This is an older truck but in great shape.. It's a 1988 GMC White Volvo with Cummins N14.
My oil pressure is now dropping... The "Dummy light" comes on at idle... There is only about 12LBs of pressure at idle. At 2000 RPM there is about 40LBS... I am not low on oil. Any idea's on what's going on... It just started happening last week...

My brakes stick on (Drag) fairly frequently now. The brake lights stay on as well (Tractor and Trailer). I usually have to apply the Parking brake and re-energize several times to get them to release.
Sticky Brake valve? I just replaced the Brake Chambers on the Tractor itself.

 

The oil....could be the oil filter is to small. More likely the oil pump is needing replaced.

The brakes...something blocking the treddle valve? Is the "Johnnie bar" partially down?

 

I don't know about Cummins, but my Detroit has two valves inside the oil pan that get worn. Mine are now. I've been living with the problem for a while because it's not threatening to the engine. Sometimes oil pressure is around 50 psi (hot) and sometimes it's around 40. I get low pressure at idle.

I was thinking of replacing bearings, but from what I've been told, the oil pressure valves should be the first things to check. They get scored, and depending on how they seat, oil pressure may be normal, or down a little.

I had brakes sticking on my "89" Freightliner. The teflon pusher for foot valve (from the bottom of the pedal into the valve) was getting tight in its bore. I didn't have to remove the foot valve. I just removed the three bolts holding the pedal, and was able to remove the teflon piece. Some fine sandpaper and some CRC corrected the problem.



stonefly

 

I'd suggest sampling the oil and have it analyzed by a reputable lab.

Levels of wear metals and contaminants in the oil can give a pretty good picture of whats going on in there.

I don't think it's an oil filter issue if it's got the stock filter setup on it, the oil pressure drop started recently, if it was a filter problem it would have been happening all along. Even with a completely plugged filter (unlikely if changed on schedule) the bypass valve will allow full flow of oil bypassing the plugged filter.

I'd suspect :

1) Worn oil pump.

2)Worn main bearings or rod bearings, allowing excessive oil to leak out of the bearings due to increased clearance. When you have more oil leaking out of bearings with excess clearance, even a new oil pump may not be able to move enough oil to maintain proper oil pressure.

3)Worn camshaft bushings, for the same reason stated above.

4) Broken spring on the oil pressure relief valve, allowing oil to get dumped thru the relief valve rather than holding pressure.

Does the engine have the correct viscosity oil in it?

We run 15w40 in our N14's.

Regarding the brake issues, sounds like a sticky foot valve not releasing all the way, lube the pins & make sure it's releasing all the way by pulling it up by hand if necessary, if it won't come all the way "released" without pulling the pedal up by hand, free it up until it does.

A hand valve partially applied would make the trailer brakes drag, not the tractor brakes.

 

I just like to say to Kranky... I appreciate the informative info here.

 

1998 ?

I would be immediately concerned if there was a sudden drop in oil pressure.

Have you had any recent unit injector failures? When was the last time the engine oil was changed?

I would first connect a "known good" pressure gauge to the main oil rifle to verify that my pressure sender is accurate- could be a bad sensor... doubt.

Sounds typical of a worn bottom end; rod and main bearings. If this has come on gradually, the 12psi @ idle is a warning that the pressure is getting low; 8PSI @ idle is when you want to hit the panic button.


As for the brakes... as kranky said, pull up on the pedal and eliminate that pivot pin first.

 

i have this same problem with my 2000 international 9900ix it has the n-14 and has low pressure at idel about 14 psi and running down the road at 1500-1600 rpm's it has about 25-30 psi i have 723,000 on the motor. now i drove 2 other trucks before this with about 900,000 on them with the same oil pressure. then one day one of those trucks i cant remeber wich one had a light and buzzer come on for low oil presure. we found it to be a faulty oil pressure sensor and aparently on the n-14 there is 2 oil pressure sending units one is easy to get to one is hard to get to. but we put a manual oil pressure gauge on it first to see what the oil pressure really was and at idel with the manual oil pressure gauge it was 25 psi at idel and at 1500 rpms was 45 psi so i would check the oil pressure with a manual gauge first.

 

The oil pump is sorta the weak link in a Cummins 855. We've replaced plenty of them. Many times, the gears will slip on the shaft. Usually when we replace oil pumps, around 750K or so, we go ahead and do the rods and mains as well.

You sure its a 1988? With an N-14?

 

I used to drive a 2001 International 9200 with an N14 C-elect Plus with the same problem. Turned out to be the oil pressure sending unit. I replaced it myself in 5 minuits with a 3/4 wrench. I was trouble shooting the problem, and this was the cheapest thing I could do myself. It turned out to be the problem. Price ranged from 70.00 at Truck Pro, to 115.00 at Cummins. I paid 90.00 a a Kenworth dealer 5 miles from the house.

 

I didn't see where you might have had something done with the starter recently. On my N-14, I had the starter replaced, and they cracked the oil pressure sender. It didn't show up at first. Took about 3 weeks and the oil pressure started giving me problems. Warm engine, low pressure, engine shutting down. It was a miserable trip from the west coast back to WI. Replaced the oil pressure sender and everything was fine again.

On mine, the sending unit was just forward of the starter, and doing anything with the starter meant it was very easy to hit it and crack it. Don't know if that's your problem or not.