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Compression test questions -cranking test vs. running/dynamic test
Topic Started: Oct 26 2012, 09:37 AM (3,282 Views)
cwatkin


I did another compression test with yet another loan a tool gage just to have a good baseline number once I have my new used engine installed and running well. I have used like 4 different gages and it seems they all vary some in their calibration. I suspect these may be cheap units since they are being loaned out and may also be somewhat abused but the testers or just worn out.

It seems like I get very different numbers each time. My new engine got like 170 psi on all cylinders before I even pulled it. This seems reasonable as the engine is running GREAT with more power, NO OIL CONSUMPTION, no oil farts, etc. The oil is also staying extremely clean which indicates there is very little blowby. The oil on my old engine would start to look dirty within 100 miles and would be coal black after 300 and this was with all my flushes/changes. It never stopped doing this and seemed to indicate blowby not residual crud being dislodged by clean oil and MMO. The new engine was also very clean and had no sludge deposits/film when I pulled the oil pan, etc. The PCV and EGR systems were also extremely clean. All signs point to this being a very nice used engine.

Here is the deal. I got another loaner gage and it came back at like 125psi or so balanced across all cylinders. This engine just seems too good to have that low compression. I then did a running compression test and got 180psi on cyl 1 and 185psi on 2-3. I ran it each time for just a few short seconds with one real quick full throttle blip each time. I never let the RPMs come up much but wanted a full throttle blip in there.

So, what is going on here? Would you trust the running/dynamic numbers or do you feel that something is likely wrong with the cheap loaner gages? Is this type of gage something where you really get what you pay for? I don't believe the non-running numbers based on the first test and how well the engine is running/the internal condition when I had parts removed. All the gages seem to vary quite a bit and I am questioning the use of loaners at this point. One of my past ones had a part fall off. I think this was part of the quick coupling but the gage continued to work like it did before.

Thanks,

Conor
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texan821
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Definitely not an expert, but I'm sure one will chime in here and either correct me or agree with me...

On the running compression vs non-running compression, my (educated-ish) guess is that it definitely matters. Think of holding a bowling ball one inch above a bucket full of water and imagine the size of the splash when you drop it. Compare the size of that splash to the size of the splash when you drop the bowling ball from six feet up. The bowling ball has the same displacement, but has greater force and potential energy causing greater compression.

At least that's how I imagine it. Hope this helps.
Edited by texan821, Oct 26 2012, 03:05 PM.
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Old Man


if that is true and a 2 minute run time = 150 compression---and a 10 minute run time = 175 comp. then an hour run time would equal 250 comp. and by the time you have finished a 10 hour trip your compression would be at least 550.

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texan821
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See :D I knew someone would correct me. Good times.

I thought he was describing a dry test. Like not running, just turning it over with a wratchet. My bad.
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cwatkin


Yes, dry test vs. running test. I am going to get another loaner gage and see if the numbers are way different.

Conor
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cwatkin


The way I look at a cranking vs. running test is this. Take an empty plastic soda bottle and place something inside to measure the pressure while leaving the can slightly loose. The pressure will be greater if you jump onto the bottle with all your weight vs. squeeze it flat with your hands.

I plan to get another loaner gage today and may get one from OReilly and Autozone so I have two different ones side by side. I will also do a wet test this time too.

My new engine runs so well, has no oil farts, no oil consumption, and no blowby past the rings. I don't really see how the compression could be that low unless the valves were uniformly burned some. The numbers were balanced between the cylinders but uniformly low with a cranking test. The timing was really retarded on this engine which I understand is not a good thing for valves. Maybe they are all weak the same amount and the valves are the weak link, not the rings.

I will report back once I get the new testers.

Conor
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cwatkin


Ok, I did some more testing on this engine and want to report back. I got two testers from two different parts stores and both were in agreement.

Dry cranking gave me 125-130 psi and a wet test made no change to the results. I got 180-185psi during a running test (running for a few seconds on 2 cylinders).

Please interpret these numbers for me. Are my valves all uniformly weak? This engine had the timing EXTREMELY retarded when I first got it and was wondering if this might have uniformly started cooking my valves. The distributor was turned almost as retarded as you could go and the numbers are uniform across all 3 cylinders.

I am considering rebuilding a spare head with bad valves as this doesn't look very hard. Would I be inviting ring issues next? I will say the inside of this engine was pretty clean and my oil looks as clean as the day I changed it after several hundred miles so I don't think blowby is an issue for these rings. There is also no oil consumption.

Any suggestions?

Conor
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cwatkin


This engine runs well but want to head off future problems. I was wondering if I should just leave it as is.

Conor
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clarkdw


Running test means nothing. It is not a test that is normally done so there is no data or basis for comparison. Any interpretation of your numbers would not be apples to apples. The other compression numbers, if done as per the how to thread, don't look great but if you are not burning oil and have no drivability issues I would call it a day and enjoy the great mileage a Metro can give.

Take your old engine and over the next year or so get it rebuilt so when needed it will be there. You have dumped enough time and money to get the car to running decent, now just do the simple regular maintenance to keep it going. Forget the flushes and multiple oil changes, etc, etc. If it is running ok, leave it alone.
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cwatkin


Yeah, that is my plan. I was thinking about doing a head, likely the one with a burned valve, then moving onto the rest of this or another engine. The engine in the car seems clean so I am not going to spend money on oil changes/flushes that are not needed. There are no obvious issues with the rings but am worried I might be looking at a re-ring job if I restore compression with new valves.

I would probably choose to rebuild the engine in the car if it comes to that as it seems to be a good runner overall and looks to have been reasonably well cared for which can't be said about my other two.

Conor

Conor
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clarkdw


Tear down the old one and check the bottom end because that is the only question mark in a rebuild. The pistons and rings should be renewed in most cases anyway.

As for your existing engine, you are right, leave it till it exhibits signs of really needing a rebuild. Looking for possible future problems will just lead down an expensive and possibly unnecessary path.
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mwebb
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FOG

running compression is hands down better than
a "standard" compression test

having said that
compression tests that do not involve the use of pressure transducers are
useless

the only test that matters is
leak down and if leak down is less than 10% on all cylinders
that is is good as you get .

the only way leak down tests do not find a problem in cylinders is if the cylinder in question has a bent connecting rod due to hydraulic lock related to water ingestion or bad head gasket or stuck open injector.
in which case leak down may still be under 10% but compression would be "low" because combustion chamber size is increased while bore and stroke remain the same

most will not agree with this
but
most do not use leak down testing so they just do not know any better ,
no one uses pressure transducers to measure compression
too bad , they have no clue what they are not able to see
Posted Image
Posted Image
back pressure in the exhaust at WOT snap BMW 328
Posted Image
jumped timing belt on a honda civic
shown by location of exhaust valve open on good and bad captures overlay-ed , same engine same car
Posted Image

moving along
intake vacuum at hot idle
measured at MAP sensor , Teed into the line before the restriction orifice
must be flutter less as in no flutter
at
20" of hg on a engine with "good" compression
Edited by mwebb, Oct 31 2012, 12:11 AM.
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mwebb
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FOG

on a running compression test
check valve should be removed from compression testor
engine at operating temperature
value will appear to be low
at idle

test at idle
then at 2k rpm
then back to idle
for WOT snap measure peak value at WOT snap and minimum value

you will have 4 values per cylinder , IF they are all pretty close , engine is good
if one set of values is not uniform
ka ka

a technical term
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starscream5000
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Got 70 MPG?

clarkdw
Oct 30 2012, 12:53 PM
Forget the flushes and multiple oil changes, etc, etc. If it is running ok, leave it alone.
This. :gp
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