Compression test & Leak down test

[dom14]

Hi Guys,
If the engine passes the compression test with flying colours, can it still fail leak down test on one or more cylinders?

In other words, if my fourby engine passes the compression test with 'flying colours', can I call it a day and not worry about
leak down test at all?

I just did the compression test(I will post the compression test results here, later today), and it came out ok.

The engine is RB30 carby model.

I'm yet to do a leak down test, but thought I might post this here and get some advice.

Thanks for any help.

[89GQPootrol]

I guess it depends on whats wrong with the car and the reasons why you are performing such tests.
the leakdown test is a much better test as it not only tells you the condition on the engine but where any of the faults might lie.

[dom14]

I guess it depends on whats wrong with the car and the reasons why you are performing such tests.
the leakdown test is a much better test as it not only tells you the condition on the engine but where any of the faults might lie.

I want to perform a leak down test because it's being said that it's more accurate than compression test to assess the condition of the engine(I read it somewhere).
It's bit smoky when idling (when warm at times) and tend to blow oil out of the rocker cover gasket(regardless of gasket being air tight).
I've been wondering whether the valve stem seals are worn out and that's causing these issues.
I thought a leak down test might give me some idea of any such leaks?!!
Since, I have no prior experience with leak down tests, I thought it may be a good idea to get some advice from the experienced folks here.
I'm guessing, somebody with some experience might be able to answer my first question?! I think that will shed some light on my understanding on leak down tests.

So, what do you think of the compression test results? Last stroke compression appears to be within acceptable area. But, I'm not too sure about the first three strokes. That's where I need some help, as well.

I'm also bit puzzled at the cylinder 1 compression reading. I thought 190 is way too good?! Or bit too much compression for an engine with a 300k+ on the clock!!! The cylinder head was machined & I did a bit of a dodgy job with grinding the valves(didn't spend too much time on it). So, I'm not too sure about the accuracy of the compression reading(s). Spark plugs had bit of carbon build up(I'll post some photos tomorrow). That may be because it has some kind of carby or ignition issue at the moment(engine misfires, stalls & cut off). It first started on LPG(so, I thought it's an issue with the LPG), then it 'spread' to petrol as well.
I think it's an ignition issue or a carby issue, not an engine issue. I'm doing these tests to rule out any engine issues and to be 100% sure.

[4bye4]

As mentioned by 89GQPootrol above it all depends on why you are doing the tests.Usually if the compression test is showing reasonable results and the engine is running Ok you would not do a leak down test. Obviously you are doing this testing for a reason. Compression tests give you a rough idea of what is happening in each cylinder, but do not identfy a problem. For example if a cylinder is low comp, it may be leaking by the inlet or outlet valves of through the head gasket or bypassing the rings. Compression tests can also vary because of engine crank speed, ie. battery condition and other variables. A leak down test assists in finding a problem by determining where the air is leaking out of a cylinder. Air leaking out of the exhaust indicates a problem with the exhaust valve. Air coming out of the carburetter indicates a bad intake valve or seat. Air going into the crankcase is leaking past the rings may indicate rings or cylinder damege. A leak where the air is going into an adjacent cylinder or into the coolant indicates a blown head gasket or cracked head.
Regarding the "high" reading in your figures, this can sometimes be caused by oil or carbon build up on the cylinder.
The bottom line is that all these tests only go so far as a diagnostic assistance. You need to decide if the results and the engine performance warrants a tear down or not. The symptoms you describe, engine misfires, stalls & cut off, don't sound like a cylinder or head problem, as you say more like carby or fuel induction issue. As you are getting this issue with both LPG and petrol, it might be worth looking to see if there are air leaks in the manifold which could upset the fuel/air ratio. Have you checked the basics like aircleaner? Does the motor use oil?
Good luck with it.

[dom14]

As mentioned by 89GQPootrol above it all depends on why you are doing the tests.Usually if the compression test is showing reasonable results and the engine is running Ok you would not do a leak down test. Obviously you are doing this testing for a reason. Compression tests give you a rough idea of what is happening in each cylinder, but do not identfy a problem. For example if a cylinder is low comp, it may be leaking by the inlet or outlet valves of through the head gasket or bypassing the rings. Compression tests can also vary because of engine crank speed, ie. battery condition and other variables. A leak down test assists in finding a problem by determining where the air is leaking out of a cylinder. Air leaking out of the exhaust indicates a problem with the exhaust valve. Air coming out of the carburetter indicates a bad intake valve or seat. Air going into the crankcase is leaking past the rings may indicate rings or cylinder damege. A leak where the air is going into an adjacent cylinder or into the coolant indicates a blown head gasket or cracked head.
Regarding the "high" reading in your figures, this can sometimes be caused by oil or carbon build up on the cylinder.
The bottom line is that all these tests only go so far as a diagnostic assistance. You need to decide if the results and the engine performance warrants a tear down or not. The symptoms you describe, engine misfires, stalls & cut off, don't sound like a cylinder or head problem, as you say more like carby or fuel induction issue. As you are getting this issue with both LPG and petrol, it might be worth looking to see if there are air leaks in the manifold which could upset the fuel/air ratio. Have you checked the basics like aircleaner? Does the motor use oil?
Good luck with it.

Hi mate,
Thanks for the reply. Yes, the motor has been using/burning oil recently. I discovered that during the last oil change couple of weeks ago. Oil looked bad too.
That didn't sound good for a motor that's been running on LPG all the time. Air cleaner looked fine when I checked it. I removed the air cleaner and tested the engine, and that didn't make any difference.

I suspected air/vacuum leak, but still can't find any leaks. I used carby cleaner spray to detect any leaks around vacuum lines, fuel lines, seals, etc etc. Only area I couldn't check was the under the manifold. That because the carby cleaner spray couldn't reach it. I'm still suspecting an ignition issue or carby issue. I'll do the leak down test today and post the result soon. I think, then I can decide whether I go for a ignition system tear down or carby tear down.

Another issue that has been with the engine for a while is that,
when I run the engine on LPG, when the engine is cold, I hear a humming noise more than half of the time. I believe it's coming from the LPG converter, but I'm not too sure. I'm wondering whether you could give me some assistance with that issue. I think the converter may need a rebuild. But, I don't want to ahead and do it without being certain.

[4bye4]

I wouldn't be too concerned about the converter humming. There are moving parts in a converter (valves) and they can make a noise when operating. When you first start the engine the expanding gas in the converter, which is what it does - converts from liguid to gas - makes the converter itself cold. Usually you have a connection to the radiator for the water to heat the converter. Anyway I don't think the converter is a problem unless it is very loud noise.
If you are running on LPG all the time consider this. There is no carbon or contaminates in LPG as there is in petrol. If you are getting deposits of carbon on pistons, spark plugs etc. it can not come from the fuel (LPG) therefore it must be coming from the engine oil. If you are seeing carbon deposites inside the cylinders and you are using oil it seems preety obvious that oil is getting into the cylinders during combustion. Rings allowing oil up from the block/sump? Valve guide seals allowing oil to seep from the head into the cylinders? Head gasket? any of these could be at fault. If say an oil ring or rings are worn, oil can get into the cylinders but you may not see a significant indication on the compression or leak down as the compression rings may be OK.
My background incidently is a engineering specialising in fitting LPG to air cooled motors, so although I know a bit about the principles, especialy LPG (I am a licensed gas fitter), I am not and don't claim to be an automotive mechanic.
Tony

[dom14]

Thanks Tony. That indeed helped me a lot.
What I always do is to start the engine on petrol and switch to LPG in a minute or so.
The reason for that is that regardless of having a cold start solenoid installed by a LPG specialist, I can't start on LPG without cranking for about 10-20 seconds.
Initially, I suspected a vacuum/air leak, though I still can't find a one.
I didn't bother to fix it, because starting on petrol and switching to LPG worked fine, so far.
If I understood you correctly, I have to do a leak down test now.
I'll post the results soon.

BTW, I had no idea LPG and air cooled engine combination. Only air cooled engine I know in cars is Volkswagen beetle and Kombi, which are hard to come by(even though I wanted a one while ago. :) ). I know, many motorbikes are apparently air cooled these days, though I don't think I'm going to see a one on LPG anytime soon. :)
I'm guessing you've been working in industrial engine area?!
Cheers