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MG MGB Technical - Back pressure? Coming out of oil filler cap.

My 1979 MGB with unknown year of engine (possibly a 1975 from some other application other then an MGB. It is however an 1800 B series engine). It has started blowing oil out of the oil filler cap. I have an aftermarket Aluminum valve cover, it has no vent pipe. (The original steel valve cover did not have a vent pipe either). Anyway, lately it has begun to spew oil out of the filler cap vent hole. When I rev the engine I can actually see and feel with my hand a stream of oil/air mist shooting up from the small vent hole in the filler cap.

Does anyone have ideas on what the most likely cause of this back pressure is?

Thanks,
John Fraioli
John F

Three possibilities; the venting arrangement on the port side of the block (one of two panels)is choked, or the vent is not blocked and the engine is blowing more gas down the side of the pistons than it should or thirdly, that some of the mundungus fitted to please US authorities (and no one this side of the pond really understands) is not working as it should.
HTH
Roger

John-
It could be simply that the vent hose from the front tappet chest cover to the induction system has come loose. Check and tighten all of its connections.

Of course, it may be more complicated than that. Sometimes the oil separator inside of the front tappet chest cover becomes clogged and causes the problem that you describe. Remove it and wash it out thoroughly, then reinstall it. You'll need to remove the manifold in order to get at it.

If that's not it, then it's probably blowby. Run a compression check. The readout of pressure for all of the cylinders should vary by no more than 10%.

Steve S.

Steve,
It may well be the blowby you speak of. I have driven this MGB on many long cross country trips these past 4 years. The car had been running poorly for the last couple of trips. I could not figure out what was causing the lack of power(I was thinking it was either carburetor or ignition but could not find any problems in those areas). After returning from a long trip last March it was all I could do to get it home. I thought maybe I had worn the cam by not using oil with ZDDP. So I decided to check the push rod height. All the rockers seemed to be raising as they should so I then checked the valve lash clearences. But, before measuring the clearences I decided to do a compression check. I found that I had zero compression on cylinders 2 and 4. Then when checking the valve clearences I found that those clearences on 2 & 4. Had tightened up, so that those valves were not closing completely. After adjusting the valve lash, I got number four's compression almost back to the reading of 1 & 3, I think number four came up to about 110 lbs. 1 & 3 was around 120 lbs. but mumber 2 only came up to around 40 lbs. The car started and ran better then it had for quite some time, so I was rather happy. I concluded that the valve on number 2 may be pitted, and I was hoping that it might correct itself thru use. In the mean time I would continue to enjoy driving the car until such time as I decided to remove the head and have the valves done. Other symptoms that I have been made aware of by fellow MG drivers behind me when taking spirited runs around twisty roads, is that I am blowing puffs of smoke quite a bit when downshifting.

So my question to you now...Is this blowby you speak of caused by worn rings or could it all be because of the possible pitted/bad valve that I have.

I did write down all the exact compression figures (before and after the valve lash adjustment), but I would have to dig around to find them if you need to know the exact numbers.

I do have a vented valve cover hanging around. Would it be helpful for me to put it on and plumb the vent pipe somewhere down below the car to at least keep the oil mist(in the mean time)from spraying up onto the underside of the bonnet and running onto the top of the engine?
John F

John-
The puff of smoke that results from downshifting is the result of oil being drawn past the valve stem into the intake ports.

Concerning the variation in valve lash clearance, you may have a case of worn tappets. The bottoms of the tappets should have a .004" dome. If this dome wears away, then the layer of hardened material may be completely gone, resulting in an uneven bottom. This will result in inconsistent valve lash measurements. To check for this without removing the tappets, take several measurements after rotating the crankshaft several times.

Blowby is caused by combustion gases leaking past the piston rings, not by a leaking valve. A leaking valve would not pressurize the crankcase.

Installing a rocker arm cover that has a vent pipe and running a tube under the car will vent the gases out of the engine compartment. However, did you check to see if the tube from the front tappet chest cover is hooked up to the induction system?

It always helps to know exactly which engine we're dealing with. The code on the aluminum identification plate located on the distributor side of the engine will tell us what we've got on our hands. If the engine identification plate is missing, there is a way to date the age of the engine block. On the right hand side of the engine block, in the area between distributor and oil filter, there are three numbers that form a circle and that are slightly raised, e.g., 30 12 71, which tells the day, the month, and the year during which it was cast (in this case 30th day of December, 1971). The top one is the day, the left one is the month and the right one is the year. In the early 1970s the month code changed to a letter, thus 12 G 3 would be 12th day of July 1973.
Steve S.

My guess is that you are looking at a ring job. RAY
RAY

Steve,
The aluminum engine ID plate/tag found rivited in the usual place has the ID# DBL 230E 52802 (see MG Driver Magazine Vol.15 No.5 for a picture and a small blip about my engine, in the picture you will see my after market valve cover..... The color of the engine is blue, I am quite certain by looking at the paint that this is the color it was originally painted. This is a mystery engine, it more then likely came from some other british application (mabye even marine or tractor). It has the 1800 cast into the block in the usual place. The valve cover that came on the engine(same blue color) is unvented. The circular date stamped into the block in the area you mention has (I believe) a 13 at the 12 O'clock position, a 3 to the left at 8 O'clock and a 5 at the four O'clock postion. I believe the engine was made March 13, 1975. The 45D4 distributer has the number 41599 a rare dizzy according to Paul Hunt (at least he originally did not have an advance curve for it, he does now). This corresponds also to '74-75 US and '74-76 Canada. I believe my car was imported to Canada because it only had one electric fan instead of two. When changing out the clutch 9 years ago, I also removed the back end plate to change the main rear seal. The gasket set I puchased to put the back end plate back on did not correspond exactly as it should have so I had to improvise a little. Also the oil drain plug is a little different than a standard MGB oil drain plug.

I have owned the car for the past 16 years, I have no history on the car prior to my purchasing it from a Japanese college student studying here in Denver. I think he purchased it from a used car lot and owned it for almost a year.

I do have a hose connected from the front tappet cover to the port on the ZS carb close to where the fuel entry is. The hose is a little cracked where it connects to the tappet cover port, I think I will replace the hose.

Ray,
I don't think the rings are that bad as I ran the compression check dry and wet and came up pretty close to the same readings.
John F

"Then when checking the valve clearances I found that those clearances on 2 & 4. Had tightened up, so that those valves were not closing completely. After adjusting the valve lash, I got number four's compression almost back to the reading of 1 & 3, I think number four came up to about 110 lbs. 1 & 3 was around 120 lbs. but number 2 only came up to around 40 lbs."

With the valve clearances changing that much you probably have some valve seat recession. The low compression reading on #2 is probably from a burned valve. See this article from Sean Brown for more information on valve seat recession.
http://www.flowspeed.com/seat-recession.htm

Actually your compression isn't good, it's on the low end unless your engine has a very lower compression ratio than the low compression MGB engine. I have owned some engines that had good compression readings, yet the rings were badly worn. A compression test isn't the best test to check the health of an engine. A cylinder leak down test provides more information.

Based on the information you have provided I would would pull the cylinder head to inspect for a burned valve and seat recession. I would also remove the valve lifters and check their condition, if they are bad you will also need a new cam. I agree with Ray you probably also need a ring job.

Clifton

Clifton Gordon

Every MGB has *two* passageways into/out of the crankcase, both these would have to be blocked for 'normal' piston action to be able to blow seals or oil out. If the rocker cover doesn't have a vent port it should have a ventilated oil filler cap as that is what allows fresh air, filtered and restricted, into the crankcase. Extraction is either by PCV valve (Feb64-Sep68) or carb suction (Oct68 on). Before Feb64 it was a very hit and miss non-positive system that just had one pipe dangling down by the bottom of the engine and another from the sie of the rocker cover to the front air-cleaner.

If you have excessive blow-by then it is going to be difficult to determine if the PCV system is functioning or not, except by disconnecting the pipe from the carbs (in this case) to the front tappet chest to see if you can feel any mild suction on it. But the filter and pipe in the tappet chest cover could still be blocked. Other than that you would need to do a dry and wet compression test. Big differences between dry and wet point to rings/bores.
Paul Hunt

John,
The puffs of blue smoke are a fair indication of the state of your engine as is the change in rocker clearances. If this was a standard HC MGB engine, the compression readings would be further indication of woes but the engine came in a range of configurations for different applications. Yours might have come from a Wolsey or Austin Maxi but equally if you car's original engine failed, the spares organisation might have supplied the first that they could get their hands on. Inspection of the head and valve gear will answer most of your questions but you can be sure that the pistons are showing their age.
Roger

OK, I still need help with this problem. First, let me say some of you mis-understood what I was saying about the valve lash clearance. It did not change on me. I just meant that I had not checked it for many years and last march when I got back from a long trip the car was running very poorly and when I checked it. I had zero compression on two cylinders and the zero comppresion was a result of two of the valves not closing completely because clearance had closed up on two of the valves. After adjusting the valve lash the number 2 cylinder came back to pretty good compression and number 3 improved from zero to about 40 lbs. So that gave me fair compression on 3 of the cylinders and at least some compression on the other one. This made a drastic improvement on the way the car ran. I did do dry and wet compression checks and there was no significant differences between the dry/wet readings.

So back to the backpressure problem. Last week I un hooked the hose that goes from the carb to the front tappet vent pipe. I unhooked it at the carb and blew into the hose, I held my hand over the opening in the valve cover oil filler cap and felt air streaming out (just the way it does when the engine is running). So I am assuming that things are clear through the tappet vent tube?

Today I decided to check the compression again, when I took the plugs out they looked like I had never seen them before. They were black (I have seen them black before) but more important the electordes were very corroded, rough and pitted, some white spots and almost entirely closing the gap with the corrosion.
I have been lead to believe that when the crankcase doesn't breath properly that moisture can build internally. I am wondering if this is what has caused the plugs to look so pitted and corroded?

I've been reading on another current tread about crankcase ventilation and I wonder if I would get better venting if I connected the front tappet cover vent tube to a manifold vacumme port using a generic PCV valve instead of to the ZS carb? Or could there be a plug in the ventilation system some where else in the engine?

I could not get the compression check done today as my
Compression gauge is not holding (working) properly.

Up until recently this engine was not been using or loosing any oil. Now it is blowing it out through the vent hole in the oil filler cap.
Thanks for any help,
Please try to explain what the problem could be in simplest terms possible if you would. I get confused when it is too technical.
John Fraioli
John F

Plugs as you describe are normally a result of water getting into the combustion chamber, from a leaking head gasket or cracked casting. Cracked castings frequently result from abusive overheating, like from exhaust valves with no clearance. As Clifton said, the closed up clearances indicate seat recession, nothing else closes clearances.

Bad venting causes moisture build up in the crankcase, not the combustion chambers. "White spots" on black plugs is usually aluminum from a melted piston - that would certainly account for the blowby.

ZS carbs will run extremely rich under several conditions, namely loose chokes and bad diaphragms. Either will dilute the oil with fuel, wearing stuff (like rings and pistons) out drastically and causing oil to go everywhere; it also makes the plugs black.

Quit wasting your time with wishful thinking - at least the head is toast, the gasket is probably leaking at least. Once the head is off, check the cam and tappets and cylinder bores at a minimum. Get someone who actually knows what they are doing, because you are in over your head - no offence meant.
Good luck!
FRM
FR Millmore

FM,
Thanks for the candid comments. I understand what you are saying. It is not really wishful thinking on my part. I am willing to do what ever is necessary to get my car running good again.

I like working on my MGB and have been doing so for 17 years. I am not a young man (57)and I have a thirst for mechanical knowledge. I will continue to work on my car as a hobby till the day I die, and I hope to get better at diagnosis and repairs as I grow.

I have some additional info to post as I have been working on the engine these past two nights. The car was still running rather strong on Sunday, but the back pressure was still concerning me. So I thought I would swap the valve cover out to one that I had that was vented, and just run a hose from the vent pipe down below the car. During the process, I thought I would remove the plugs and do another conpression check, since it had been 6 mos since I last checked it. I bought a new compression check gauge, since the other one had quit working after 32 years. These were the readings I got. last night. Number 1 cylinder 132 dry, 140 wet. Number 2, 40 dry, 90 wet. Number 3 132 dry, wet 177 &number 4, 98 dry and 115 wet.

So, I then check the valve lash. I last adjusted it last March. Thought I would check it while I had the valve cover off. I went thru and all the valves and made sure they were set to .016 (cold). Then put the valve cover on and went to start the engine. Very hard to start and when it did fianlly start with lots of starting fluid I had had very severve lack of power and backfireing both exhaust and carb. It idled pretty smooth, but backfired when reved. Backed the car out of the garage to try and take if for a test run. could hardly move forward without dying and it was all I could do to get it back up the slight incline back into the garage.

I don't know what I could have done to make it run so poorly when it had lot's of zip on Sunday. I also took the dip stick out while it was idling and saw oil coming out from there as well. I understand blow-by would cause oil to come out the valve cover oil filler vent. But is it not a different problem causing back pressure to come out the oil dip stick tube?. I know I have a compression problem in number 4 and worse in number 2 cylinder, but I would like to know what is causeing this backpressure before I precede to have the engine rebuilt.
John Fraioli
John Fraioli

John..
The oil coming out the dipstick hole means to me that your engine is on it's last throws. The rings on the two cylinders that are low are most likely history. My suggestion would be to remove and rebuild ASAP. Continued flogging will lead to more serious problems such as broken pistons and possibly a ruined cylinder head caused by pieces of piston hitting it when the self destruct.Not only that but you could destroy the block itself. A broken piston leads to a connecting rod flailing about at 3500 RPM or more and poking holes where there should be non.
THis info is gleaned from over 60 years experience in working with all sorts of engines. Marine / aircraft/motorcycles and automotive.
Sandy Sanders
SANDY SANDERS

John,

Yes, the oil coming out of the valve cover and the dipstick tube are most likely the same problem.

All of your dry compression figures are low. Two and three respond to oil, so they have leakage past the rings. Number 4 is still low with oil, so likely those valves are leaking.

There may be a blown head gasket that is contributing to some of the problems, but is not likely to be responsible for all of them.

It certainly appears to me that you need yank engine out and rebuild it. If you like, you could pull the head first for a bit more diagnosis, but I think it is all going to have to come out.

Charley
C R Huff

My wife said to me this morning, "Did you mean to leave your dipstick on the shelf in the garage?" (I had been adjusting the distributor and the dipstick gets in the way.)

So I had driven over 100 miles with the dipstick removed!

There were a few spots of oil around but nothing significant, so I guess that's how it should be.
Geoff Everitt

Not a good idea though, on all but the earliest engines the PCV system will have been sucking unfiltered air through the dipstick hole, and will have been weakening the mixture. The normal source of air into the crankcase is via a filter and a restriction.
Paul Hunt

John-
I think you have pretty well established that you have multiple problems, all of which say "rebuild me now".
Leyland tractors had blue engines.
A good possibility for the loss of power, hard start, and backfiring is that you may have switched the plug leads around whilst fiddling.

FRM
FR Millmore

FRM,

You hit the nail on the head. I had all the wires off while checking the compression. I looked in the manuel to check the firing order. However, I had put them on the distributer cap clockwise instead of anti clockwise. Also, I started the number one lead one terminal off also. Once I determined the position of the rotar while the number one cylinder was at TDC, and went around anti-clockwise then she started and ran.

Also, thanks for the info on the Leyland Tractor. So now I am quite cetain that was my engines original application.

I will probalby take the engine out in the near future and take it into a machine shop for rebuilding.

I'm, glad I still have my 72 B-GT that I can drive while the 79 roadster is down.

John Fraioli
John Fraioli

This thread was discussed between 15/08/2009 and 21/09/2009

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