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Compression problems on a z12xep

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  • Compression problems on a z12xep

    Hope someone has a solution for this as I have run out of ideas. Let's start by saying hi all. I am a first time poster. So hope all who read this is well and all is good with you also nice to meet you.

    I am a builder who likes to hold a spanner at night.
    I have a corsa with a z12xep with 96000 on it. I bought it as a non runner and figured as I have replaced the head gasket on them several times before this should be easy enough. But this little fella don't want to behave. I have low compression on 3 and 4 cylinders . After have the head on and of at least 6 times now I still have low compression on 3 and 4. I have replaced the rings on all 4 cylinders and found a broken ring in 4 when removing the pistons. I have bought 2 sets of bolts and 2 head gaskets as I was told you can't use the gasket and bolts again. Let me also add into the mix that I sanded the head down myself with wood glass and sandpaper. I still had low compression on 3 and 4 so figured that all my hard work. Never sanded a head before so a lot of figuring out and errors made on the way to a true flat head. I was convinced that it was me so sent it to the shop. New bolts new gasket and skimmed by machine. Still low compression on 3 and 4. 4 more than 3. 115 on 3 75 0n 4. Took off again and checked the valves by putting spark plugs in and filling the valve chamber with Petrol and no leaks. Any input would help thanks

  • #2
    Got a corsa sxi 2011 trying to do hand brake cable and brake shoes but euro part in York keep giving me wrong parts please show me the right cable and shoes


    • #3
      Srsmith 119 Welcome to the forum ,, but yours is a new and unrelated question. Please start a new thread .

      Godzuky 2006 welcome to the forum. I will comment as I am on , but if you can wait a bit longer Walksall and restorer may be able to suggest more.

      Your method of skimming a possibly warped head by hand using a flat surface is a technique known of old, (usually using perfectly flat float plate glass and lapping paste) that might still work for some less precision flanges and things. But not recommended for a modern precision cylinder head. Even very basic low compression side valve cylinder heads from the 1930's were normally machined flat when required. , although I have used the float glass method to briefly clean the face up and check for any low points on a 1930's cylinder head. Even if you manage to get a perfectly flat surface it might be in the wrong plane. eg too high at one end,too low at the other. Hopefuly now you have had it machined it will have corrected any errors you may have made. .

      I would not have used petrol for checking the valves. Something less volatile and liable to evaporate such as paraffin ,white spirit or diesel might have been better, and safer.

      Do you know the original cause? I suspect the car has overheated. Although the first to suffer is the head gasket and a possibly warped cylinder head there may be damage to the piston, rings or bore. Or all three. This damage might not be immediately obvious to the naked eye, without some expertise ,precision measuring equipment , and crack testing of the bores and piston. Number 4 cylinder seems to be the one thats most marginal for cooling and often the first to suffer overheating damage. The broken ring might be an indicator of this . . It could have broken when you removed the piston, by chance, but bore wear or damage might have made this more likely. And if its been broken for some time,it may have caused damage to the bore and or piston .This might just be accelerated bore wear rather than more obvious physical scratches and gouges . etc. And what caused it to break in the first. place?

      When you replaced the rings were the bores precision measured and check for damage, wear . Even new standard rings (or correct overbore size) will not make a good seal in a worn or damaged piston or bore. The replacement might be a fair fit in bores 1 and 2, but not in 3 and 4

      Also did you ensure the ring gaps were staggered?

      It might be worth checking carefully for cracks in the piston, bore and head, that might not be easily visible to the naked eye. You can buy crack testing kits ,but if you want to do it on a budget you could try wiping oil on the target areas, then wiping it off with a rag (but dont use a solvent) Then puff some talcum powder on the area. The theory is the crack will retain some oil , that will show as a slight trace in the talcum powder. Its not guaranteed to show all cracks, but might reveal a problem that you hadnt noticed before.

      Have you tried doing a wet compression test? Put a small amount of oil down the spark plug hole, Just enough to form an oily seal between the piston rings and bore. If this now gives a noticeable ,but temporary ,improvement in compression it tends to suggest compression is being lost there.

      If you need to pay to have the have bores checked using professional equipment, and possibly having a rebore,new pistons, yet another new new gasket etc,etc , it can get to the point where it may be easier and cheaper to find a good replacement engine.


      • #4
        I agree with all what Bugman is saying. He explains
        things far better than me
        I suspeIt that the engine over heated ,and yes it's normally no 4 that goes down due to it being furthest from the water pump. As Bugman said did you space the gaps out on the piston rings. Did you put a new ring down inside the bore and measure the gap in the end of the ring. That would of told you if you had wear in the bores.
        Another thing that could be causing the low compression is the alloy piston go oval. I had some pistons once that were 0.018 thou out of round .
        also how are you checking the compression. If you haven't started the engine up. The oil pressure won't have filled the hydraulic lifters, so the valves aren't fully shutting off until the oil pressure is right up and the oil light has gone out on the dash.
        Last edited by Restorer; 19-07-2020, 07:42 PM.


        • #5
          Restorer - You've made my day ! I thought that there was only me that still worked in 'proper' measurements.

          Just a thought to add to what has been said.
          Is there any chance that there is some sort of obstruction in the inlet manifold?
          If there was, it might just be possible that the air supply to cylinders 3 &4 are obstructed. - Its a long shot, but as the car was sold as not running, it might be that someone has (for example) stuffed rag in there whilst doing some other work.

          Unless the bores are VERY worn, I wouldn't really expect much noticeable difference if you had failed to stagger the ring gaps.

          One (crude) way of measuring bore wear between top and bottom of the cylinder, is to use a piston ring, measuring the gap with feeler blades when the ring is at the bottom, then near the top of the cylinder, but within the part of the bore covered by normal ring travel (Obviously, the very top of the cylinder never sees any wear)



          • #6
            Glad I made your day Walksall. Always worked in imperial. None of this ,1 ,2 crap 4 thou and 8 thou to you and me


            • #7
              Hi folks. Thanks very much for the response to my post. The update is over the weekend I did more compression tests wet and dry and took the head and piston 4 out and found one of the new rings I put in was bent slightly. When put in the cylinder on it's own did not make a complete seal against the cylinder wall. I am convinced I bent it putting it in as I am sure I would have noticed it bent when fitting it. Any way happy with the compression on all 4 125 and above I buttoned it up and expected a little fight to get it started but didn't think It would turn into a battle. I am getting spark plugs are getting wet compression is good. And it just does not want to even try to start. I have tried easy start and nothing except oil shooting out the dip stick holder and back firing out the air intake but nothing in the way of even trying to come to life. I think I should mention that when I have said in my previous post I have done several of the head gaskets on these z12xep engines. I owned them bought all parts needed and paid any expense. I normally did have help from a friend with more knowledge than I settings the timing on cars. I am convinced I have set it right. Piston nearest the timing chain is set to tdc and the two cam are locked in place with a spanner. Then tighten up the two sprockets. Again any input would be gratefully appreciated. Also thanks again for the very informative reply. I really appreciate the time you spent to reply. On another note I skimmed the head down by hand more for the reason that I believe I could do it by hand and it worked. I think t might have worked only I was losing compression elsewhere and was convinced it was the head so sent of to get skimmed then got the same compression when put together again with new bolts and gasket. I believe that it is very possible to achieve it by hand but the whole start to finish procedure time and cost. There is no comparison to getting it done in 30 min at a cost of 48 pound. Thanks again


              • #8
                Are the spark plugs wet with petrol or oil??
                If its petrol and you are happy with the timing, I'm guessing that you haven't got a spark - in which case, all the easy start in the world won't help.
                If there isn't a spark on ANY of the cylinders, then the problem is something common to them all (the plug to the coil pack for example)
                Oil from the dipstick tube is a little worrying, as it could indicate crankcase compression, probably meaning that the piston rings still aren't sealing BUT, before becoming too disheartened, check that there isn't too much oil in the sump.

                With regards to obtaining a flat surface on the cylinder head, it is quite possible to do it by hand, and particularly easy with an aluminium head.
                Rather than rubbing away with emery cloth, I much prefer the engineering approach, this involves the use of a 'flat' engineers scraper, a surface plate (or a sheet of PLATE glass - the stuff that shop windows and good quality mirrors are made of) and a tin of engineers or mechanics blue (an alternative name for the same stuff)
                Scraping a surface flat is a skilled task, but one that can quickly be learned.
                You can make a scraper out of an old flat file (about an inch wide), grinding the teeth off on both sides near to the end. The end needs to be ground more or less square to the teeth, but it is an advantage to grind a shallow radius on the end, finish (and re-sharpen) with a fine oilstone.
                The plate glass needs to be larger than the size of the head - you can obtain it from your local glass merchant. - Use an oil stone to remove the sharp edges and corners.
                The engineers/mechanics blue can be obtained at your local engineers merchant.

                With the head supported on the bench, clean up the face with emery cloth.
                Apply an even THIN coat of blue to one side of the glass.
                Push the blued glass up and down the length of the head, this will leave blue on the high spots only (this is the reason why the blue should be applied THINLY or you will get false results - the thinner the better in fact.
                Take the handle of the scraper in one hand and rest the other hand near the cutting edge, scraping away the blue and a small amount of metal under it - do this 'till the blue areas have all gone.
                Use the glass again to leave more blue high spots, repeating the scraping process 'till you get an even blue surface all over the head.
                This process is time consuming, but if done correctly, you can be sure of a flat surface when you have done.

                Sorry about the lecture, but I couldn't do it with fewer words.