Oil pressure abnormalty @ warm idle

[DigitalSC]

Hello everyone.. This is my first post here, and I would like to first greet the community!

Now, for my question. I have a 1993 Thunderbird SuperCoupe with a 3.8L Eaton supercharged engine & 5 speed transmission; a factory package. The vehicle has 93,000 miles, I bought it with 87,000. I recently performed some maintenance work to this car, and it did not run for about 2 months. In this time, many items under the hood were disassembled, inc the air intake. I changed the oil for the third time in my ownership, both times I used Mobil 1 synthetic 5w30.

Now here is the problem. On the day the car was complete, It was started and left to idle to check its operation. I let the temperature achieve normal levels, it was idling perhaps 10-15 minutes at this point. As soon as it warmed & the electrical fan enguaged causing the RPMs to drop, the oil pressure sending unit turned on the low pressure warning light.

The sending unit on this car is a switched electrical device, sending a warning when oil pressure drops below 7 psi.

My first thought about this problem was a sensor issue. However other events led me to believe otherwise. For one, the sensor is relatively new, having only about 7,000 miles on it. Number two, the drop is only registered when the car has achieved nearly full operating temp, at idle. As I drive the car like this, it sometimes increases to just above idle. I can also cause the event to occur by dropping the RPMs @ idle when warm, using either the clutch or the A/C. This happens a lot when I take off from a stop.

Now I have not been able to take a mechanical reading of the oil pressure due to various circumstances, but the regularity of this problem really leads me away from believing a sensor issue is the base. The engine otherwise runs normally with full power. The lifters are also hydralic, and demonstrate no abnormal behavoir or noise during this event. I can see oil traveling through a gulley under the cap during this time also.

My mechanic and several others believe the problem lies in bearing wear. The clearances have probably increased, resulting in the volume of oil being pumped lowering. However, he went on to say with the present symptoms this by itself does not mean the engine is in danger.

This being the case, my question is essentially, why did this occur so suddenly? Previous to the maintenance downtime, I had logged about 6,000 miles on the car, and changed the oil twice in this mileage w/ Mobil 1 5w30. It never occured once during this time. It seems to me this type of bearing wear is something that should be occuring over thousands of miles, and probably then abnormally, not suddenly resulting in a problem like this.

One other thing which I have left out, the environment was one in which sawchips were in the immediate vicinity. The intake was open for a period of time, though I did attempt to ensure no foreign debris entered.

I would appreciate having your opinions on what might have caused this condition with my engine so suddenly..

Thanks for your time reading my long post, it is appreciated!

[ December 28, 2003, 07:09 PM: Message edited by: DigitalSC ]

 

[DigitalSC]

I would like to make a clarification in that post;

I changed the oil twice in that 6k period, both times using Mobil 1 5w30 & a m1 synthetic oil filter.

My last oil change after the maintenance saw a switch to a Motorcraft oil filter.

[ December 28, 2003, 07:10 PM: Message edited by: DigitalSC ]

 

[Drew99GT]

The only thing you can do to accurately get an assesment of the situation is replace the electiric sending unit with a mechanical oil pressure gauge or get a real gauge for the electric sending unit. Even if some saw chips got into the motor, they'd be burned and sent out the exhaust and little harm would be done IMO. If the pressure truely is low, then see how it rebounds with rpm etc. If your idle pressure is at least like 4-5 psi, and rebounds nicely with an increase in rpms, then I'd say run slightly thicker oil until it just plain dies, then do a rebuild. Oh, and many people are going to say Mobil 1 is on the thin side, which it is. Depending on your climate, I would run a 15w40 heavy duty motor oil and see what that does to oil pressure. Are you sure the motor isn't completely sludged up or something and limiting oil flow? Good luck.

 

[DigitalSC]

The climate is currently Illinois winter, and the car is outside. Additionally, I thought about a thicker oil, but the manual indicates nothing more than 5w30 or 10w30 and I have not been able to find technical data which would indicate to me that thicker oil can increase volume in a system with worn bearings.

At this point, I would like to assume it is in fact low pressure, for reasons previously stated.

My main problem is the sudden occurance of the problem. I can't help but think there is a very specific reason for it.

I am unsure about sludge. None is visible under my valve covers, but then again neither is anything but the springs. Again, though, I would think sludge would be more obvious. These engines normally run at temperatures between 215 to 225 degrees, if that makes any difference.

Thanks for your response..

[ December 28, 2003, 07:12 PM: Message edited by: DigitalSC ]

 

[sciroccoGTX16V]

i would check the oil pump for wear. Ignore the oil reccomendation, the supercharged engine thins the oil

 

[Propflux01]

quote:

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Originally posted by DigitalSC:
My main problem is the sudden occurance of the problem. I can't help but think there is a very specific reason for it.

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Sounds to me like you either bumped something loose, or your 7000 mile-new sender took a dump on you. that is more common than you think, But you will never know until you install a direct-pressure guage on it while warm and get an actual pressure reading. For the turbo oil recommendation, If you want to go with the Mobil, which, like the earlier post mentioned, is thinner, try going to the Mobil 10w30 instead of the 5w.

 

[geeeman]

Try Mobil 1 0W40...it is thicker at operating temperature (for a while, at least...!) and still flows good when cold. You will probably see no difference between 5W30 and 10W30; both are the same thickness at operating temperature.

Don't be scared of a 40 weight oil (or 50, for that matter), especially in a higher mileage car.
Many mechanics preach 30 weight for the first 50 000, 40 weight for the next 50 000 and 50 weight oil 'til it stops...!

 

[Drew99GT]

With worn out bearings, if that's what you have, your not getting less volume of oil though them; in fact, you may be getting more actual volume of oil though them but there is too much clearence to create pressure that actually keeps things seperated and keeps oil between the bearing shells and the journal. That's why a thicker oil helps in those circumstances. That's another reason why the higher mileage oils are blended on the thick side of their respective grades.

 

[Gary Allan]

Since you say that there's no lifter noise I would tend to follow the theory (TimH and I think Drew) that the switch is defective and is prematurely turning on the warning light.

The bearing theory is premature. You don't really know what oil pressure you're actually getting at the sendor/sensor. Go through the PITA stuff you have to do to get an electrical/electronic/mechanical gauge installed and get some real data to digest. You may just be victim of misinformation.

 

[Ken4]

You need to install an accurate oil pressure gauge and check if it falls within specs with the engine fully warm. Extremely low oil pressures mean your bearings and/or oil pump is worn.

 

[SHOZ]

I would get a different brand of oil that the Mobil 1 5w30. Seems to have a bit of a hot oil viscosity problem.

The senders are cheap and I would try another. But if the problem continues it is either the rod bearings or oil pump. Rod bearings are easy if you can get the pan off while in the car.

 

[DigitalSC]

Ok everyone thanks for the informative responses thus far. I have acquired a Napa dash-style mechanical oil pressure gauge, and will soon be checking pressure with it.

Additionaly, I have read Bob's oil pump technical writeup. It seems to indicate that excessive clearances between either rod or main bearings manifest themselves as symptoms of low pressure as RPMs increase or less pressure for more RPMs, respectively.

It goes on to say that a bad relief valve in the oil pump would cause a problem similar to mine, low pressure during idle & low rpms, normal pressure as RPMs rise.

I wonder if there are ever exceptions to these cases, and how exactly a stuck pump relief valve would affect pressure.. that is, would it be low mainly at idle or for a significant range of the RPM band until hitting the normal open state of the valve?

Will update with pressure check results..

 

[Gary Allan]

quote:

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and how exactly a stuck pump relief valve would affect pressure.. that is, would it be low mainly at idle or for a significant range of the RPM band until hitting the normal open state of the valve?

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The assertion is that the valve is stuck in a partially open position. This gives you somewhat normal pressure as the pump's output is greater ..but substandard after warm up where the oil is thin and the pump output is low. Sludge, varnish, scoring, spring wears out ...the bottom line is that its bleeding oil back to the suction side of the pump when it shouldn't.

In short ..I think that my answer is "Yes".