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Old 06-12-2019   #1
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Another oil explosion

On driving back from my dyno tune I had a massive oil explosion in the engine bay. This has happened before and I've posted about it. But I may have some more clues now.

Here's some details that might help someone point me in the right direction.

I had just started a longish (1 hour) drive home. I'd recently done an oil change ready for the dyno tune. The dyno work had been done 2 days before.

I noticed that the oil pressure gauge on my Abarth style dashboard array had gone off the clock. It was reading a maximum of 8 bars. I had noticed this before but assumed it might be an intermittent problem with the oil sensor. However, I was expecting a problem this time, since I'd had this oil explosion before.

I couldn't pull over straight away. Before I could stop I noticed the pressure return to normal. When I stopped the engine bay was covered in oil – dripping out through the vents, as if there has been a rear end explosion – no curry involved

So the two things must be related. Huge oil pressure then a sudden drop and a big expulsion of oil. I've felt in the past that the dipstick was the source, but it was still in place, though could have been loose. Hard to really know where this is coming from. But what would cause a build up of oil pressure like that? (Note: I have the 3.5L Abarth sump.)

I also have an aluminium rocker cover, not fitted yet. The plan is to fit that so it can be screwed down more firmly and eliminate that as the source.

Any thoughts?
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Old 06-12-2019   #2
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Re: Another oil explosion

Quote Originally Posted by timgarman View Post
On driving back from my dyno tune I had a massive oil explosion in the engine bay. This has happened before and I've posted about it. But I may have some more clues now.

Here's some details that might help someone point me in the right direction.

I had just started a longish (1 hour) drive home. I'd recently done an oil change ready for the dyno tune. The dyno work had been done 2 days before.

I noticed that the oil pressure gauge on my Abarth style dashboard array had gone off the clock. It was reading a maximum of 8 bars. I had noticed this before but assumed it might be an intermittent problem with the oil sensor. However, I was expecting a problem this time, since I'd had this oil explosion before.

I couldn't pull over straight away. Before I could stop I noticed the pressure return to normal. When I stopped the engine bay was covered in oil – dripping out through the vents, as if there has been a rear end explosion – no curry involved

So the two things must be related. Huge oil pressure then a sudden drop and a big expulsion of oil. I've felt in the past that the dipstick was the source, but it was still in place, though could have been loose. Hard to really know where this is coming from. But what would cause a build up of oil pressure like that? (Note: I have the 3.5L Abarth sump.)

I also have an aluminium rocker cover, not fitted yet. The plan is to fit that so it can be screwed down more firmly and eliminate that as the source.

Any thoughts?
Hi Tim, Where does your rocker box vent to??? I appreciate that originally the vent fed into the air cleaner. Do you have a flame trap fitted in the pipe?? If so is it clean?? Is your oil filler cap a good fit?? Is the rocker box seal still ok or leaking?? When this happened were you driving at speed and was the oil level about half way on the dip stick?? You could have a build up of pressure in the crank case or possibly a blockage in the oil system somewhere??
Ian.
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Old 06-12-2019   #3
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Re: Another oil explosion

Hi Ian

I had filled to half way on the dipstick. Oil filler cap is a very tight fit.

Currently it's the original rocker cover which vents to the FZD carb. I'll check this is clear.
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Old 06-12-2019   #4
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Re: Another oil explosion

I would tend to agree with Ian Tim, it sounds as if you have a blockage somewhere in the system (a piece of dirt/sealant etc) that is blocking a 'return to the pump' passage somewhere. If this is so, it could lead to the gauge showing very high pressure prior to the oil 'exploding' everywhere. When did you last clean out the filter? Could a piece of 'muck' from the filter be re-circulating in the oil-pressure system. Would it be worthwhile draining the engine oil (and checking through it) and then re-filling the engine with flushing oil to try and displace any muck that is blocking the system?
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Old 07-12-2019   #5
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Re: Another oil explosion

Suggests to me that the oil pressure gauge is reading correct and big pressure build up for some reason. Is the engine free of oil leaks apart from these explosions.? I can think of any seal that would blow under pressure then reseal under normal running.
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Old 07-12-2019   #6
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Re: Another oil explosion

I'm only aware of some minor drips normally.

Have recently drained and refilled with oil.
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Old 07-12-2019   #7
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Re: Another oil explosion

The usual cause of very high oil pressure is a sticking oil pressure relief valve.
Unfortunately, to access this for inspection, will require removal of the timing cover and oil pump - I wouldn't rush to do this until every other possible cause has been checked out.

I'd agree with Bleeding Knuckles about checking that the oil breather system is not blocked.
Some of the Fiats used to have a 'flame trap' inserted into the rubber breather pipe that connects the valve cover vent to the air cleaner. This flame trap looks like a spiral of wire mesh and usually has a little ring on one end to allow it to be extracted - if this flame trap becomes choked with carbon/sludge etc, it can result in some of the sudden oil blow-out issues you've experienced. It won't cause excessive oil pressure though.

Maybe the oil being pumped to the cylinder head is not able to return quickly enough to the sump under some operating conditions e.g. sustained high revs?

If the oil pressure relief valve is sticking, resulting in very high oil pressure, then maybe under this condition, too much oil is then being pumped to the valve gear and is not able to drain back quickly enough to the sump -this build up of excess oil in the valve cover might then blow out somewhere.
Although, if there was too much oil build up around the valve gear, I'd expect some to be drawn down the inlet valve guides, resulting in significant smoke from the exhaust.

The reason the breather pipe/hose goes from the valve cover to the air filter is for emission purposes i.e. to draw any oil fumes back into the engine to be burned. On worn engines, I used to run a simple pipe from the valve cover down towards the ground so that oil fumes weren't drawn into the engine - excessive fumes could adversely affect engine idle/pick-up. If you want to try this and also avoid a mess, you could run the breather pipe into a catch bottle. (if a lot of oil is expelled, you can use the contents of the catch bottle to top up the oil level ).

Al.
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Last edited by F123C; 07-12-2019 at 04:06.
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Old 08-12-2019   #8
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Re: Another oil explosion

Here's a detail I forgot to mention.

Firstly, it’s a reconditioned original engine, rebored to 594cc.

Here’s the detail. It has a lightly ground camshaft which is not the original one. It was supplied to me I think by Toshi (correct me if I'm wrong Toshi – it was a while ago).

When we came to fit it, we realised that it was from a later model and didn’t fit. The end of the shaft that runs through the oil pump gears and connects to the camshaft is a different shape. So we found a work-around. Order the gears for the later model, push out the shaft and re-insert into the existing gears. Fitted fine. But could this be an issue?
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Old 09-12-2019   #9
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Re: Another oil explosion

Hi Toshi/Dave here currently from 300kms up the road in Rotorua & yes it was from me.
I have encountered problems sometimes when changing camshafts and the drive dog on the oil pump shaft is too tight to fit into the dog inside the end of the camshaft. Measurements showed an interference of about 0.010" or 0.25mm max so I sorted it by a very light grind to the fuel pump dog. I have come across one engine where this fit was overlooked and the timing chain cover forced home which sheared off one of the camshaft drive dogs which remained as a floating foreign body within that small space and the camshaft still continued to drive the oil pump.
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Old 09-12-2019   #10
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Re: Another oil explosion

Excess oil pressure is not a breathing problem.
The oil pump is capable of producing significantly more pressure than needed, and this is controlled by the pressure relief valve. Excess pressure is almost certainly due to this valve sticking, because any blockage furher along the line would cause the valve to open further.
It only takes a tiny bit of debris to casue the valve to stick. It will need removing and carefully inspecting and cleaning.
The pressure valve will have drain of some sort to allow the relieved oil to go back to the sump, or into the intake side of the pump. Check this is clear.

The reason the pressure suddenly drops again is due to the loss of oil. Then when there is insufficient for the pump, pressure drops, the valve may then return to its at rest position and all seems normal again.

If I understand correctly, you are using different gears in the pump body. If you still have the originals, compare them. The 'new' gears may produce more pressure, and the relief valve cannot release it fast enough.
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Old 09-12-2019   #11
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Re: Another oil explosion

The large and very strong spring of the pressure-relief valve is a very simple mechanism which as @portland_bill has explained, should allow any surplus pressure to escape; thus keeping oil pressure within fairly tight limits.

Whilst a small amount of debris might keep it from closing properly, this would cause a loss of oil-pressure.

It would need something quite big, sufficient to jam within the coils of the spring, in order for the sealing disc to be prevented from opening. This would lead to unregulated oil-pressure.

But I think even this is very unlikely because even if the spring could still achieve a small amount, even of skewed movement, some oil pressure would be released.

It seems unlikely that any part of the pressure-release valve could seize, even on a temporary basis, since by definiion, the whole thing is permanently bathed in oil.

But given how relatively straightforward it is to remove the timing-cover in-situ, it seems logical to do that and then strip the whole thing down for an analysis of whether the parts used are the correct ones and that they are correctly assembled.
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Old 10-12-2019   #12
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Re: Another oil explosion

Quote Originally Posted by fiat500 View Post
It seems unlikely that any part of the pressure-release valve could seize, even on a temporary basis, since by definiion, the whole thing is permanently bathed in oil.

I would totally agree if I hadn't witnessed it first hand. I was rebuilding a VW aircooled lump a number of years ago and it was clear that servicing of any definition had rarely been carried out. a full strip was being undertaken and low and behold the biggest pain in the butt thing was to get an oil relief piston out, at one point I was going to scrap the case! It was only after much cleaning soaking and copious amounts of compressed air it finally moved, everything was cleaned and it was then fine.


The engine was in a running car when I got it also and despite the usual oil leaks it wasn't blowing oil out anywhere which in this instance I would expect it to be pushing past the crank pully which is the usual favourite as it doesn't have an oil seal...
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Old 10-12-2019   #13
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Re: Another oil explosion

Quote Originally Posted by The Whitakers View Post
I was rebuilding a VW aircooled lump a number of years ago and it was clear that servicing of any definition had rarely been carried out. a full strip was being undertaken and low and behold the biggest pain in the butt thing was to get an oil relief piston out,
That surprises me and is another prompt to get the thing stripped.

I'm not actually sure how the Fiat 500 release-valve works but from the diagrams it looks more like a disc is pushed against the end of the camshaft by the spring. If that is the case I suppose the disc could stick in place.

I'll have to tear one apart some time and learn what's going on. I've never felt the urge to strip the oil-pump on any of my engines and also never suffered any problems in that area (yet!)....co-incidence???
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Old 11-12-2019   #14
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Re: Another oil explosion

I'm still wondering where the oil exited the engine?
OP said the oil filler cap and dipstick were still in situ, no mention of any breather pipe blown off, just that oil was coming out of the vents.
And as Toshi 975 has said, if a seal blows, it doesn't re-seal itself.

How does engine oil drain from the top of the engine?
How is excess crankcase pressure released?
Are these 2 functions using the same path? e.g. one of the 'pushrod' tubes?

With an engine of 499cc displacement, the pistons are pushing this volume of air upwards as they move from BDC to TDC, this creates a 'vacuum in the crankcase, so air has to be allowed to flow in. As the pistons move from TDC to BDC, they're attempting to push the same volume of air downwards into the crankcase - this 'pressure' has to escape. So, there's a movement of air in both directions through the crankcase breather system.

Now, if someone enlarges the engine capacity from e.g. 497 to 594cc the swept volume has increased by approx. 20% so the engine breather system now has to cope with a 20% increase in air flow. But can it?
Many have increased their engine capacity without encountering any breather problems, so presumably the breather system can cope with the extra air flow.

However if the oil pressure increases dramatically e.g to 8 bar, this will presumably pump extra oil up to the valve gear, oil which has to drain back to the sump. Could it be that the combination of extra crankcase air flowing upwards into the breather system while the extra oil is trying to drain downwards is too much for the breather system to cope with, resulting in these oil explosions?

I agree with others about excess oil pressure usually being due to a faulty oil pressure relief valve system (either valve stuck closed or relief passage blocked/restricted), but in this case it might be due to the modifications made to the oil pump to fit the uprated camshaft.

Some Fiat engine have the oil pressure relief valve mounted on the side of the cylinder block, so easily removeable to check, some have it built into the oil pump, iirc on the 500 it's on the crankcase behind the oil pump?

On most of these valves I've seen stick (open or closed) it was due to varnish/sludge build up, not wear or a faulty spring. And as the Whitakers has said, they can be a pig to remove - on the hollow piston type I've used a pair of external circlip pliers (i.e. the type that opens outwards when you squeeze the handles) or an expanding 'Rawlbolt' (concrete anchor).

Excessively high oil pressure is undesirable - afaik 10 psi pressure per 1000 rpm is sufficient. It can cause overheating of the oil, frothing of the oil (not good for bearings) and puts great strain on the oil pump drive components.
On any engine that has been run with excessive oil pressure being generated, it would probably be a good idea to carefully inspect all the oil pump drive components - e.g. I've seen spline drives shear in service some time after the original high oil pressure problem was rectified.

Al.
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Old 11-12-2019   #15
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Re: Another oil explosion

Quote Originally Posted by fiat500 View Post
That surprises me and is another prompt to get the thing stripped.

I'm not actually sure how the Fiat 500 release-valve works but from the diagrams it looks more like a disc is pushed against the end of the camshaft by the spring. If that is the case I suppose the disc could stick in place.

I'll have to tear one apart some time and learn what's going on. I've never felt the urge to strip the oil-pump on any of my engines and also never suffered any problems in that area (yet!)....co-incidence???

It was only because this particular lump was so gunged up internally that I did it as normally its something I do not strip out either...…..In fact when I rebuilt my current 650 lump I never stripped the oil pump assembly off either...…...food for thought there...…..
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