So a few weeks ago, i started to notice that my oil pressure gauge would drop down on the low side as the engine would get hot, not completely on zero, but close too it. Never heard any unusual sounds, so i just figured i'd run an engine flush, and change the oil to a 15w-40, and of course a motorcraft filter.

So i drove my truck after the oil change, and the pressure when to about half of the pressure gauge. I drove it around town, just enough to get it to running temperature. By the time i got home, my gauge was back down to low oil pressure. But no weird sounds coming from the engine.

My question is, is the oil sensor/ gauge faulty, or should i look further into this? Note, that my battery voltage gauge, and coolant temp gauge are not working right now. I know that people usually jump to the conclusion that the main rod and bearing are worn, but i'm pretty positive oil is getting to my valve cover, and that being the top of the engine, there has to be some pressure?

I guess i'm a bit lost on what to do, or where to start to diagnose the problem. Help?

 

You may be okay... don't jump to any conclusions, yet!

The best answer is to get a mechanical [aftermarket gauge] to test the actual oil pressure. The factory gauges are prone to less than accurate readings.

The ICVR [use the search feature] controls voltage to the fuel gauge, temperature gauge and oil pressure gauges. If all three gauges read very low or very high, the regulator may be faulty [not uncommon].

My son's 1984 F150 was showing very low oil pressure, but there were no noises and the other gauges worked normally. A mechanical oil pressure installed directly in the block [replacing the sending unit] revealed normal oil pressure. A new Motorcraft oil pressure sending unit was installed and everything was then back to normal.

 

so i started it today, first time in about 3 weeks, and there is a noticeable amount of noise from the lifters under the valve cover. kind of a clacking noise.

so i guess their is something with the oil pressure after all...

 

First time to start after sitting for 3 weeks? Did the noise stop after the engine ran for a while?

 

i didn't let it run for to long after that. I figured oil wasn't getting up there, so i didn't want to wear anything out, if i didn't have too.

 

Just checking the basics... did you put 6 quarts of oil in when you did the oil change?

 

yes, 6 quarts exact, and it is well above the safe level on the dipstick

 

How many miles are on the truck?

I ask because we use oil PRESSURE gauges in cars, not oil FLOW gauges. Flow gauges would make the instruments more expensive and intricate. So we use pressure instead.

When the engine was built at the factory, it had say a rod and main clearance of .006" (I'm not sure what it is, I am using this as a point)

As you put miles on the engine and change the oil etc, the metal on the bearings DOES get wiped away very slowly. Thus increasing the clearance on the bearings.

So the oil pressure gauge says everything is OK at .006" but now you are at .008" and your worried. The pressure goes DOWN but flow goes UP. If you hit the gas and the pressure comes UP, then you are fine.

My mothers 1998 Jeep 250 L6 has 210,000 on it and she actually replaced the oil pump for this exact issue. At idle low pressure. Once she moves pressure is up.

Start the truck and let it idle so the oil gets to the lifters. If the clacking goes away, the next oil change, I would go to a higher weight oil.

I.e. from 10W30 to 10W40 or 20W50 if the weather permits.


P.S. Mine does the same thing. Start cold high oil pressure then lower once he is warmed up.

Classic Inlines Performance Parts - Understanding Oil Pressure

 

i get what you are saying, the only thing is, i've already went to a 10w-30 to a 15w-40. As far as the miles, i have no clue. My speedometer isn't working, and i haven't had the truck very long. Previous owner didn't know either. No telling how many times it might have rolled over, but if i had to guess, i'd say somewhere up of 200,000.

 

I would go to auto zone or the like and buy one of those 3 gauge sets.

Voltmeter
Coolant temp
Oil pressure

Search the net as to where to tap the block for oil. Meaning is there a boss already drilled. And see what the pressure is on that gauge.

Actually you can just remove the electrical one and screw the mechanical there. I have no clue what the "normal" range is. I was thinking of doing this as well.

 

I went out, and let the truck warm up, and you can't hear any noise after that. Also, the oil pressure begins to rise again around 2500-3000 rpms. I also opened the oil cap, and there is plenty of oil near the valves.

 

I think that you are OK. A truck with that many miles will have that symptom. I do like the mechanical gauge with an actual readout rather than a range in the NORMAL. Heres a decent set on the cheap.

Dave

http://www.summitracing.com/parts/atm-2397/overview/

 

Better, when you remove the old oil pressure sender, get a T-fitting at your FLAPS or Ace or wherever which you can screw into the block, and into which you can screw the senders for both your old gauge and the new gauge.

 

On the oil, I might be inclined to go with a straight 30wt in a situation like this, since the cold wx is over. A 15-40 multigrade is a 15wt oil with viscosity index improvers to get it to an effective 40wt when hot. But VI improvers "wear out" and the oil degrades to its basic 15wt. This should not happen within the recommended oil change period (can be in the 7000mile rage nowdays), but a couple of times I've seen some weird things I couldn't explain, which were fixed by a change to straight 30wt. But 67Tempest's suggestion of 20W-50 sounds right as well.

Unless you are using a diesel oil like Chevron's Delo 400 or Shell Rotella, you probably should add supplemental zinc (Rislone is one brand) at any oil change, in any but the newest generation of engines. You could even do what old-timers did, and add some "top cylinder lubricant," sort of a 2-stroke oil, to the gasoline to get a little extra lube to the guides and the top ring.

As 67Tempest indicates, you can often get by without a whole lot of indicated oil pressure. The old rule of thumb for minimum safe oil pressure was 10psi for each 1000rpm. If you get eight pounds at idle, you should be okay, according to the usual experts.