cylinder compression problems

Factual Questions
1

The other day my old man performed a compression test on the cylinders of my car and concluded that cylinders 1 & 4 had compression problems…

The only thing that has him confused is ‘why?’… He said something about the valves possibly not sitting right and crossed out worn rings because the engine was not burning any oil… What other things can cause loss of compression on an engine???

Regardless, he says that he’s going to have to lift the head off the engine which I’m not looking too forward too :)…

2

I might also add that there’s a backpressure problem on the engine as well which is powerful enough to force oil up into the air filter and out the sides of the oil cap when the engine is driven hard…

3

How did you determine that there was no oil burning?
A catalytic converter will hide a lot of problems.
You may have two seperate problems.
The problem with the air filter may well be valve guides.Or something as simple as a stuck PCV valve.
You don’t mention what year engine. Built for racing? Does it have all the pollution control crap on it?
If you have checked all this stuff out Dads probably on the right track.

4

The car is a 1984 Nissan Pulsar (the engine is a N12 I think)… It doesn’t have a catalytic and last week when we tuned it up my old man adjusted the tappets (or something along the lines of that). He checked the PCV valve the week before and it seemed to be working fine.

He thought that the backpressure problem might be developing in the sump but is there any chance that the compression problem is linked to the backpressure?

The car is 17 years old with 153,000k’s on it. It beats me why he wants to keep it running since the engine is starting to get tired but his opinion is that the body and everything else is fine (no rust) owing to a good service history by the previous owners.

The engine has obviously had problems before because he commented that the head had previously been lifted (and possibly replaced) but this was most likely because the timing belt had failed and fouled the valves.

He seems quite enthusiastic about overhauling the head since he worked engines quite a bit when he was a teenager, but, the problem with the compression and most certainly the backpressure has him confused and he’s been asking around to work out what the problem may be…

Anyone got any ideas? :slight_smile:

BTW, I think the problem with oil being blown into the airfilter may be exagerated since the airfilter is probably doing its best to SUCK whatever it can in as well so the problem may not be THAT severe…

5

Sounds like Dad is having fun and perhaps trying to teach you something about engines???
I’d let him have fun and try to learn something.A little bit of grease never hurt anyone:)

6

Mickazee

Not good but it might be something as simple as the crankcase breather being blocked.

When the air in the crankcase heats and expands then some of it some goes out the breather and as it cools and contracts it draws in air, this in turn brings in moisture which condenses and should the breather become blocked it will expand far more than the air will, causing the over pressure.

IIRC some crankcase breathers are actually ball/spring valves, others open the valve in other ways, I’ll need to get back to you on this.

Its possible I suppose that there is a tiny leak of coolant into the crankcase.

If you had duff rings then you could get back pressure but you seem to have discounted this, its the only single thing that would account for both problems, so it could only be the inlet/outlet valves and seats that are burned away.

7

Why not?

8

Cool, thanks for your help ppls! :slight_smile:

My old man is still pretty adament about pulling the head off so whatever the problem, he plans on correcting it :slight_smile:

9

I hope you didn’t remove it, because AFAIK it is illegal for that model year of car to have the catalyst removed.

Well, yes. I mean, when combustion gases blow by the rings, they end up in the crankcase and sump, and have to be vented. I saw a car that blew by so much that if you held your hand near the oil fill cap it felt like a hairdryer. The fact that it does not burn oil does not absolutely mean that the rings are good, but it does sure points to them being good.

Ah…a cracked/leaking head gasket could, I suppose, pressurized the crankcase, if somehow the leak was between the cylinder and an oil return passage. But I have never seen or heard of that ever happening - it would be hard to imagine.

Your air filter guess could be accurate.

10

In reference to the catalyctic, this is in Australia so the laws are probably different… I don’t see how it could have a catalyctic converter anyway because the car originally ran on leaded petrol and the previous owners had it tuned to run on leaded a little further on down the track.

Correct me if I’m wrong, but, doesn’t leaded petrol destroy catalytic converters??

Anyway, my old man came home from work and he asked around to see if anyone could shed some light on the problem. A mechanic told him that it could still possibly be the rings but I still find that doubtful since it’s not blowing any smoke.

My dad is really confused about this because if the valves were not sitting right or the seats were burnt out, the engine would be misfiring…The funny thing is, it’s running like a new swiss watch after the tune up… Other than the loose tappets which he adjusted on the weekend the only after visible problem with the engine is the backpressure and a little knocking due to the timing being too far advanced (which should be corrected soon)…

Would it be wise to leave it since it seems to be running fine or should we get to the bottom and have it all corrected??? What damage can this type of stuff cause?

The compression problem could of been there for quite some time, but, the backpressure problem has only come into play about a month ago due to my own stupidity…

[EMBARRASING STORY]
A month ago I drove from Sydney to canberra which is a long distance trip (the 2nd the cars had). It’s handed long distance fine before but on the way home there was a stretch on the federal hiway that was quite long and downhill. I decided to open it right up and 160km’s an hour later I eased back only to have the car shudder… This is what I think happened:

  • At 160k’s an hour with the engine going so hard, backpressure obviously developed and about half the engine oil was sucked into the airfilter.
  • It choked the carb temporarily which made it shudder (no air)
  • …and ran super rich the whole way home…

My old man was NOT pleased… but, he was quite cordial about it later owing to the fact that he did some pretty stupid shit when he was a teenager too…

[END EMBARRASING STORY]

When I checked the engine oil at a service station, it was barely registering on the dipstick so I put in about 3 litres (4 litre sump). That actually overfilled it so the car didn’t lose as much as I thought it did. (mind you, the excess oil had come out the air intake and splattered everywhere and burnt onto the exhaust manifold)…

Obviously I did some damage to the engine during my moment of great stupidity and I was thinking along the lines that I may have glazed the bore or something?

Does this help shed any light??? Thankyou very much to the people who have given me suggestions, greatly appreciated! Feel free to verbally abuse me about my great ‘160k’s an hour’ I deserve it ! :slight_smile:

11

This really does sound like a classic case of dead
rings. Does your dad know the trick about pouring a little
bit of oil in the cylinder and doing the compression test
again? If the compression numbers go up, that’s usually a
sign the rings are gone.

Low compression is caused by one of two things:
dead rings or bad valves/valve seats. (Well, okay,
dead head gasket could do it too.) Try putting a capful
of oil in the cylinder and doing the compression test
again.
-Ben

12

I guess let’s go back to your OP - how do you know the engine is not burning any oil? More and more this sounds like bad rings, rather than anything else.

If you are losing so much oil that is sucked into your air filter, how do you know oil is not also being burned? I mean, how are you able to measure it with any accuracy?

And I did see a Toyota once that “burned no oil”. That was because about a quarter of the oil was actually unburned gasoline that had gone past the rings and condensed, with the light hydrocarbons havy all evaporated away.

I understand about the catalyst, and yes, you are correct about the effect of lead on the catalyst.

13

If your father “adjusted the tappets,” there is a possibility that an error was made and one or more valves are unable to seat properly.

I personally would re-adjust the valves and then re-test the compression.

Also, someone mentioned squirting a little oil through the spark plug hole. This is a good test for weak piston rings.

Whatever the problem turns out to be, I urge you to enjoy the experience with your “old man.” I wish my father had been as willing to establish a bond.

14

My car’s motor’s head gasket is bad and a couple of cylinders have very bad compression. When it runs it consumes both water and oil. I need to get a new car pronto!

15

I said I’d get back to you but all too late since ModernRonin2 has already posted the best way to prove the rings.

On that sort of mileage worn bores and tired rings are likely to be the main suspects.If so the engine will be coming out.

If you are going to have the head off you are a goodly chunk of the way to taking the rings and pistons out, and if you go that far its worthwhile going to the bottom end and checking the shells out too.

There isn’t much point in reworking the head only to have to sort the rest of it in maybe 20000 miles if this is a car you intend to keep, it will save you on gasket kits if nothing else.

…and while your at it why not check out the clutch,if you are lifting the engine out to work on the bottom end anyway.