5.7 Merc Oil Pressure

[300sflyer]

I think I have an oil pressure problem... It?s a 98 5.7 Merc EFI [throttle body] engine, with about 500 hours on it. It seems to run just fine, except for this problem. This boat is new to me, and I do not have any history on it.

When the engine is cold, I have about 35-40 PSI showing on the gauge when I first start it up. As the engine warms up, it begins to drop. When the gauge is showing 160F, it now reads about 20PSI at idle. When I increase the RPM to 2000, [at the dock not in gear] it goes back up close to 40PSI. After taking the boat out on the water, and getting it fully warmed up under load, I am still close to 40PSI while at 3500 RPM up on plane, however if I then bring it back to idle, the gauge is now showing close to zero, and I even had the engine alarm come on. If I increase the RPM slightly, [up to 1200] the alarm would go off and the gauge would then read about 10 PSI. I did not hear any lifter noise from the engine at all so there must still be some pressure there even when hot at idle. I swapped out the oil pressure gauge sending unit, and there was no difference. The fresh oil [Mobil 15w50] is right on the full mark on the dipstick.

Any suggestions on how to further diagnose this?

Thanks

 

[ENSIGN]

Re: 5.7 Merc Oil Pressure

I would start by checking the oil pressure with a mechanical gauge,But it sounds like the engine bearnings and oil pump are worn.

 

[rpaldrich]

Re: 5.7 Merc Oil Pressure

Check the mercruiser manual for your engine, #24. It can be found here: http://boatinfo.no/lib/mercruiser/manuals/mercruiser24.html#/36

PG 1B8 It says that at 2000rpm the min pressure is 30psi and at idle the min pressure is 4psi. Your 2000rpm pressure is fine. Look through the manual and see how the alarm circuitry is wired? Maybe your gauge is bad and that is causing the alarm to go off without reason (if the alarm gets its value from the gauge which I don't know if it does)

The cold engine pressure is fine---that is normal, as you heat up the engine the oil heats up as well and becomes more viscous and you will see a lower pressure.

BUT---check out page 1B17. It has a bold face section talking about what oils NOT to use. Paraphrase "The use of oils other than quicksilver 25w-40 or a good quality marine 20w-40 or 20w-50 are specifically not recommended. I'd suspect since the wrong viscosity oil is in the engine, it is dropping below 4psi at idle, triggering the alarm because the oil is too thick. IMHO change your oil and you won't have the alarm any more.

 

[rpaldrich]

Re: 5.7 Merc Oil Pressure

But it sounds like the engine bearnings and oil pump are worn.

I don't think this is the first conclusion the OP should make...it sounds like it is a good engine with the wrong viscosity oil in it. Every engine that uses oil as a lube will have varying PSI's across the RPM range. Check merc manuals...its the same across the years. My '87 at 2000RPM says it can vary from 30-55psi.

 

[pennyduke]

Re: 5.7 Merc Oil Pressure

It was very strange to read your post because Friday I had the exact same problem. I agree with ENSIGN that the bearings and oil pump may be wore out. I had my oil changed a few days prior and Friday after losing oil pressure it looks very thin with a silver color tint. I have a 1996 5.7L Vortec on it's way to the machine shop Monday to replace my 1993 5.0L. I'm going to make it a 383stroker and use a mild RV cam. Not knowing the history of your motor, I would start looking for a replacement or rebuild.

 

[300sflyer]

Re: 5.7 Merc Oil Pressure

Thanks for your replies guys!

I should add, that this is a twin engine boat, with the same mobil 1 15-50 in the other engine, with no issues. The pressure in it never went below 20 psi.

 

[joewithaboat]

Re: 5.7 Merc Oil Pressure

Thanks for your replies guys!

I should add, that this is a twin engine boat, with the same mobil 1 15-50 in the other engine, with no issues. The pressure in it never went below 20 psi.

Having a ton of engine experience mostly with SBC i can tell you that you dont have an "oil or viscosity problem".

You are using one of the best oils and the viscosity is fine.

With 500 hours you likely have worn rod and main bearings " increased clearances". I have seen them run for years like this. It will probably spin a rod bearing eventually. If the two engines have the same hours on them, the worn one might have had a slight imbalance or out of spec crank or something to cause increased wear. Not uncommon.

Look at the last video at the bottom of this page.

http://www.melling.com/Aftermarket/TechInfoVideos.aspx

If you want to help it out you might add an external cooler as dropping the oil temp would help. Even adding a bigger filter to allow the use of an additional quart of oil might help.

 

[rpaldrich]

Re: 5.7 Merc Oil Pressure

Why would you make this a harder problem than it needs to be? Every time Don S or Bond O or AChris posts, they talk about the mercruiser manual and following it.

According to the OEM Mercruiser Manual for the OP's engine.....his pressures are fine! Minimum of 4psi at idle? Couldn't it just be his gauge isn't accurate enough at low psi? Also according to the manual, it specifically cautions against using a motor oil other than 25W40, 20W40 or 20W50?

So what am I missing here. I'm not a merc mechanic and I don't have years and years of experience, but the manual says he is running the wrong oil. Viscosity decreases as temperature increases (so he'll have higher pressure at low temps and lower pressure at high temps so that part of his question is normal,) and except for idle he is well within the parameters of the spec sheet. His idle pressure might have to do more with using a different weight oil or a bad gauge than his engine is worn out? Or am I out to lunch with all that?

 

[joewithaboat]

Re: 5.7 Merc Oil Pressure

Why would you make this a harder problem than it needs to be? Every time Don S or Bond O or AChris posts, they talk about the mercruiser manual and following it.

According to the OEM Mercruiser Manual for the OP's engine.....his pressures are fine! Minimum of 4psi at idle? Couldn't it just be his gauge isn't accurate enough at low psi? Also according to the manual, it specifically cautions against using a motor oil other than 25W40, 20W40 or 20W50?

So what am I missing here. I'm not a merc mechanic and I don't have years and years of experience, but the manual says he is running the wrong oil. Viscosity decreases as temperature increases (so he'll have higher pressure at low temps and lower pressure at high temps so that part of his question is normal,) and except for idle he is well within the parameters of the spec sheet. His idle pressure might have to do more with using a different weight oil or a bad gauge than his engine is worn out? Or am I out to lunch with all that?

Sticking to the oem manual is always best, especially for inexperienced people. I would think the o/p is mainly concerned because he has 2 identical engines which he treats the same and one has much different oil pressures than the other.

Other than the fact that the mobil1 doesn't have any moister inhibitors in it, it is better quality oil than the Mercruiser branded oil. Unless the boat sits unused for long periods of time its not an issue.

In any of these above oils you wont see much difference on the gauge between 40w and 50w when hot. It should be a little higher with the 50w.

Double checking with a good quality mechanical gauge is a good idea.

 

[Silvertip]

Re: 5.7 Merc Oil Pressure

The rule of thumb for small block oil pressure is 10 PSI/1000 rpm. At idle, the only spec necessary is that the engine have "some" oil pressure. Since the boat is new to you and y0u have no history, anything is possible. But I certainly would not pull the engine until I had gathered a little more information. Up here in the tundra we run 5W-20 oil in the winter and the oil pressure gauges read the same as they do with 5W-30 or 10W-30. The engine will be just fine with 15-50 Mobile 1.

 

[300sflyer]

Re: 5.7 Merc Oil Pressure

With the gauge [trying 2 different sending units] and the oil pressure alarm coming on at idle as well, [another separate sending unit in the oil system] there is no doubt in my mind, that the oil pressure is low at idle, once fully warmed up by putting a load on it. As was suggested, I will put a mechanical gauge in there to confirm this. These test runs out on the lake were done on a 55F day, with the water temp at about 45F. I would only imaging this will get much worse once the air and water start to heat up during the summer.

With respect to the oil and its viscosity, there are a million and a half threads on this site about oil. Having read quite a number of them, I would bet my last $ that the Mobil 1 15-50 is not the real problem here. I'm thinking it's bearings as well...

Assuming the pressure is indeed low at idle, I am wondering about 2 possible solutions to help me get through the season, and then yank the engine in the fall to check it over. The first one is to replace the 160 thermostat with a 140 so it runs a bit cooler, and second is to increase the oil viscosity even more, [thicker oil = higher pressure] by adding perhaps a litre or so of 80-90 oil after draining some out first?

 

[rpaldrich]

Re: 5.7 Merc Oil Pressure

Ok got it...Learn something new everyday Good luck I hope it isn't anything that keeps you off the water for too long

 

[joewithaboat]

Re: 5.7 Merc Oil Pressure

With the gauge [trying 2 different sending units] and the oil pressure alarm coming on at idle as well, [another separate sending unit in the oil system] there is no doubt in my mind, that the oil pressure is low at idle, once fully warmed up by putting a load on it. As was suggested, I will put a mechanical gauge in there to confirm this. These test runs out on the lake were done on a 55F day, with the water temp at about 45F. I would only imaging this will get much worse once the air and water start to heat up during the summer.

With respect to the oil and its viscosity, there are a million and a half threads on this site about oil. Having read quite a number of them, I would bet my last $ that the Mobil 1 15-50 is not the real problem here. I'm thinking it's bearings as well...

Assuming the pressure is indeed low at idle, I am wondering about 2 possible solutions to help me get through the season, and then yank the engine in the fall to check it over. The first one is to replace the 160 thermostat with a 140 so it runs a bit cooler, and second is to increase the oil viscosity even more, [thicker oil = higher pressure] by adding perhaps a litre or so of 80-90 oil after draining some out first?

I would try the good mechanical gauge first. Oil pressure will vary depending on were you take it from as well.

T stat might help a little.

Be careful mixing brands and types not all are compatible with each other.

I think 50w is the thickest from Mobil1, Amsoil makes a strait 60w racing synthetic.

You are using, arguably, the best oil available. If that doesn't protect you, you have a problem that can't be fixed with changing oil types.

You are also using the cheapest engine on the planet to replace/rebuild. If it is running well otherwise, id run it till it blew up. After all you have two, so you wont be stranded. Lol

 

[300sflyer]

Re: 5.7 Merc Oil Pressure

If it is running well otherwise, id run it till it blew up. After all you have two, so you wont be stranded. Lol

Ever try steering a twin screw 32' 13500 lb boat through a series of locks, with just one engine, and no power steering? [Power steering is on the problem engine]

I wonder if this stuff would help? http://www.lucasoil.com/products/display_products.sd?catid=7&iid=25&loc=show

 

[joewithaboat]

Re: 5.7 Merc Oil Pressure

Ever try steering a twin screw 32' 13500 lb boat through a series of locks, with just one engine, and no power steering? [Power steering is on the problem engine]

I wonder if this stuff would help? http://www.lucasoil.com/products/display_products.sd?catid=7&iid=25&loc=show

As to the twin engine thing... No i have no experiance with having to drive one like you mentioned without the power steering... yikes

As to Lucas products... I use the injector cleaner/upper cylinder lube in all of my vehicles a few times a year and have used the transmission additive in 2 high mileage transmissions that i still have and use daily. I believe in both of the products.

Anytime i notice a hard or erratic start/misfire its time for the injector cleaner. Always seems to go away after. Comes back in a few months.

The transmissions were both shifting oddly and shuddering, upon using it stopped. One it even cured the small leak it had for quite a while.

Generally I'm not a believer in miracle cures, but these seem to work as advertised. All three vehicles i am referencing have in excess of 150k miles on them. Two have been with us from new. One low mileage used. I do all of the maintenance on them.

 

[300sflyer]

Re: 5.7 Merc Oil Pressure

Sounds like you have had very good luck with their products. I will give their oil treatment a try this coming weekend, and then post the results next week.

 

[dan t.]

Re: 5.7 Merc Oil Pressure

Yes the Lucas oil stabilizer will help, its a stopgap at best but chances are good that with it you will make it thru the summer.

 

[capecodder116]

Re: 5.7 Merc Oil Pressure

First thing I would do is install a mechanical gauge. It's very possible your problem is either the existing gauge or sending unit. It might be just that simple...Good Luck..!!

 

[300sflyer]

Re: 5.7 Merc Oil Pressure

Given that I have tried two different sending units with the same results, the temp gauge does read normal when you first start the engine [cold], and that the alarm comes on [separate sending unit controls this] at idle once it is fully warmed up, I'm 99% sure it is a bearing issue. I will still be putting a mechanical gauge on it to confirm this.

 

[300sflyer]

Re: 5.7 Merc Oil Pressure

A mechanical gauge confirms the low oil pressure. I then took out one litre of oil, and added the Lucas oil treatment. It seemed to help a bit, however the gauge still reads pretty low at idle once warmed up, and the alarm still came on occasionally. Hopefully I make it through the boating season without it spinning a bearing, and then install a longblock in the fall.