Strange oil pressure sender readings

Irvin

Hi all,
I have a custom built 76*85.5 type 1 engine with a VDO 0/5 bars pressure sender on it.
The motor has a few thousand km ans runs great, cyl heads are around 160/170° on the motorway, oil pressure around 3 bars, perfect.
However every once in a while at idle when the motor is cold the pressure suddendly drops from 4+ bars to 0 bars until I rev up just a little bit and it comes back to 4 bars. It never happens more than two minutes after I start the motor.
Do you have any idea on what is happening? I don't think it's a wiring issue as this wouldn't go away by reving up the motor...
Thanks.

TheAmazingDave · Posted: Sat Jul 04, 2015 3:00 pm Post subject:

Did you keep the oil pressure dummy light? If so, does it come on at the same time?
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OLD VW NUT · Posted: Sat Jul 04, 2015 5:04 pm Post subject:

This sounds like the pressure regulator dumping cold oil back into the sump - a safety feature in that it prevents the oil cooler from getting cold thick oil at high pressure which could burst it.

What happens if you don't rev the motor?

You didn't mention if you have an external filter added to your build. Which oil are you using?
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Irvin · Posted: Sat Jul 04, 2015 11:51 pm Post subject:

Yes the idiot light comes in but is wired to the sender too.
I never tried to do not rev up the motor, I have no external oil filter, the oil is Motul 300V 15W50.

OLD VW NUT · Posted: Sun Jul 05, 2015 2:30 pm Post subject:

Try a lighter oil for your next oil change - such as 5w30 - 10w 30 or even straight 30w and see what happens. 50w is a bit on the heavy side - VW recommended straight 30w for most climates for ACVW motors. They must have known something.
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Irvin · Posted: Sun Jul 05, 2015 2:35 pm Post subject:

OK I'll try that but why did it suddenly appeared a few weeks ago ? I've used the same oil with this motor for 18 months and I had no problem this winter.

FreeBug · Posted: Sun Jul 05, 2015 2:39 pm Post subject:

Where is your full-flow filter (I assume you have one) in relation to the oil level? Is it placed high up?

Irvin · Posted: Sun Jul 05, 2015 10:34 pm Post subject:

I have no full flow filter, I can't find any explanation for this pressure drop...

earthquake · Posted: Mon Jul 06, 2015 10:55 am Post subject:

A lot of the problems I have seen with Electric gauges is from using Teflon tape on the senders threads, some times you don't get a good enough ground on the sender.

Casey
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Irvin · Posted: Mon Jul 06, 2015 2:21 pm Post subject:

I think I'll remove it and use the standard pressure switch for a couple of days, if the idiot light still comes on the problem is real and comes from the motor, otherwise I'll live with it.
Thanks for your advices.

Kjell Roar · Posted: Mon Jul 06, 2015 2:49 pm Post subject:

I Think Motul 300V is among the best oils you can put in your engine. We cured camshaft problems just by switching to that oil, on Honda motorcycle engines. But I think they make it in 10W40 too, that is better in a VW engine.
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philthy · Posted: Mon Jul 06, 2015 2:49 pm Post subject:

I have always liked putting an actual gauge on the engine, tee'd off from the switch
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Irvin · Posted: Tue Jul 07, 2015 12:50 am Post subject:

ozzo · Posted: Tue Jul 07, 2015 6:13 am Post subject:

In your shoes i ll remove and clean the relief piston next to flywheel and then change oil. Maybe one point thinner.
Maybe you are at the edge to get the relief piston open. At higher temp no problem as warmoil become thinner. When cold maybe it slowly increase pressure and some friction keep the piston to close. When you increase rpm at cold you press more the spring which generate a higher force that will close the piston when u release gas

Irvin · Posted: Tue Jul 07, 2015 7:57 am Post subject:

I'll do that too, it does not take a long time to clean the piston.
Thanks.

videoguy009 · Posted: Mon Oct 19, 2015 12:57 pm Post subject:

I know this is an older post but do you have a vdo dual pole sender?
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Irvin · Posted: Mon Oct 19, 2015 1:03 pm Post subject:

Yes I do but it was working properly, the oil level was a bit low and the oil too heavy so it was not flowing back to the sump fast enough when the motor was cold.

videoguy009 · Posted: Mon Oct 19, 2015 1:10 pm Post subject:

I just installed a two pole sender with the idiot light on one pole and the gauge on the other.When I was driving,all of a suuden the oil light would come on and the gauge read 0.I installed a tee and used one pole off of the dual pole for the gauge and a stock sender for the idiot light.No problems since then.I am running Rotella T 15w40.

Read this from another post.
The problem with the 2 pole senders is the VDO 2 pole idiot light trips at 8psi, but the stock idiot light switch trips at 2.5-3psi. On a lot of engines the light will be off at idle with the stock sender, but on with the VDO 2 pole.

VW engines idle at relatively low oil pressures compared to other engines. It's not bad, it's just the way they are.

So a lot of guys run a tee and run the stock idiot light switch for the light, and VDO 1 pole sender for the gauge.

I can't tell you how many guys install a 2 pole VDO sender, then have heart attacks with the idiot light on when the engine idles.

It does work well.
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Irvin · Posted: Mon Oct 19, 2015 1:13 pm Post subject:

Yes I knew that trick, but the problem was not coming from there. Thanks anyway for your answer.


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