Oil pressure on start up

bimbill

bimbill

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I have a symptom that I have had trouble resolving and hoped to find some help from the experts here. The issue is, since completing my 3.5 installation, on cold start it takes about 6 to 8 seconds for the oil pressure light to go out. This is after the car has sat for several days. Some background:

I am using an M30b35 from a 1990 seven series. Using the excellent advice and documentation on this forum, I retained the B35 oil pump but attached the E9 M30 pickup and used the E9 pan. I have the L jet intake system with E21 intake runners and a Tii brake booster. With this arrangement I couldn't use the upright oil filter housing from the B35 because of interference with the brake master cylinder. So, I opted to use the B34 filter housing and cannister. I made sure to plug the hole in the block that isn't used and must be blocked with this arrangement.

My question is, does this sound like a typical situation given my combination of parts? I did some internet research and found one thread that suggested this delay in the light going out is not unusual for M30s that have the pressure switch at the back of the head. There was also some reference to failed check valves but I couldn't find consensus in that regard.

I did connect a mechanical oil pressure gauge and once oil pressure is established all indications seem normal. 55 psi cold, 18-20 at idle hot. I appreciate any thoughts.
 
just curious about htis i checked this morning, after sitting for one week, started from cold, 3 seconds for the light to go out

when start from warm engine 1 second

btw, my oil pressure when hot engine goes from 30 psi iddle to 65 psi at >4000rpms

oil pump is brand new over an M30 stock engine rebuild on 2012
 
Similar with 3.5.

If you are using the location to read the pressure (sending unit) that was stock for a 3.5 which would be at the back of the cylinder head then it's last in line for pressure to build.
 
m5bb said:
If you are using the location to read the pressure (sending unit) that was stock for a 3.5 which would be at the back of the cylinder head then it's last in line for pressure to build.
Click to expand...

I don't agree with that explanation - oil pressure should propagate through the engine at the speed of sound (in oil) resulting in a negligible delay between oil pump and back of head.

Here's another way to look at it: If hydraulic pressure propagated that slowly, your car would always pull to the left on braking, since the calipers on the right side are last in line for pressure to build.

I think the delay in building oil pressure is simply due to the pump losing its prime when the car sits for several days.
 
And the infamous problem with the b35 oil filter housing check valve which caused extra long waits for pressure because the filter canister would empty each time. Air in line is not your friend
 
I always crank the engine without touching the accelerator (thereby not priming with fuel through the accelerator pump or re-setting the choke), until my light goes out / gauge reads some pressure before then introducing gas, which causes the engine to fire immediately.

I've done this with every car I've owned as I don't like the idea of bearings taking a pounding with no oil pressure on cold start-ups.
 
sfdon said:
And the infamous problem with the b35 oil filter housing check valve which caused extra long waits for pressure because the filter canister would empty each time. Air in line is not your friend
Click to expand...

Agreed. I have a b35 with the upright oil filter housing, but am fortunate to have a good check valve. The oil light goes out almost immediately after startup for me.

-S
 
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