HELP! Blew oil cap sky high...

[02Anders]

Hello all,

 

Pardon my absence from the forum - life has simply been too busy lately.
For that same reason, my Verona red '73 BMW 2002 has also been stood for a while.  Drove her about 1000 miles in early November after which I treated her to an oil change and tucked her away for winter.
Yesterday the sun was shining and I was in need of a bit of automotive therapy, so went to the garage, checked all fluids and connected the battery.  It's a stock 2-liter with the downdraught Solex and the mechanical fuel pump.  As always when she's been stood for a while, I gave her just a tiny amount of fuel straight into the carb to help her fire up.
Next thing that happened gave me quite a fright and is something I've never previously experienced - nor have I even heard of it happening.
Turned the ignition key and the engine turned over fine but just didn't catch.  Tried again a couple of times and then a huge POP! from the engine and the oil filler cap was blown right off the engine so violently that it hit the ceiling of the garage with quite some force!!  There was no backfire through the exhaust, just the loud POP from the engine bay.  Needless to say, I'm now worried if it's caused any internal damage, so I didn't try to turn the engine again...

 

Clearly, there's somehow been a build up of pressure within the engine and it's let go at the weakest point i.e. the oil filler cap.  Can the tiny amount of fuel I poured into the carb somehow have worked its way into the crank case and ignited down there?  Has anyone in here ever experienced something like this before?

 

My thoughts are to change the oil and oil filter - both the see whether any small metal parts come might come out with the oil (I sure hope not!), but also to ensure that there isn't any volatile fuel left in the oil and crank case.  Then I'll do a compression test.  While I'm at it, I'll obviously check the spark plugs and if necessary fit 4 new ones.  At this point, if all is well, I guess I'll tempt fate and try to start her again...

 

For the record, the engine has about 80,000 miles on her and in the 29 years I've driven her, she's always been the most reliable car I've ever owned!  The engine is - or was? - the sweetest M10 I've ever had the pleasure of driving.

 

What say you guys?  Have any of you heard of or experienced this happening before?  And do you have any further advise that I should consider?
All input is much appreciated as I'm a little worried right now.  It's been such an excellent engine, I really hope I don't have to start taking it apart...

 

 

 

[Conserv]

It’s always good to hear from you, Anders, but I’m sorry to hear the reason for your query! ? 

 

I, admittedly, haven’t a clue. But I’ve never allowed my cluelessness to interfere with my life! Thus, if it happened to me, I’d probably pull off the coil wire and verify that the engine turns over normally with the starter. If that went normally, I’d probably reconnect the coil wire and try starting it again, perhaps with a rag or tape over the oil filler aperture. And I’d be ready to shut down if I heard anything out of the ordinary. My reasoning would be that if any real damage was done by the explosion, it’s already occurred.

 

But.... before you take this as meaningful advice, please know that I wouldn’t hesitate to simply crank over a 20-year-dormant engine, as long as there was oil in the crankcase. So I’m a madman compared to many on this forum... ?

 

Best regards,

 

Steve

 

[dlacey]

Crankcase is normally full of oil mist and combustion products (very little oxygen). In this case, with the engine standing for some months, I guess air could have permeated the crankcase. A dash of fresh fuel down the carb throat could've got into the crankcase breather tube (is there a flame trap in that line?). Then as you cranked it there was a misfire that ignited the mixture in the inlet manifold, breather and crankcase??? Never heard of blowing the filler cap off any engine before...

 

As for next steps, I wouldn't worry...these engines are bulletproof: just keep cranking until it coughs to life and let it run to regain its composure.

[02Pilot]

Pull the crankcase vent line off the valve cover before you proceed (assuming you've replaced the oil filler cap). After that, I'll add my vote for crank it over and see what happens. If something significant broke you'll know quickly.

Edited February 1, 2021 by Andrej

[Schnellvintage]

I would remove your spark plugs in addition to unplugging the positive wire on the coil and crank over the engine to hear any odd sounds before trying to fire it up again.  At the same time do a cold compression test..   

 

[02Anders]

Thank you Gentlemen for your thoughts and suggestions.  Much appreciated...

 

I'm probably just being overly cautious here, but I'm worried that the explosion might have damaged a piston ring.  After all, it was enough to blow off the oil filler cap!!  If a piston ring is indeed broken, bent or damaged, cranking the engine could scare the cylinder wall.  Then I'd suddenly be in real trouble!
At the same time, I must confess that I don't particularly fancy the thought of dismantling the head and turning the engine by hand while watching the pistons slowly moving up and down...

Edited February 1, 2021 by 02Anders

[Einspritz]

I offer this: The gas fumes were sucked into the vent line into the valve cover and to the top of the head from the air cleaner; when the backfire occurred the flame front moved into the head area and an explosion occurred, popping off the cap.

 

That said, I don't think anything drastic has happened.

 

Speaking from experience on backfires wherein my Brother was severely burned from a backfire as it traveled up into the can holding the gas being poured down the intake, the vapors exploding in the engine compartment, I can say "don't do that", or do that cautiously. (not "hold my beer, I'll take care of this")

 

AND have a fire extinguisher handy right next to the car, preferably similar to Halon or Halothane, and that you know it is serviceable.

 

So, pull the plugs, crank the engine over a few times to make sure the cylinders are vented and the plugs aren't too wet, then try again.

 

Perhaps using spray engine starter (Ether) might be more useful as it is already a vapor even in the cold.

 

3 hours ago, Conserv said:

But.... before you take this as meaningful advice, please know that I wouldn’t hesitate to simply crank over a 20-year-dormant engine, as long as there was oil in the crankcase. So I’m a madman compared to many on this forum... ?

 

Yup.

[tzei]

You may have popped crank seal(s) or not. This will be evident soon enough when you get it running.

[Hans]

Are any parts missing from te cap? I'd at least pull the valve cover and see what's in there. Very odd. That's a lot of pressure. Is the vent tube clear? You'd think that would pop first. Then a compression test.

[visionaut]

Hi Anders. Great to hear from you, sorry about your difficulty...

 

I agree with Ein, in that the explosion most likely occurred under the valve cover to be able to blow the oil cap off... so unlikely you’ll find damage down below.

 

It could be as simple as the vent line working in reverse, and enabling a carb/air cleaner backfire to exhaust into the valve cover.

 

But I’d also look into whether you might have a slow fuel leak from your carbs, which might allow for fuel vapor to end up accumulating up within the cylinder head, trapped under the valve cover.

I would have thought your vent line should have been able to vent any accumulating vapor to atmosphere out the air cleaner.

 

If there was combustion under the valve cover, I’m surprised it popped the cap versus exhausting out the vent line...the typical path of least resistance. The oil cap is twisted on and retained by its flanges - to blow off means it had to be pressurized enough to also then turn to pop off (unless it wasn’t on tight, post oil-change), so I’d also check if the vent line is OK.

 

Like others, I wouldn’t be afraid to crank it again after doing a quick run-thru check of fuel/spark/timing.

 

Good Luck!

 

Tom

 

 

Edited February 1, 2021 by visionaut

[MitchaPaLoOza88]

Just going to think out loud for a second.

 

So the pistons and the valve cover space (volume) are pretty well sealed off from each other. The only direct way for the two volumes to mix would be if you have like a massive piston/ring failure (air/fuel blows down). Otherwise the intake and exhaust valves are the only in and outs for the air in that space. I would put a piston failure as a fairly low probability from a car that "ran when parked" responsibly.

 

How is your Valve Cover Vent routed? If it goes to the intake, sometimes you get like the cough cough backfires that don't ignite and then you get the bang. The cough backfires may have pushed a little fuel air in the valve cover if its connected to the intake and then when the backfire ignited. (Pop) Oil cap imprint on the ceiling.

 

 

Side story: My buddies Silverado 1500, he wired the dizzy wrong. When he went to start it, loud bang and a few plugs and whatnot flew off the engine.

[jimk]

5 hours ago, 02Anders said:

I gave her just a tiny amount of fuel straight into the carb to help her fire up.

It got more than just a tiny bit of fuel.  Don't need more than a teaspoon full to get things going.

[Conserv]

8 minutes ago, MitchaPaLoOza88 said:

 

... Side story: My buddies Silverado 1500, he wired the dizzy wrong. When he went to start it, loud bang and a few plugs and whatnot flew off the engine.  And then he took the 1500 out and plowed 18” of wet snow...

 

Fixed that for ya... Well, that’s what I’d do... ??

 

Best regards,

 

Steve

 

[dang]

Although I doubt yours is a major problem, we got in a Mercedes to our yard one time that the owner said had a bad head gasket.  We cranked it over and the filler cap blew off, put a dent in the hood and flew about 50 feet.  We were lucky no one was standing in it's path.

 

My guess is cylinder pressure over an explosion. 

Edited February 1, 2021 by dang

[TobyB]

Yeah, I was going to guess that some mud dauber wasps (or something like that) made a nest in

your crankcase vent, and then blowby blew by.

 

BUT that would have caused a leak, not a bang.

 

So, yeah, let's go with a mild backfire firing back into the valve cover, AND just the right

fuel- air mix in there to ignite.

 

It's not going to hurt much- your engine does this ON PURPOSE in the combustion chambers 

ALL THE TIME, so pistons, rings, etc, etc are fine.  Seals- well, maybe, but the cap blew off first, so all 

the energy went out as flow-I suspect actual pressure in the crankcase never got all that high.

 

UNLESS there was fuel vapor distributed throughout the crankcase, (even more unlikely than the initial explosion,

as fuel/air only burns between 9 and 20:1, and it take GALLONS of air to light off a TEASPOON of fuel.

So the explosion is SEVERELY oxygen- limited.  

What you probably would have seen on a super- high- speed camera was a small flash as the lid came off,

followed by a big flame as the vapor came out, finally got enough air, and combusted.

 

I'm in the 'so not a big deal' camp on this one.  If you want to be cautious, change the oil

in case fuel's leaked into it in storage and diluted it, but honestly, I let cars sit 6 months and don't even

think  about it besides making sure the battery's not flat.

 

Have at it- you'll be fine.

 

t

confidently