Wow, I haven't been on these forums in a long while....

Anyways, I have a school bus with a Ford B700 chassis. The engine, a 1985 8.2L Detroit Diesel Allison, is a very reliable one. Lately I have noticed very low oil pressure readings. I hear no excessive noise from the engine and there is PLENTY of oil, but the oil pressure still appears low. When I first start up the engine, the oil pressure is in the middle of the guage (a simple stock guage with L on one side and H on the other) and after it warms up it will settle down just above the red section of the L side of the guage. It has always done this since I bought it so is this normal? Sometimes you will see it pass down into the red section and come back up later... Is there anything I should be checking? Or do you think it's just a faulty guage? Is there any way to install a second, calibrated guage with numbered readouts?

Thanks for your help!

 

Does it vary directly with RPM's? I know that Ford put dummy gauges in some vehicles, but I dont know if this is one of them. I would suggest putting a mechanical gauge on it and see what the numbers are.

 

Dummy guages.... are you referring to the ones that just have "L and H" on them or "C or H"? If so, yeah, this chassis has them. How would I go about putting in a mechanic guage? Would, for some odd reason, too much oil cuase this problem? I plan on changing the oil soon (been 2,000 miles) and getting it back down to the right amount.

 

The temperature gauge is not a dummy gauge. It reads actual temperature differences, unless it's broken.

What I was talking about, is when they use a pressure switch and a resistor to place the needle in the middle of the gauge when it has pressure. Either you have pressure or you dont. I think they use 7psi for the cutoff. If it's above that, the pressure switch connects and the resistor places the needle on the gauge. Below that, the gauge bottoms out. Really, the placement of the gauge reflects the resistance of the resistor and the voltage in the electrical system. This means your oil pressure gauge reading only means you have pressure; it has no bearing on what that pressure is.

That's why I say to hook up a mechanical gauge. How to do that on a detroit diesel I have no clue. Most of them should be the same however, and instructions should be included. Usually there is a port on the oil filter adapter to hook up gauges.

Good luck!

 

What year is that DDA?? The old 70s models did not hold very much pressure at idle such as the old 318 2 stroke Detroits. I think the heavy trucks got fully functional gauges by the way. It could be a sending unit for the gauge as well.

 

I would expect the engine to be a 1983-1985 model. The Ford chassis was completed in April of 1985, and the school bus body added on in May of 1985. Any idea how I might add on a guage or find the current sender? (On a side note, why are they so expensive??) Also, would I be able to verify if I have adequate pressure if I took off the oil cap to see oil squirting around in there? Would that give me at least the assurance that I have enough oil pumping that it won't seize up on top? Thanks for your help!

 

If I remember correctly, the DDA 4-strokes have hydraulicly operated injector's. So if it runs and idle's smoothly, it should have at least the minumum allowable oil pressure. If not more. Your description does sound like Ford's "idiot-gauge". So if you need to know the oil pressure for sure, you could connect a temporary local mounted mechanical gauge.

 

Well I ran it the other day and didn't see oil squirting around underneath the oil cap, but the parts moving around in there (which was kinda cool to see, hehehe) did look to have new oil on them and didn't look to be struggling to move. When I did look under that cap, the oil guage showed a decent amount of pressure. The pressure only began to fall when I was moving it through the driveway and the engine was warming up. I have made plans to change its oil on Saturday....do you think that'll allow better oil pressure? It was changed 2,000 or so miles ago, but that was also four years ago. The engine runs plenty, but the bus doesn't necessarily move.

 

What weight of oil have you been using? 15w40 'CD' was the only multi-grade oil recommended for the DDA 4-stroke. The only other oils recommended were 30 weight and 40 weight, meeting API 'CD' specification.

 

Ok...so I changed the oil, oil filters, and fuel filters as well as reduced the oil level to the proper level. So far (haven't used it on the road yet) the oil pressure has increased a LOT! Yay! So what about having the crankcase overfilled causes low oil pressure? And what about the new oil caused better oil pressure?

 

Sounds like the old oil was pretty well used up. What weight oil did you use, and do you know what was used in it before?

Having the crankcase overfilled too much isn't good for your bearings, since if the crank whips the oil it can force air into the rod bearings, as well as foam the oil. Unless it turned the oil to foam down to the oil pickup, I dont see why it would cause low oil pressure immediately though.

 

When I bought the bus four years ago, I shortly thereafter changed the oil with Wolfs Head 15W-40, an oil that was recommended to me. It's only been 2000 miles later, but I'm sure I should get into the habit of changing it at least biannually to fend off condensation and the such. I used Wolfs Head 15W-40 again this time, but like I said, used the correct amount. The dipstick is hard to get readings from (esspecially snice the oil is so clean now compared to the black), but the oil pressure was still quite high. It never got into the red area like it did before and it took a long while for it to drop, presumably because it warmed up.

 

Ok, well I'm still quite confused about my bus. I changed all the oil and filters (including the fuel filters, which is cool) and it seems to have solved all of my oil pressure issues. The gauge will be quite high (3/4 across the gauge towards the H section) at start up and drops only to about 1/4 across after it’s been running hard for a half hour. I think that's a great thing, right? I put in 3 gallons of 15W-40 diesel oil from Wolf's Head (4 quarts per gallon, right? So that makes 12 quarts). The Ford manual says 10 US quarts for the 8.2L DDA, but then has a note that says to add 2 US quarts for Full-Flow filters. I have no idea what a Full-Flow filter is, but I guess an extra two quarts won't hurt anything if I don't have those filters. There is a second note that says to add 6 US quarts for By-Pass filters, but that only applies to the 10.4L CAT. So my problem, the dipstick still gives weird readings. When we had it overfilled, it would show on the dipstick, but the black color of the oil helped that. When I check it now, it's still the yellow-new-oil color but looks as if it’s giving low readings. The stick looks oily until a bit above the high mark, but you can really only see oil accumulated just below the L of the Low mark. Is this something I should worry about? I'm confident I'd get better readings once the oil turns black again, but by that time, any damage from low oil might already be done. These weird readings are what eventually led to the gross overfilling of the crankcase at the last oil change. I ran the bus for about 40 miles at 55 mph this past Sunday (took it to Easter brunch ) and it ran great and everything, I'm still a bit worried though. Any help or suggestions are greatly appreciated.

 

Any takers? hehe

 

I lost my dipstick on my 8.2 I was wondering if you could tell me how long it is and how far up the full mark should be