compression testing

[Davezj]

Hi All,

I have done some stipping and rebuilding today to assess a head gasket leak to water. my intent was to strip until all plug removed and crank the engine to see which plug hole the water came of then i would know which head had the problem. then do compression test to see how bad it was.

I set out to do a compression test on all six cylinder, so to make the access better i removed the inlet manifold but left the throtle body in place. removed the crank sensor connection to stop spark and fueling.

removed all plugs and cranked the engine, the starter came on but the engine did not turn over.

(Moved my tool box and put my back out with the car still in bits, not a good situation to be in)

couldn't figure out why it was not turning over.

So put the plugs back in and inlet manifold on, crancked the engine and it turned over fine. whats the crack!!!


Car now back together and runs ok. so today was a bit of a waste of time and i got to put back out for free.

Any ideas??????

Cheers Dave

[Nick Mann]

Maybe because of the crank sensor? I honestly don't know - I have turned the car on the starter with the rocker covers off!!

[Davezj]

removing the crank sensor is in the procedure for compression testing in the manual. This cuts the fuel and spark so detonation does not occur.

Cheers dave

[White Lightning]

That sounds odd.

What do you mean by "the starter came on but the engine did not turn over" - are you saying that the starter motor was trying to turn the engine over and couldn't ??

[Davezj]

No it was as if the starter was turning with high pitched motor sound but it didn't engage with the engine. maybe it was something to do with open vacuum line that were connected to the inlet manifold?

[richy rich]

hum stater motor should move the gear forward when spinning.(IE engage)

it has two wires to it power and switch wire nothing else

so i don't understand this 1

[The Vee]

You had the plugs out so there will be no compression. Therefore the engine offers very little resistence to the starter which would have been spinning the engine quite freely and effortlessly. In this situation all you will here is a whining noise.

[Davezj]

I had not thought of that.
but to check which one of the cylinders was letting in water i put numbered newspaper sheets over the plug holes to see which one had the water spray on it, and the paper didn't move not even a flutter.

i had assumed there would have still been a compression stroke for each cylinder every other revolution of the engine. which would make the contents of the cylinder shoot out of the plug hole or at least move the newspaper.

or am i missing something.

as a kid when i cleaned out the pistons with redex (as you did back then, wouldn't do it now) by pouring it in to each cylinder at the same time cover with a rag and cranking the engine with coil diconected, as directed on the bottle. It used to shoot the rags out like a pea shooter.

I am usually pritty good with cars but by this time my back had given out and i still had to put the F**ker back together. so i was not in my best diagnostic frame of mind.

in a four stroke engine, each suck,squeeze, bang, blow, cycle takes two revolutions of the crank, and each piston is off set to get smoother power delivery. But it still takes two revolution of the crack to get back to where the first pston started from in the suck, squeeze, bang, blow, cycle.

So the air drawn in to anyone of the pistons should have been expelled on each of the compression strokes if the engine was turning, even with the inlet manifold removed.

PS has anyone had any dealings with "Bal" as i am in the process of getting a head gasket kit from him. he seems a nice enough person and has made over 1000 posts. any comments or does anyone know him.

one more thing with the engine running and the oil cap removed do you see down the oil cap opening (with a tourch) a jet of oil comming up from the head and hitting the oil cap neck. i hadn't noticed the before but mines does it now.

Cheers for any input you can muster, i know this post is an odd one but any comments are appreciated. (Spelling is not my strong point)

[Nick Mann]

There is no compression with the plug out. But I would have thought there would be enough air movement to unsettle a few pieces of paper.
I still can't say why the engine wasn't doing as expected.

Bal has participated in a group buy that I organised, plus been to a couple of events. I have found him to be completely trustworthy.

If I remove my oil cap with the engine running, oil gets thrown out of the hole! Not liking oil all over the engine bay, I have never had the cap off long enough to look at where the oil is being squirted from!!

[richy rich]

as above on all points ^^^^^^^^^

[Davezj]

Cheers it's nice to know on all responces.

Cheers for all the help

Dave

[pitslayer]

i always leave the plugs in when doing a compression test, my golf would turn over properly with 2 spark plugs in....so i do one plug out tester in...plug back in repeat.......check theres no vacuum pipes disconected, ive not looked at my engine in detail, so wouldnt know...but again with the golf references lol....it had a pressure pipe on the back of block, if that pipe came off , there was no air pressure whatsoever in the block, car wouldnt run, and would barely turn over untill it was reconected