Oil pressure on rebuilt engine

[RudidSA]

Hi all, I just rebuilt a 3.0L engine for my 944 turbo and this car has fought me with every thing I've tried to do so I wanted to test the oil pressure before I started it up so I removed the DME relay and I'm turning the engine over with the starter motor but I'm not seeing the oil pressure rise on the dash, I know oil is flowing through the engine because there is oil in the filter and in the head, but I'm not seeing any pressure on the gauge. I recall seeing the pressure rise on the gauge before I pulled the old engine out so I'm wondering if my memory is faulty and my expectations are wrong, can someone help with what I should be seeing? is the engine turning over too slowly to have the pressure rise on the gauge? and how can I confirm if this is accurate?
Any help would be appreciated!

[Tom]

It's been a while, but when I first cranked my motor I'm fairly sure I did the same and did get pressure on the gauge. Dumb question, but are you sure the sender and gauge are working? Does the needle move at all when you turn on the ignition?

[RudidSA]

Not a dumb question at all, I've also thought it could be the sender, though it's brand new, I was thinking of testing with the old one just to be sure. To your second question, the needle does move just a little bit which makes me think the sender is fine, I'm worried I'm going to damage my freshly rebuilt engine from turning it over too much.

[Tom]

Try putting a little air pressure in the dip stick (5-10psi) while cranking and squeezing the AOS outlet hose to build a little pressure in the crankcase. Bad or disconnected senders usually peg the gauge or leave it dead, so I'm guessing that's not it....

[usury]

I forget what happens when the wires on the sender are switched to the wrong posts.

One post provides variable resistance based on oil pressure. (Don't remember if resistance increases with oil pressure or if it decreases.) That wire feeds the gauge.

The other wire shorts to ground when there is low oil pressure. This triggers the central warning light.

Could your sender wires be swapped?

[four0four]

I forget what happens when the wires on the sender are switched to the wrong posts.

One post provides variable resistance based on oil pressure. (Don't remember if resistance increases with oil pressure or if it decreases.) That wire feeds the gauge.

The other wire shorts to ground when there is low oil pressure. This triggers the central warning light.

Could your sender wires be swapped?

It'll peg the gauge at >5 bar, at least it did last time I made that error



Also, just because there's oil *flowing* doesn't mean it's built pressure. The filter housing is basically the first thing after the pump, you've got to fill your cooler (well, maybe - I forget if the thermostat allows any at all thru when cold) and lines/channels. For how long are you cranking it? I know mine took a slightly uncomfortable amount of time, and I'd even packed the pump w/grease.

[RudidSA]

I feel a little better because all the comments are along the lines of what I was thinking, definitely aware of the fact that there is a difference between flow and pressure, but I'm assuming that flow means the oil pump is working, unless there is a way for it to underperform? I'm fairly sure I got the connections on the sender right because the connections are different sizes and I recall paying special attention because the wires were cracked and I repaired them, the fact that the needle moves when I crank the engine makes me think there is some pressure and the sender isn't completely wrong. I cranked it 3 or 4 times for 30-40 seconds each. though cranking a newly built engine with no oil can be tough on the nerves, so it could have been 20 seconds.

[usury]

When I rebuilt the bottom end on mine, it took maybe 15 seconds of cranking (with the coil unplugged and DME relay removed) before there was pressure on a mechanical gauge I had adapted to the sending unit opening. My GF watched the gauge for me.

I don't think it should take too long.

[four0four]

I only mention it to drive home that it really needs to make it all the way to the bearings in order to start to move the gauge.

Might it be worth throwing a mechanical gauge on there? You can usually take the schrader valve out of a cheap compression tester and use that (maybe with whatever included adapter is necessary for M18x1.5? threads).

Edit: beaten by a second!

[RudidSA]

Thanks everyone for your help and suggestions, I went and got an external oil pressure gauge to test with, I decided to pour some oil over the cams and the head to test one more time before I used the external gauge and now the oil pressure goes up to 5 bar when turning it over, it seems a little high but I seem to have found a couple more oil leaks so I think it definitely got oil pressure now. Now I need to troubleshoot why the tach seems to think I'm getting to 3k RPM when the starter is turning the engine over.