No compression at all, apparantley

[Chrisso]

Short story: bigport 4age appears to have no compression on any cylinders.

Long story: The engine was rebuilt 12 months ago, but hasn't been run as I was dealing with the megasquirt ecu and other non-car issues. The head was reconditioned, ported and 256* 8.1mm cams were fitted. The bottom end had new bearings, seals, rings, and new gaskets were used all round. The intake was changed to itbs.

I was trying to get it running today as the ecu is now working great. There's great spark and plenty of fuel, but it wouldn't start. I ran a compression test and the dial basically didn't move. This is probably the last thing stopping me from having a running car again.

Questions: What are the possible causes of no compression? [I would have thought rings, valves and head gasket]

Any suggestions other than pull the engine apart and start again to double check?

Thanks,
Chris

[wilbo666]

Cam timing?

Cheers
Wilbo

[oldcorollas]

did you hold the throttle wide open while doing the test?

[Screamn_Sleeka]

holding throttle open or not wouldnt lead to NO compression,

since you DID have a cam change, wilbo looks to be on the money, take the cam cover off, take the engine to TDC, figure out which valves SHOULD be open/closed and see what the cam is actually doing, sounds like the exhaust and intake cams have been set up the wrong way.

[oldcorollas]

leaving throttle closed will lead to virtually no compression, as there is virtually no air. depending on the compression tester, it may not register anything at all, hence asking the question

i guess other option is that someone forgot to put on the timing belt

[fixeruperer]

with your cams, looking at the front of the engine, are the dowels for the cam gears nearly 180 degrees apart ?



theres like 15-20% differance with open or closed throttle, i doubt it would make the diff to cuase the guage to hardly regisiter.

[Screamn_Sleeka]

nah mate, air still gets through if you dont open the throttle, its not sealed.
My friend was testing his compression and told me he was getting really low, like 80psi, so i went to have a look and it was cause he wasnt holding the throttle open, so you do get a perssure reading, its just low.

-like fixeruperer said!

[oldcorollas]

ok. my experience is different but it depends on the gauge

edit: it is running a PCV to the intake now or not?

[xistant]

throttle open or not, when checking for compression i thought absolute zero sparkies in,
which would make having the throttle open or closed near void as air would get in via the other cylinders yeah?

[thechuckster]

timing belt borked

[jeffro ra28]

If cam timing is ok check valve clearance.

[JustenGT8]

You have checked the gauge is OK i assume? I've never had a zero reading no matter how badly smoked a motor was?

[lexsmaz]

As Justen said, check your gauge ..

As a very rough measure try putting your finger over the plug hole as you get someone to crank it over ... You will soon find out if its nothing ..

As oldcorollas said the throttle definately needs to be open especially now as its running ITB'S ... It wouldn't measure zero, but it would certainly be quite low ...

[Chrisso]

Okay, thanks for all the suggestions. It's not quite zero, but struggles to see 15 psi.

The timing belt is new and on.
Checked with throttles opened and closed no difference.
Going to check with a second gauge but the finger test yielded the same result.
Checked the valve clearance and appears okay on the exhaust side. I haven't checked the intake yet.
The PVC is currently open (venting to atmos).
Cam timing was set using the markings on the cam pulleys.

So it looks like I need to take the covers off and check the cam timing again and then check the valve clearance. It seems that this is the most likely cause. I only get time to play with the car on weekends so I'll check it out on Saturday and let you know who was right.

[WDE_BDY]

Have you put oil down the bores? Sitting for 12 months definitely won't help the rings seal at all. Try that before pulling stuff apart.

Callum

[frostyadonis]

when i got my bigport head overhauled, i assumed the notches on the cams meant
(top) but i was wrong. the exhaust cam notch should be at the bottom (180deg)

the cam gears can be installed both ways on the cam

check this before you remove the head in fustration

[af300e]

leaving throttle closed will lead to virtually no compression, as there is virtually no air.

That'd make idling a problem...........

[oldcorollas]

intake changed to ITB's, no idle bypass, no PCV, and throttle stops not set properly can lead to very low compression...

the actual compression at idle is very low... the cylinder is nowhere near filled... so if you did a compression test when idling, it would still be well down.
http://www.rbracing-rsr.com/calcinjpulse.html
no idea on accuracy, but they suggest 10-15% VE at idle..
that means the cylinder is 10-15% full.

assuming 10:1 CR, and cylinder is 10% full..... that means the compression at idle is actually.... standard air pressure (roughly) = no movement on gauge at all...

edit, more mathematically..
assume cylinder has 9X volume, and CCV is 1X. total volume = 10X and so CR = 10:1
10% VE = 0.9X air going in.
0.9X/1X = 0.9 compression.. the pressure in the cylinder is LESS than atmospheric at TDC!! you will get negative reading on gauge.

with 15% VE
1.35X compressed to 1X size = 1.35 compression
0.35x14.7 = 5psi reading on gauge.

when that amount of air and fuel is burned, it will increase in volume maybe 3 times, and still exter pressure on piston.... just enough to overcome friction and keep enging spinning at idle speed...

or something like that....
so.... what compression will you get with throttle closed?

[af300e]

intake changed to ITB's, no idle bypass, no PCV, and throttle stops not set properly can lead to very low compression...

the actual compression at idle is very low... the cylinder is nowhere near filled... so if you did a compression test when idling, it would still be well down.
http://www.rbracing-rsr.com/calcinjpulse.html
no idea on accuracy, but they suggest 10-15% VE at idle..
that means the cylinder is 10-15% full.

assuming 10:1 CR, and cylinder is 10% full..... that means the compression at idle is actually.... standard air pressure (roughly) = no movement on gauge at all...

edit, more mathematically..
assume cylinder has 9X volume, and CCV is 1X. total volume = 10X and so CR = 10:1
10% VE = 0.9X air going in.
0.9X/1X = 0.9 compression.. the pressure in the cylinder is LESS than atmospheric at TDC!! you will get negative reading on gauge.

with 15% VE
1.35X compressed to 1X size = 1.35 compression
0.35x14.7 = 5psi reading on gauge.

when that amount of air and fuel is burned, it will increase in volume maybe 3 times, and still exter pressure on piston.... just enough to overcome friction and keep enging spinning at idle speed...

or something like that....
so.... what compression will you get with throttle closed?

Perhaps, in theory.
I've seen a friend (incorrectly I know) test comp on closed throttles. 2t/3t hybrid, approx 10:1 static cr, 280 and 300 advertised duration cams, no idle bypass and idle openings already set, 225psi reading.

If there was just 1atm pressure in the cyl at cranking, why is it so hard to turn a motor over tdc with the plugs in?

EDIT: Ok, negative pressure, so vacuum, the engine is trying to crank on it's own??

[oldcorollas]

because when you turn the motor over slowly, there is time for the air to fill the cylinder.

those numbers before are idle speed.. cranking speed will have higher VE tho
how much higher i don7t know, but VE at hand speed = 100% minus the overlap amount (how much air goes out before intake closes)

edit: IF you couls spin up the motor, and measur the amount of power used.. it shoudl be hard initially, as lots of air gets into cylinder... then it should decrease as the VE goes down with less time for air to get into cylinder....that make sense?

225psi with large overlap cams and closed throttle? there is something seriously wrong with it cams set wrong?

edit: bu tyeah, IF you have a PCV valve which allows air into the manifold, and a large brake reservoir, then you can get some compression on gauge, maybe up to 100psi or so... more if you haev lots of air getting past the throttle...

but original poster has swapped to ITB's, no mention of setting up PCV or brake res, or setting throttle stops or throttle bypass for idle... so i assumed worst case scenario (well, worst case before suggesting something mechanical was borked )