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Compression fault finding
https://auszookers.com/forum/viewtopic.php?f=5&t=54323		 Page 1 of 1
Author:  bumstein [ Tue Jun 20, 2017 3:58 pm ]
Post subject:  Compression fault finding
Hi all.

'93 Sierra G13BA motor, its always lacked a bit of power uncharacteristic with my other Sierra's. So it's off the road getting some love and I find the 2nd cylinder has much less compression than the others. By 'much less' I mean between 100-130PSI and the others 170-180.

So reading a bit and as far as I can tell it comes down to:

1- rings or bore worn
2- valves not closing/sealing properly
3- valve stems or guides worn
4- rocker gap adjustment
5- weak valve springs
6- head gasket

Now I've already done the rocker adjustments however I will triple check my work.

wondering if anyone has some tips on fault finding or excluding the other potential issues?

I found somewhere that the bores have a habit of elongating and may best be checked by a machine shop. Otherwise any other tips or potential causes?
Author:  just_cruizin [ Tue Jun 20, 2017 4:12 pm ]
Post subject:  Re: Compression fault finding
Oil or bubbles in the coolant will eliminate head gasket.

Blue smoke on start up after still for a while will indicate stem seals

Weak springs I thought would be uncommon as they would need to loose a lot of strength
Valve sealing or rings would be hard to differentiate I would think
Author:  sideways [ Tue Jun 20, 2017 4:28 pm ]
Post subject:  Re: Compression fault finding
Squirt some oil down the bore and see if that lifts compression. If it comes right up then it's piston/bore related. I'd probably back the rockers off half a turn just to check, back them off to much and the valve wont be open long enough to fill the cylinder properly. I'd probs retouque the head bolts whilst you're there, just to check.

It's not going to be 3 unless that's causing 2 but that's super unlikely unless a guide has sunk in the head. It'd run terribly if it was 5.
Author:  bumstein [ Tue Jun 20, 2017 7:01 pm ]
Post subject:  Re: Compression fault finding
Cheers fellas!!

There's no oil in coolant or coolant in the oil. But I'm not 100% certain that this completely rules out head Gasket or not?

No blue smoke on startup that I've ever noticed.

Good idea about oil in the bore!!

Will have another look soon and see what I can report back..

I'll retorque head bolts while I'm there and back off the rocker gap just a tad, probably set them to their maximum cold setting.

Failing these it might have to wait until I've the opportunity to remove the head and investigate some more
Author:  MrRocky [ Tue Jun 20, 2017 7:14 pm ]
Post subject:  Re: Compression fault finding
Overtorque the head bolts a lil ;)
Author:  fordem [ Tue Jun 20, 2017 9:00 pm ]
Post subject:  Re: Compression fault finding
bumstein wrote:
I found somewhere that the bores have a habit of elongating and may best be checked by a machine shop. Otherwise any other tips or potential causes?


I can't help but wonder - how does a hole get longer?

Bores wear "oval" because of the increased combustion pressure on the "thrust" side, but it's extremely unlikely that any one bore is more worn than the remaining three, and it would be quite unusual, though not impossible, to see that much of a variation due to worn bores and/or rings - I don't see worn valve guides as having a significant impact on compression numbers, the low rpms will give the valve springs plenty of time to "centralize" the valves as they seat, so my short list would be as follows ...

- valve clearances
- leaking valves
- head gasket
- broken/stuck rings

A compression test, dry followed by wet (with oil), is usually the first diagnostic test, the second is a "leak down" test, where compressed air is introduced via the spark plug hole, and that is usually quite helpful at pinpointing where the compression is being lost - you'll hear the air escaping, into the intake, exhaust, crankcase or cooling system.
Author:  bumstein [ Tue Jun 20, 2017 11:36 pm ]
Post subject:  Re: Compression fault finding
Haha just a smidge eh Mr Rocky?

Yeah, oval rather than elongated.

Ok, I'll do the dry then wet test and see what that shows up. And the leak down test, that might well be jolly helpful! Thanks Fordem
Author:  BAZZOOK [ Wed Jun 21, 2017 11:13 am ]
Post subject:  Re: Compression fault finding
Been told by a machine shop suzuki bores are very tough. So prob not worn.
burnt or cracked valve usually exhaust, cracked head. Cracked ring.
My son had a cracked piston in his wide track but you could hear the clatter
Author:  bumstein [ Wed Jun 21, 2017 11:34 am ]
Post subject:  Re: Compression fault finding
Cheers for that!!

I'd have thought that if I were looking at a ring or piston problem then the car would use some oil, but it uses very little. Still!! I will report back my findings. Good to know it isn't likely the bore
Author:  MrRocky [ Wed Jun 21, 2017 2:03 pm ]
Post subject:  Re: Compression fault finding
You can also drop the sump and check for ring debris my last 1.3 zook had half a piston in the sump but still ran ok if not a little smokey
Author:  Gwagensteve [ Wed Jun 21, 2017 2:23 pm ]
Post subject:  Re: Compression fault finding
Agreed - its would be unusual for one bore to wear more than the others, however, broken ring is a possibility.

A burnt valve or poor valve sealing would make a pretty distinctive noise IMHO, and I agree that a leak down test will detect these

Sierra bores DO ovalise. its the reason most cheap rebuilds last no time at all - its imperative to bore G motors with a torque plate installed. A quick hone will see them using oil again rapidly. The bores also "walk" once the head has come off which is part of the reason I would never bother to rebuild a G motor unless you're actually trying to build a competition motor and are prepared to spend all the money.

93 might be in the realm of the piston skirts failing and/or pistons scuffing bores. the G13BA's in SJ80's were renowned for this but you could generally hear it on startup. Dropping the sump will confirm this if you have a chunk of skirt in the sump. With the sump off you should also be able to eyeball the bore/bores for scuffing. I'd suggest this is most likely.

Steve.
Author:  bumstein [ Wed Jun 21, 2017 5:57 pm ]
Post subject:  Re: Compression fault finding
Well!! Thanks for input Mr Rocky and Steve!!

Just having a play now. I haven't had the chance to do a leak down test but a dry and wet test has been done and the results are interesting.

The first thing I did was remove the rocker cover and recheck the gaps. They are both sufficient enough that I'm confident that the valves are closing properly. A 0.17mm gap gauge fits into the intake gap, and a 0.2mm gauge into the exhaust while stone cold and an ambient 16.5 degrees, thus I haven't adjusted them.

Dry test and I got 160-170psi in cylinder 2!! So significantly better than yesterday. What?? Why??

Yesterday I reinstalled the compression gauge twice in case the seal wasn't the best and yielded the same results. What's changed? Nothing that I'm aware of.

So I thought I'd try the wet test anyway. 200psi. This is only on cyl 2 but I will test the rest anyway.

With 190'000 km on the clock I'm not surprised if there would be a little bore wear in each cylinder. What says everyone else, would this be a usual dry to wet difference in a G13BA with 190k on the clock?

Also I had the sump off last week to check the oil pickup. Did the timing gear and water pump at the same time.

I didn't notice anything unusual in the sump but then again I wasn't looking for it at that stage. If I dropped the sump now it would merely be clean oil in there.

Will wet test the other cylinders while I'm at it and report back the difference.
Author:  bumstein [ Wed Jun 21, 2017 6:09 pm ]
Post subject:  Re: Compression fault finding
Other cylinders similar difference dry vs wet. Around 20-40psi difference but difficult to tell exactly as the battery is getting weak
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