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MG MGA - Blowdown test

I wonder if anyone can enlighten me on this one. I tried to do a blowdown test but was unsuccessful. Here is how I did it, maybe I missed something out.
I removed the number one plug and screwed in a purposely made tyre valve instead. I turned the engine over to bring the number one cylinder to TDC on compression stroke, and applied an air supply to that cylinder. When I pressed the trigger to get to 20 psi, the engine turned over and the inlet valve opened just enough to let air pass through it. I engaged 1st gear to lock the engine and the same thing happened. Tried the same procedure on number 3 cylinder and again when air pressure was applied the engine turned over just a little but enough to slightly open a valve. Obviously the test had failed and I was unable to determine whether I had a leak either in the valve train or in the piston rings. What did I do wrong?

Frank
F Camilleri

You need to position the crankshaft EXACTLY at TDC, use the timing mark if necessary. Then you can apply all the pressure you like and it won't turn.
Barney Gaylord

Thanks Barney, will give it another go tomorrow when I will do exactly as you explained.

Frank
F Camilleri

" EXACTLY at TDC"
True, but it must be truly exact, and that is not likely. And if it is only a degree or three off, it will not move until it has a lot of pressure on it, when it will move quite quickly. This presents the possibility of the chopping off of things in the area - like hands. If it is in gear, you will bury your grill under the back bumper of your Rolls, or run over your kid or...

Put it dead on TDC, in top gear, brakes locked up, and wheels chocked. Treat it like a loaded gun.

I will note that if turning it "just a little but enough to slightly open a valve" opened the intake, then the engine turned backwards. Forward rotation will open the exhaust. In any case, in order to open a valve the engine has to turn a considerable amount, like typically at least 30-40 degrees depending on cam. If a valve opens in less than that with a stockish cam, look for rocker adjustment errors or cam timing issues. Worn timing gear can leave (all valve but particularly..) inlet events late, which is often a cause of weak performance and oil burning; this would also mean the amount of back turning required to open the inlet might be less than expected.

I'm assuming that you are looking for a problem here.

FRM
Fletcher R Millmore

You can do a traditional leak down test if you want. I do it a bit different. If you leave all the plugs in - chock the wheels,put in gear, TDC, etc as others have advised. What I do is apply a small bit of air like 10-15 lbs and then go listening. If it hisses out the tail pipe-exhaust valve or seat, if it hisses out the intake manifold/carbs - intake valve or seat. If it hisses at the valve cover (where you put in the oil) good bet you have bad or broken rings or a good bit of wear in the cyl. You can even put a rubber ballon on the opening and if it inflates -Viola a leak.
BRIAN WARMUTH

Brian--that won't work! except when #1 is at TDC (which you confirm with the timing mark). How are you going to know if #4, #3, #2 are at TDC with the plugs in?

Additionally, the rubber balloon probably won't blow up if your road draft tube is not plugged.

--Or am I missing something? Probably!

--Jack

JM Morris

Well guys, I followed Barney's advice to the letter and I'm pleased to say that when I tried the blowdown test again the engine DID NOT move a whisker. However, when I applied air pressure to the no. 1 cylinder via the plug hole, air did not hold in the combustion chamber. It must have been escaping from somewhere, and it was doing so at a very fast rate, so much so that it didn't give me a chance to listen for hissing of escaping air. What is this telling me? Am I right to assume that I have a bad leak, either from the valves or from the piston rings. I might add that all four pistons together with their rings are brand new, only fitted about 1K miles ago. The pistons are AE Hepolite with five rings not four. As for the valves and valve seats, these were machined by an engieering firm and adequately lapped in by myself. I'm baffled. Any suggestions from anyone?

Frank
F Camilleri

Most likely answer--you were 180 degrees out.

--Jack
JM Morris

I am in learning mode here. What is the purpose of a blowdown test that is not achieved with a standard compression test?

Steve
Steve Gyles

Steve,
This from the web:

The neat thing about a leakage test (as opposed to a compression test) is that it's faster and easier to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor would point to a leaky intake valve. Air coming out of the breather vent or PCV valve fitting would tell you the rings and/or cylinders are worn.

Cheers,
GTF
G T Foster

Thanks GTF. Most interesting. I see Frank made a hybrid adapter. I presume there are purpose built airline adapters for this procedure?

Steve
Steve Gyles

JM - You are right, you have to take the plug(s) out for each cyl in turn that you are testing and get that particular cyl at TDC on the compression stroke (so that both valves are properly and completely closed).
GT - see my post several above yours.

I've been doing this type of test for years on every type of car engine and find it quick and easy way to get in the ballpark with diagnosis of potential or real problem areas.
BRIAN WARMUTH

" I see Frank made a hybrid adapter."
This implies that there is a picture someplace, but I don't see one. I've noticed this several times lately on different threads - is it just me or do other folk notice this?

I make the adapter by brazing an air fitting to a dead plug body. Most useful and at higher pressures like 100psi will show up blown head gaskets etc.

FRM
Fletcher R Millmore

Replying to Jack Morris when he says that "most likely you were 180 degrees out...", I had aligned the timing marks exactly at TDC on the compression stroke. To make absolutely certain that I was on compression stroke, I removed the distributor cap and metaculously checked that the rotor arm was pointing directly at the no.1 plug high tension wire inside the cap. Would this put me 180 degrees out? Please enlighten me if I did it wrong.

Frank
F Camilleri

Frank--if that's the case, then another possibility might be that the cam timing is so far off that a valve is open even though you're showing TDC.

Beyond that--I can't imagine that a lapping error or ring installation error would allow that much air loss that fast.

What is your air supply like? Are you using a relatively large tank with 80-100 lbs pressure, etc.?
JM Morris

Frank--P.S. (additional to last post):

"Would this put me 180 degrees out?" Obviously, no, unless your distributor is 180 degrees out. :)

--Jack
JM Morris

Jack thanks for your comments. You mention cam timing might be off. How do I do a thorough check of this, and most importantly, if the timing is indeed found to be off, can you possibly explain, in some detail please, how it can corrected.
Thanks

Frank
F Camilleri

You should first do a standrd compression test. If it passes, no need to mess around further.
Art Pearse

FRM here is a pic of the adapters (x3) which I had fabricated. I use adapter A to get the piston exactly on TDC. A small lamp is connected between a power source and earth, and turning the engine slowly by hand until the piston touches the end of the adapter and the bulb lights up. Unscrew the adapter until the lamp goes out, and again turn engine, very slowly so that the lamp comes on again. If the light does not come on this time, that means that the piston is on its way down. Turn the engine slowly anticlockwise so that the piston is pushed up again and stop turning as soon as the lamp comes on. That should now be TDC. It is a little finicky job but I found that it works for me.
Adaapter B is the one I use for the blow down test, and C is used as a piston stop for setting timing, etc.
Hope my explanation is easy to follow.

Frank

F Camilleri

Frank,

When using your adaptor there is a quicker way than that you describe.
You can use either your adaptor A or C. Turn the engine clockwise until the piston touches the adaptor, make a temporary mark on the timing cover in line with the pulley timing mark. Now turn the engine counter-clockwise until the piston touches again. Make another mark on the timing case. TDC is halfway between the two marks.


Mick
M F Anderson

Mick, your method of finding TDC is of course spot on. I have used that method in the past many times. But it involves having to crawl under the car a few times to make the marks, etc. With the light bulb method the crawling is avoided. I only have to get under the car (very slowly) once to check the timing marks Today, that's a big plus for me as I suffer from a bad back.
Again, using a piston stop the way you describe to determine the exact TDC is very accurate and highly recommended.

Frank
F Camilleri

Art, I have this morning carried out a compression test on all four cylinders, on a hot engine and with the the throttle wide open. These were the results:-
No.1 cyl. = 145 psi
No.2 cyl. = 155 psi
No.3 cyl. = 159 psi
No.4 cyl. = 150 psi.
In my humble opinion these figures tell me that the compression on all cylinders is acceptable. However, I'm not so sure about the difference in compression between each cylinder. Is this telling me something? Is the difference too great, i.e., more than 10%, which would result in uneven idle? As a matter of fact, I find that the idle is not as smooth as I would like it to be. Any comments or suggestions please.

Frank
F Camilleri

Frank, looks OK to me. The differences could be due to combustion chamber volumes not equal, carbon build up, or just plain measurement accuracy. You obviously have no serious problems with the valves , gasket or rings. You could repeat with a measured squirt of engine oil in each.
Art Pearse

Thank you Art for your comments. I am rather puzzled at the difference in chamber volumes. The engine has only done a few hundred miles since the last headwork. New pistons were fitted and all the carbon build up (very little at the time) was removed. I don't believe that in such a short space of time carbon build up has accumulated so much that it would upset the combustion chambers volume. I'm not so sure what you mean by 'plain measurement accuracy'.

Frank
F Camilleri

Ok, the carbon theory does not fit. You can only measure the pressure to +/- about 2-3 psi,(experimental arror) so to get a good picture you need to repeat it a few times before concluding you really have a 10% difference. The cyl head volumes are not going to be accurately matched unless they have had that treatment.
Art Pearse

Frank--how is it that you were able to do a compression test but were unable to get results on the leakdown test? What turned out to be the problem?

--Jack
JM Morris

Jack, that question also crossed my mind, but the answer is I haven't the slightest idea. No problem with the compression test but when I tried the leakdown test, pressurized air did not compress inside the chamber. It was like inflating a badly punctured tyre. I am still mystified by all this. Could it be that I was using a rather small air compressor (50 ltr)? I use the same compressor for spray painting, inflating tyres, etc., no problems there. I'm pretty certain that it can handle 20 psi. BTW I tried the test only on the No.1 cylinder, then I gave up. Maybe I should try this test again, more slowly, and on all four cylinders.

Frank
F Camilleri

Trick to the blow down test is having both valves closed at the same time with piston at TDC. Camshaft rotates half speed of crankshaft, so two turns of crank for one turn of cam. If crank is on wrong rotation, at TDC both valves will be partly open, so no pressure. Solution is to turn the crank exactly one rotation to get the cam in proper orientation to have both valves closed.
Barney Gaylord

Hi Barney, When I did the test I only tried it on No.1 cyl. As I had mentioned before I followed your previous instructions to get the piston exactly on TDC. Both rockers of no. 1 cyl. were loose which indicated to me that both valves were closed. From your last post, am I to understand that, after bringing the piston to TDC, as I did, I now had to turn the crank another one rotation to have both valves really and truly shut?

Frank

F Camilleri

If both rockers wiggle with a little clearance, then it is in correct position. No clue why it would hold no pressure at all. This is why we do the blow down test, to see where it is leaking. When it holds no pressure it is definitely leaking somewhere big time. Listen to hear where you get the hiss, tail pipe, carbs, or oil filler port.
Barney Gaylord

Thanks Barney. I'm also at a loss as to why it's not holding any pressure. If you have seen my previous post the compression test figures are quite acceptable. Maybe I did not do the blowdown test properly. I shall have another go as soon as I can find the time.

Frank
F Camilleri

Frank, could your adapter B be leaking?
Art Pearse

ART, although I very much doubt it, that did cross my mind. I will test it and keep you posted. Tks.

Frank
F Camilleri

ART, I tested the home made adaptor and found that it did not leak in any way. It's perfect. Thanks for your interest. I shall put everything on hold for now, the whole engine is coming out sometime in September, so I will be in a much better position to do all the necessary tests. The only test I'm not so familiar with is the cam timing. But, above all, if the cam timing is out I haven't got a clue on how to put it right. Any suggestions anyone?

Frank
F Camilleri

Frank, cam timing is checked by setting the #1 inlet valve clearance to .060 ins and then it should just be opening at TDC. If it is off, then you should check the sprocket marks line up, then maybe even check the TDC pointer (use a piston stop) and also chain stretch and sprocket wear.
Art Pearse

Art, thanks a million pal. That is exactly what I need to know. I did have some idea of how it's done but I hadn't done that check for years and my memory is fading badly these days. As I mentioned in an earlier post I shall be lifting the engine out come September, when all the necessary checks will be done with greater care.

Frank
F Camilleri

Frank that's assuming you have a standard cam, 16 deg BTDC inlet opens
Art
Art Pearse

Art, very shortly after I bought the car, I replaced the camshaft with a brand new one. I bought it as a standard part. I also replaced the tappets and pushrods, and the rocker shaft. The rockers were rebushed. All these parts were completely shot. I did order a stock cam at the time, which I fitted as it came. You are telling me that with a standard cam the no.1 inlet valve will START to open at exactly 16 degrees BTDC. Well, I'm hoping that the shaft was indeed standard, but I did not check it at the time. I suppose there is a way of checking to see that I'm running on a standard cam. Would this be the way you explained in your previous post? If not, would you be kind enough to explain the procedure to me. I shall be very grateful.
Many thanks

Frank
F Camilleri

Frank, first set the inlet clearance to .060", instead of the usual .015" Then using a long breaker bar, rotate the engine until the inlet just starts to open, you can detect this by spinning the pushrod with your fingers, it will stiffen up as the pushrod rises. Then if all is well, the crank pulley mark should line up at TDC. If not, you have to calculate the difference from the offset. Mine turned out 4 deg late. I then checked the pulley TDC mark using a piston stop and corrected it, only 1.5 deg late which I put down to chain and sprocket wear. Don't forget to put the inlet clearance back to standard!!
Art Pearse

Tks Art, I will be doing these tests in September when I will have the engine out of the car. I guess it will be a lot more convenient working on the bench.

Frank
F Camilleri

Sorry, one last question if I may. In your last post you said to set the valve clearance to.060" instead of the usual .015" My engine is a std 1500 cc. The usual valve clearance is .017" with the engine hot. Are we talking of different size engines here?

Frank
F Camilleri

Frank, its in section A15 of the WSM, which refers to the 1500 and the 1600. The 1600 MkII is section AA, but no extra info. So I think it is right, and does not matter what running clearances are used.
Art
Art Pearse

Art, thank you very much for your much needed advice. I think I know exactly what you mean. I shall follow all that you said when I take the engine out in September.

Frank
F Camilleri

This thread was discussed between 30/06/2010 and 22/07/2010

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