Mercruiser 3.7 (470) wont turn all the way

[QBhoy]

Happy New Year.
Engine is turning on the starter so I could do I a compression test:
1: 115psi
2: 125 psi
3: 130 psi
4: 90 psi

This is done with a cold engine, so not very reliable, just for indication.
I will replace fuel and oil so I can start the engine and do a proper compression test with warm engine.

My question. I dont like to lower compression on the last cylinder. From your experience, I this are really bad sign. Or too early to say for an engine that rotated first time after 5 years?

From the last time I did a compression on the thing my friend had issues on…when it was in good health, it had fairly high compression compared to your result. Don’t quote me on it…but I think they were all about 160/170 psi when it was running ok. I think you must have a valve related issue. Or is there any sign of bubbles in the header tank, when you turn her over ?

 

[Benny67]

Hi. Read through the whole thing just there. Are you aware of the infamous issues with these engines ? If there is any doubt about its mechanical health, throw a good used 3.0 in there instead. If that 3.7 isn’t broken now, it will be soon…then again and again after that. Disaster of a thing.

3.0 won't match the power the 3.7 produces.

As far as the fact you claim of it breaking in the near future, that only holds true to the person who doesn't maintain their vessel.

 

[QBhoy]

3.0 won't match the power the 3.7 produces.

As far as the fact you claim of it breaking in the near future, that only holds true to the person who doesn't maintain their vessel.

There is maintaining and there is maintaining a 3.7. I get it. Some guys like to persist with them and spend more time tinkering on the trailer than actually using it. But most guys just want to turn the key and rely on their boat…with just the usual maintenance

 

[Benny67]

There is maintaining and there is maintaining a 3.7. I get it. Some guys like to persist with them and spend more time tinkering on the trailer than actually using it. But most guys just want to turn the key and rely on their boat…with just the usual maintenance

I've owned twin 3.7's for almost 10 years now. I've done the usual maintenance and they've served me well. I don't see any reason to claim their worse than any other similar powerplant. In actuality I'd say their better than anything in the market for the size, weight and design.

 

[nola mike]

There is maintaining and there is maintaining a 3.7. I get it. Some guys like to persist with them and spend more time tinkering on the trailer than actually using it. But most guys just want to turn the key and rely on their boat…with just the usual maintenance

Seriously, this isn't the case. A 30 year old engine is a 30 year old engine. What are you tinkering with on a 3.7 that you won't be with a 4.3 or 3.0? Nothing. You talk a lot about this, please back it up. Any 3.7 by this point has an alternator, a 4" hx, and likely cam seal repair. So what's failing that isn't failing in your underpowered forklift engine?

 

[Grub54891]

Seriously, this isn't the case. A 30 year old engine is a 30 year old engine. What are you tinkering with on a 3.7 that you won't be with a 4.3 or 3.0? Nothing. You talk a lot about this, please back it up. Any 3.7 by this point has an alternator, a 4" hx, and likely cam seal repair. So what's failing that isn't failing in your underpowered forklift engine?

I currently am running three 1985-1985 3.0 MerCruiser's. Not the same as that motor but they are flawless. It's not the age, it's the maintenance. Keep in mind that I got these used, and don't know the history of them. If the op wants to fix up what he has he will have a good motor for his use.

 

[QBhoy]

Seriously, this isn't the case. A 30 year old engine is a 30 year old engine. What are you tinkering with on a 3.7 that you won't be with a 4.3 or 3.0? Nothing. You talk a lot about this, please back it up. Any 3.7 by this point has an alternator, a 4" hx, and likely cam seal repair. So what's failing that isn't failing in your underpowered forklift engine?

I’ve backed it up plenty times previously. Have a read back. Would take me an age to write out again.

 

[Thomas1981]

If got the engine finally running, Just the fuel pump is 'overflowing' through the sight tube. So I need to replace this. As a result mixture is too rich and engine is smoking. For the rest engine sounds good, runs even on 4 cylinders.
I ran the engine for a few minutes to warm up and did a compression test.
1 110 psi
2 100 psi
3 100 psi
4 100 psi
I know these numbers are too low. But since all cylinders have almost identical compression and the engine only ran 300 hours I wouldn't expect a total warn out engine. Maybe a failure on 1 or 2 cylinders.

See my earlier post. With cold engine compression on cylinder 2-3-4 was higher. Probaly this was because I injected oil through the spark plug holes which created extra sealing on the piston rings.

With a new 95Ah battery starter has a hard job rotating the motor, should be an indication of proper compression, or a bad starter....

Any thoughts, does I pay of to continue with this engine.

 

[Thomas1981]

I made a mistake, I did the compression with closed throttle.
Engine already cooled down but now with throttle open I read 125 PSI on all cylinders

 

[Grub54891]

*possibly* if you run it more the comp will come up. Just a guess though.

 

[Scott06]

I made a mistake, I did the compression with closed throttle.
Engine already cooled down but now with throttle open I read 125 PSI on all cylinders

If this is accurate while on the lower side this should run ok. For testing you can take the overflow tube off the fuel pump and run it to a bucket until you get the replacement fuel pump, just don't throw your cigarette butts in there...
Also verify you aren't getting fuel in the oil, Not sure if a marine pump will do that but certainly on a car it will push gas into crank case

 

[Scott Danforth]

A good motor is 140-150 psi
A worn motor is 125 psi
A motor with 100 psi may run, but it wont develop power

 

[Thomas1981]

If this is accurate while on the lower side this should run ok. For testing you can take the overflow tube off the fuel pump and run it to a bucket until you get the replacement fuel pump, just don't throw your cigarette butts in there...
Also verify you aren't getting fuel in the oil, Not sure if a marine pump will do that but certainly on a car it will push gas into crank case

Thanks for the tip regarding the fuel pump. Do I need to the replace the fuel pump or can I just replace the diaphragm?

 

[jhamblett]

Thanks for the tip regarding the fuel pump. Do I need to the replace the fuel pump or can I just replace the diaphragm?

Diaphragm

 

[Scott Danforth]

if you can get a rebuild kit with checks and diaphragm, that is all you need. however those have been NLA for a while

 

[Scott06]

Thanks for the tip regarding the fuel pump. Do I need to the replace the fuel pump or can I just replace the diaphragm?

There's two diaphragms the overflow is in between them. If it bolts together vs crimped then yes in theory you can rebuild it or send it to a company to rebuild. Issue would be if you can ID what pump it is and also find a rebuild for what now is at least 30 years old....which is why replacing is usually done.

 

[Scott Danforth]

the original pumps were Carter pumps

 

[Thomas1981]

Replacing it is. A new pump is 250-300 dollar, I need 8M0073435.
There are 50 euro Chinese version of this pump. Does anyone have experience with this?

3854858 Fuel Pump Kit Engine Parts Replace 861676A1 for Mercruiser 165 | eBay

Part Number: 3854858 42725A3 18-7282 861676T09 8M0073435 861676A1 18-35437 9-35422 509407. We'd like to settle any problem in a friendly manner. It's just take you 1 minute. 1 Set Install Washer. Color: Silver.

www.ebay.de

 

[Scott Danforth]

Replacing it is. A new pump is 250-300 dollar, I need 8M0073435.
There are 50 euro Chinese version of this pump. Does anyone have experience with this?

buy this one, not chinese junk

Sierra Fuel Pump-Mc 3.7L Gm2.5L&3.0L - 18-7282 for Mercruiser Stern Drive

Key Features:Includes Mounting GasketAdditional Details:Flange I.D.: M60337Fits: 4 Cyl. (2.5L, 3.0L & 3.7L) EnginesUse With:18-0889: Fuel Pump Gasket/18-7784: Fuel FilterCross ReferencesOMC Stern Drive 1407Fitment InformationMercruiser120 GM 1983-1986 120hp 4 stroke 2.5 liters 4 cylinders 153...

www.iboats.com

 

[Benny67]

Seriously, this isn't the case. A 30 year old engine is a 30 year old engine. What are you tinkering with on a 3.7 that you won't be with a 4.3 or 3.0? Nothing. You talk a lot about this, please back it up. Any 3.7 by this point has an alternator, a 4" hx, and likely cam seal repair. So what's failing that isn't failing in your underpowered forklift engine?

Back up what?

The 2 motors I have work well, burn little fuel and haven't given me a bit of trouble.

Pal, I don't know what you're issue is but I just don't have issues with these motors that some of you seem to have.