Author Topic: Muppet's guide to oil pressure and common pressure problems on Calibras  (Read 5518 times)

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Offline Bad Kid

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Right I've seen a few people asking about this lately, so here goes. I'll keep it simple

What is oil pressure?
As the engine spins, oil is forced through very small spaces to give it pressure. Just like squeezing the end of a hose pipe to make a powerful jet of water.

Why is this needed?
As the engine rotates, the pistons and rods are moving up and down the cylinder bores as the fuel is combusted. This energy is transmitted to the crank. Where the rods join the crank, they are obivously pivoting very quickly. You can't just have two metal surfaces doing this. You couldn't even just lubricate them with oil. They have to be physically held apart with oil pressure.

The best way to think of this is like a water slide. You slide so freely because you are separated from the slide by a film of water. Without this oil pressure in an engine, bearings rub against the crank.

Here you can see a conrod


Parts 9 to 12 are the bearings, that are separated from the crank by the oil pressure, enabling them to float and therefore have little or no wear when they have pressure.

Common problems

1. The oil pump gets it's oil from the sump, which is basically a bowl of oil at the bottom of the engine. It picks it up through a pickup pipe, which can become blocked and therefore starve the pump.

2. The oil pump has a built-in pressure relief valve. In simple terms - if it's stuck and can't move, you will have no oil pressure. To rectify this, remove the 24mm nut on the front of the oil pump and a spring will fall out. Next, you need to pull the valve out from the top of this bore, use a hook of some sort. Clean it up with wet+dry, then refit and ensure it isn't sticking.

Following a loss of oil pressure your bearings may be damaged due to running dry. Most of the time you can just replace them with standard items. However, to check properly you need measure the crank journals properly using calipers, to get their correct sizes. You can then compare these measurements to the tolerances (as found in Haynes manual) to determine whether you can just put new bearings in

If the crank is excessively damaged or oval'd, it will need a grind. This is where an engineer grinds the journals down by a specified amount. You then buy bearings to suit this new size, and you're left with a perfectly round crank journal with the proper gap round it.

Here is a crank I had polished (polishing can remove minor imperfections without affecting the sizing)


An easier method that using calipers is to use plastigauge, which is basically something you put in place of a bearing and tighten the big end cap down over (fig 7/8 in diagram). This then indicates the bearings you need.
NURBURGRING CHALLENGE :-D

Bovva

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Re: Muppet's guide to oil pressure and common pressure problems on Calibras
« Reply #1 on: September 28, 2009, 17:17:33 PM »
Top guide as always mate.

Offline carl

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Re: Muppet's guide to oil pressure and common pressure problems on Calibras
« Reply #2 on: September 28, 2009, 17:35:58 PM »
nice one....
bodybuilding thread! the thread for winners!
http://www.clubcalibra.com/index.php?topic=34651.msg449718#msg449718
**Carl's calibra project**
http://www.clubcalibra.com/index.php?topic=34701.msg450951#msg450951
You argued with an idiot, I brought you down to my level and I beat you with experience.

Offline sealeysonic

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Re: Muppet's guide to oil pressure and common pressure problems on Calibras
« Reply #3 on: September 28, 2009, 17:48:54 PM »
very intresting chav...nice one m8

Offline Ash,

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Re: Muppet's guide to oil pressure and common pressure problems on Calibras
« Reply #4 on: September 28, 2009, 21:31:17 PM »
Then what can cause just *Low* oil pressure and how can it be diagnosed?

Offline Bad Kid

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Re: Muppet's guide to oil pressure and common pressure problems on Calibras
« Reply #5 on: September 28, 2009, 21:47:54 PM »
Then what can cause just *Low* oil pressure

Worn bearings / engine internals (bigger gap = less pressure)
Old oil pump
Excessively thin engine oil

Quote
and how can it be diagnosed?

A gauge will tell you the pressure. To increase oil pressure you can thicken the oil up a bit, or there are ways of modifying the pressure relief valve (using shims or washers) to adjust the pressure

NURBURGRING CHALLENGE :-D

Offline Trigger

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Re: Muppet's guide to oil pressure and common pressure problems on Calibras
« Reply #6 on: September 28, 2009, 22:15:15 PM »
Topic moved to appropriate section

Offline Bad Kid

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Re: Muppet's guide to oil pressure and common pressure problems on Calibras
« Reply #7 on: September 28, 2009, 22:18:30 PM »
Ta Trig :)
NURBURGRING CHALLENGE :-D