I'll take low compression for $100 Alex.....(question)
 

A conundrum if you will - Let's say you have a car with 'low compression'

I did, about 60k miles ago. Way back when at 200k miles. It measured about 50-80 per cylinder (except for the burnt valve on a dead cylinder- that was a "zero")

Mechanic said... "It's dead Jim...."

So I drove it home (cause thats how I roll :) ). Coming home over the mountain, I noticed a pickup truck ahead of me blowing all sorts of smoke and stuff.

I wasn't.

"Hmmm...." I thought. Got home, pulled the head, found the burnt valve, and the cylinder walls had original honing marks from when they were made.

Hmmmm.... How would a car with no compression have zero wear on the cylinder walls?
My working theory was - "perhaps the rings never even seated"
So I fixed the valve, and enjoyed the car another 60k miles. Now 260K miles

In yo' face mechanic

NOW.....

I can't get good PCV suction in the engine. Just redid the system. There WERE some leaks in the system- but- Engine runs worse. than before I redid the system.
Still no suction.
Perhaps the rings are worn out? Perhaps I did something wrong?

On one hand, the compression test from long ago sucked, but... the car has some variable timing which might affect compression, or maybe compression is compression-

But... there's not one bit of oil or debri or crap on the rear of the car which all my other cars had when the engines really did wear out. No oil plumes or friends laughing at me. My long since gone 944 literally shat oil on a coworker when I started it once and she was walking behind it- that's a bad sign.

No signs of a rear end nature here.

No "Colon Blow"

What gives?

The compression test 60K ago was done cold. Bad reading. Ignore it.

Trouble shoot for today. What does a leakdown show? I will confess I know nothing about PCV failures.

I believe I've also heard that you can do a test, then add a little oil to each cyl and test again. If compression goes up with the oil, then that shows that it's the rings. I've not done it myself, it's just something that I've heard. It makes sense, but I don't know if it has merit or if that would be the case for any engine bad rings or good.

Do you know what the engine SHOULD be showing?

^^^ leakdown test needed.

What is the car?

What motor is this? Some are more persnickety than others. There are small block chevy engines in '60s pickup trucks, and then there are 2.0L turbo engines in modern cars.

Steve suggests diagnostic steps, and I agree. Yes, oil on rings helps seal them for testing. Heck, when you do a leakdown test, you can listen to the tail pipe (suggesting the air is leaking past valves) and listen to the crankcase/breather (suggesting air is getting past rings). You can pop new bearings, rings and valves in a motor.

Or just get another one. What motor is this?

This is for xc70 volvo 2005. Another esteemed pelican recommended I dump it and just buy another- they are a dime a dozen- but I'm too far in.... :)

Thanks (for new leakdown test idea). I might do that. My gut instinct is that after redoing the engine, both cams might be in a retarded position during cranking until they can get oil pressure and be directed to advance by the computer, might be lagging behind. They've got about a significant degree oil activated advance when fully advanced. I'm scared I will just get another low compression reading- especially with my 'high tech' HF gauge.

I might try dumping some oil into the cylinders to get some lubrication for the test, but the CVVT system really leaves leaway for interpretation. Out of 360 degrees, is seems to be about 80 degrees of play in the preload.

When I redid my PCV system, the intake was shredded, to the point I could put my mouth over the holes and breath comfortably through the leaks. Now there are none, but still no love on negative pressure.

The real kicker here is that the engine is struggling at idle and dying/surging... (it was before I messed with it- and that's WHY I messed with it) and after "my" fix, now backfiring, I think in the intake. I have since replaced injectors and fuel pressure regulator, and Ive got a new fuel filter and pump, and new plugs, so the intake side should be good.

I'm going to change O2 sensors this weekend- but- even if that fixes the idle/surges, that doesn't address the lack of vacuum.

I am coming to terms with pulling the head, dropping the pan, and re-ringing it, but want avoid if possible.... Thanks all. :)

Hmmm. I wonder if the ecu has learned to live with the vacuum leaks and needs a reset…

^^^ great minds think alike! :D:D:D

The other problem, is now, I can't get the engine to throw a code to know where to look, which makes me think I've done something redneck to screw it up...

But.. it did throw one code. "check engine light" no signal...... which is weird and has no internet google references, and I could only relate that to an ecu, but the engine still runs too good for an ecu crap out. (for now)

Also is it possible that an electrical connection isn’t connected or got damaged from your recent surgery?

^ This I'm thinking and will research this weekend. My incredible mind cannot contemplate such oversight from myself, but I will be quietly double checking things this weekend.

To replace the upper 02 sensor, I have the privilege of going back through several pertinent connections

Another revelation from my amazing carnac brain- the knock sensors had the insulation knocked off them. Dumb arse me thought they were grounds, but, they must have been knock sensors. Those will be addressed.

I'm going to go reseach this... Maybe some 20-50 from the p-car oil stash could fix all of this :)

ugh, variable valve timing, fantastic for us, but ouch, that could be a huge pain if it interferes with classic troubleshooting.

Sounds like you need a new Retro Turbo Encabulator.
Or short of investing in a new one right off, get a leak down test.

Your post is a little confusing, to me anyways. The 50-80psi seem low but that is a turbo engine correct? How are you testing PCV vac? I'm thinking timing belt possibly jumped a tooth or cam stuck full retarded or full advanced?

I had a TR6 with worn rings and excessive blow-by that was pressurizing the crankcase and getting oil and crap up into the intake. That car was ancient by 2005 standards, but maybe with the low compression you have a blow-by issue. Just a wild a.. guess.

Volvo engine? Valves not rings is the typical problem in my experience. At a similar mileage my 850 engine bores looked like new. The valves, not so much.

Leakdown test will tell you very quickly.

Also, the Volvo PCV system is a total PIA...and I'm thinking that's not your real problem. Those engines usually run and idle regardless of the PCV condition.

Did you try the Volvo glove test?

Well, I'm going to try the compression test this weekend. Thanks for all the feedback.

One thing I didn't mention- was when I fixed the valve at 200k, I did the whole head, so the valves BETTER be good! :D

What codes is it throwing??


Why not do a proper diag on it? Start with the basics, codes, work through the test plans associated with these codes.

This. Your PCV system is likely shot.

Did you drop the sump and clean the pipe from the pcv canister? I had an XC70 do the same thing.

https://www.matthewsvolvosite.com/forums/viewtopic.php?t=53246

Yeah, I just replaced the entire PCV system, and dropped the pan and cleaned out the lower internal pcv ports in the engine. Unless I hooked something up wrong, or broke something in the process, there's no suction.

At idle, the rubber glove trick is producing more positive pressure than it did before I changed the system. The engine runs so rough right now, I cannot test the engine at speed, I run and turn it off it backfires so bad. Before, at idle, it was neutral. Not- some positive. Certainly no negative. I'm going to dig into some stuff this weekend, and redo the compression check, but I'm scared of ring blowby. The head- I'm hoping is okay, having a valve job 60k ago.

If the compression is that bad you only need your thumb over the hole...

:) I'm going to ease into this. Just spent 8+ hrs wrenching on this sucker and I get the privilege of sorting though it again.

The scuffs all over my hands/arms haven't healed yet from last weekend. Arghh!!!!!

If it's that bad, pulling the spark plug wire off will make zero difference to how the engine runs.

And a laser temp sensor will tell you if a cylinder is firing or not.

Check to see if your exhaust has any flow through it.

Cleaned the banjo bolt into the manifold?

:) The old banjo bolt one way valve was jammed and the round metal pcv attachment piece the banjo bolt goes through (and has the copper washers on each side), was clogged 100% with carbon and the plastic tube from it to the crossover pipe was broken. I don't know if the plastic tube was pre-broke, or if I snapped it during removal, but it was trashed. The access on that is a proper *****, so I loosened the manifold, and yanked the entire assembly (manifold and all) up so I could access that mother effin' banjo bolt from hell. That might have snapped the plastic line from the banjo to the x-over.

Somehow I must have clicked on 4 banjo bolts for the order, because I have 4 new ones. :) I did use the nicest looking one.


Today, I went and did something that needed to be done to cross off the checklist, but not necessarily something that makes sense from a pcv or low compression concern.

Earlier, I was getting a fuel trim bank malfunction, that can mean a lot of things, so I replaced the upper 02 sensor.

The car runs ****ty, but a lot less ****ty than it did. Before, it was backfiring so bad through the intake, I just had to turn it off. Now, it runs like it's missing on a cylinder. Still crappy, and struggling, but it runs well enough to get the feel it's missing.

I know the feeling. It feels like it is running on 4 out of 5 cylinders.

No code though. Tomorrow- I'm checking timing marks, compression, and ignition and fuel injection. I do have new plugs, new injectors, new fuel pressure regulator, new fuel pump, new fuel filter, and I rewired the injectors due to a fire several years ago. I need to recheck the wiring to make sure I don't have a dead injector.

Clearly, I'd like to get a vida on this, but, with the fire damaged wires (now replaced), and the older pcv system (now replaced), there were things I needed to go through before a dealer code read would reveal anything worthwhile. They would just laugh me out of the building. They quoted something like $500 for the o2 sensor, so I was just like "let me spend $100 or so for the part, slap it on, and make one less thing worry about. I've got the lower 02 sensor too, and will put that on (just because) after doing all the other stuff. I'm in a parts throwing mode, but I think, getting to the end of that, and they were things that can't hurt to replace anyway. The misfire gives me hope.

ok. thanks. dumb question, do you point it at the exhaust manifold? block?

today, the car was running good enough to warm it up- well, not good at all, but it wanted to run, not blow up like last week. God, that was painful. There was moisture/water vapor coming through the exhaust. Seemed normal, but I did not try to quantify it.
Also, when I replaced the upper o2 sensor, no oil in the exhaust downstream of the turbo. it was whitish greyish smokey dry like the exhaust fins on a turbo.
I'm hoping no clogged cat at this point.

Are the intercooler pipes all connected ok? Intake pipe to the turbo? All the one way valves ok? Just sounds like maybe a big vac leak somewhere.

Block, or exhaust very close to the block.

Thanks. My laser thermometer is mia at the moment. I put it somewhere I wouldn't misplace it. I did get out though and tinkered (very cold outside).

Compression with a HF gauge- 120 across the board. 180 with an ounce or two of oil. Hmmm....

Compression isn't your main problem. I think at 120 dry it should run better than what you're experiencing.

The good news, it's not a major engine issue. The bad news, it's something else....cat maybe? I don't think it's PCV...massive vacuum leak?

That's what I'm thinking. I'm going to hunt down the laser temp sensor and make sure everything is firing.

Clogged cat, failing ecu, bad injector rewire short, or failing coil are all on my suspect list. Dang thing still not throwing codes!

Oh well 120 is much better than I was expecting.

I'm going to throw on the downstream O2 sensor, not that I expect much. I was surprised what a difference the upstream made.

Problem is, it's cold as heck outside. Not fun to work in.:D

Unplug injectors or spark plugs one by one.....you might have a dead cylinder from spark or fuel. But from what you've described it's likely bigger.

Even on four cylinders it should run.

Oh... on your suggestion of vacuum leaks- never underestimate the impact of a loose intake manifold :D

There's that pia manifold bolt crammed inbetween the manifold and the t-stat. Now, I would never leave that loose, but if I did, that might make a difference. I also cleaned any electrical connection I messed with.

Car now starts and idles cold normal. Still some missing on warm idle low RPM, but, it runs and has negative pressure in the case.

I'm still going to check injectors and spark plugs as you mentioned. Slow steps getting better.

The low idle mis was the reason I went down this rabbit hole. Now with the new pcv system, and all the other stuff, I can figure it out.

Also, never lose an intake manifold bolt. Those fuch ers are 7mm, and impossible to find...

Truth^

Okay- I replaced the downstream O2 sensor- also, went to HF and bought a metric crowsfoot to tighten the pia bolt the last bit.

viola.

success. Smooth idle both cold and warm low rpm.

I dropped the crowsfoot into some dastardly crevice, went fishing with a magnet, and came up with a 10mm socket that went missing in 2018. The crowsfoot is still in there. I will go fishing again tomorrow.

Only weird thing is the right side of my instrument cluster went dead. No fuel tank level. No CEL. I'll deal with that later.

Thanks everyone for input/ help.

Great work, Leaky!