Mercruiser 170 3.7L engine knock

[bill2014]

Hi everyone, two years ago I bought an 86 Stingray 18ft bowrider with the 170 horse 3.7 mercruiser. When I first took it out on the water it didn't take long for me to tell that it needed attention. Leaked oil like crazy was running hot and not running well, had water coming in. Long story short this past winter I replaced the Oil pan gasket as well as the rear main seal and fuel pump. I also did a new gimble bearing, bellows, u-joints, shift cable, raw water pump, and completely rebuilt the carb and even fixed the infamous cam shaft seals on the engine water pump (closed cooling system). This last summer it ran great! No oil leaks, no water leaks, no over heating, tons of power, only one thing..... with a load on the engine and above 1900 rpm there is what seems like some type of knock or tapping noise. It is loud enough for me to hear while driving. The boat will not produce the noise out of the water, there needs to be a load on the engine. It is tough to determine where the noise is coming from but it is driving me nuts! When I did the oil pan I did not notice anything out of the ordinary. Each cylinder has about 150 psi according to my compression tester and other than this noise it runs great. I tried to mess with the timing to determine if it was detonation and although this changed how loud the noise was it did not fix the issue. If anyone has any insight into what could be going on that would be much appreciated. I have a video of the boat in the water making the noise and a video of the boat out of the water I can post on YouTube if people are interested. Thank You

 

[Bondo]

Ayuh,.... Welcome Aboard,.... Post a link to yer videos,...

 

[bill2014]

The video with the engine knock in the water. Don’t mind the rubber band on the linkage. It was an on the water adjustment.


https://youtu.be/fni70GI41nQ


Video without knock out of water

https://youtu.be/Xu_7Y-GkqCU

 

[QBhoy]

Hard to tell listening on my phone. But likely related to water ingress in the oil previously ? Big ends etc.
Presumably fresh oil in it and no sign of water in sumo or on underside of oil filler cap ?

 

[bill2014]

Hi QBhoy , yes actually, before I was able to fix the water and oil seal on the cam shaft some coolant got into the oil. I am not sure how long it was like that but not very. When I changed the oil pan gasket and everything I obviously added fresh oil. What do you mean by big ends and sumo? I haven't heard those terms before. Thanks for the input!

 

[QBhoy]

Big ends would be the main bearings.
Sumo was a poor iPhone autocorrect for Sump.

 

[kenny nunez]

I bet that the weep hole that is supposed to let you know that the pump seal is leaking got plugged. That is probably how the water got into the oil.

 

[Scott06]

Kind of reminds me of this video. https://m.youtube.com/watch?v=HRyOwnhQwgE

This 5.7 ultimately needed a head gasket ticking wat water in the cylinder. 3.7 s are known fir blowing head gaskets when they over heat, which you mention. Might be worth pressure testing the cooling system and looking for water/antifreeze on the plugs. To me doesn't have that deep rod knock death noise but I could be wrong (believe it or not it's happened before...)

 

[bill2014]

Thank you for the advise everyone! I think Scott06 is right! Now that I have heard that video with the exact same sound and same conditions it seems like it all makes sense. I have noticed signs of a bad head gasket as well now that I put everything together, such as a small amount of white smoke on start up. I think I will pull the head this winter and also check the valves and ect. My last question is will I have to have the head and block planed or do you think they will be okay? Thanks again

 

[stonyloam]

When you pull the head, you might consider having hardened exhaust seats installed. Not too expensive.

 

[Bondo]

My last question is will I have to have the head and block planed or do you think they will be okay?

Ayuh,.... Have the head checked for flatness, 'n mill only if necessary,....

The block is an open deck design, aluminum, with iron cylinders,..
Not sure the process for inspectin' it, but I'm Sure it's covered in the Merc manual,...

 

[Scott Danforth]

follow the mercruiser procedure in the manual. use a genuine mercruiser head gasket

 

[stonyloam]

Make absolutely sure to retorque the head bolts after running the engine up to temperature the first time after the head install. Keep track of which hole the pushrods come out of. There is a small plug on either end of the head for the hoses for the thermistor smog equipment on a 460 Ford V8. They can rust out and let exhaust from #1 or #4 blow out. Make sure they are in good shape. If rusty, you can get them from your local Ford dealer. Ask for ?thermistor plugs?. Good luck!

 

[bill2014]

Hi again everyone, sooo.... I went ahead and replaced the head gasket this spring and had the head all checked out and much to my disappointment there is still a knock. At this point it must be a rod or main bearing. I was just curious if it is worth rebuilding this motor, getting a different used 3.7. The motor runs great now besides the noise, I have been using it for the past few weekends with no issues other than that if I run it above 3000 rpm for an extended period of time it is slow to turn over when restarting. My plan right now is to use the boat for the rest of the summer depending on if the engine holds up and address it in the fall. What does everyone think? Thanks again for all the input!

 

[smcneilpe]

I have my dad's '88 Stingray with a 3.7LX Merc remanufactured eng and the one it replaced sounded like yours, same symptoms. Analyzing oil tells a lot. If you can get a pan in the bilge and drain out the plug, using a strainer and coffee filter, you can look at the chips. The chips are tough to get out through the dip stick. Generally, Rod and main bearings are softer (leaded steel), crank material, sleeve, rings harder and more magnetic. If large enough chips, does an Exacto blade cut into it? The rod and main bearings are sometimes Babbitt, or at least a soft steel and a knife cuts into the Babbitt layer, not the crank steel. If you see chips verse fines i would be concerned my crank may not clean up at regrind if planning to rebuild verses engine exchange.

If you can cut open the oil filter without creating chips it also tells a lot. Companies also specialize in oil analysis. I analyze my own at work.

Have you tried SAE 40 oil? My oil cap said to use 10W30, but the older school motors like the 470 series really like SAE 40. That will quiet worn bearings but will not help a really damaged crank and rod bearing. I don't like rod noise, and I have sent them through a block on outboards then you don't get your core exchange fee, and hopefully no one gets hurt.

Good Luck
Scott

 

[bill2014]

Thanks for the info Scott, I took the boat out yesterday and I hate to say it but the motor is cooked. Started knocking like never before and on my way in it decided to call it quits. From what it sounds like these motors are very challenging to rebuild as they are an aluminum block. If anyone has any input on what my next steps should be, either a remanufactured motor, used 3.7 or possibly a 3.0 conversion, that would be great. This motor has certainly been a disappointing experience. I hate to get rid of the boat as I have already gone through the outdrive and the rest of the boat and trailer are in good shape. Thanks for the help again!

 

[stonyloam]

470’s are about the same as any other engine to rebuild, just need to follow the manual EXACTLY when you put her together. Best bet would be to pull it and take a look at the bottom end, see what is wrong and decide from there. Either rebuild it, get another 470, or find a 4.3 V6, rather than a 3.0. You would be disappointed with a 3.0 and your outdrive gear ratio is wrong for that engine. The 4.3 will give you a bit more power and it uses the same outdrive gear ratio as the 470. Me, I would rebuild the 470, even if it needed a new block or crank. You have already taken care of two of the major concerns with the head work and alternator conversion. Good luck!