Oil pressure question

A

Arnold

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Oct 14, 2003
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12330
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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.
 
Nothing wrong with 25 psi. It does thin out when hot. Do you have oil coolers?
What grade oil? Is this anew result?
 
what weight oil are you using and after a sustained run the oil pressure may drop a bit from the oil thinning out
 
I think is 25-40 oil.
I feel better now. Thanks for your comments.
 
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
 
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
 
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 didn’t 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 it’s 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.
 
I had a mechanic check with the mechanical gauges and the reading is the same.
Could it be the oil coolers?
 
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.
 
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.

Click to expand...


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
 
What did the mechanic that checked with a mechanical gauge say about it?
 
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 it’s 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 don’t worry about it.
It would be nice to see higher oil pressure but 25 psi at 3000 rpm isn’t terrible and with 1000 hours on the motors it is what it is.
 
I always add STP for the ZDDP at my oil changes as well. Especially important in flat tappet engines as mine are.
 
The mechanic says not to worry, but I do worry.
I will try the STP.
Thanks again for your comments
 
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