You are here:

MG Car Repair/Gas in Oil?


QUESTION: hi Barrie - I have what I suspect to be a vacuum leak or exhaust manifold leak on my MGB - that is, I can hear hissing when the throttle opens. I changed my oil after just 1000 miles and it was pretty black and dirty, smelled of gas. I have no flooding issues, or overheating, and compression is 140 psi all cylinders, but I do notice exhaust smell when idling in place.... Are there some tests that can tell me if I have a leak or gas is getting into oil sump?   
thanks PW

ANSWER: Hi Paul.

I cannot see any connection between the hissing noise and the smell of gasoline.  I think you have two separate problems.

I would start by checking the oil level on the dipstick.  If it is ABOVE the full mark and the fresh oil smells of gas, then you have a problem with gas in the oil. The fuel tank on the MGB is below the level of the engine oil, but if you park `nose down' on a steep slope, then gas can syphon from the tank via the front carburettor and into the engine. It is a known design fault. The solution to this problem is to make sure that you always park 'nose up' on a steep hill.

To find an exhaust leak I sometimes use an old tennis ball to block the tail pipe with the engine running.  If there is a leak then it will hiss very loudly and you will soon find it.  The manifold-to-exhaust-pipe seals often leak on an MGB.

It could be that you have an air leak at the intake side of the engine, usually one of the carburetter gaskets.  I would use an emissions analyser to check the CO2 level in the exhaust.  If you don't have access to a test machine, then go to a local garage and ask them to check the emissions for you.  The CO2 level should be well below 4 percent.   

---------- FOLLOW-UP ----------

QUESTION: thanks Barry! as matter of fact I do park nose down and will reverse it. As for the exhaust manifold seal - I will try your tests and if it is leaking, would you recommend using any of those epoxy sealer products? or better to remove and replace gaskets? ( I am nervous about removing those bolts without damaging the studs )

If you stop parking nose down then the gas in the sump will evaporate when the engine gets hot, so you may find the oil level drops.  Keep an eye on it.

You cannot re-seal the manifold rings satisfactorily once they have blown.  Do it properly.  If the rusty steel nuts are frozen, cut one face off them with a small hacksaw or the edge of an angle grinder, then remove the C-shaped piece that will be left behind.  Fit two new exhaust rings with exhaust sealant paste, then re-fit the exhaust with 6 BRASS nuts (5/16 ANF).  You will never have trouble undoing them in future.

MG Car Repair

All Answers

Answers by Expert:

Ask Experts


Barrie Jones


MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.


I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

MG Car Club.

Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

M Phil, C Eng, BSc (Eng).

©2017 All rights reserved.