So another issue popped up

[Trap Jaw]

As I've already posted on here, we have a "new" boat with the 3.7L Mercruiser. Took it out on a shakedown and everything seemed OK. Went fishing for a few hours close to the launch and everything was fine. Last Friday we ended up about 8 miles from the launch and after running in I went to troll into the chanel and noticed that the oil presure was only about 10psi. I wipped around because I didn't want to have the motor seize up in the chanel. I rev'd up the motor a few hundred rpms and it came back up around 25-30psi. As soon as I backed it down to idle it went right back to 10psi. In talking to a few people they said not to worry, but they are all basing their oppinion on car motors, which don't need to run at idle for hours on end. As a note, the oil presure is around 40-50psi when cold at idle.

So the question for all the 3.7 owners, is this the normal operating presure?

Also, what viscosity oil are you running. I thought I was OK with 10w40, but in researching I saw as high as 20w50 and even SAE 60 recommended.

I'm starting to get concerned that an engine swap is in our future.

[Diversion]

Your oil might be too thin, and when it gets hot, its going to be even thinner resulting in the low pressure reading. I run 25W40 Mercruiser oil in my 5.7 Mercruiser, and is the same oil I have ran in all my Mercruisers over the years, which were 3.0 4.3 and now the 5.7.

[tree top flyer]

normal. i had mine rebuilt last year and of course i forgot to upgrade the pump and it does the same dam thing. I put staite 30 in it one year and that made a diffrence, wouldnt recomend it if your not willing to change it with the weather. I would try diversions advice with the 25w40. let us know how u make out?

[snupytcb]

it is normal for the oil pressure to drop alot when it is warm. might be time for an oil change though. i run hd30. try a bottle of motor honey if it does not get any better. how many hours on the boat?

[jimski2]

Boats run at higher RPM's longer than automobile engines. They are not used in freezing weather hardly. So high viscosity oils are needed and recomended.

[L&M]

A straight name brand HD30 oil in this climate will do you well.

[Trap Jaw]

Not sure of the hours on the motor, no hour meter. I'd venture a guess and say, ALOT! Guessing it's still the original motor, so pushing 30 years old.

As for the oil, it was brand new, just changed it before running out that night. Grand total of maybe 45 minutes on that oil change.

OK, so I need to change the oil, again This will make change #3.

What I really want to know is what kind of oil pressure do these 3.7's put out at idle warm. If this thing is remotely whoop'd I want to know and swap it out.

[FSNmachine]

My old boat with a 165 straight 6 always did that. We trolled for 8 to 12 hours a day for hundreds of hours every summer for years and it never hurt it. Oil pressure may be low but as long as the top of the motor is getting oil its not a problem. I ran hd30 oil in it. Sold it this spring and new owner is running it the same way.

[S Mick]

Diversion hit the nail on the head......

My Merc manual lists 5psi at a "HOT IDLE" as the extreme low side of operation for the 3.7. That being said my 94' 5.7 Merc has 40 psi at cold startup,( it also lists 5psi as the bottom number psi) and I have seen it go as low as 12-15psi at a hot idle after a 6 mile running at times 3800-4000 rpm (nearly W.O.T.). After running a good ways at 3200-3500 (normal operation), I will see anywhere between 20-30psi after coming off plane and down to an idle to troll, but I gain 8-10 psi back after 10-15 minutes of trolling (as oil cools). I am running 20W/40, and I wouldn't go any lower than that in weight. Remember, boat motors are under servere strain at all times, and should not be compared with automotive engines although they they share many of the same internal componenets. If you would place your vehicle at the bottom of a steep incline, and chain another vehicle behind it and run it at near full throttle for miles, this is close to what a marine engine operates at each time it is on plane. Also, be sure to check for any obstructions in regards to the inline oil cooler/heat exchanger. If you are not cooling your oil properly under normal operation you will see very quick breakdown of your oil regardless of make/weight. As with any mechanical item, rememeber, excessive heat is an enemy.

In short, I would change over to a good 20W/40, and check your oil cooler for any sort of blockage, any take her for a run. Good idea to mark your guage at both cold startup and hot idle to make it easy to read. (I have all my guages marked for range of operation)

Going forward, running an oil temp guage will also be a big benefit, especially in early detection of a problem as you're running, as once the pressure ABNORMALLY drops, the damage is usually done.

[borderline 350]

Dear Trap Jaw,

Listen to Hank....L & M.... Straight 30 weight viscosity will give you the correct protection in gasoline powered I/O's used for trolling. None of the Merc/OMC I/O's have engine oil coolers. When you pick up and run in at the end of the day @ 3000 to 3500 RPM for 15 to 30 minutes, the oil temperature will skyrocket!! Oil viscosity & the oil pressure will drop dramactically!! WHEN YOU BRING IT DOWN FROM CRUISE SPEED!!! Remember this!! Do not let it idle....run at 1000 to 1500 RPM (for two minutes) so the hydraulic lifters won't collapse. The oil will cool down and your oil pressure will return to normal if you heed this advice. All the Mercruiser Inboards have engine oil coolers. That's why they will last 4000 to 6000 hours before major overhauls. Most of the I/O boats used for fishing are underpowered and very heavy due to the number of people and the amount of gear. Change the oil every 60 hours of trolling and use a zinc additive also. DO NOT USE FRAM oil filters, use a high quality AC/Delco or Mercruiser oil filter. I have seen the FRAMS disintergrate internally and cause high oil pressure readings! And worse! Fishing boats eat engines regularily!! Use the proper oils and stay away from running 4000 RPM on long runs!.....Respectfully Submitted....Jet Boat Bill

[Trap Jaw]

Thanks Mick. You described exactly what happened. Now I don't feel so bad. Other then I have to change the oil again. Well, on the bright side, the engine is getting a good flushing.

Thanks for all the input. I'll post after I get it changed again and what happens.

[S Mick]

None of the Merc/OMC I/O's have engine oil coolers.

Bill, not to get off subject, I know the inboards have coolers, but I am pretty sure the I/O big block 7.4 has an external inline oil cooler/PS cooler as well?

[borderline 350]

Dear S Mick,

Good proofreader! Yes, the big block I/0's do have a combo PS/Engine oil cooler and the SS FI 5.7 do have a combo unit. They still aren't big enough to handle 4000+ continuous RPM running in a heavy boat. I blistered the main bearings in a pair of 5.7+ SBC's with these combo coolers after 20 minutes @ 5000 RPM. I have since then gone to a 2 inch by 12 inch length oil cooler. I haven't blistered a bearing since going to the 12 inch length. ....Respectfully submitted,.......Jet Boat Bill

[Trap Jaw]

Just wanted to give an update.

I changed the oil and went to Valvoline VR1 racing 20w50. I just couldn't bring myself to grab those bottles of SAE 60. So the lake was too rough and couldn't get it out to run under load, so we ran it with the muffs. The cold oil pres. was better at around 60psi. After 45 minutes varying between idle and 1500 rpm the oil pres. was holding at 40psi. Quite an improvement to what we had the last time I ran it. Hopefully this has solved everything.

Just wanted to chime in Mick and Bill. This motor does have an oil cooler installed. Otherwise I have no idea what that can is with two radiator hoses and two hoses going to the oil filter.

[borderline 350]

Dear Trap Jaw,

I am glad you changed the oil and got back adequate oil pressure. Please look carefully at those oil hoses and what they are connected to. My best guess unseen on your 3.7/470 older Mercruiser (198??) is that these are power steering oil cooler connections. These engines did not come with an engine oil cooler. THEIR is a Mercruiser optional engine oil cooler kit for these engines for inboard apps., but I haven't seen one in this area. ........Enjoy your fuel economy with your rig, you definitely will appreciate that!!!......Good fishing & Catching!.....Jet Boat Bill

[ray koziatek]

Just for a little heads up for some of you newer guys ,from some one who reads most every post ,has a lot of mechanical knowlage,and has even cought a few fish over the years.... If borderline 350, or Hank of L&M repair give ya some marine engine advice your lucky as these are 2 of the best marine engine guys that their is....So if they say it you can pretty much "take it to the bank" they are right 99.9% of the time .... But as times change its always a good thing to look for "fresh ideas" just in case.............................................................................they dont respond to a post of a fellow "Boater In Need"............................................but they have been here for free advice for 1000's of questions .................So

THANKS TO """ALL"""" WHO HELP THIS SITE BE A GREAT RESOURCE TO SOOOO MANY!!!!!

Ray K.

[muskybob]

Just for a little heads up for some of you newer guys ,from some one who reads most every post ,has a lot of mechanical knowlage,and has even cought a few fish over the years.... If borderline 350, or Hank of L&M repair give ya some marine engine advice your lucky as these are 2 of the best marine engine guys that their is....So if they say it you can pretty much "take it to the bank" they are right 99.9% of the time .... But as times change its always a good thing to look for "fresh ideas" just in case.............................................................................they dont respond to a post of a fellow "Boater In Need"............................................but they have been here for free advice for 1000's of questions .................So

THANKS TO """ALL"""" WHO HELP THIS SITE BE A GREAT RESOURCE TO SOOOO MANY!!!!!

Ray K.

Quit schmoozing, yer still gonna get hit with the storage fees.

[Trap Jaw]

I do apreciate all the help. This is the first I/O we've owned in 12 years. And you know how things vaporize from your memory when you don't need them.

Oh, and Boarder. It does have an oil cooler on it. I put the smily there for a reason. It's hooked up to the top of the oil filter. I kind of did that as toung in cheek.

[dcso]

go with Hank!

thanks Hank if I didn't before on the top!!!

Ted 21ft bayliner trophy w/5.0