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  1. #1
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    Default overheating recommendation 5.7 alpha 1

    hi all:

    I come once again to the well of knowledge here.

    I'm looking for some advice on diagnosing my symptoms and confirmation / suggestions on my course of action.

    background.
    27' tiara 1987. repowered in 2001 with Mercruiser 5.7 alpha 1 gen 2. with 600 hours and 16 years on motors.

    manifolds and entire engines are original 2001 wet joint manifolds in .

    I purchased the boat this past april and put 100 hours this season and the motors ran great.

    my original plan was to replace the manifolds etc this fall but on my last outing my starboard engine ran hot enough to trigger the overheating alarm when I was running WOT. ( I always run WOT for about 5 minutes every cruise to keep them in shape).

    after cooling down the starboard motor ran normal temp and my normal cruise speed but began getting warm if I pushed it.

    when the engine overheated I noted the following. both manifolds, elbows, thermostat housing, etc were equally hot, no one spot hotter than another.

    the outdrive was fully serviced in July with new OEM water pump, hoses and bellows, oil etc. including new OEM thermostate

    after the overheat I noticed a lot of steam coming from the exhaust of the starboard motor.

    I've decided to end my season and do the manifolds, etc now.

    my question is this. what is the most likely cause of the overheating?

    I expect my problem is upstream of the manifolds because the temperatures were equal everywhere. where would you look? and is my thinking sound?

    my plan is to replace all manifolds, all hoses, elbows and the water circulating pump and thermostate...and to flush the power steering cooler.

    what else should I consider?

    thanks for your help

    I shut down

  2. #2
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    Default Re: overheating recommendation 5.7 alpha 1

    fresh water cooled or raw water cooled??

    how old are the exhaust elbows?

  3. #3
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    Default Re: overheating recommendation 5.7 alpha 1

    start by inspecting and replacing the raw water pump

  4. #4
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    Default Re: overheating recommendation 5.7 alpha 1

    Raw water cooled manifolds and elbows have never been changed since new in 2001 (yes I know, way beyond due) and were already part of the plan to change before my recent overheating incident.

    the overheating problem developed suddenly. Prior to and all season, temp remained extremely consistent even under heavy load. Now temp is normal at idle and cruise speed of 3000 RPMs but will creep up in heavy seas or when pushed harder

    changing the water circulating pump is currently part of the plan

    since motors are raw water cooled there is no raw water pump other than what's in the outdrive

  5. #5
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    Default Re: overheating recommendation 5.7 alpha 1

    Assuming the work done in July was complete, I'd bet on the exhaust elbows being at end of life....

    no need to change the manifolds unless they are thin or distorted.....I doubt the circulating pump is a contributor unless it is noisy....

  6. #6
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    Default Re: overheating recommendation 5.7 alpha 1

    New Manifolds are ordered with the elbows.

    I still don't know where else to look. What would cause the whole motor to overheat? If an elbow had failed or manifold water passage got clogged I would expect a localized hotspot rather than equal temps

    is this thinking wrong?

  7. #7
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    Default Re: overheating recommendation 5.7 alpha 1

    What about the flapper valve?

    dont these motors have a flapper valve in them, in the exhaust "Y"?

    if that failed could it cause overheating? How can I check for this ?

    also what about the power steering cooler? Is this something that needs to be replaced or serviced on some timeframe?

  8. #8
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    Default Re: overheating recommendation 5.7 alpha 1

    1).... poor flow in the manifolds/elbows will result in engine overheat. Lack of a hot spot not definitive.
    2)....I went 9 seasons on my FWC engine w/dry joints before doing elbows that were flushed regularly with fresh water
    and THAT was considered... "pushing it"
    3) Steaming... poor flow of cooling water or a blown head gasket
    4) " 5 min @ WOT to... keep them in shape". IMHO that's about 4 min and 45 sec longer than justified. Cruising @ 80% or less of MAX WOT RPM all you need to "keep your engine in shape". After all, you don't run your car or truck at redline in a lower gear for 5 min to keep you vehicle " in shape"????. But, dude, it's your boat!!!! I've talked directly with factory marine engine manufacturers on the whole RPM issue and they and an independent engineer who did consumer engine life tests, agree.... engine life at WOT is measured in tens ( 30 to 50) of hours of life. Given that a new engine can easily cost $10,000 (and above!!!) installed that's $200/hr operation. FWIW, I run my boat WOT 3 times a season ( @ launch, mid season and pre haulout) and then only for 10 to 15 secs, and that's just to check engine state of tune and cooling system health. Been doing this every season since I bought my first boat in 1964. I've only ever replaced 2 engines, one of which hung a valve ( a very very used, new to me boat) and another that was a 1969 raw cooled ( sea water) engine in 2007, the season after I bought it.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  9. #9
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    Default Re: overheating recommendation 5.7 alpha 1

    Maybe I'll reconsider the WOT strategy��

    Its actually a bit too fast at WOT for my comfort.

    i thought i was doing good for the motors

    what is considered normal cruising speed for these motors?

    boat will plane at 18 knots 2600 rpm

    i normally cruise comefortably at 22 to 24 knots at around 3000 rpm (depending on seas)

    WOT is 4600 rpm at 38 knots

    idle is 4 knots at 650 rpm

    all speeds are gps speeds

    the speedo gauge driven off a water pickup in one of the outdrives reads about 3 knots lower

  10. #10
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    Default Re: overheating recommendation 5.7 alpha 1

    again, overheats start with a bad/worn/damaged raw water impeller. Start there first
    Before you reinstall the lower ,put muffs on and see if there is any leakage, water leaking out = air getting in

  11. #11
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    Default Re: overheating recommendation 5.7 alpha 1

    Quote Originally Posted by outobie View Post
    Maybe I'll reconsider the WOT strategy��

    Its actually a bit too fast at WOT for my comfort.

    i thought i was doing good for the motors

    what is considered normal cruising speed for these motors?

    boat will plane at 18 knots 2600 rpm

    i normally cruise comefortably at 22 to 24 knots at around 3000 rpm (depending on seas)

    WOT is 4600 rpm at 38 knots

    idle is 4 knots at 650 rpm

    all speeds are gps speeds

    the speedo gauge driven off a water pickup in one of the outdrives reads about 3 knots lower
    ASSUMING (!) that 4600 RPM is within the specified MAX WOT RPM for that engine ( check decal on flame arrestor) then 80% of that RPM , i.e., 3680 for good life, 75% (3450) for better life. Speed is whatever that hull will do with the load... GPS reads speed over ground, speedo pickup is speed thru water with somewhat indifferent accuracy and varies with current direction relative to boat.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  12. #12
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    Default Re: overheating recommendation 5.7 alpha 1

    Correct WOT range according to decals and the manual is between 4400 to 4800
    so I'm dead center of that range

    is my normal cruise speed of 3000 rpm too slow? Ie is there a sweet spot that's best for fuel economy and longevity?

    as I check for leaks on the outdrive water pump, where should I be looking?
    is there a documented procedure someone can point me to?
    Last edited by outobie; 09-12-2017 at 04:00 PM.

  13. #13
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    Default Re: overheating recommendation 5.7 alpha 1

    being a GEN II you may se a trickle from the slinger seal on the driveshaft,and maybe a bit from the water outlet tube but nowhere else

  14. #14
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    Default Re: overheating recommendation 5.7 alpha 1

    Re: cruise @ 3000.... no that's fine! engine life continues to improve as cruise RPM drop to 60%. I run about the same RPM on my boat and find the 3000 rpm is a nice speed. My rule of thumb is boat on plane, nose drops and plus 50 rpm is minimum cruise and per some folks here that is about the most economical.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  15. #15
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    Default Re: overheating recommendation 5.7 alpha 1

    Quote Originally Posted by Bt Doctur View Post
    being a GEN II you may se a trickle from the slinger seal on the driveshaft,and maybe a bit from the water outlet tube but nowhere else
    Not to be stupid but
    how do you check those areas which are inside the drive? I was under the impression that the motor needed to be running and on muffs to check fire keaks

  16. #16
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    Default Re: overheating recommendation 5.7 alpha 1

    There are none ,your checking the lower units water pump by itself before you install the lower unit.
    from the top of the SS pump body is a guide sleeve for the copper tube. From there are machined holes in the casing directing the flow thru the drive, thru the gimbal housing, thru the hose to the fitting inside the boat and then to the engine. In some cases that hose can collapse internally and restrict the water flow.

  17. #17
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    Default Re: overheating recommendation 5.7 alpha 1


  18. #18
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    Default Re: overheating recommendation 5.7 alpha 1

    In July I my mecury dealer serviced both of my drives.
    new bellows new water pump new seals new hose with OEM mercruiser parts.

    ill pull the drives and check again but I'd really hope the service life of these parts is longer than 2 months. The old hose and bellows lasted 16 years and were still performing well when replaced

    i was reading through the shop manual and it said you can test for reduced flow from the outdrive by disconnecting the intake hose from the outdrive and measuring volume into a bucket...Idle the motor at 1000 RPMs for 15 seconds and you should get 4 quarts of water

  19. #19
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    Default Re: overheating recommendation 5.7 alpha 1

    Re:.... 2 month service life.... 20 to 30 seconds running dry is full service life. Some random (short) time bumped into a sand/gravel/mud bank also "full service life".
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  20. #20
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    Default Re: overheating recommendation 5.7 alpha 1

    No argument on that
    doesnt apply here though
    boat is wetslipped... outdrives haven't been out of water since installed
    boat is run in deep water shallowest draft is my slip which is 9 foot

    temps are are cool and steady at idle up to cruise speed... only starts getting warm when run hard.

    parts are all ordered, should be here next Friday so next Saturday I'll dig in.

    gonna start with manifolds, elbows, circulating pump and all new hoses and tubes and thermostats on the inside.
    Will also inspect and clean the power steering cooler and do a compression test.

    then run it to see what happens.
    ill pull the drives when I pull the boat for winter... next month if the above effort solves my overheating issue or sooner if not

  21. #21
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    Default Re: overheating recommendation 5.7 alpha 1

    All well and good but did you check the inlet holes for any growth

  22. #22
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    Default Re: overheating recommendation 5.7 alpha 1

    There was some that I got after with a pressure washer and scraper

    then we painted it with an antifowling paint for outdrives

  23. #23
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    Default Re: overheating recommendation 5.7 alpha 1

    Did you check "inside" under the water pump SS base .They grow there too

  24. #24
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    Default Re: overheating recommendation 5.7 alpha 1

    Yes I assumed that's what you meant
    when we replaced the water pump in July I took the pressure washer to all the internal passages I could get to and a scrapper or thin screwdriver for everything else. It has some growth but not terrible

  25. #25
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    Default Re: overheating recommendation 5.7 alpha 1

    started digging into the exhaust system today. so far I removed the port elbow and manifold and several hoses and drained the block.

    no evidence of water intrusion in the manifold all gasket and sealilng surfaces thick and dry. did find some internal rust flaking causing partial obstruction of the elbow...maybe 15% obstruction...also discovered that half of the flapper valve on the port side of the Y pipe is missing.

    I ran out of time to look for the missing chunk but I suspect that will be a contributing factor in my overheating condition. if I can't find the missing piece, what are my options? will I be afforded access to the bottom of the Y pipe with the outdrive removed? or is it more likely to be lodged inside the outdrive? is there a common location that these chunks usually end up?

    thanks
    John

  26. #26
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    Default Re: overheating recommendation 5.7 alpha 1

    Quick question on doing this job in the water.

    I hadnt considered waterline. When I removed the exhaust manifold, elbow etc, there was water just about up to the top of the Y pipe. As the boat rocked a little some water sloshed over the top and into the bilge. Had the top of my Y pipe been 3 inches lower I could have sunk my boat when removing the system.

    i reinstalled the tubes on the y pipe to give me a few more inches.

    now I'm thinking about the intake hose from the fitting on gimbal housing to the power steering cooler. This fitting is below the water line. Will removing the hose from this fitting allow water to rush into the boat?

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