compression test

Post by **j_robi** » 27 Nov 2006, 11:06

did a compression test last night on my 1985 1.9dg, was about 9 on all 3. and 7 on one (topped this up with oil, went up to 14)
what would you suggest doing in terms of the engine? is it on its way out?

Post by **j_robi** » 28 Nov 2006, 09:54

is compression a good indicator of engine life expectancy?

Post by **OLD ONE** » 28 Nov 2006, 10:22

I think you should seriously consider getting it sorted..... Not good with that variation

Post by **tonytech** » 28 Nov 2006, 10:57

Well the cylinder reading low is obviously in need of some work.
The oil trick isnt a very useful guide to piston ring wear on the dg as the engine is on its side, so the oil could be sealing a valve problem as well as ring wear.

Someone who knows better will be along soon.

T

Post by **T'Onion** » 28 Nov 2006, 11:23

is it down on power ? using oil ?
i've just looked at the bentley and nominal is 10-13 bar and minimum is 8 bar so your no.3 is only just under , also the max differance between the cylinders is 3 bar

if it running ok then run it and keep an eye out of another engine , or rip it out and rebuild it .

Post by **j_robi** » 28 Nov 2006, 11:29

thanks
in the haynes it says new= 11-14, wear limit=8

its not using lots of oil, runs ok
open to suggestions

Post by **T'Onion** » 28 Nov 2006, 11:36

run it .until it bleeds

. theres a few on fleabay at the mo .. and a few breaker/traders on here are flogging them ..

Post by **HarryMann** » 28 Nov 2006, 12:58

Filling it up with oil is not the idea, as TonyTech says, not a brill idea, you want to spin it over with the plugs out after putting (a bit) of oil in there, before doing the comp test maybe..

Even then a leakdown test might tell you more, watch the pressure decay and listen in the exhaust and then inlet for hissing. If its goes down a lot faster than the others, and you can hear a slight hissing, then its probably the valves. Otherwise rings. You might have sealed them with an overdose of oil?

But those pressures are a bit marginal, especially if you can't get a nice reliable even idle, or fuel consumption is way off.

Providing carb and ignition are all good, a reliable slow and even idle and strong pulling at low revs are a good indication of nice compression pressures.

Post by **j_robi** » 29 Nov 2006, 15:08

the idle is ok, and fuel consumption really good
using ok amount of oil and water

what do people tend to get on their 1.9dg compression test?

Post by **HarryMann** » 29 Nov 2006, 15:37

125 psi (~8.5 bar) is what you would be looking for on any modern engine really (that is, not an old low compression or sidevalve engine) and always, fairly even across the cylinders. +/5 psi would be good enough. 175 psi would be exceptional for this type of engine.

Below 100 (6.8 bar) would not be good for sure.

Always controversial, but an oil flush or some mono-molecular additives can often free-up sticking gummed rings, if that is the problem. Don't of course use STP or PTFE additives.

Stick with it I'd say, and change the oil and filter a bit more frequently than normal. Once its spinning fast the compression leaks don't have as much time to escape so the test is simply an indicator, and depending on the source of the problem, may have less effect than imagined.

If it is using more oil than it perhaps should, that's of course another sign that piston/ring/bore may not be in A1 condition. Don't let it overheat.