low oil pressure and engine running hot

[Virginia]

Sorry, but this is going to be a little long but I think I need to give some history.  Back in the summer I helped a friend move a mattress and box spring.  Nothing heavy.  Now, he's about 300lbs.  We were driving to his house and had been for about 30 minutes when I noticed that the truck was struggling a little to go up some hills.  Nothing major, just over passes.  I can't imagine that he would have caused that.  Then I noticed that my oil pressure needle dropped to 0 and stayed there for a few seconds but the engine temp was normal as rain.  I pulled over and checked the oil and yes, it was low.  Maybe 3 quarts low.  I added and went on our way.  We got to his house and unloaded the mattress and box springs then when I started the truck and stepped on the brake it went completely to the floor.  No pressure no nothing.  I did notice that the engine compartment smelled a little like wire melting but I couldn't find anything wrong.  I had the truck towed home.  I came out the next day and the brake worked just fine and has ever since.  

 

What I've noticed since then is that my oil pressure seems lower than it used to be and that if I'm driving on the interstate at 65 to 70, after a while my oil pressure needle starts fluctuating all over the place.  I check the oil and it's fine and the engine temp is fine. So, I'm thinking the pressure sending unit is bad.

 

Yesterday, driving home and after only about 8 minutes of driving, the engine temp is creeping up.  never actually gets in the red but right on the edge.  2 minutes later I'm home.  In the driveway, I pop the radiator cap and add water until it's full.  While the truck is sitting there idling, the temp needle gets a little into the red.  I shut it down and let it sit over night.  I check the oil level and it's fine.

 

This morning while driving to work in about 35 degree weather, the engine temp gets hot fast and by the time I get to work, it's right at the edge of the red.  Normally, since I only drive 3.5 miles to work, the engine would just barely even be warming up on a day like this before I got to work.  The other thing I noticed this morning is that at one point, the heater stopped blowing hot air, the temp guage crept up some then the air turned hot again and the temp guage dropped just a tiny bit.  The oil pressure was operating at about 40 psi while driving at 2500 rpms.  At a stop, the oil pressure would drop to about 25psi.  I've never heard the engine ticking even when my oil pressure needle dropped to 0.  I do hear a slight ticking when I first start it in the morning but that only lasts about a second then it starts running quiet.

 

I don't know if these two things are related.  oh, when I popped the radiator cap in my driveway the water coming out was nice and rusty.

 

this truck is a 91, 6 cyl, 4.0, 5speed 4wd with about 189K miles on it.

 

I have no idea where to start to fix this.

 

NEW:  I just went down and was able to spin both cooling fans with the motor off.  I'm thinking that should happen.

[yxmj]

1- change the oil and filter....check the oil p then...sometimes repeated overheatings cause the oil to lose viscosity ....give it some new oil and at least you will know it is new and full.

2- first guess on the overheating is your thermostat is not opening.....if not that then your water pump is gone......change the t-stat...and flush the rad....see where you stand after you do those easy fixes

[1987Comanche]

Sounds like a few different things going on.  Does the temp gauge only creep up when the truck is idling or moving slowly?  If so you may have a bad fan clutch on the mechanical fan.  T-stat may be hanging up as YXMJ suggested.  Are you sure the gauge is accurate?  If you can buy/borrow an infrared thermometer you can pop the hood and confirm the accuracy of the gauge (my Harbor Freight one was under $30).  Shoot the T-stat housing then upper and lower rad hoses.

 

The oil press gauge bouncing erratically does sound like a bad sender.  They're easy/cheap enough to replace so that's easy to try. Hot idle oil pressure of 25psi is great assuming the gauge is accurate.  If you can borrow a mechanical oil pressure gauge you can verify OP when you change the sender.  Perfect time to do so as you'll have the sender out already. 

 

  Do you know if the lower rad hose still has an anti-collapse spring inside?  A lot of people don't swap them from hose to hose and the hose sucking shut can cause odd symptoms.

[Virginia]

The temp creeps up even when driving at speed, not just when idling or going slowly.  I have an infrared thermostat so can check that in the morning.  I assume I need to let it warm up.  Don't know about the lower rad hose but will check.  Will also do the easy things first and work up and will find an oil pressure guage.  Will keep you posted.

[cruiser54]

Change the oil and filter as suggested. What brand oil filter are you currently using?

 

I'm also betting your radiator is clogged up since it gets hot on the highway even with cold ambient temps. New radiator is only $100. 

[Virginia]

I use Valvoline 10w30.  Changed oil, put in new thermostat and new oil pressure sending unit and this appears to have rectified the problem.  Haven't taken it out on the interstate yet but will tomorrow.  Drove around the cithy for a bit and the temp guage reached about 190 then stayed stead and the pressure guage needle stayed steady around 35 while driving at 1500rpm.  No jumping around.  It was about 62 ambient temp when I cranked it and the pressure guage went to just over 40 then dropped to about 25 at idle after the engine warmed up.