fox889 Posted October 28, 2011 Report Share Posted October 28, 2011 Just looking through the various paperwork received with my newly acquired '6' & have noted that in 2000 she went in for a service & either the previous owner or the garage asked for a compression test, these are the following results: 65 70 80 65 65 100 followed by "compressions very low" from the garage written on the receipt. This was some 11 years ago & some 7000 miles ago & as far as I can see there's been no engine work carried out to rectify the low compressions. When I bought the car, she runs very rough on low revs/tickover (looking at checking the butterflies etc) & did mention to the previous owner that this could be a valve problem, he didn't come back & say the "head's been off & done" so one has to assume the low compressions are still here! She starts well, runs at 60mph as smooth as anything & there's no smoke from the exhaust, a little 'puff' from the rocker cover but that's it. Over to you 'guru's of the 6'. Look forward to your views. Quote Link to post Share on other sites
robnob Posted October 28, 2011 Report Share Posted October 28, 2011 Sounds a bit low to me!! - I would have thought 140 - 180 ish in an ideal world and... to be equal within 10psi of each other Are you doing this with throttle open or closed? This can make a difference - should be open?, and again will make a difference if cold or warm... Putting a little oil (table-spoonful of) into each bore will temporarily improve the ring seal and help determine whether this is a valve or bore issue.... Quote Link to post Share on other sites
john.r.davies Posted October 28, 2011 Report Share Posted October 28, 2011 There's no such thing as "high" or "low" compression readings, as compression guages are not precision instruments and are never calibrated. The person who did the readings may have been able to compare with recent readings from similar cars, but we'll never know that. What the gauge can show is different compressions across the block, casued by poorly modifed chambers with varying volumes, or much more likely worn piston rings/bores. It can also indicate poorly sealing valve seats, and the test that robnob describes discriminates those, as the oil won't seal a valve, but will a piston ring. The list you quote shows very consistent readings, except one that is much higher. But it is a decade old and of unknown provenance. I suggest that you buy yourself a compression gauge (less than £25) and repeat the test yourself. Thus: Ensure a well charged battery! Disconnect electrics to ignition and, if you have PI, the fuel pump. If carbs, disconnect the fuel supply to the mechanical pump. Take out all the spark plugs. Connect the guage to No.1 plug hole, and turn the engine on the starter, with the throttles wide open. Turn until you have a consistent reading on the gauge - about four or five turns Record the reading. Repeat for the remaining bores. Try to be consistent in the number of turns and keeping the throttles wide open. You may like to consider that as a practice run, so recharge the battery and do it again! Look for consistency across the block. If they are not consistent, repeat the test using the oil as Robnob says. All bores will have an improved reading. If the low pressure bore is due to worn piston rings then the pressure will be restored to the same, or almost, as the unworn bores. If the low pressure is due to poor valve seating, then the reading will not improve. Come back if you wish, with the new list! John Quote Link to post Share on other sites
BlueTR3A-5EKT Posted October 28, 2011 Report Share Posted October 28, 2011 Just a couple of points- Before doing the compression check, ensure/set all the valve rocker clearances as per the service manual for your engine type. Is the car carb or PI? That may determine how high a compression ratio you may expect to record. Cheers Peter W Quote Link to post Share on other sites
Peter Cobbold Posted October 28, 2011 Report Share Posted October 28, 2011 You dont give the measurements' units - they are conventionally in psi. But of your garage was reading a scale calibrated as 'kg per sq inch' the readings would be fine , around 140psi. But 'kg per sqinch' is an unusual unit - but then the readings are unusually low. As John says DIY it. Quote Link to post Share on other sites
fox889 Posted October 28, 2011 Author Report Share Posted October 28, 2011 God I love this site, leave a question & there's replies flying everywhere, thanks very much, appreciated! All being well & weather permitting, I'm taking the 6 out to my mates garage where he has a lift to have a gander 'underneath' just to see what I've bought & time willing, we'll try & do a compression test! I'll come back over the weekend with the figures for your perusal! Thanks again. Nick. Quote Link to post Share on other sites
fox889 Posted October 29, 2011 Author Report Share Posted October 29, 2011 Weather held off so down the garage we went with intrepidation, but a wry smile has come on my face, as the following compression (dry) test results are (psi): 100 110 110 105 105 130 These were done dry & at nearly full running temperature. I'm quite pleased & somewhat releived at the above, which I think shows that she's in not too bad a shape for 75k & as far as I know, an untouched engine! I'm just about to post yet another thread on here, so grab a coffee & I'll see you in a mo! Thanks again Nick. Quote Link to post Share on other sites
Peter Cobbold Posted October 29, 2011 Report Share Posted October 29, 2011 Could be a low comression head - was car imported form USA? Quote Link to post Share on other sites
fox889 Posted October 29, 2011 Author Report Share Posted October 29, 2011 No, genuine UK car! Quote Link to post Share on other sites
rpurchon Posted October 29, 2011 Report Share Posted October 29, 2011 No, genuine UK car! check the serial number on the head,it could have had a diffrent head fitted 15 - 30+ years ago ? richard Quote Link to post Share on other sites
Nick Jones Posted October 29, 2011 Report Share Posted October 29, 2011 Throttle open or closed? If closed, then the high reading (1) indicates the one where the butterfly is more open than the others...... Nick Quote Link to post Share on other sites
fox889 Posted October 29, 2011 Author Report Share Posted October 29, 2011 sorry, should have said earlier, throttle fully open. Quote Link to post Share on other sites
squirrel Posted October 30, 2011 Report Share Posted October 30, 2011 if ever you are in any doubt about your compression readings or not sure about what is causing low compression problem. do a cylinder leakage test,its far more accurate because its not affected by compression ratios or battery/starter motor issues etc.. you can buy them from as little as 70 or 80 pounds these days. they fill the cylinder with compressed air and you can hear where you are losing pressure. if rings are leaking you hear it coming out of the dipstick tube/oil filler cap, if exhaust valve is leaking you hear it in your exhaust pipe,if inlet valve is leaking you hear it in the inlet manifold,if your head gasket is leaking you get bubbles in your coolant. anything up to 20% loss is acceptable. Quote Link to post Share on other sites
john.r.davies Posted October 30, 2011 Report Share Posted October 30, 2011 Nick, Those readings, bearing in mind my point about lack of calibration, are exactly the same as the old ones. Did you try an oil test? JOhn Quote Link to post Share on other sites
fox889 Posted October 31, 2011 Author Report Share Posted October 31, 2011 Nick, Those readings, bearing in mind my point about lack of calibration, are exactly the same as the old ones. Did you try an oil test? JOhn I shyed away from doing a 'wet' test as I thought the dry readings were quite acceptable for a bog standard 75k engine, rightly or wrong! Quote Link to post Share on other sites
Tony Millward Posted October 31, 2011 Report Share Posted October 31, 2011 Nick, if you are happy with those readings then there is not much more to say. However....John is correct, you really need to do a 'wet test' with a bit of oil down the bores. If there is wear ( and your readings indicate this ), it would be good to know where considering you posted the question in the first place...don't be shy.... Cheers Tony Quote Link to post Share on other sites
john.r.davies Posted October 31, 2011 Report Share Posted October 31, 2011 There's something about No.6. Either it's out of step, or all the others are. But as Tony says, it's your car! John Quote Link to post Share on other sites
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