tidbit. Low pressure pump is Walbro with "made in USA". High pressure is marked made in Czech. I noticed on ebay a Crotian pump is $70. Mercruiser is closer to $400. But is it the same pump in a Mercruiser box?
     

    Attached Files:

    Last edited: Jul 24, 2021
  20. Jeffrey King

    Jeffrey King Member GOLD Sponsor

    32
    Feb 16, 2021
    2007 340 DA
    8.1s Horizon
    Hi Everyone,
    Need advice & feedback
    I was running the boat yesterday. I was out for a few hours a various RPM's. i was almost home and got the boat up to 3100-3200 RPM's. after about 5-10 mins at that RPM, the port engine shutdown. i mean completely off. gauges stuck at their last position.
    i throttled down the stdb engine and started limping home. I tried a few things, but no response.
    I then hit the emergency battery cutover and the port engine start switch and the gauges responded and when back to an off position.
    I could still not start the engine.

    After another 5 mins limping home, i realized that i was still in drive. i put the port engine in neutral and i was then able to get a response when i moved the starter switch to on. However, every time i turned on the port engine, i got a constant alarm.
    after another 10 mins, i tried it again and the alarm had cleared so i tried restarting the engine. It started right up, but the temp was at 179. once the engine ran for about 1-2 mins the temp dropped back down quickly to 162 and i went home using both engines around 1500 RPM's

    Impellers changed 4 months ago but the port engine water pressure is lower than the stbd engine at all RPM's Pressure at idle is at 1.8. i do not remember the exact RPM's ( i think around 2800), but the stbd engine water pressure was 15ish and the port was 10-11.
    I need to pull the sea water pump housing to check but either i chewed through an impeller or the housing is scored badly.

    My real question is... is it normal for the gauges to freeze like they did & i thought in guardian mode the engine would simply drop to idle? this engine turned off completely.

    Has anyone experienced this ?
     

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