Ok, I have Googled this quite a bit but I'd like to get this all down on one thread, rather than jumping from site-to-site all the time.

Anyway, cossie engine was missfiring, comp test shows no.3 cylinnder lack of compression, not zero but quite low.

Before this happened, the head was changed because of a faulty head gasket, and it turned out the head was knackered, so changed for another one. When everything was reassembled the car wouldn't run properly and comp test showed no.3 cylinder was down.

Now, my first port of call going by internet teachings is to re do the comp test and put some oil in no.3 to see if the results are better, thus would show that it is a ring/piston problem.

However, I suspect that it's an issue with the head, headgasket or valves, so this is where I need guidance on any non invasive ways to diagnose this before I take it apart. I don't want to have to take the head off and find no issues, which then means I will have to get the bottom end stripped just because of an unknown cause, so any fault finding tips you can share would be great.

*The car in question has been sitting for a long long time and has not seen the road since 99/00

 

If you use the oil in chamber method and its still low, then a valve or spring problem could be likely.

 

Possible burnt exhaust valve?? Is it doing the classic chuff chuff??

Do the tissue paper trick at the exhaust end it see if it's sucking back.

 

I haven't had a chance to see the car, this is all reserarch before I return home on thursday.
From what I have been told the head is rebuilt and looks 'as new', but the car never ran properly after this was put back on, so if I choose to accept this as correct then probably an assembly issue rather than a valve if it hasn't been driven etc.

I don't know what you mean about the tissue paper trick, so I will look into google for that, this is all pretty new to me in terms of diagnosis for this sort of thing.

I will get a battery and check the car over then try using oil on cyl 3 first, just need to get a comp tester sorted out.

 

With the head off,this is a bit crude,level the pistons,poor a measured amount of fuel into each bore,if the 3rd cylinder drains the quickest,its a fair bet its the rings damaged or stuck in some way.

 

What ever is up with it rings/valve/head gasket the head has to come off so you may as well just take it off lol

 

do the same with the ports,take the camshafts out,fill the ports up with fuel,and you will clearly see how bad they leak out of the valves

 

agreed

 

I see what you're saying and I agree that ultimately the head will have to come off, but I want to diagnose it first before it's stripped rather than send the head away to be fixed to find there's nothing wrong with it, then have to get the bottom end done as well. I just want to properly check it over whilst it's all assembled.

Thanks for your help though, baptism of fire buying a car you've never seen, i guess I'll know more when i see it next week

 

detted number 3 piston is the most likely problem on a yb
cylinder leakage check is the way forward and an endoscope / borescope

 

Check for a hole in the piston.

 

Cheers, I was going to do that as well before the comp test as I have access to one, that may save a lot of messing about if it's obvious. I do have it on pretty good authority that the issue wasn't there before the head was changed, but it's wise (and cheaper) to keep an open mind to quickly get to the bottom of it.

 

I suppose the fitter could have bent the valves fitting the head

 

Yep I guess anything is possible, I think the work was done about +10 years ago and the car basically hasn't moved since, so I'll take anything I'm told with a pinch of salt and just see how I get on.
It does sound true what I have been told, the head looks new from the photos I've been sent, and why anyone would put a new head on a fucked bottom end wouldn't make sense, but I'm sure it has happened before.
I might manage to call you and annoy you about it!

I also forgot to mention it's a 4x4 head on a 2wd block, but I don't think that makes a great deal of difference.

 

it will alter the comp ratio slightly but wouldn't alter 1 cylinder more than the others,
best way to do it is to do a cylinder leak test if you have access to the req'd equipment
this would give you an idea where the compression is escaping to and point you in the right direction

 

check the inlet manifold gaskets also!

 

that would cause a boost leak, not a drop in compression

 

The compression test itself is largely the diagnosis, so exactly what were the readings ? And what do the spark plugs look like ?

And of equal importance...history. Under what circumstances did the problem appear ?

 

Agree with Tony. Cylinder leakage test is the best way forward.

 

What you been buying now Rog?

 

Thanks for the input folks, I'll let you know what I find when i get a chance to have a look, probably next weekend when I get back home.

The history of what/how it happened is just what I have been told by the guy I'm getting the car from, but I know him well and he has known the car for at least a while so thats all I can say already posted above. Not overly bothered as I bought it like this so it's no big deal.

Bob, strangely enough I've been missing that last white rwd saph I had, last week I was bored and thought I'd look for another one, I happened to mention it to my mate on text and he immediately sent me a photo of a saph in his garage and said "Will this do?", the deal was done like that, so I havent' even seen it yet....

All I know is it's an E plate white saph, hasn't been on the road since 99/00!!!

 

Bob, strangely enough I've been missing that last white rwd saph I had, last week I was bored and thought I'd look for another one, I happened to mention it to my mate on text and he immediately sent me a photo of a saph in his garage and said "Will this do?", the deal was done like that, so I havent' even seen it yet....

All I know is it's an E plate white saph, hasn't been on the road since 99/00!!![/QUOTE]

Youve got a thing about buying a car blind, hopefully its as good as the Cab turned out to be. You need to tar your front garden.

Hopefully the 106 will be back home in a few weeks time, its all done bar wheels and Gaz has done a blinding job.

 

Yup, it looks an ok car, but yes I think you're right, something needs to go soon, it's becoming a right pain in the arse.
it's a cossie, what could possibly go wrong...

Think i'll only be home for a week and away again, so lots to do. Can't wait to see the 106, it's been a long time coming!!!!

 

Compression tests tell you nothing other than the compression value, if there is non or its low, it still has told you nothing.


It's the only way before stripping

 

Shit news mate

 

It only tells you nothing if you know nothing and cannot interpret the readings.
Don't blame a useful diagnostic method for your own inability.

 

Please do enlighten me then in your ultimately superior knowledge of compression pressure.

I have a theoretical four cylinder si engine that is now 24 years old has unknown mileage and has a miss-fire.
We have carried out a compression test and the results are as follows
Cyl1 -150psi, cyl 2 - 140psi, cyl 3 - 125psi, cyl 4 - 155psi.

You have carried out a wet test and only achieved a 5% variation.

So with your ultimate diagnostic capability you can interpret them result to tell me the internal engine problem yes?

I will await your superior knowledge over mine with your answer.

 

Well I didnt conduct the test, I didnt see how fast compression was rising etc. I havent inspected the spark plugs nor heard the car running.
A test with a small variation could mean many things. A test with a large variation could mean very different things, and a test with zero compression usually means something very specific.
But any fool knows that a compression test is only one part of a diagnosis.

In your case neither a leakdown or compression will be specific enough to pinpoint the problem.
And of course you dont state what sort of misfire, nor do you state which cylinder. So throwing out make believe figures with no history behind them is just stupid and far removed from what the OP has actually asked.

If you have ever actually done tests and repaired engines, you would know this. The OP did not say there was a small variation, he said there it was very low, but not zero compression on one cylinder.. Again, the actual results are very important and in a case like that often the diagnosis combined with some history and basic visual inspections is very easy.
Which is exactly what I asked for. Not some made up bullshit.

Back to your specific question...the problem with trying to diagnose remotely, is getting useful information from the fool who is supplying it, and thinks a diagnosis can be given from such little information.

It could be anything from the test not conducted correctly, to worn/sticking rings, maybe poor valve sealing from a variety of reasons, or even a partially blocked inlet tract, which again could be a few things..

 

So now a fool can do a compression test, and like I stated above, and thank you for backing me up, compression tests are pointless for diagnosis other than has it has it or differentiated amounts.
Whenever you post, you always come across trying to belittle people and their own experience/professionalism.
For your information I teach this to people day in and day out and spent over 20 years diagnosing and repairing engines.

Just to finish, and my apologies to the op for the off shoot, I did state the vehicle had a missfire and a cyl leak tester would have pinpointed the issue without fail, but then you knew that im sure.

Regards
Mark

 

And equally your leakdown test is pointless on its own. And as I said, you didnt say which cylinder had a misfire, nor give a clue as to whether it's fuel, spark or other misfire problem. The misfire could have been on a good compression cylinder which would have meant either a leakdown or compression numbers were actually pointless for your specific problem.
That doesnt mean a compression test is useless, if you're a teacher then you should know this.

A leakdown can only give a clue as to a valve or piston sealing problem. A compression test can give insight to more than that as it relies on also getting air into and out of the cylinder in order to give a result. So IMO in many cases is far more useful than a leakdown test.

In the case of the OP, a compression test sounds very valid, and the exact results will give a good diagnosis. Again, you say you teach this stuff, you should be very well aware of this.

And back to you claiming a leakdown can pinpoint a misfire. So how can a leakdown pinpoint a failing plug, lead, coil, dodgy injector, worn camshaft/valve clearances too large ?
All potential causes of a misfire, and none of which a leakdown test will identify

But I guess you knew that too ?

 

A compression test can at least tell you where the problem is as in number 3 cylinder for example, or if 2 adjacent readings are low then probably point to the head gasket between those 2 cylinders. If it's a zero reading on one cylinder then I'd expect something like a non sealing valve or melted piston.

A leak down test should be able to point you towards inlet valves, exhaust valves, pistons/rings head gasket depending where you can hear/see the air leaking from. I've never had to do one yet, so won't claim to be an expert on this subject.

 

Steve - maybe you haven't noticed that this topic is called "lack of compression - how do I diagnose it" and that is what I and Markk are referring to and is why we aren't talking about mssifires etc.

 

And as Ive said. You diagnose by looking at the plugs and detailing the exact compression readings and also any history leading to the failure.

And based on this

I would say the problem does not lie with the head. But again without specific readings much harder to say.

But he has clearly said it misfired before with the old head, a new head was fitted and it still misfired, and it was down on compression on cyl 3 with both cylinder heads.

IMO it shouts of a ballixed piston, actual compression readings would confirm this. A leakdown isnt really going to tell you anything else, other than perhaps letting you hear all the air getting dumped into the crankcase.
But a "low, but not zero" compression reading has pretty much told you that already.

One way another the head needs to come off at a minimum.

it would also be interesting to know why the previous head was deemed scrap.

 

i thought i read it as, its had a missfire since the head was changed, not before
but i may have read it wrong
if it was there before then i'm with you on the melted piston, but if its just since the head was changed then more likely a leaky valve
bu either way the heads coming off to be sure

 

Sounds like it's been a bit lean, detonated and blew the head gasket and f**ked the side of the piston. The HG failure may also have damaged the head, although again, it would be interesting to know why the old head was deemed scrap.
Piston likely to have melted a little and/or trapped rings, or broken ring lands. Often neither of which leaves an easily visible sign on the crown..

Although at the same time, there is usually some sort of indication on the plug.

 

Thank you, I refer myself to Jean-Luc Picard in your avatar Stu pmsl

 

Yes, but you Mark did raise a fictional scenario based around a misfire, and I responded to that.

A facepalm is indeed warranted if someone trying to educate people on diagnosing engine problems is saying a compression tester is useless compared to a leakdown test. Which is very far from the truth.

Because this statement is just complete BS.

 

I'm sorry, I thought this thread was about lack of compression Stevie.
you enjoy the rest of your evening, I'm not about to lose any sleep over this.