Odd Oil Pressure Indicator Reading

[akoldnav]

I have very odd oil pressure readings on my 1988 F250. I have an aftermarket Westach Oil Pressure Gauge that is really great. Now the problem:

Cool oil shows over 40 lbs of pressure
Warm oil shows 18 lbs at idle and appx 35-40 at cruise.

But at higher curise (over 4 lbs of boost) the oil pressure reading drops off to below zero. I actually tried standing on the brake and just advancing the trhottle in drive and at 4 lbs of boost the oil pressure just went to below zero. If I let off of the throttle the oil pressure comes right back up to 40 lbs. I have changed out the sender and that changed nothing.

The oil pressure senders are connected to the top of the turbo, and I suspect that the spinning shaft has a sort of suction effect that is not allowing the oil to enter the "T" fitting that comes out the side of the top of the Banks Sidewinder. Does this sound possible?

Where would be a better place to put this sensor.

help

akoldnav

 

[82fordtruck]

I wonder if your turbo is bad. I believe there is a better port somewhere by the oil filter, but not sure on that one.

 

[towcat]

Tapping off the turbo feed line is generally a bad idea since it is a high oil consumption item. There's two easy to get to(somewhat) ports. I presume you are using the port off the side of the motor, so the last one is the one off the back. The earlier trucks(83-86) had a remote hose for the oil pressure sender, that makes it easier to get to the port or for testing purposes, plum directly there. Do clean off the area as much as possible since alot of crud does gather back there.

 

[Agnem]

I'd have to agree. Get the oil presure sender away from the turbo. It's not offering enough resistance to the flow of oil when it's really spinning. Connect the gauge to one of the two other readily accessible ports.

 

[82fordtruck]

Do they really feed that much oil into the turbo? I guess the restriction in that line is somewhere in the block then, not the turbo?

 

[LUCKY_LARUE60]

I have a real problem with what was said that at higher RPM's that the oil pressure gauge drops to zero, dose that mean the oil pressure at the turbo at the higher RPM's lack oil pressure?

 

[82fordtruck]

well - bearings don't been pressure - just flow. As long as they have that.....

 

[akoldnav]

I don't think I am loosing any oil or pressure at that location, what I am not doing is flowing oil into the port for the two oil pressure senders that I have on a "T" at the top of the turbo. I do think the turbo shaft and bearings might be making a pumping or siphoning effect causing all the oil on the up side to be under a slight vaccuum when the turbo is really spinning, such as when it is making lots of boost.

For the port at the back of the engine, are you talking about the one that was pluged off for the turbo install?

akoldnav

 

[highest_vision]

I have a real problem with what was said that at higher RPM's that the oil pressure gauge drops to zero, dose that mean the oil pressure at the turbo at the higher RPM's lack oil pressure?

I agree. You should be able to plumb you gauge into what ever spot you want to and not see a drop like that. That drop is indicating something. The gauge is effectivly deadheaded, so it uses no flow, only showing pressure. If it is showing no pressure, you can assume little to no flow as well at that point.
James

 

[akoldnav]

I didn't make the problem clear. The loss of oil pressure at the gauge sensor is not related to engine RPM at all, but rather turbo boost. It doesn't matter what RPM I am showing on the tach, if I put the truck under load and the boost rises, the oil pressure at the sensor drops. I know the truck still has oil pressure because I have driven it for 100 miles showing no oil pressure on the gauge, but she still runs and runs well.

Where are the other points that I could use for mounting a gauge sensor. I know of one on the passenger side, top of the engine in the rear, but my turbo blocks that one. Is it ok to run a hose from that location to another location for an oil pressure gauge? The sensor screws into a 1/2 hole.

Thanks for all the help, but I still need to know where I could mount the oil pressure sensor other than at the top of the turbo.

akoldnav

 

[Agnem]

The faster the turbo spins, the more oil it's going to pass. To draw an analogy, I have two oil presure sensors on the Moose Truck. One is in the block, and the other is at my bypass oil filter. When cold, I see about half the presure at the bypass filter, as I do at the block. The reason is this. The presure that develops in the block is due to restrictions in the flow of oil. All the bearings, and even the turbo are drawing oil from these passages at various rates, but the volume of oil present at the given presure is sufficient to keep the presure from droping off. At the bypass filter, it is a different story. First of all, the total volume of oil that is available, is only what is being passed through the hose to the filter. 100% of that oil is going to one place, that being the bypass filter media. The media itself, offers some resistance to the flow of oil due to it's restrictive design. This allows me to measure some presure, which is in effect back presure, or presure built up from the flow volume exceeding the capacity of the filter to flow. I usually see about 40 pounds in the block, and 20 at the filter. Now when the engine oil gets hot, and the engine is idling, I see the normal greatly reduced presure that we all see at the block, and I see NO presure at the filter. The oil is now thin enough to go thorough the filter media with less resistance, and the volume of the flow equals the volume being passed, so there is no back presure to measure. This is basically what he is seeing at the turbo. I'm no hydraulic expert, or scientist, so maybe someone more learned would like to convert what I've said into whatever the facts may be, but I stand by my explaination dispite any want of technical terminology.

 

[82fordtruck]

The faster the turbo spins, the more oil it's going to pass. To draw an analogy, I have two oil presure sensors on the Moose Truck. One is in the block, and the other is at my bypass oil filter. When cold, I see about half the presure at the bypass filter, as I do at the block. The reason is this. The presure that develops in the block is due to restrictions in the flow of oil. All the bearings, and even the turbo are drawing oil from these passages at various rates, but the volume of oil present at the given presure is sufficient to keep the presure from droping off. At the bypass filter, it is a different story. First of all, the total volume of oil that is available, is only what is being passed through the hose to the filter. 100% of that oil is going to one place, that being the bypass filter media. The media itself, offers some resistance to the flow of oil due to it's restrictive design. This allows me to measure some presure, which is in effect back presure, or presure built up from the flow volume exceeding the capacity of the filter to flow. I usually see about 40 pounds in the block, and 20 at the filter. Now when the engine oil gets hot, and the engine is idling, I see the normal greatly reduced presure that we all see at the block, and I see NO presure at the filter. The oil is now thin enough to go thorough the filter media with less resistance, and the volume of the flow equals the volume being passed, so there is no back presure to measure. This is basically what he is seeing at the turbo. I'm no hydraulic expert, or scientist, so maybe someone more learned would like to convert what I've said into whatever the facts may be, but I stand by my explaination dispite any want of technical terminology.

Mel - I agree completely. This is what I was trying to say earlier, but not very well. Bearings don't need pressure at all. In this case, there could be a restriction in the block or the beginning of the hose to prevent too much engine oil going through the turbo. When not using the tubo much, there may be another restriction at the bearings in the turbo that disappears when it spools. Thus, you would see flow, but not pressure.

We build hammermills at work, and use bearings about 24" in diameter and 6-8" thick. We use a hydraulic pump to feed each one, and the idea is to get only enough pressure to lift the oil to the top of the bearing. The bearing is not filled with oil, and all the oil flows out as fast as it can go through the bearing. We do not use too much, as the oil froths up and heats up. There is no pressure going to it, it drips in through a sight guage on top. But the flow is 2-3 GPM. In our case, the restriction is the valve in the line that lowers the line pressure to about 30-40 PSI. When it goes to the bearing, it just drips in.

 

[akoldnav]

That is exactly what I belive is happening here. So where should I put this expensive oil perssure sending unit so my westach gauge works properly.

akoldnav

 

[towcat]

with your turbo in the way, can you get to the port on the top of the motor and install an extension hose? I have a couple off 83-86's where the oil pressure sender is relocated to the firewall near the heater box. that should give you enough room. otherwise, T it on the other side of the line where it comes off the block to the turbo feed hose.
let me add a little to Mel's descriptions. Oil pressure is not a consistent item in a motor. IIRC from my machine shop days, there is a 10 psi drop for every foot of travel from the oil pump. The size of the passages also affect the psi drop too. The passage that you tap the turbo feed line is roughly 1/4 in. the line is roughly 1/8 in. The opening into the turbo is 7/16 in. There are groves machined into the turbo impeller shaft to help guide the oil into the bearings. So, when the turbo is running, the pressure drop is pretty healthy. The designers were not as concerned on the pressure since that would mean you would be pumping the oil past the bearings in the turbo and going into your intake or exhaust, they want the flow volume instead. Same with how our oiling system is designed. Due to the cooling jets, our motors have a 1in oil suction pipe from the bottom of the pan going into the pump. They were more concerned with volume over pressure.
everybody confused yet?

 

[akoldnav]

Thaks again for all the help folks.

akoldnav