What Causes Petrol to Leak from the Exhaust

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George
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#1 What Causes Petrol to Leak from the Exhaust

Post by George » Sat Aug 15, 2020 4:31 pm

Hi Everyone,

I have a 1972 S3 Roadster which I imported from California, I have converted it to RHD and just completed a full restoration save for the bonnet that will shortly be resprayed.
The car starts first time with a little choke but I am getting fuel dripping from a joint in the exhaust, it is definitely fuel because the spill evaporates after 10 mins. I have not run the car for more than a couple of minutes due to this leak.
I would be most grateful if anyone can offer any causes and solutions for this problem please.
Many thanks
George
George
1972 S3 V12 ROASTER

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jagwit
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#2 Re: What Causes Petrol to Leak from the Exhaust

Post by jagwit » Sat Aug 15, 2020 9:01 pm

If the engine is running on all cylinders it CAN NOT BE FUEL.

It CAN, and MOST LIKELY IS water that condensated to the cold exhaust pipe.

Hot humid exhaust gas results in moisture condensating against the cold exhaust, causing droplets to form, which gets blown along the exhaust until puddles form, or it gets blown out the back, if you rev up the engine.

It may even smell of fuel because with a cold start, the mixture is very rich.
Best Regards
Philip
Jag: 72 E-type V12, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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George
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#3 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Sat Aug 15, 2020 9:29 pm

Thanks for the reply Philip, when I first spotted the leak I was hoping it was water vapour but alas water vapour doesn’t evaporate as this does.
George
1972 S3 V12 ROASTER

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bitsobrits
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#4 Re: What Causes Petrol to Leak from the Exhaust

Post by bitsobrits » Sat Aug 15, 2020 9:46 pm

I agree with Philip that if the engine is running well, the leaking liquid cannot be fuel.

For fuel to leak from the exhaust, it would have to be coming from a very flooded carburetor, with the result that the three cylinders fed by that carb wouldn't/couldn't be firing at all.

You don't mention if the leaking fluid smells like fuel or is flammable, which is a better test than rate of evaporation. And you need to consider that condensed water leaking from an exhaust joint would be quite warm and therefore would tend to evaporate more rapidly than water at ambient temp.
Steve
'65 S1 4.2 FHC

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#5 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Sat Aug 15, 2020 10:05 pm

Hi Steve - yes the leak does smell of petrol but I have not tried to ignite it, probably will tomorrow. The leak is barely even warm because as I said I have only run the engine for a couple of minutes and if it was water it would take a lot longer than 10 mins to evaporate it.
George
1972 S3 V12 ROASTER

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lowact
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#6 Re: What Causes Petrol to Leak from the Exhaust

Post by lowact » Sat Aug 15, 2020 11:31 pm

If it is petrol, maybe a fuel line leak onto the outside of the exhaust, dripping off when it reaches the joint?
Regards,
ColinL
'72 OTS manual V12

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#7 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Sun Aug 16, 2020 10:16 am

Hi Colin - no its definitely not a fuel line leak.
I have now collected some of the leak into a small paint can lid and ignited it and it does burn. However when I ignited it whilst at arms length wearing welding gloves I expected it to burn with a whoosh (as petrol does) but in fact it just had a slow burn with flames about 2 or 3 inches high. The liquid is black and my 1st thought was that it could be oil but it does not have the consistency of oil and obviously oil does not evaporate as this does.
If anyone has any ideas on what to do next I would be most grateful.
George
1972 S3 V12 ROASTER

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Heuer
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#8 Re: What Causes Petrol to Leak from the Exhaust

Post by Heuer » Sun Aug 16, 2020 11:01 am

Anti-freeze?

"I therefore chose to conduct testing to determine if pure antifreeze or antifreeze mixed with water in a 50% solution (coolant) would ignite or burn. I found that both would auto-ignite (ignite outside the presence of a spark or flame) on a hot surface. Surface temperatures of 650 degrees F to 750 degrees F would cause auto-ignition. Vapors at lower temperatures would ignite from a spark or flame. Although the concept of a water-based fluid being flammable may be difficult to comprehend, one must recognize that coolant is actually glycol based and only mixed with water. Raised to a high enough temperature, the glycol is still flammable even in the presence of water vapor”.

https://www.jcmmachineandcoatings.com/u ... r%E2%80%9D.

Suggests it could be a blown cylinder head gasket.
David Jones
S1 OTS OSB; S1 FHC ODB

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George
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#9 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Sun Aug 16, 2020 11:14 am

Hi David,
Thank you for your input. Firstly I am amazed that anti freeze burns and without your article I would never have thought of that, so it looks like a head gasket change for me.

Many thanks.
George
1972 S3 V12 ROASTER

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christopher storey
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#10 Re: What Causes Petrol to Leak from the Exhaust

Post by christopher storey » Sun Aug 16, 2020 11:25 am

George : much too fast, if I may say so, particularly as head gasket changes on the V12 are quite a complicated job. First of all, have a look at all the plugs on the affected back . Then, check the compressions on the affected bank . Then, get one of the coolant testing kits which show the presence of exhaust products in the coolant , and check the coolant. If these show an HGF then a change is called for. If not, then you need to look elsewhere, and certainly I would then do nothing without having run the engine fully up to temperature for a significant period

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jagwit
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#11 Re: What Causes Petrol to Leak from the Exhaust

Post by jagwit » Sun Aug 16, 2020 11:31 am

bitsobrits wrote:
Sat Aug 15, 2020 9:46 pm
For fuel to leak from the exhaust, it would have to be coming from a very flooded carburetor, with the result that the three cylinders fed by that carb wouldn't/couldn't be firing at all.
Just to be clear, the two carbs on one bank feeds into a common plenum from where the two branches feed the front 3 and rear 3 cylinders. So the rear carb does not EXCLUSIVELY feed the rear 3 cylinders and likewise the front.
Best Regards
Philip
Jag: 72 E-type V12, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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jagwit
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#12 Re: What Causes Petrol to Leak from the Exhaust

Post by jagwit » Sun Aug 16, 2020 11:52 am

christopher storey wrote:
Sun Aug 16, 2020 11:25 am
would then do nothing without having run the engine fully up to temperature for a significant period
The only fluids in the car is :
- Coolant
- Fuel
- Engine oil
- Gearbox oil / ATF
- Brake fluid

If its not fuel and its not (combustible) condensate, it MUST be one of the others.

I recon the next thing to do is find which of these it is. Engine oil, ATF and EP90 all have unique smells. Brake fluid also but is perhaps not so obvious as the others.

Just to be clear, are you 100% certain it comes out from inside the exhaust? IOW, it does not originate from somewhere ALONG the exhaust or from above it?
Best Regards
Philip
Jag: 72 E-type V12, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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George
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#13 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Sun Aug 16, 2020 12:00 pm

Philip - yes I am certain it is coming from the exhaust. There is a joint with a clamp just before it enters the 1st exhaust box and I can see it coming out of one of the slots in the pipe. There is no trace above this slot nor anywhere else on the pipe or near the pipe.
George
1972 S3 V12 ROASTER

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#14 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Sun Aug 16, 2020 12:08 pm

Chris - I did answer your post but it seems to have disappeared into the ether. I will follow your advise but it will take me a while because I will have to purchase a compression tester and the coolant test kit.

I don't really want to run the car up to temperature because as soon as the exhaust gets hot I will have a major fire.
George
1972 S3 V12 ROASTER

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#15 Re: What Causes Petrol to Leak from the Exhaust

Post by AussieEtype » Sun Aug 16, 2020 12:48 pm

If the engine runs and the fuel is coming from the exhaust really the only place it can come from is a rich fuel mixture and it is unburnt fuel coming out the exhaust ports - maybe not too rich that it stuffs up the engine running but maybe similar to the choke being on a bit. My guess is that the excess fuel mixture is condensing out as it cools and is pooling somewhere.

However if this is happening, the fluid will be burnt oily black colour as it is partially burnt. If the fluid is clear then you have a simple good old fashioned oil leak that is dripping down onto the exhaust.

In either case I would get it fixed or carry a a big fire extinguisher with you.
1971 Series 3 E-type OTS

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#16 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Sun Aug 16, 2020 3:37 pm

Just had my son in law visit who is a car guy and he agrees it is definitely petrol coming from the exhaust but we are now thinking it is something to do with the vacuum lines that is making it too rich which is inline with some of the comments I have had.

I removed all the a/c equipment and the emissions stuff from the car during the rebuild and so I am now thinking it could be something open or missing on a vacuum circuit so I will check this out whilst waiting for a compression tester.
George
1972 S3 V12 ROASTER

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Bob.
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#17 Re: What Causes Petrol to Leak from the Exhaust

Post by Bob. » Sun Aug 16, 2020 4:38 pm

Hi George
Vacuum issues will only lead to weak mixtures, not rich. Have you checked that none of the carb float needles are sticking open. Remove air cleaners, turn ignition on to run fuel pump and look into each carb. (Don't start engine). Is fuel visibly squirting up from a main jet ?
Bob
'71 S3

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#18 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Sun Aug 16, 2020 4:44 pm

Hi Bob,
Many thanks for your suggestion I'll try that tomorrow and report back.
George
1972 S3 V12 ROASTER

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#19 Re: What Causes Petrol to Leak from the Exhaust

Post by andrewh » Sun Aug 16, 2020 6:36 pm

I would bet on a hugely rich mixture somewhere. I had exactly this on my pre HE XJS and it turned out to be a couple of injectors stuck open. Because of the smoothness of the V12 its really hard to identify a few pots being under par. PLUGS! that should be the tell tale. I ended up buying a set of top of plugs testers and worked my way around the engine to find poor sparks as well. One other thing to be aware of is the different emission fitments of a California V12 to a UK 12. If you have got rid of the excess emission pipes and fitments you may have not done the conversion quite correctly. I know from my XJ12 on Strombergs, that there are lots of bits that are not common between countries and various pipes blanked off etc etc. A good look at the parts book should help.
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#20 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Sun Aug 16, 2020 6:56 pm

Hi Andrew,
Your suggestions are gratefully received and I will check them out tomorrow.
Many thanks
George
1972 S3 V12 ROASTER

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