Low compression on all cylinders?

[EFI_LC]

The thing thats interesting is that every cylinder is reading the same psi which makes me think either theres 6 problems that are identicle or one problem thats affecting 6 things.

The one thing that is definite is that its not sealing as the evidence is in the oil so my first question is why is that happening, has it got a major problem which is also causing the 75psi readings or is that a minor side issue.

If the cam is rulled out it only leaves a hand full of items left.... if the inlet valves are not opening fully or at the correct time then your compression will be lower, what ever it is its happening x6

Maybe the rings are upside down, I am interested in the outcome.

[Toucan]

Did you have throttle wide open during test? But that will only bring them up a bit, not enough to give your low result

[fuel]

yeah and did you crank the motor over for long enough until the gauge stops moving higher? usually 15-20 cranks/10 seconds or so.

I don't know nissan engines very well (or at all) but are there any differences in the heads, ie do some heads have bigger combustion chambers than others? Perhaps it could be a head/piston mis-match and the compression ratio is stupidly low.

also if you slap the engine back together you could perform a leak-down test to find out where the lost compression is going to (either out past the rings or through the valves or even head gasket etc).

[Unclejake]

How can you go past the obvious to me of rings/pistons?

75 psi across all 6 cylinders

100 when oil added

I don't know these motors but a 25psi increase with oil down the bore does not absolutely mean the gas is escaping past the rings due to a machining error. I too would recommend the head and timing gear back on and a leakdown test before you pull the engine to bits.

Has this engine sat for a long time? If so then there could be rust on the valve faces and a few 'stuck' rings. Both would improve with running the engine for a while if you were not too fussy about the long life of the motor.

Lastly - did you do the compression tests with the throttle open or closed?

[KKtrips]

most obvious to me sounds like the cambelt has skipped a tooth or two on the inlet side..

other possible scenarios -

are the pistons reaching top of the bore - ie short RB30 rods and pistons on a short stroke RB20 crank (this is an example - I don't actually know about piston/rod/crank ratios in RB's)

is the engine turning over good and quick when compression testing - if it a sluggish battery it could be bleeding it off through lack of piston to valve speed..

a rag stuck down the inlet pipe (or tape accross the inlet ports and preventing any air getting in)

the valves are all staying open drom crap under the valve seats (very unlikely as all 6 are even)

other scenarios are possible but dam they are far fetched and you can rule out the majority of these because it is the same on all 6.

the same identical issue happening to all 6 simultaneously is almost lotto sized odds..

I would be concentrating on issues that share components on all 6 cylinders like cams and crank and the less likely block and head

[Requiemk]

engine appeared correctly timed. both cam gears matched to factory marks aswell as engine in tdc when i pulled off head, i didn't remove crank pulley. I noticed both cam lobes were facing correct place at tdc

block is identical to other which is 5metres away bores are same size.

There are 2 types of pistons for these engines and both are non turbo high comp types which wield a healthy 170 to 180 mark when tested, have done so with old engine, which is the same as this one which is giving me problems

Tests were done with throttle not wide open which skipped my mind in the rush of things

Although in the past i tested first rb at both and results were close/same/cant remember but nothing to raise an eyebrow at.

Engine turns over speed is fine

engine will run but bogs down when given to much throttle and wont stay running for to long.

intake fine, removed manifold and no obstructions in head.

the engine has sat for a good 6 months but turned over every month or so?

to note 3 other rb engines in my shed have practically no piston movement at all compared to engine in question

[KKtrips]

Between when you bought it (I assume it was running fine then) and when you ran it this time - what has been done to the motor (head removed?, cams changed? other?)

[Requiemk]

i can only guess to whether it was running fine based on sellers information

Nothing inside engine has been touched since purchase just turbo manifolds injectors etc

[KKtrips]

is the engine block out of the car?

[KKtrips]

if so pull the pan off and drop a couple pistons out and check for damage/burnt rings

I fear you have been ripped off by some asshole..

[Requiemk]

damaged/burnt rings

I fear you have been ripped off by some asshole..

likely, bores mint as fuck which is wierd

[KKtrips]

post pics of pistons once you pull them out

[ProZac]

Ive seen significantly lower results from compression tests done with the throttle closed.. Not to the degree your getting, but enough to definately try again with WOT.

[fuel]

yeah and I've had a low reading on just one or two cylinders for not winding it over long enough, when I had thought I had wound it over enough times to get the highest reading.

[CaMpylobacter]

I've had bogging problem on a carb fed sohc engine that was a tooth out on the cam chain. triple check that shit. it may need advance/retard on the cams that's not set up right. full open on bottom dead centre doesn't hold much weight, really.

burnt rings i would think on almost any engine wouldn't be even compressions.

[Requiemk]

Now im doubting myself whether i did see it timed up, but it was. I don't pull apart engines without checking timing.

This is so gay i have gut feeling its not the rings, will put head back on and preform leak down test tomorrow, i should have done prior to but i eliminated other options in my head

- All the valves/springs cant be half fucked to give even low compression

- headgasket can't/shouldn't leak evenly across each cylinder

- rings/piston/bore

the wot thing is on my mind to as well......

[Requiemk]

yeah and I've had a low reading on just one or two cylinders for not winding it over long enough, when I had thought I had wound it over enough times to get the highest reading.

results did even out i give it about 4-5 seconds generally.

[kyteler]

Too modern, that's the problem

Go get the KH230 and an L28. Problem solved.

[KKtrips]

Interesting reading for you perhaps

http://www.oldschool.co.nz/phpbb/viewtopic.php?f=2&t=21721

My technique for compression test is this.

1 - Get engine up to operating temp

2 - Crack off spark plugs a quarter turn each

3 - Run engine for 30 seconds at around 2-3000rpm

4 - Remove the plugs (being an 8 plug you will need to leave 1 plug in per cylinder)

5 - Disconnect the distributor to eliminate the spark

6 - Jam the throttle wide open (have someone sit in the car) and make sure the throttle stays WOT until the entire test is complete (opening and closing the throttle between compression checks causes bore wash and can alter the compression from first to last)

7 - Start on number 1 and thread the compression tester in till it is firmly seated

8 - Crank Engine over so the engine turns over 20 times (10 compression strokes)

9 - Write down the result from the gauge (generally between 100-180psi) for each cylinder

10 - Back to stage 7 but move to next cylinder and continue

11 - Compressions shouldn't alter more than 10% between worst and last (up to 20% at most)

[Requiemk]

I took the 330 for a sweet trip to the beach today at lunch, highlight of my day.

All information is good. i will adjust my testing in future, but in this case i have achieved a good double compression easily on same model engine.

Any thoughts opinions are awesome might seem like im stuck on the same ideas but im just going off past experiences