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Old 06-13-2022, 01:03 AM   #1
DS42
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Default Piston Soak, then compression loss #4

I have a 2001 Saturn SL2 DOHC with 242,XXX miles. My wife is the original owner, and I've used for a commuter car for the last 10 years or so. After deciding I could no longer deal with the lack of A/C on hot summer days during my 45 minute commute, I recently bought a new commuter, so time to sell the Saturn.

The Saturn, as usual, has spent the last 10-15 years burning oil. I've gone through a few EGR valves over the years due to the them gumming up from the oil burn. The check engine light was on again due to the EGR code, so before I sold it I bought a new one to replace it, and figured I'd do the plugs as well. Before this, other than the EGR code and burning oil it ran just fine.

Since the plugs were out, I thought "hey, lets do a piston soak and see if it will stop some of the oil burn for the new owner." I did an overnight soak with some B12, some MMO to follow it up, then a bit of oil in each cylinder to re-seal the rings in case they were stripped by the B12. Remove excess fluid, new plugs, crank it over. The car started up after, but I noticed after it warmed up a bit that it ran rough, which it hadn't before. I thought maybe it was just loosened crud that hadn't fully been blown out. I changed the oil and got a fuel system flush done, but it still ran rough and almost stalled out when coming to a stop.

I did a compression test and got about 150 on 1-2-3, and 60 on cylinder 4. I put a bit of oil down the spark plug hole and did another crank, and got 90 on #4, so it looks like a ring issue there. I never did a compression test prior, so I don't have a pre-soak reference.

Did my piston soak potentially unearth a problem that was being held together by sludge, or did I miss something afterward to fully finish the job? The car was parked at the curb during all this work so it was on a slight tilt when I put the oil back in to re-seal the rings, is it possible that part of the ring just didn't get coated properly? I somehow doubt it, since I've driven it a bit and I'd think that would get oil up there into anything I missed.

This thing is going away either way. I'm just wondering if there's something I forgot to do (or can do) that will get the engine running as it was before, so I can make it a viable sale to someone, or if it's getting sold to the junk yard.
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Old 06-13-2022, 12:55 PM   #2
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Default Re: Piston Soak, then compression loss #4

What altitude are you at? 150 is pretty low... and those were the "good" ones.

I would run it some more, then take compression again. I think most likely is that some crud from the piston crown has gotten mashed into a valve face/seat and may work its way out.

I doubt there is anything wrong with the compression rings, they never seem to wear, since these beasts are so "well lubed".

Are you holding the throttle wide-open when cranking to take compression readings?
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Old 06-13-2022, 05:39 PM   #3
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Default Re: Piston Soak, then compression loss #4

Sea level, or near to it (Bay Area). I didn't hold the throttle open while taking the readings, it was just straight cranking it over. Maybe that's why the readings were lower overall? I know spec for that engine is 180+.

I may try driving it some. It was just around the block a time or two after the oil change and fuel system flush, so maybe I'll jump on the freeway and let it run some high RPM's to blow out anything that got stuck. I thought rings since the compression went up after I dropped some oil in the cylinder, but there could be something stuck to a valve as well.

I forgot to mention that this isn't the first time I've done a piston soak on this engine, but it is the first time I used B12 in addition to MMO. I also shot some B12 down the EGR port during the soak to try to loosen / clean out some of that crud and extend the life of the new EGR. I know the EGR is fed from #4, so maybe something from that gummed up a valve.
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Old 06-13-2022, 05:56 PM   #4
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Default Re: Piston Soak, then compression loss #4

Iíve rebuilt plenty of S series engines with less miles than that and the carbon is nearly as hard as the metal surrounding it. A metal pick after the piston has been soaking in Chem-dip for days isnít even enough to free the stuck rings. The carbon was likely helping keep the compression up, but itís time to rebuild the motor. Use Silvo-lite pistons and itíll never burn another drop of oil again.
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Old 06-13-2022, 07:12 PM   #5
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Default Re: Piston Soak, then compression loss #4

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Iíve rebuilt plenty of S series engines with less miles than that and the carbon is nearly as hard as the metal surrounding it. A metal pick after the piston has been soaking in Chem-dip for days isnít even enough to free the stuck rings. The carbon was likely helping keep the compression up, but itís time to rebuild the motor. Use Silvo-lite pistons and itíll never burn another drop of oil again.
That's kind of what I was thinking, and worried about. Since I've already bought a replacement car, I have no interest in sinking time and money in to this thing that I most likely won't get back selling it, so it'll probably be off to the junk yard with it, unless someone really wants a "mechanic's special" sale.
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Old 06-13-2022, 09:46 PM   #6
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Default Re: Piston Soak, then compression loss #4

If you're in Michigan, I'll pick it up and rebuild the engine no problem.
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Old 06-13-2022, 11:32 PM   #7
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Default Re: Piston Soak, then compression loss #4

SF Bay Area? What part?

Yeah, not holding the throttle wide-open can affect compression readings a lot. Run it for bit, then take compression again.
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Old 06-14-2022, 01:00 AM   #8
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Default Re: Piston Soak, then compression loss #4

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SF Bay Area? What part?

Yeah, not holding the throttle wide-open can affect compression readings a lot. Run it for bit, then take compression again.
East Bay, out in the Tri-Valley area. Close enough to San Jose / Oakland / SF to visit when I want, but far enough away to avoid the crazy that populate those places.

I'm off for the next couple days, so I'll run it and see what happens.
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Old 06-14-2022, 04:35 PM   #9
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Default Re: Piston Soak, then compression loss #4

Walnut Creek here.
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Old 06-22-2022, 05:09 AM   #10
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Default Re: Piston Soak, then compression loss #4

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Old 06-22-2022, 10:02 AM   #11
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Default Re: Piston Soak, then compression loss #4

#4 can be a burnt exhaust valve. Gets a small v burned at the edge and there goes low compression. Put at TDC for that cylinder and blow air into spark hole and see how much you feel or hear at the tail pipe. Use another cylinder to compare sound. This is a cheap trial vs your alternative.
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