As I've said recently, we all know the oil pressure gauges are prone for mis-readings due to the sender clogging up. But I was wondering what the reason for a healthy reading on start up and then gradually dropping to more or less 0 would be. My pea brain can't work it out.

 

Thanks.

Oil pressure is higher when the oil is cold since its thicker so has a harder time sqeezing through the galleries. As the oil warms up to temperature it thins out and you see the pressure fall. A worn guage will read lower across the whole range but while the pressure is higher will register a reading....

Also, the heat may affect the component in the gauge, leading to a less reliable reading when it gets hot (and old).

Im guessing my guage/sender is probably on its last legs, get virtually 0 reading on low revs/idle when the car is hot. does shift up a little when I gun it so I guess I dont have anything to worry about in the short term anyway! :) now where's my credit card, need to order some new guages!! :D

Mine works great until the engine is at normal running temp then nowt........

 

Mr GReddy some gauges please ;)

group buy !! :D

same hear, reads nothing when hot, lucky i have an after market one

Simple fix for this:

 

Remove the guage cluster, remove the stop pin on the left hand side of the oil pressure guage, drill a new hole for the stop pin around the 30psi mark and superglue it in there. This will ensure your oil pressure guage always runs correctly. Incidentally, the same method can be used for increasing your boost, same procedure but on the boost guage.

 

So how many parts have we managed to fix by masking the fault? We had the det sensor, then the idle valve (I'm taking mine off and throwing it away this weekend) and now faulty guages :D

lol ... No more wrong gauge readings ever .... :rofl: :rofl:

cutting a hole in the pipe/throwing away the gurgling AIV ?? does that count? What the hell do those things actually do anyway?

An engine oil is in fact thinner at start up, and thicker when warmed up.

 

Basically heat sensitive polymers (VI Improvers) are added to a thin base oil and as the engine warms up, these uncoil to give the oil the upper sae viscosity 30,40,50 etc.

 

This should explain the science for you.

 

VISCOSITY INDEX IMPROVERS

 

As a lubricant basestock is subjected to increasing temperatures it tends to lose its viscosity. In other words, it thins out. This leads to decreased engine protection and a higher likelihood of metal to metal contact. Therefore, if this viscosity loss can be minimized, the probability of unnecessary engine wear will be reduced.

 

VI improvers are polymers that expand and contract with changes in temperature. At low temperatures they are very compact and affect the viscosity of a lubricant very little. But, at high temperatures these polymers "expand" into much larger long-chain polymers which significantly increase the viscosity of their host lubricant.

 

So, as the basestock loses viscosity with increases in temperature, VI improvers “fight back” against the viscosity drop by increasing their size. The higher the molecular weight of the polymers used, the better the power of "thickening" within the lubricant. Unfortunately, an increase in molecular weight also leads to an inherent instability of the polymers themselves. They become much more prone to shearing within an engine.

 

As these polymers are sheared back to lower molecular weight molecules, their effectiveness as a VI improver decreases. Unfortunately, because petroleum basestocks are so prone to viscosity loss at high temperatures, high molecular weight polymers must be used. Since these polymers are more prone to shearing than lower molecular weight polymers, petroleum oils tend to shear back very quickly. In other words, they lose their ability to maintain their viscosity at high temperatures.

 

Synthetic basestocks, on the other hand, are much less prone to viscosity loss at high temperatures. Therefore, lower molecular weight polymers may be used as VI improvers.

 

These polymers are less prone to shearing, so they are effective for a much longer period of time than the VI improvers used in petroleum oils. In other words, synthetic oils do not quickly lose their ability to maintain viscosity at high temperatures as petroleum oils do.

 

In fact, some synthetic basestocks are so stable at high temperatures they need NO VI improvers at all. Obviously, these basestocks will maintain their high temperature viscosities for a very long time since there are no VI improvers to break down.

 

Hope this helps,

 

Cheers

Simon

OK, soooooo ... why do manuals always suggest that you change the oil when warm/hot, so that it flows out better ? Or is this just a historical thing from the dim and distant past before decent multigrades were developed ?

isn't that to do with getting the oil out of the oil cooler

OK, soooooo ... why do manuals always suggest that you change the oil when warm/hot, so that it flows out better ? Or is this just a historical thing from the dim and distant past before decent multigrades were developed ?

 

Your bang on, when the oil is warm it flows a lot better. You will find you will get more out if you do this.

 

Cheers

 

Simon.

don't know if the z's have thermostatic valves on the oil coolers my old 16v astra did

z has a pressure relief valve that only lets oil go to the cooler when the pressure exceeds about 50 - 55psi. So the cooler is only called on at decent rpm i.e. when you need it. At low running pressure nothing goes to the cooler and doesnt need to.

Your bang on, when the oil is warm it flows a lot better. You will find you will get more out if you do this.

 

Cheers

 

Simon.

 

But you said :-

 

An engine oil is in fact thinner at start up, and thicker when warmed up.

 

So am I misunderstanding something or is there a terminology difference between thicker/thinner and viscosity ? So colder means thinner on startup and warmer means flows better (= lower viscosity or thinner) :confused: