Technical Oil pressure goes dramatically low with hot engine

[Mozys]

Good day,

I have a Fiat 500 F and have an issue with oil pressure going extremely low (even warning light come on while on high rpm) but only when engine is hot. When cold it runs at 2.5 Bar but then after a run as the engine gets hot the pressure starts to go very low. I have a new oil pump. I have investigated everything but dont seem to find the issue. Can someone assist with what might be the cause?

 

[bugsymike]

Blocked oil strainer in sump, worn bearings in engine, wrong grade of oil for your climate?
Is it possible to direct more cooling air to engine and sump area?

 

[Mozys]

Blocked oil strainer in sump, worn bearings in engine, wrong grade of oil for your climate?
Is it possible to direct more cooling air to engine and sump area?

Thank you for your reply.
I have done all bearing in the engine, brand new, using Castrol Classic 20x50, also checked the oil strainer and was clean. If the strainer was blocked (will check again) would I have enough pressure when cold?

 

[bugsymike]

Thank you for your reply.
I have done all bearing in the engine, brand new, using Castrol Classic 20x50, also checked the oil strainer and was clean. If the strainer was blocked (will check again) would I have enough pressure when cold?

Was the crank reground with oversize bearings to match or just new original size bearings?
Does engine sound increase when hot?
Partially blocked strainer restricts flow so may aggravate situation when hot.
Pressure will always read highest starting from cold engine.
Was oil filter /slinger /pulley totally cleaned etc.
Is there a blocked / faulty bypass valve.
It's years since I stripped a Classic Fiat 500 engine so perhaps other Forum members can give more specific advice.

 

[Mozys]

Was the crank reground with oversize bearings to match or just new original size bearings?
Does engine sound increase when hot?
Partially blocked strainer restricts flow so may aggravate situation when hot.
Pressure will always read highest starting from cold engine.
Was oil filter /slinger /pulley totally cleaned etc.
Is there a blocked / faulty bypass valve.
It's years since I stripped a Classic Fiat 500 engine so perhaps other Forum members can give more specific advice.

I have installed a completely refurbished crankshaft with respective new bearings
Sound does not change
I will re-check the strainer again but im pretty sure its clean.
Oil filter/ slinger was completely clean
Im not sure about the by-pass valve, and to be honest not sure of it, dont know location (is that inside the pump? spring loaded?)

I very much appreciate your assistance because im really running out of understanding the cause of it. Thank you @bugsymike

 

[Goldnrust]

A hot oil pressure loss would suggest a bearing clearance or bypass valve type issue to me, rather than a blockage.

Have you measured the bearing clearances yourself? Is it possible that you've got a crank that's been ground undersize and got standard size bearings fitted (or a similar mismatch)?

 

[Mozys]

A hot oil pressure loss would suggest a bearing clearance or bypass valve type issue to me, rather than a blockage.

Have you measured the bearing clearances yourself? Is it possible that you've got a crank that's been ground undersize and got standard size bearings fitted (or a similar mismatch)?

The crank was ground to the new bearing 0.20 and all was new. I have replace the oil pump, anf therefore by pass should be good but you never know. I will replace the oil pump again just to have something to check.
How can i check if the by pass valve is opening?

 

[bugsymike]

I have installed a completely refurbished crankshaft with respective new bearings
Sound does not change
I will re-check the strainer again but im pretty sure its clean.
Oil filter/ slinger was completely clean
Im not sure about the by-pass valve, and to be honest not sure of it, dont know location (is that inside the pump? spring loaded?)

I very much appreciate your assistance because im really running out of understanding the cause of it. Thank you @bugsymike

A hot oil pressure loss would suggest a bearing clearance or bypass valve type issue to me, rather than a blockage.

Have you measured the bearing clearances yourself? Is it possible that you've got a crank that's been ground undersize and got standard size bearings fitted (or a similar mismatch)?

I would say that is a good point, when on a Mazda course years ago they had something called "plastigauge " I think which was like tine strips of plastescene which waas put on the shell bearing and bolted into place then removed and measured for how much it had squashed using a gauge , this would give the tolerance beteen the shell bearing and the crank journal.
In the past I had a original mini crank reground and the guy admitted he had made a mistake and had to regrind the crank and get a much larger oversize to correct it.
When rebuilding an engine after torquing each bearing cap etc. I would turn engine a little to make sure it was actually showing that there was a resistance that it was "gripping " in effect.
Re bypass opening , it is more if it is not shutting so allowing pressure to seep away I think.

 

[Goldnrust]

The crank was ground to the new bearing 0.20 and all was new. I have replace the oil pump, anf therefore by pass should be good but you never know. I will replace the oil pump again just to have something to check.
How can i check if the by pass valve is opening?

I'd do some more investigating before just buying a new pump.

I'm not entirely sure of the bypass in the 500's design. I think it relies upon the spring loaded section of the oil pump, where it pushes against the cam shaft, being able to push back when the pressure gets too high. Might be worth checking there's nothing obstructing that join between the oil pump and the cam shaft? Like a lock washer tab that's sticking out too far or something.

I have read of the core plugs in the centre of the crank becoming loose, or even coming out, might be worth making sure they appear tight and are not leaking oil, as that would also be somewhere to loose pressure?

Thinking aloud, there are also some aluminium plugs that blank off where the oil galleries were drilled, there's no chance one of these has been removed for cleaning and not replaced is there? I'm thinking of the ones that are inside the crank case, and would just drain into the sump if they were missing.

The bearing design on the 500 means plasitgauge could only help with the big ends, not the mains, so you'd need some accurate measuring tools to help if you investigate this route. Presuming you trust your engine shop, this sounds an unlikely issue as you've had the crank reground to match the bearings supplied.

 

[bugsymike]

In theory I would agree with @Goldnrust re engine shops, but over the years different ones have not lived up to expectations.

 

[the hobbler]

I'd do some more investigating before just buying a new pump.

I'm not entirely sure of the bypass in the 500's design. I think it relies upon the spring loaded section of the oil pump, where it pushes against the cam shaft, being able to push back when the pressure gets too high. Might be worth checking there's nothing obstructing that join between the oil pump and the cam shaft? Like a lock washer tab that's sticking out too far or something.

I have read of the core plugs in the centre of the crank becoming loose, or even coming out, might be worth making sure they appear tight and are not leaking oil, as that would also be somewhere to loose pressure?

Thinking aloud, there are also some aluminium plugs that blank off where the oil galleries were drilled, there's no chance one of these has been removed for cleaning and not replaced is there? I'm thinking of the ones that are inside the crank case, and would just drain into the sump if they were missing.

The bearing design on the 500 means plasitgauge could only help with the big ends, not the mains, so you'd need some accurate measuring tools to help if you investigate this route. Presuming you trust your engine shop, this sounds an unlikely issue as you've had the crank reground to match the bearings supplied.

You say that you fitted a new oil-pump---was it a 500 pump or one sold as 500/126 pump.The reason forasking is that the pumps are different and NOT interchangeable. The 126 pump is slightly deeper, and when the engine heats up, if the pump is a 126 pump in a 500 timing-chain cover, the pressure-relief valve in the pump will come into operation earlier, and stay open longer when the engine is hot. When fitting new oil-pumps, they MUST match the engine (or timing-chain cover)---500 pumps for 500 timing-chain cover; 126 pumps for 126 timing-chain covers. The "problem" is that the timing-chain covers ARE interchangeable!

 

[Mozys]

You say that you fitted a new oil-pump---was it a 500 pump or one sold as 500/126 pump.The reason forasking is that the pumps are different and NOT interchangeable. The 126 pump is slightly deeper, and when the engine heats up, if the pump is a 126 pump in a 500 timing-chain cover, the pressure-relief valve in the pump will come into operation earlier, and stay open longer when the engine is hot. When fitting new oil-pumps, they MUST match the engine (or timing-chain cover)---500 pumps for 500 timing-chain cover; 126 pumps for 126 timing-chain covers. The "problem" is that the timing-chain covers ARE interchangeable!

Im actually not sure, i assume its a 500 but will check with supplier. That could very well be the cause.