View Full Version : Low Oil Pressure - When Hot and Idle

12-06-2007, 06:58 PM
Solicitating ideas of cause:

I have an 88 MS Prostar w/ the 351 in it - and when the engine is warming up - the oil pressure seems to be good - middle of the dail/etc..

After riding for about an hour or two (pretty constant, but the engine being turned off between riders to suit up) - the oil pressure drops big time.

When idling the gauge says nearly 0 - but at speed it's still in the middle of the gauge.

I figured I would replace the sender unit since it's cheap and easy to do. If that doesn't work - any ideas out there? Is the only other real cause the oil pump - I figured since it was mechanical - being hot shouldn't effect that as much (but maybe not).


Forgot to mention - I have been running plain 30 oil in the engine (not 10W-30/etc...)


12-06-2007, 07:26 PM
If that does not work, try a static oil pressure gage direct from engine. It it is still low then look at the pump.

How many hours on the motor?

12-06-2007, 07:58 PM
How many hours are on the engine? Has the oil been changed lately? Do you hear any unusual noises while you have the the low readings? Any knocking sounds are bad news. As the bearings wear, more space is opened up for oil to leak out and this drops pressure and allows metal to metal contact to occur. A general rule for oil pressure is a minumum of 10psi of pressure for every 1000rpm.

Along with the earlier suggestion to check the sender and gauge, the other obvious check should be your oil level.

12-06-2007, 08:08 PM
I would be very concerned if there was any noise at idle. As in shut off now. If that is not the case there are other pitot tube options to be explored. Coule be low oil level. Could be gummed oil pump. Could be plugged pick up, or gummed pick up. Ever flush the engine?

I have flushed my engines with regular oil on the dummy lake and then changed the oil again only to find it was as black as before...

Good luck -Steve

12-06-2007, 09:58 PM
Thanks for all the good feedback.

The engine has about 1600 hours on it.

I change the oil regularly - and after the first time I noticed it - I did an engine flush and change the oil to help reduce issues it oculd have been.

The engine sounds absolutely the same as it is any other time at that RPM.

Any other suggestions - I guess the first step is to check the sensor. Does the sensor ever go out just when it is warmed up/hot?


Ski Bum
12-07-2007, 01:03 AM
1600 is geting up there when the engine is cold the tolarances are tighter. when it get warm they losen up (main rod and cam Bearings) Droping oil pressure. Try a thicker oil 20-50 or if its real bad 50 weight racing oil might buy you more time befor a rebuild. Check the sening unit first and install a manual gague on the block on an oil gally first.

12-07-2007, 02:41 PM
hey man I work on these for a living and first thing is put a mecanical gauge set on the thing make sure its really the problem, if it is see your mc dealer, if not try removing the oil pressure sender and clean up the threads on it sometimes its just a bad ground to the block, and most of the time its not the gauge. Also that motor has got a ton of hours on it, it might just be time to replace that worn out peice of iron, a cylinder leak down tester is a great tool to use to determine the condition of the motor.

12-07-2007, 06:17 PM
Thanks for all the responses. I'll try them out - first step to check if it's really an issue.

Just to give a little more information - this doesn't occur until after about 2 hours of running. Think I said that in above post - but just in case.


88 PS190
12-07-2007, 07:43 PM
Its strange that it takes 2 hours before it happens.

your engine reaches full opperating temp LONG before that.

And unless the engine is overheating later I don't think it should really be giving you that problem. 1600 is quite a few hours but its not THAT many. I have seen plenty of engines still running strong with way more hours than that.

They have given you quite a few good ways to trouble shoot it.

mechanical pressure gauges are cheap, and a good way to find out what's going on.