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 Oil pressure question
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Author Previous Topic: Diesel Question - Should I run the other way? Topic Next Topic: air conditioner manual  

Arnold

RO# 12330

Posted - Aug 10 2018 :  17:41:32  Show Profile  Reply with Quote
On my 2000 7.4 mercruisers with about 1,000 hours the oil pressure is normal (45 psi) when the engines are cold but, after about 15 minutes of running at 3,000 rpm the pressure drops to a very low level (about 25 psi). The water temperature stays normal. Any idea on what it could be?
Both engines act the same. Thanks in advance.
Arnold

Homeport: San Francisco, CA

pdecat

RO# 842



Posted - Aug 10 2018 :  17:52:48  Show Profile  Reply with Quote
Nothing wrong with 25 psi. It does thin out when hot. Do you have oil coolers?
What grade oil? Is this anew result?


Bruce



Homeport: Gulf Coast FL Go to Top of Page

alien1952

RO# 32487

Posted - Aug 10 2018 :  17:53:18  Show Profile  Reply with Quote
what weight oil are you using and after a sustained run the oil pressure may drop a bit from the oil thinning out

aka Bt Doctur

Homeport: nj Go to Top of Page

Arnold

RO# 12330

Posted - Aug 11 2018 :  16:05:13  Show Profile  Reply with Quote
I think is 25-40 oil.
I feel better now. Thanks for your comments.


Arnold

Homeport: San Francisco, CA Go to Top of Page

Sandy

RO# 1159

Posted - Aug 11 2018 :  18:23:36  Show Profile  Reply with Quote
If it was my boat I would feel that while 25 PSI at 3000 rpm might be minimally sufficient, it seems awfully low & particularly for 25W-40. I would expect at least 15 PSI higher would be more normal.

I would check oil level to see if it is rising a bit perhaps with gasoline contamination to "thin it out"?

And even though you report both engines have those same gauge readings, I would be inclined to confirm with a mechanical gauge temporarily replacing the senders.

Have the OP readings been higher at cruise rpm in the recent past, or before the last oil change or sender work? How long ago was the last change?

Are your dash voltmeters reading 13+VDC at cruise rpm?

Just in case: Is this by any chance a boat with gauges at dual stations? (That would require dual-station senders with 1/2 the resistance of single station senders. So a single station sender would have higher resistance than it should for American OP gauge, and would read lower than actual PSI. )

https://fariabeede.com/site_manuals/IS0085d.pdf

You might also check this Google-return post on the same issue which indicated oil weight and wrong type filter reduced oil pressure reading which were brought up with perhaps better choices for each.
http://www.marineengine.com/boat-forum/showthread.php?398691-Merc-7-4L-Oil-Pressure



Sandy

Edited by - Sandy on Aug 11 2018 18:32:03

Homeport: The Vineyard Go to Top of Page

32carv

RO# 24150



Posted - Aug 12 2018 :  11:02:04  Show Profile  Reply with Quote
It does seem odd that if you had an oil pressure issue that it would happen in both motors at the same same time with the same symptoms. How long since you changed the oil? Any chance the wrong weight oil was used?
Jim



Homeport: Sacandaga Lake Go to Top of Page

alk

RO# 5508

Posted - Aug 12 2018 :  11:23:36  Show Profile  Reply with Quote
Surprisingly the spec on oil pressure at idle for a 454 is minimum of 4 psi ! Yes, 4. I had to look it up in a few sources, because I didnít think it was right first time I saw it.

Mine was dipping down well below 20 at warm idle, which was worrying me despite the spec. So I dumped in a can of Lucas high mile engine restorer, or whatever itís called. Have not seen the gauge below 30 since I put it in. I change the oil In this boat by sucking it out the dipstick tube, not sure how well that will work with this molasses mixture; but at least I have good pressure now.



Homeport: PA Go to Top of Page

Arnold

RO# 12330

Posted - Aug 12 2018 :  14:55:18  Show Profile  Reply with Quote
I had a mechanic check with the mechanical gauges and the reading is the same.
Could it be the oil coolers?


Arnold

Homeport: San Francisco, CA Go to Top of Page

Gregory S

RO# 2620



Posted - Aug 12 2018 :  20:47:25  Show Profile  Reply with Quote
Doubt it's a problem with coolers as long as no water in the oil. How's the oil level? Too full causes aeration and gives low readings.


Homeport: Norfolk, Va Go to Top of Page

Sandy

RO# 1159

Posted - Aug 12 2018 :  23:55:56  Show Profile  Reply with Quote
quote:
Originally posted by Gregory S

Doubt it's a problem with coolers as long as no water in the oil. How's the oil level? Too full causes aeration and gives low readings.



Definitely true regarding too-full , but that situation is more likely to essentially cause loss of all pressure and trip the alarm.

If RW flow through the oil cooler was insufficient, the oil would overheat to some "degree" & thin and pressure readings would be low. But I'd expect elevated coolant temps too, due to RW flow restriction anywhere in the RW circuit, or suction air leak or RW pump degradation.
Are the tops of the exhaust risers too hot to keep your hand on?

The linked thread mentioned wrong or restricted oil filter has caused low pressure readings. You aren't using Fram are you?


I'd change oil & filters again and use different good brands , maybe AC Delco, Wix or NapaGold( also Wix) filter? If you change a filter 1st and try a run you would know if that made a difference.

https://www.perfprotech.com/blog/tech-specs/mercruiser-74lbigv8-marine-engines-specifications

http://www.austincc.edu/wkibbe/oilfilterstudy.htm


Sandy

Edited by - Sandy on Aug 13 2018 00:25:06

Homeport: The Vineyard Go to Top of Page

Gregory S

RO# 2620



Posted - Aug 13 2018 :  08:16:05  Show Profile  Reply with Quote
What did the mechanic that checked with a mechanical gauge say about it?


Homeport: Norfolk, Va Go to Top of Page

JVM225

RO# 28365

Posted - Aug 13 2018 :  09:51:19  Show Profile  Reply with Quote
As long as there is no sign of gas, water or antifreeze in the oil:
Just change the oil and filters at regular intervals. Keep using the 25 w 40, and put in an additive like STP or the Lucas that someone else mentioned to make sure you always have a nice thick film on all the internals.
I like the STP because itís cheap and it adds back in the ZDDP that the Feds made the oil companies take out. Never used the Lucas, but read the label, it probably does the same.
Keep using the motors and donít worry about it.
It would be nice to see higher oil pressure but 25 psi at 3000 rpm isnít terrible and with 1000 hours on the motors it is what it is.


2002 Sea Ray 410 Sundancer
95 Eastern 22'
05 Maxum 18' Bowrider
C6 Corvette Convertible
68 GTO


Edited by - JVM225 on Aug 13 2018 09:53:32

Homeport: Farmingdale NY Go to Top of Page

Gregory S

RO# 2620



Posted - Aug 13 2018 :  10:02:31  Show Profile  Reply with Quote
I always add STP for the ZDDP at my oil changes as well. Especially important in flat tappet engines as mine are.


Homeport: Norfolk, Va Go to Top of Page

Arnold

RO# 12330

Posted - Aug 13 2018 :  13:50:13  Show Profile  Reply with Quote
The mechanic says not to worry, but I do worry.
I will try the STP.
Thanks again for your comments


Arnold

Homeport: San Francisco, CA Go to Top of Page

pdecat

RO# 842



Posted - Aug 13 2018 :  15:36:48  Show Profile  Reply with Quote
Is this a new issue? When did it start?

Bruce



Homeport: Gulf Coast FL Go to Top of Page
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