MYSTERY DROP IN OIL PRESSURE AT IDLE

[Highway by the Sea]

I bought a 1973 Plymouth Satellite Sebring Plus with a 400 big block that had been stored inside a storage unit for 22 years.  I replaced everything on the motor that it could possibly need, except the oil pump.  My mechanic absolutely refused to replace it, claiming that it was fine.  I told him that I did not care if he thought that it was fine, and that I wanted it replaced anyway just to be safe.  He refused.  I had noticed from the day the vehicle ran for the first time in two decades that when I start the car the oil pressure is fine.  Then when I am traveling down the road it is still fine.  But when the oil gets good and warmed up after 30 or so minutes, the oil pressure slowly drops to about 5-9 pounds (only at idle).  If I shift into neutral at idle the pressure climbs up again.  And when I am traveling down the road it stays normal.  The engine runs great, and there is no smoke, noise, or unusually high oil consumption, so I am at a loss.  Could it be the oil pump?  I thought that a bad oil pump would always register bad pressure.  I started putting a quart of Lucas oil stablizer in during my oil changes, and that seemed to slow the pressure drop significantly.  I am getting mixed opinions from various mechanics about the oil pressure.  Some say that a drop in oil pressure at idle is normal for the older cars.  Others say that it is abnormal. Any ideas?

[Ghoste]

A drop in oil pressure at idle is absolutely normal.  Now, a drop to only 5 lbs at idle is a little extreme to me.  What is the pressure at higher rpm or when cold?  Are you sure the guage is reading accurately?

[Highway by the Sea]

This car had aftermarket racing auxiliary gauges in it when I bought it (which I hated).  Since I had them removed and got all the factory ones either replaced or working, I can no longer tell you the exact number of pounds of oil pressure.  However on the factory oil pressure gauge stays around the midway point (sometimes higher) while traveling down the road.  When the car had both factory and auxiliary oil pressure gauges working at the same time, they were always in sync with each other as to the oil pressure.  The auxiliary one was attached to a tiny hose that measure oil pressure, and the factory one is attached to the factory sending unit.  What would normal idle pressure be in numbers?

[no318]

It should have atleast 10-15 @ idle.  If it is rebuilt with correct bearing tolerances probably 15-30 ball park.  Usually the oil "light" (if equipped) turns on @ around +/- 5 psi.

The best thing to do would be to install (or have a shop install) a mechanical oil pressure gauges (like it used to have) and drive it until hot.  Then you can see what it is actually reading.  That tells you if you have an oil pressure prob. or a gauge/sending unit/circuit problem. 

You really aren't going to know for sure until you check that.

[Nacho-RT74]

I just really worry when tappets start to sound LOL.

[Highway by the Sea]

I hope it does not come to that!  I am having a new oil pump installed today as I am typing this.  I hope it works.  If it does not then I guess I will have a compression test done, then have them pull the oil pan to examine the bearings.  Fingers crossed!  I am hoping to avoid another auxiliary oil pressure gauge.

[aifilaw]

this may be of no interest to you, but if the engine was rebuild, in the old days (70's,80's,90's) engine builders and racers would bore out the oil passages which increased the volume but dropped the velocity and pressure so they could use an aftermarket pump. More than once they would pull them off and then sell the engine with a stock one.

[squeakfinder]

I think this is typical of an older engine. My thinking is, the engine is a little worn and loose where the cam and crank come in contact with the bearings (babbitts). So the oil flows past much easier.
  My sons slant 6  76 Scamp only gets 4 to 5 pounds of oil pressure at idle when warmed up, but I don't think its ever been rebuilt.

[Steve P.]

  I am hoping to avoid another auxiliary oil pressure gauge.

You can always mount a manual gauge inside the glove box. It will be out of sight until you want to see it.

[firefighter3931]

Engines with los of miles will have looser bearing clearances due to wear. A quick fix for low oil pressure is a high volume pump and/or a higher viscocity oil. Shell Rotella 15/40 works well. 


Ron

[66mopar]

I had the same problem with a sb chevy this summer. It ran great and had good oil pressure moving down the road but at idle the pressure went down to about 5 lbs.  I first thought the gauge was bad but when I pulled the engine a part to install a new oil pump I quickly found the real problem. The rod and main bearings were completely gone.   

[Todd Wilson]

Sounds to me like the old boat anchor 400 is tired.  Might do like FireF says and put some heavier oil in it. Summer time and a mopar big block I wouldnt want to use less then 10w40 anyways. The 15w40 may help. I used to run 20w50 in my 440 truck when I was pulling a lot with it in the summer.



Todd

[SeattleCharger]

 

[Highway by the Sea]

I sure hope that the bearings are not worn out!  I will know if the new oil pump works better by tomorrow (I hope).  Midas has had my car for three days now, but they are also rebuilding my steering mechanism.  And SeattleChargerDog:  I used to have a 74 dodge charger (notice my screen name is SANS 74 CHARGER).  I got sick and tired of it staying broken all the time, and I found more rust throughout the body than I cared to deal with, so I sold it.  But the Plymouth Satellite is also a B Body Mopar, so I guess they are like brothers or cousins or something!  I can not find a decent Charger for a price that I can afford on my salary.  I found my good-condition Plymouth on a lucky fluke.  I NEVER see these anymore, and I have only seen a 1969 (General Lee) and 1973 charger in my city in 6 years.

[SeattleCharger]

      I didn't say anything about what kind of car you had.    But I can relate about the rust issues, that is the one thing that has hindered my enthusiasm about fixing up my 68 charger, rust in lower back quarters and valance, trunk pan, I got a good deal on the car, it is all there, but the body work to fix rust spots is lame.   I wouldn't mind a super bee or roadrunner or coronet, b bodies, although I definately first choose my charger, now I have spent a ton on going back to school, so I have to wait some more to fix up car.   There is a car like your new one I see around w. seattle here every once in a while, it is brown with mags, daily driver, sort of souped up looking, cool car,   

[dkn1997]

couldn't you change the spring in the pump to bump up the pressure?

[Highway by the Sea]

dkn1997: I may try this if the new one does not work.
SeattleChargerDog: I was referring to the fact that I joined a Dodge Charger board with a Plymouth, so I am like an outsider!  You have a 68 charger?  Those are great!    I think the car in the "House of Wax" movie was one of these.  It was in cherry condition!    Exactly how bad is the rust on yours?

[Steve P.]

Many of us here DO NOT have Chargers..   

[SeattleCharger]

dkn1997: I may try this if the new one does not work.
SeattleChargerDog: I was referring to the fact that I joined a Dodge Charger board with a Plymouth, so I am like an outsider!  You have a 68 charger?  Those are great!    I think the car in the "House of Wax" movie was one of these.  It was in cherry condition!    Exactly how bad is the rust on yours?

A Plymouth is a Mopar, and any Mopar is cool in my book.      And steve p., that coronet is way cool too.  glad we are all on the site,

  Here's mine, some in drivers door, dent behind gas cap,  back lower quarters and valance,  trunk pan is pretty bad, some rust/bondo hole on right rear of trunk lid., dent behind driver front fender and a small one on front of door higher up,  a little bit around top of front and rear windows, rear bumber has dent, but I realize the car is fixable and a good starting point I guess,  could be worse,  these pics sort of make it look better than it is,    the car stance is like that because the engine is out of it now and dissassembled.

[Highway by the Sea]

That is a beauty!  I was thinking of getting the same wheels for the Plymouth, but decided to keep the factory magnum 500 mags.  Many auto body shops in my city will not even touch rust.  When you mention fixing rust, they look at you as if you just spoke Greek.  I hope you get that rust problem fixed!

[Todd Wilson]

Many of us here DO NOT have Chargers..   

Many of us DO have Chargers!   



Todd

[Highway by the Sea]

The Plymouth is out of the shop with a new oil pump and the same thing still happens at hot idle.  I am going to drive this car until the engine goes, then install the 440 that a friend is selling me for $400 and use all the new parts from my 400 on it (carb, water pump, oil pump, etc.

[Steve P.]

Did you ever try a manual gauge?? When I had my shops I alllllways put a known good manual gauge in play before changing an oil pump.  You would be surprised how many bad gauges, wires and sending units I have changed when people swore they had a bad oil pump..

[Highway by the Sea]

Yes, I tried a manual gauge.  There was one on it when I bought the car (which I removed).  It agreed with the factory one, and they were hooked up differently i.e. factory one to a sending unit, the manual one to a plastic tube.

[supserdave]

IF it doesn't knock or rattle at idle, don't worry about it. Sounds like you have a back up plan anyway if it does go.

Your motor is just loose from a lot of miles of use. I would up the oil to atleast 10w40 though.