S

smudge67999

Active Member
hi i have a disco1 3.9v8 78000 miles. been working on the engine and fueling side for some time. now engine drives good starts good but has a very slight miss when ticking over and is still heavy on juice 6/8 mpg driving car at 1000/1500 rpm. so only thing i have not done is a compression test so i completed one today and here are the results. starting with the n/s from front
first 3bar-49psi 3bar 49psi 3bar 49psi 4bar 51psi
now for the o/s from front first 5bar 74psi 4bar 55psi 4bar 55psi 4bar 55psi..... so would you say its rings on the n/s and maybe valves on the o/s plus what should my readings be for a 3.9v8 1996.disco. please
 
:eek:

Not surprised its drinking fuel with compression figures like that! I would be expecting

- For all readings to be equal
- Add another 100psi at least onto the lowest figures for an older engine.

A healthy engine I would be expecting that at least.

I went ahead and made neat table... easy to read. :eek: (in psi if it isn't obvious)

doMQtj5.png


I've seen something like this before, just asking just in case. Did you...

- Do the tests with the engine warm?
- Hold the throttle wide open during each test
- Take all the spark plugs out, rather than the one you had out that you were testing



If you did all of those things, then I would say your engine isn't healthy! Without taking the engine apart you can't tell if its one thing or another. Could be a very worn cam with very worn rings and very worn valves.

To test if it were the valves you can take off the heads and fill the combustion chambers with petrol, the petrol shouldn't drain out.

Buttt I reckon you might want to take that engine out and give it a well deserved rebuild. ;)
 
hi thanks for that info. only thing i dident do is open the throttle and i took 1 plug out at a time.y can this make a diffrence.
 
Yes it will make a difference, holding the throttle wide open will allow the engine to suck more air in, more air in the same volume gives you a higher pressure. Only taking 1 plug out at a time will cause the engine to crank over far far slower than it would if you had all the plugs out at the same time, piston is moving slower and less air gets in again.


fanatic said:
Did you crank with throttle wide open
Click to expand...
I already said that. :p
 
Its not supposed to cure anything, its used as a test. When you've done a dry test you'll get a certain reading. When you put some oil in and if the compression readings improve then that tells you if you have worn piston rings.

Say having a reading of 160psi dry and 170psi wet would indicate worn slightly worn piston rings.
 
I would be surprised if it ran at all with compression figures that low.

I'm sure I've posted the compression values for a 3.9 before but coz I'm feeling helpful here they are again.

These are for a 3.9 V8, engine warm (normal operating temperature), fully charged battery and good starter motor.

Wide open throttle - 150 - 160psi for 8.13:1CR and 170 - 180psi for 9.35:1CR
If compression is appreciably less than the above figures or varies by more than 10% piston rings or valves may be faulty. Low pressure in adjoining cylinders may indicate head gasket failure.

After doing the test "dry" put a small amount (say a teaspoon) of oil in to each bore through the spark plug hole and do the test again. If the compression result rises suspect rings, if no change then possible valves or HGF.
 
well looks like my engine is unhealthy. compression test wet test. you can basically add 1bar to the above dry tests no diffrence when redone the dry test with all plugs out and throttle open.
 
hi kev ive done the wet test and just about 1bar diffrence takes it to 75psi. no change on the dry test from previous test ive done. but the way the car starts and performs you wouldent think any thing wrong. only reason i did this test was because its bad on juice and i had exhausted all other possibilitys it could be.
 
what was your reason for doing compression test ,are cam lobes worn ,does it burn oil ,breath heavy ,smoke badly
 
no it dont burn smoke. breathers are fine. reason for compression test is because its been heavy on petrol 6/8 mpg. i took plugs out today and the drivers side are black sooted. the nearside if half brown half black. now i have been working on this for months to get the fueling right and exhausted most of the problems asociated to bad fueling even brought after market lambas at £80.00 each. if you look through this post you will see my compression tests i did the other day.dry test. i have redone them but wet test and a slight increase of 1bar
 
please note any engine will show a higher psi reading on a wet test as when compared to a dry reading as a rule of thumb plus 10 percent is not a worry and would be expected on rebuilt unit .one of the important things is all readings should be within 10 percent of each other.check the haynes manual for the expected readings for your vehicle and note pistons
of different compression heights were used on some markets
 
Are you saying the wet test increased the pressure of all the bore's from around 50 to 75?
Its still really low
 
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