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MG MGB Technical - Rough running, fouled plugs, and white smoke

Here goes...

Information on engine:

A rebuilt cylinder head was put on in April 2002

Rebuilt carburettors were put on around Nov/Dec 2002

Electronic Ignition (Lumenition Magnetronic) was added in January 2003. Also, new ignition leads, rotor, and cap.

Compression is good in all cylinders. This was checked after the symptoms appeared and shows about 170 psi in each cylinder.

The bottom end was rebuilt at some point in its life as oversize pistons were noted (don’t recall what size) when changing the head. Otherwise, history of the engine is not known.

Information on symptoms:

For some time I experienced backfiring while descending downhill and using engine braking (mixture too rich?).

A couple of months ago I noticed that the engine was running a little roughly between 1000 – 1500 RPM. Otherwise, idle was fine and once underway it was not noticeable.

Then a couple of weeks ago I experienced a sudden loss of power and extremely rough running across the entire RPM range. It felt like the engine was only running on two cylinders. I tried a number of things, none of which helped (new rotor, cap, leads, fiddling with the carburettors). I also suspected the PCV valve might be somehow at fault but with the engine running a sheet of paper gets lightly suctioned onto the open oil filler on the valve cover which I understand indicates that it is working correctly.

The rough running problem was finally cured by putting in new spark plugs. The old plugs looked as follows: No. 1 and No. 4 were normal, No. 2 a bit more carbon than I’d like to see and No. 3 also had carbon but with a slightly “wet” (oily?) gloss. There is still a little bit of miss-firing and when I took out the new plugs to take a look at them after a test run they all appeared to have become quickly coated with more carbon than normal.

Now for the most worrying symptom that has not gone away. Upon hard acceleration in first gear I will sometimes get a cloud of white smoke from the exhaust. The strange thing is that it does not seem to do this consistently. It seems more prone to this after idling – for example at a stop light and then if I pull away hard I can catch a glimpse of the smoke cloud in my rear view mirror. This is something I’ve never had before. The engine has never burned oil but when I checked the oil level was at the “min” mark (however, this might also be due to the fact that it’s been six months since I last changed the oil).

Any theories on what caused all this and what problem I may have causing the smoke?

Thank you.
Boris

Boris,
This is just an idea, however your carbs may be set to rich. It's happened to me in the past and after leaning the carb out there was no more smoke. However, I'm sure you will get some great ideas from the BBS people in the know. Good luck Lee
LH Keyes

Hi Buddy,

Sounds like you will be replacing the head gasket soon. Cylinders 2+3 went first and now there is no compression,and the white smoke is really steam. Do you have any water left? Maybe the head is cracked somewhere. FWIW Alan
Alan

Boris. The spark plugs on cylinders two and three will, normally, show a somewhat richer mixture than one and four. Also, the excessive use of the choke can lead to more rapid fouling. Good compression across the board tends to indicate the head gasket is sealing the cylinders. When the head gasket goes between cylinders two and three, there is, commonly, low compression in those two cylinders. White smoke is usually an indication of coolant leakage into the cylinders. This can be caused by either the head gasket or cracks in the head. Leaking coolant, going into the cylinders, can also cause the spark plugs to mis-fire.

I think the first thing I would do would be to examine the level of coolant in the radiator, then have it pressure checked. While the system is pressurized, you can remove the spark plugs, then rotate the cylinders up to where you can see them using a small flashlight. Often, with a cracked cylinder head, you can see coolant on top of the pistons in such cases. Les
Les Bengtson


Could the smoke be brake fluid passing through the servo into the inlet manifold?
Boy Racer

Sounds very similar to my experience of a blown head gasket. I had a big backfire one day, and then a couple of months later I was driving when I lost power, engine was running rough and great plumes of white smoke (steam) were coming out of the exhaust.

On removing spark plug number two, steam was shooting out like a kettle boiling. The car was still drivable once the revs got going, but I got it towed anyway to my mechanic. Inspection revealed a failed head gasket.

Best of luck!

James
James

Smoke: black is fuel,blue is oil. white is coolant look at it carefully and you will see a tint Ric
R E L Lloyd

Compression can read good and you can still have a coolant leak into cylinders 2 and/or 3. I had the same thing happen recently. After checking compression and finding it good, I looked for the rough running problem everywhere else. Also did not have a significant loss of coolant, which leads one down the garden path right nicely. I finally did a leak down test and found much larger leakage from cylinders 2 and 3. As I was redoing the test, I finally heard the bubbles in the coolant recovery tank whenever I would pressurize either cylinder 2 or 3. Based on the above, I would guess that your are going to have to replace the head gasket. When you do, be sure to have the head and the top of the block checked for flatness and consider replacing the head studs with ones from ARP or other high quality studs.
Good luck - Dave
David DuBois

Thanks everyone for the suggestions. It certainly gives me new areas to investigate. It would have taken me a long time to suspect the head gasket given the good compression I have across the board. Sounds like I may have the same problem that David had.

So far my coolant level is still completely full. I installed a coolant recovery tank some time ago and almost never have to top up the coolant.

I usually remember to push in the choke as soon as the engine warms up so I don’t think that could be the main reason for the problem of fouled plugs.

I’ll try what you suggest Les and to look into the cylinders to see if there is any sign of coolant. I imagine some sign of coolant may show up in the oil pan and on the dipstick as well?

I sure hope that my relatively “new” head is not cracked and that it’s only a blown gasket.

Boris
Boris

Boris,

Water leaks into the engine usually result in very clean plugs - even spots of rust on them.

I would look at the brake servo first - fluid results in white smoke - disconnect the servo vacuum pipe and look inside it for fluid.
Chris at Octarine Services

Boris,

I had exactly the same symptons a few years ago and just as Chris said it was my brake servo leaking. Have you noticed the fluid in your brake master cylinder dropping lately? When I took the servo unit out it was full of fluid, this had been sucked into the manifold thus causing the white smoke.

Paul
PR Hughes

I should have mentioned – my car is a 1968 mgb roadster. There is no brake servo (vacuum assisted braking) and I don’t think brake fluid could have made it into the manifold otherwise. So that as a possible cause of the white smoke is probably ruled out.
Boris

Boris,
I have a '69 MGB-GT with similar problems. It smokes periodically. Generally, it only smokes after it has been driven a while. I notice it especially after I've driven it at say 45 - 55 mph constantly for a while and come to a stop. Starting out again from that situation is when I really notice it smoking more. I believe my smoke is more bluish in color. I am also using oil and my MGB is not marking it's territory (it's not leaking on the ground). I've decided the problem is probably the valve guides so I will probably have the head rebuilt soon. Plug residue is tan in color but a heavy build-up accumalates on them fairly regular. If you find out what's causing your problem please post results. FWIW mine has been back on the road about a year now after setting for 6 or 8 years. New valve seals helped but did not cure problem.
Paul
Paul S. Canup

Boris, there is a tool we can purchase on this side of the pond that reads exhaust gas presence in the radiator. It is a very effective test for a bad head gasket, even with the smallest of leaks. Place it where the radiator cap should be, and the blue fluid turns yellow if any exhaust gas is present. Everyone should own one!!!
Jeff Schlemmer

I think that the problem has been located. Chris’ comment that water in the engine results in clean plugs made me think that maybe it wasn’t water after all as my plugs were certainly not clean! The tint of the smoke was also an important clue. But this was hard to really notice when glancing in the rear view mirror while speeding away - the smoke may have had a blue tint after all. The suggestion about brake fluid getting into the manifold was actually heading in the right direction – but as it couldn’t have been brake fluid it must have been something else. The something else was indeed oil, and it was coming through the PCV value which had a small rupture in the diaphragm. So it looks like my problem was that oil was periodically being sucked into the manifold through the hole in the PCV valve diaphragm. This caused the inconsistent bouts of smoke and eventually fouled the plugs causing the rough running. The PCV valve diaphragm has been replaced by my local mg specialist and so far no more smoke. He also mentioned the test that reads exhaust gas presence in the radiator. I’ll have to see about getting one of these for my diagnostic collection.

Well, I hope that’s the end of this problem. So until the next one, thanks to all again for your suggestions.

Boris
Boris

Vapours are supposed to be sucked from the crankcase through the PCV into the inlet manifold, that is how the Positive Crankcase Ventilation works, but a ruptured diaphragm could well cause excessive suction which might suck droplets of oil up as well.

I have used the exhaust gas leak detector, in fact someone from California brought it over for me on a visit and I'm just about to send it back! I wasn't able to find anything like it in the UK, if you were closer you could borrow it.

Paul S - worn valve guides/valve stems cause blue smoke when blipping the throttle after idling for a while, I can imagine that new seals can help to reduce it for a time.
Paul Hunt

This thread was discussed between 17/11/2003 and 25/11/2003

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