Odd oil pressure readings - help

[soutex]

I noticed very odd oil pressure readings this week and don't know when it started. I have a 351W bored to 427, 20-50w oil, Motorcraft oil filter, water temp was 205 degrees. Pressures: Idle, 35 psi. 2000 rpm, 65 psi. 3000 rpm, 34 psi. 4000 rpm 60 psi. I pulled the oil sender unit and hooked it up to a calibrated air source and recorded pressures for each tick mark on the Auto Meter electronic gauge all the way to 75 psi. PSI Numbers above are the corrected figures based on the pressure test. The oil has less than 10 miles on it when tested.

Whats going on here? Do I need to worry about this?

 

[Rusty67]

What kind of oil pump do you have in there ?

 

[soutex]

Melling HV standard pressure. I am going to call Melling and ask their techs about it. If I were going to see a fall off of pressure it would be more likely at 4000 not 3000.

 

[Hack]

Could be a poor ground or electrical connection causing variation in the oil pressure readings. Another possibility is a bit of teflon tape or other debris got into the sender and caused incorrect readings. If the high volume pump is pumping the pan dry I would also expect that to worsen as the RPM increases. Does the oil pressure gage always give those readings at those RPM levels or did you just check once?

 

[soutex]

Good idea

The pressures readings are somewhat consistent. On my 3rd run 2 days ago I got pressure drop off at 2800 rpm thru 3000 and starting to rise back to normal at above 3500 and proper pressure at 4000. Melling told me screen height closer than 3/8" from the bottom might do it. They also recommended putting in another quart of oil. They said they had never seen anything like this. I have not thought about Teflon insulating the electronic sender. Sure could cause something weird to happen. The car is a beast and gets a lot of vibration at higher rpms. I'll pull the sender and run the Teflon higher up the threads so I get at least half the threads in contact with the pipe. I'll pull the pipe too and check it.

Thanks. If that doesn't do it the next step would be to get a mechanical gauge and check it with that.

 

[Rusty67]

I somehow thought you already hooked up a mechanical gauge to it. I would have already sugested that. I should have read the first post more carefully.

The mechanical gauge is the way to go. Screw that bad boy right into the block and see what happens.

 

[soutex]

I have been bogged down with other events but I did get it on the road yesterday for a 50 mile cruise. Prior to taking off I added another quart of oil for a total of 7 quarts in a stock pan. Helped some (2psi) but there is still a dip at 2800-3000 rpm. Did not go below 36 psi. At about 3200 the pressure started climbing up. Traffic kept me below 3500 so I don't know what the terminal pressure was. Next comes the mechanical gauge. I want a good one. Who makes em?

 

[Rusty67]

7 quarts in a stock pan ? Is this a 302/5.0 or a modular motor ? Check your dipstick. It seems to me you have your oil pan over filled.

 

[Slobin3d]

7 quarts in a stock pan ? Is this a 302/5.0 or a modular motor ? Check your dipstick. It seems to me you have your oil pan over filled.

I'm guessing that the pump is pulling the oil faster then it can drain back. starving it of oil and causing the drop in pressure, or that's the theory.

 

[PoppyMod]

I'm running all auto-meters.

 

[Hack]

I have been bogged down with other events but I did get it on the road yesterday for a 50 mile cruise. Prior to taking off I added another quart of oil for a total of 7 quarts in a stock pan. Helped some (2psi) but there is still a dip at 2800-3000 rpm. Did not go below 36 psi. At about 3200 the pressure started climbing up. Traffic kept me below 3500 so I don't know what the terminal pressure was. Next comes the mechanical gauge. I want a good one. Who makes em?

Adding a quart is not a bad method for checking this. Ford actually recalibrated the 351C CJ oil dipstick to add an extra quart. My understanding is the capacity of the pan isn't larger, they just added a quart of oil.

It still seems to me that if you are running the oil pan dry it should be worse as rpms increase, not worst at 3,000 rpm. I don't understand that.

I think autometers work ok as well.

 

[soutex]

It is nuts, I know. I discussed this with Melling and they don't get it either. They suggested the extra quart. I didn't see any problems after running 50 miles, except for the dip in pressure. It could all be related to the Auto Meter electronic gauges. My gas gauge is off by 1/4 tank and the oil gauge is off at all levels above 10 psi by as much as 15 psi. The pressure drop is consistent every time I run it up. I asked Melling about cavitation and they said no. "Their pumps don't cavitate". After I get a chance I'll go with a mechanical. I am shying away from Auto Meter for the moment until I can see some test data on a particular model gauge.

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[Rusty67]

Is it possible that there is a wiring problem ? Maybe something funny with the alternator puting out funny voltages at a certain RPM ? Mechanical gauge is the next step.

 

[PoppyMod]

Hi, Also, perhaps give AutoMeter tech a call and get their take on it. I've used these gauges for a number of years and have seen no crazy swings in their operation.
Goods Luck!