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Test engine compression with engine removed


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16 replies to this topic

#1 bearbalu

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Posted 04 December 2004 - 03:35 PM

Can compression test be done with engine removed? I am worried if I did something to hurt the engine.

I tried doing compression test - opened the throttle body and put a compression tester tube on spark plug holes. Cranked the pulley with a breaker bar clockwise a few times for each cylinder. However, only cylinder 4 registered around 70. Cyclinder 1 did not register anything and 2 and 3 registered around 10. Intake manifold is still on, Exhaust manifold has been removed and so has been the air chamber. Timing belt is still on...I had drained engine oil which I realized after a couple of cylinders, I poured it back in, but it didn't make a difference.

I removed the engine to do for blown head gasket/clutch/timing belt job. I know blown headgasket would leak compression, but I could drive on freeways - so it must have had reasonable compression. Would cranking with a breaker bar not register any compression?

Thanks

#2 frag

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Posted 04 December 2004 - 04:09 PM

A compression test is usually done with the engine near operating temp.
Moreover, I think the speed at which you can crank an engine manually is not enough to register true compression. Third, you need a little oil on the cylinder walls and rings: what there is after a running engine has just been shut off. For all those reasons I think it will be very difficult for you to measure the true compression values of your engine when out of the car. Putting a little oil in each cylinder would probably give you relative compression measurements : you would find out if all the cylinders have the same compression or not.

When I replaced my timing belt, I used a compression tester to find TDC on Cylinder no 1 and I remember that the compression registered at TDC (I was slowly turning the engine over with a breaker bar) was way lower than what I knew the real compression value was.
That's the extent of what I think I know.

#3 bearbalu

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Posted 05 December 2004 - 11:21 AM

Gilles,

Thanks. I suspect that it is the manual cranking with breaker bar. What threw me off is that cylinder 4 registered 70, while others whimpered. If it is lack of oil, wonder if it is just an accident that when I took the engine out, only cylinder 4 had some oil in it.

How does one put some oil into cylinders? Through spark plug hole?

Balu

#4 bearbalu

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Posted 05 December 2004 - 11:24 AM

If you can't test compression outside engine, how do people validate the health of engine when they rebuild it?

Balu

#5 frag

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Posted 05 December 2004 - 05:08 PM

If you can't test compression outside engine, how do people validate the health of engine when they rebuild it?

Balu


That's a question for Emily.

#6 frag

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Posted 05 December 2004 - 05:15 PM

Waiting for Emily, I think one could get a fair appreciation of an engine's cylinders health by putting a little oil in each cylinder (thru the spark plugs holes. It is also recommended that all spark plugs be removed for a compression check) and then connecting the starter to a good battery.
I'm just speculating here cause I've never had to do that.
Good luck!

#7 bearbalu

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Posted 06 December 2004 - 02:16 AM

Gilles,

Thanks. Engine is already out, I can't use the starter motor! I did get the plunger to put some oil into spark plug holes, will try out tomorrow and crank it with a breaker bar. I spoke with a couple of folks today - with conflicting opinions - one claimed you could do the test with engine out, another said otherwise!

BTW, I had loosened the crank bolt before taking out the engine- 5th gear trick! I am glad I haven't removed the flywheel yet - undoing the crank bolt after cranking a few times can be tough! I am just wedging the flywheel with screwdriver to loosen the crank bolt.

Balu

#8 1993 Legacy

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Posted 06 December 2004 - 04:28 AM

Hi,
Try doing a leak down test. It will show you the state of your engine maybe even better than a simple compression test. And, you do not need to crank the engine or anything similar for a leak down test.

My 2c

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#9 frag

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Posted 06 December 2004 - 09:31 AM

Gilles,

Thanks. Engine is already out, I can't use the starter motor!

Balu


Sorry! Stupid of me not to have remembered the starter is bolted to the trans not the engine. :(

#10 bearbalu

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Posted 06 December 2004 - 01:17 PM

For leak down test, I guess I would have to take the entire engine to a shop without dismanting heads/valves...If I start working on my head gasket, it would be too late to do it...

I assume leak down would be tough in garage...

Balu

#11 ron917

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Posted 06 December 2004 - 01:39 PM

A leak down test is dead simple to do in your own garage, and with the engine out of the car.

A google search found a short explanation: http://www.type2.com...neg/leaktst.htm (the site is about VWs, but info applies to any vehicle)

You can buy a leak down tester at any automotive tool supplier.

How to build your own leak down tester: http://www.morrowmar...kdowntester.htm

Of course, you need an air compressor. If you don't have one, now is a good time to get one. I don't know how I lived without a compressor and air tools.

#12 bearbalu

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Posted 06 December 2004 - 11:49 PM

After squirting some oil and manually cranking, I got following readings.
Clearly oil helps, but I suspect manual cranking is liable to give erratic results.

Cylinder 1 : Before 0, Now: 45 one time, 60 second try
Cylinder 4: Before 70, Now 45,80,120
Cylinder 3: Before 10, Now 105,100
Cylinder 2: Before 10, Now 75, 90
Balu

#13 slo5oh

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Posted 07 December 2004 - 02:52 PM

How about some history... What have you done?

If you have new (piston) rings then you will get #s all over the place until the new rings seat. I wouldn't use thick oil in the combustion chamber. In the past I've used WD-40 to keep the cyl walls moist. I usually crank an engine over 2 to 3 times to get a good reading for each cylinder.

What made you want to check?

#14 bearbalu

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Posted 07 December 2004 - 03:02 PM

No compression test history. No ring or engine overhaul jobs yet - has just 86k miles on it.

I blew head gasket and just wanted to do some compression test before taking the heads out.

Have you done your tests in past with Engine out and manual cranking? Or with starter motor?

B

#15 slo5oh

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Posted 07 December 2004 - 04:29 PM

No compression test history. No ring or engine overhaul jobs yet - has just 86k miles on it.

I blew head gasket and just wanted to do some compression test before taking the heads out.

Have you done your tests in past with Engine out and manual cranking? Or with starter motor?

B


I've done it both ways on a ford 302. If memory serves me I was pulling between 175 and 180 on the engine stand... Had to have someone hold the engine to keep it from tipping over, stand and all.

#16 bearbalu

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Posted 02 January 2005 - 03:02 AM

To complete the loop here, I put the engine back in and measured compression on cylinder 1 and 2. On both cylinders I get to around 200 psi after 6-7 cranks with starter motor. I get to around 150 after 2 cranks. I would conclude that doing it on stand is not reliable. Car runs great!

#17 DDad

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Posted 02 January 2005 - 12:59 PM

Well, I do know on Aircraft engines you test compression cold- but its done differently than normally done on cars. Suspect thats how they check them when they rebuild them.
Requires a adaptor that screws in the Spark plug holes- set and "hold" the cylinder tested at TDC. Connect adaptor to airline with a measurable PSI- - place cylinder under pressure- see how much leaks down. If leakage is excessive- re pressurize and start listening for air hissing
In the intake- bad intake valve
At the Exhaust port- bad exhaust valve
In the Oil filler- bad rings
On a Subaru engine I'd probably alos add listen in upper Rad neck= for cracked head/Head Gasket




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