Oil Pressure Gauge Only Works When Engine Is Warmed Up.

[ajohnson928]

When I got my MJ it already had the full gauge dash cluster switched in but the previous owner hadn't bothered changing the senders.  I installed the proper oil pressure sender the other day, took a test drive, got appropriate looking pressures and called it a night.  The next morning the I take off to work and the gauge just sits right above 40 and doesn't move.  BUT... when I get off the interstate near work the gauge is working properly again.  I've watched it for the last few days and it's only working properly when it's warmed up.  Any ideas?  Specs of my MJ in the signature. 

[Geonovast]

Was the sender new?  Have you verified that your oil pressure it good with a mechanical gauge?

[ajohnson928]

Sender was brand new.  The shop didn't have a mechanical gauge I could borrow.

[Eagle]

Have you considered that hot oil is thinner than cold oil, so there's higher pressure when cold and lower pressure when hot? Your gauge may just be telling you what the oil pressure is ... which is sort of the point.

[ajohnson928]

I have considered that.  But the thicker colder oil should still change pressure some based on engine revs.  When cold, the gauge is frozen solid at just above 40, when hot it fluctuated between just above 20 to just below 40.  I am planning to find a mechanical gauge to nail down the gauge accuracy but it looks and feels like it just isn't working at all until it's warm.  

 

Then again, it might be working perfectly and I'm just expecting it to work differently.

[oldseddie]

You could have a weak relief spring in the oil pump limiting the pressure to 40 lbs. If it was excessive bearing clearances it would vary with engine speed.

[HOrnbrod]

I have considered that.  But the thicker colder oil should still change pressure some based on engine revs.  When cold, the gauge is frozen solid at just above 40, when hot it fluctuated between just above 20 to just below 40.  I am planning to find a mechanical gauge to nail down the gauge accuracy but it looks and feels like it just isn't working at all until it's warm.  

 

Then again, it might be working perfectly and I'm just expecting it to work differently.

 

This is correct. You may have a sticking gummed up meter movement which is common with these gauges. Try disconnecting the wire from the the sender, key ON, while someone observes the gauge. The needle should go smoothly and quickly full scale. Then touch the wire to ground. The needle should bottom out. If it does not, remove the gauge from the cluster and clean the movement with a quality electrical contact cleaner, like the stuff Radio Shack brand. Reinstall making sure the brass mounting nuts are screwed tightly against the foil and try again. This should get it operating correctly.

[ajohnson928]

The gauge does it's full sweep check on start up as it should.  I'll give this a try though.

[skidoo_j]

Just for reference.  at start I sit at 40 and stay at 40 for 10 minutes or so while warm up and easy drive until coolant reaches temp.   then i'll get to 60 once running.   When back to idle when hot I'm' at 20 psi.      Honestly I don't know if mine has issues or not it's kinda always been a question in my mind.   

 

 

 

When i was using a fram filter it would take a 2-3 minutes at idle for pressure to drop below 40.   I'm now using napa gold with ams oil for the last 4 oil changes and once oils hot i drop to 20 seconds after i get down to 800 rpm.