Low Compression

[GH05T]

Ok so wasn't happy to find my compressions are as follows (front of motor back)

130

130

130

112

112

112

Rb20 det

Is my head stuffed? I'm assuming so, is it easier Bette to get a new head of have mine reconed? How do I find out exactly whats wrong with it so I don't waste time ad money?

[shaund]

Assuming you've not got issues with timing or bent valves, it may be just a very tired motor with rings that are badly worn.

Other than low compression, are there any other symptoms or problems noises? coolant in the oil? oil in the coolant? lots of moisture/water coming out the exhaust? If you lift the oil filler cap is there milky crap on the underside?

If its a reasonably stock engine, go for a wrecker motor. It may not be worth rebuilding. RB20 wrecker motors are cheap. its probably a good opportunity to do an RB25DET conversion.

[GH05T]

Oil is clean coolant is crystal. Nothing out the exhaust.

Plent of mods

1bar boost

Td05 gtr injectors coilpacks Nistune. Claimed 210rwkw an only 120.000km so probably a hard life from the last owner (who didn't build the car)

Backfiring on high boost soni have removed the boost controller and running .5bar

Where do I start diagnosing after a simple compression test?

[blind_elk]

Guessing you did the test by the recommended method - engine up to temp, throttle wide open.

Repeat the test, this time adding a small amount of engine oil to each cylinder. If the compressions improve, then you have worn rings. If not, then it is likely worn valves. If worn valves, get a leakdown test to point to inlet or exhaust or both valves.

[GH05T]

Yup I did,

Will do the oil test and report. How muh oil? Like a drop or a splash

[GTScotT]

to be honest its just a tired motor as said above, but it doesnt mean you need to do ANYTHING.

if its not burning oil or mixing things where it shouldnt be, just leave it alone. main thing is that its consistent, not ONE bad one showing theres a serious issue... when they are all consistent it shows the motor is intact but a little worn.

years ago my best mate had a CA that was abit like that (only worse).. He put a disco on it, boosted and tuned it to a good 190kw, thrashed it for ages, sold it to another mate who put slicks and a locker and ran a 12, who then sold it to a friend of a friend who competetively drifted it for another 4 years (evidence on NS if you really need it) and then sold the CA to a bloke with a dato in working condition (literally nothing wrong). drifter boy put an SR in it............. 3 of them to be exact within the first 12 months.

so yeah. unless ur pedantic dont worry about it.

[DVS JEZ]

Recommend doing as GTScoT says but fix ur missfire, try a new set of spark plugs gapped to .8mm boost it back up and get the tune checked to be sure

[sriver killer]

engines are the most FORGIVING part of a car and you really need rough as guts to hurt them.

even though compression doesn;t look too crash hot more than likely she will keep on going for a very long time! as thats what they are designed to do

[GH05T]

Well I would like to be pedantic but ATM I can't hold over .4 bar happily, misfires and struggling warm starts.

[GH05T]

Also I have done most of the basic things already to address the misfire unsuccessfully. That's why I aspects maby the head, also why I turned the boost right down.

So theirs no easy way (unless it's rings) to freshen up the head?

[SargeRX8]

Put some oil in the cylinder and test again, if numbers rise noticeably, head is fine and rings are worn.

[Bsa]

Just drive it till it dies it's very hard to kill a rb20 tho. I have a head and block for sale if you want a new rb20 was making 260kw before being removed from the car

[xfixiate]

This is what you need to do:

1. Ignore all the posts above. Using oil can mask valve issues as well.

2. Get a "Leakdown" test done. This is where they bring each cylinder to TDC, and take the spark plug out and put in compressed air.

3. Remove oil filler, radiator cap, all spark plugs. Handbrake up, out of gear.

4. Rotate engine to bring first cylinder to TDC firing and inject compressed air into sparkplug (continuously) using a compressor.

5. If air blows out the rocker vent / dipstick / you have f*ed cylinders/bores/rings

6. If air blows out of coolant, you have head gasket problems or a cracked head

7. If air blows out of inlet side, you have bent/worn inlet valves.

8. If air blows out of exhaust, you have bent/burnt exhaust valves.

9. If air blows out of an adjacent spark plug, you have blown gasket (or air is coming out from valves being burnt/bent, but u will hear it in the pipes as well).

10. repeat for all 6 cylinders.

if the results are consistent, the you have a consistently worn engine. So go looking for fuel/electrical issues.

if you have one pot with stuck valves, then that will create misfires.

If al the above looks good, you should also look for a stuck PCV valve and blown gaskets around the throttle bodies. you should be able to hear a pfft pfft noise...

Good luck.

BTW I made my own leakdown test fitting. Brazed a compressor fitting onto the bottom half of a spark plug..

[GH05T]

Very interesting I will try that when I get the opportunity.

I did have the chance to do a wet compression test today.

For a start I got a new battery. My figures may have been skewed by my old battery. (we needed to piggy back it and. Have it out cos it would only hold up properly to 3-4 cylinders before flattening)

So new battery

The test we performed a few times

These are the figures for the three runs front to back of motor.

These arethe test results...

-Dry test a few weeks ago -old battery-

-Wet test

-Test again no more oil

-Yet again no more oil

            Dry.    Wet1.   Wet2.   Wet3. 

Cyld  1 - 130.    140.  140

Cyld 2- 130.    135.   135

Cyld 3 - 130.   132.  131

Cyld 4 - 112.   115.  120  125

Cyld 5 - 112.     125.   135

Cyld 6 - 112.     145.  145.   135.

I retested the lowest and heighest again.

I have noticed a bend of figures lowering in the middle so it seems to me to be a banana head

The 112-145 I'm hoping I a testing issue...

The only blackened spark plug again was no1 consistantly blackened?

Edited February 12, 2012 by GH05T

[Dose Pipe Sutututu]

Just drive it till it gives then worry about it, after all the motor is at least 20 years old

[AngryRB]

I dont think your compression is that bad , my brother used to have a rb20det out of the r31 and that have about 150psi in most cylinders ,

you could put a new set of rings on or convert to an rb25, sounds like your coilpacks need replaced, you should still be able to run a bar of boost even with worn out motor.

[GH05T]

But those figures, do they suggest a warped head? Seing as the figures bend... Lower towards the center?

I think I will get a proper leak down test next. See what that returns

Edited February 13, 2012 by GH05T

[Bsa]

Dude is a rb20 just buy a new one for 400 bucks

[GH05T]

I don't exactly have the time tools space or knowledge on how to test it to make sure I'm nt buying new problems and of course do the engine swap

[AngryRB]

But those figures, do they suggest a warped head? Seing as the figures bend... Lower towards the center?

I think I will get a proper leak down test next. See what that returns

if you have the ability and time, you could take the head off and have it checked and have a bit shaved off , that would raise the compression by itself, take half a mm or 1mm off and you'll up to over 160psi or more

but if it were me id get an rb25, check the compression , then you'll be on another level and more potential obviously

i wouldnt waste money on a 2L.