Oil Pressure Drop At ~5500rpm

[McDude 33]

Pretty much as the title says, when pulling to the redline, everytime I hit ~5500rpm the oil pressure needle drops.

 

I have a number of ideas:

- it could be the oil is losing viscosity and at the higher rpms I'm getting cavitation - the oil has been through some very hot weather at the ring and could be totally goosed

- it could be the pump is worn and I'm getting cavitation

- It could be the sensor goes out of range as the pressure rises with rpm and is eroniously reading low

 

Any ideas?

 

The engine is a fairly recent Skip Brown rebuild so the next step is to ring Paul. Just wondered if anyone had any thoughts before then.

 

Cheers!

[MrG 3]

I'd give them a call so they know there's a possible issue with it regardless of what the cause could be.

 

I had a similar thing on my old TVR, when I got to 4500rpm the oil pressure dropped like a stone but when I released the accelerator it went back up? They said it was electrical and this went on for a few weeks until they drove it, as soon as they saw what was happening it went in, and resulted in an engine build? Failed oil pump was the reason and not electrical.

[Anthony 1,003]

Check the crank pulley bolt is tight, and put a known good oil pressure sender on before assuming anything untoward.

[macaroni 17 1 Cars]

What viscosity oil are you using?

 

On my 1.9, I get the same thing with 20/50. When I used 10/40 the pressure was fine, but it burnt oil after long periods at very high speed. It didn't burn the oil with the thicker oil though.

 

So the head is coming off to have the valve stem oil seals replaced.

[Tom Fenton 1,542]

Before anything else I'd get a mechanical oil pressure gauge hooked up and see if you really have a problem or not.

 

Then you also need to judge whether the drop is slight, and is being caused by the relief valve lifting as the pump produces more pressure at high speed.

[macaroni 17 1 Cars]

So I/we may not have a problem then?

Maybe it is the relief valve bleeding some pressure away.

 

Sorry to hijack your thread McDude, it might help you too!

[James Stewart 1]

I had something very similar, but at about 4000. Good pressure until about 4000 then it would drop and become erratic. By 5500 rpm there was no oil pressure according to the needle, but the light wasn't on either. The problem was just the spade connector going onto the sender - the connection was breaking down, presumably due to the frequency of the vibrations in the engine at the above revs.

 

One new spade connector and it was back to normal showing good and steady pressure again.

 

Try that first and hope it works!

 

James.

[macaroni 17 1 Cars]

I have removed the spade connector and cleaned the contacts, to no avail.

 

My problem is that if I use thinner oil, the pressure stays high, but it smokes after prolonged fast speeds.

[McDude 33]

I'm using 10W/40 Shell Helix oil - I have no problems with smoke.

 

I spoke to Paul and he reckons it is highly unlikely that there are actual oil pressure issues and recomended I clean the contacts up. I'll try and get after it this weekend and let you know progress.

 

As for the relief valve, it should be operating all the time above idle so if it is jamming at all then you would get over pressure in the oil as the revs rise.

[Tom Fenton 1,542]

I wouldn't bother trying to clean the female spade connector on the end of the wire, it is a fruitless exercise as you can't get inside it to clean it well enough. Cut the bugger off and crimp a new one in place it what I do.

[GLPoomobile 958]

This sounds exactly like the problem that reared it's ugly head on my Mi last year, 1/4 of the way through a 500 mile journey home With no oil pressure warning light (think the bulb has blown), I was properly s*itting myself!

 

Was on the motorway doing about 80 when the gauge started dropping like a stone, then fluttering about, then dropping, then staying off etc. Was around 4500 to 5500 on the tacho I guess. I ended up driving home at 60 mph on a very light throttle. Anything over that, or a little bit of throttle, and the gauge would be going haywire. Having completed the journey and still having a bottom end in one piece, I don't think I had actual pressure problems. I still can't confirm anything though as it's been off the road pretty much since then (due to be re-born by the end of the month). But what I can say is that the female terminal was quite loose, and the wiring, particularly around and down wind of the old brown plug, was in a bad state. So I'm banking on that being the cause. Now I have a completely new loom, plus additional gauge and warning light, all will become clear soon enough.

[jackherer 543]

If the needle moves suddenly and/or jerkily it is definitely an electrical fault, the sender simply isn't capable of responding that fast to actual changes in oil pressure.

[GLPoomobile 958]

That is a very, very sensible point! I can't believe that never occurred to me before (yet again I overlook the obvious and logical answer ).

[macaroni 17 1 Cars]

I don't think my problem is electrical, as the pressure doesn't drop with thinner oil.

So, if the relief valve wasn't working, what would the symptoms be?

Would it force oil past the valve stem oil seals during a fast drive?

[McDude 33]

Well my problem was electrical!

 

Played around with the spade connector and it will show oil pressure all the way through the rev range now. Funnily enough the indicated oil pressure is lower now than before showing what a total peice of crap the sensor/calibration is.

 

Just some old car electrical woes.

 

Many thanks for the (as always) expert advice.

[Rich_p 23]

I can never get my pressure needle to go above the 2nd line, there seems to be no issues with the engine itself so I can only assume it's a problem with the sender?