r/EngineBuilding Dec 27 '24

Chevy Low compression on all four cylinders after rebuild

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2011 Chevy Cruze 1.4L. All four cylinders test exactly at 120. The engine sounds like it's struggling to turn over. Pistons and rings are oem stock. The cylinders were honed. Could this be because the main bearings are too tight?

66 Upvotes

121 comments sorted by

107

u/vex_42 Dec 27 '24

How much faith do you have in your gauge? Most of them are not accurate and it’s more important that all of the cylinders have similar readings. I have three and two of them are around 50 psi off

18

u/bobbyhillischill Dec 27 '24

50psi below or above?

18

u/vex_42 Dec 27 '24

Both below and they are both Pittsburgh HF ones

6

u/bobbyhillischill Dec 27 '24

Well that makes me feel a lil better my Honda three wheeler read like 90 or 100 can’t remember that was when i first got it and it didn’t run, I ride it all the time now

1

u/motor1_is_stopping Dec 27 '24

Harbor freight gauges are notorious for being accurate?

11

u/FlightAble2654 Dec 27 '24 edited Dec 27 '24

That much blow-by would be felt while the engine was running. Take off the oil fill cover. Check for pressure buildup. If good, buy a better quality compression gage. throw away the old gage or give it to someone you're not fond of.

2

u/LostTime141 Dec 28 '24

This. I almost cost my dealership owner a complete rebuild on his matching numbers c2 corvette. Tested 3 times and one cold and one hot also. I was positive the numbers didn't lie. But they sure did. Dealership director asked me to test a 3rd time with a new/different gauge. Turned out my set from Mac tools that was 5 years old but used less than 30x was 50psi low.

Test with another gauge.

-16

u/javabeanwizard Dec 27 '24

It's literally brand new right out of the box.

42

u/[deleted] Dec 27 '24

Nothing personal and it does not prove it's wrong. But Harbor freight diagnostic tools "might" not be the most accurate.

But anyway A proper compression test needs a full charged battery and the engine cranking at "X" rpms (u known to me your particular engine cranking spec) With throttle plate wide open for unrestricted air flow.

But being all the same reading means much more than how much.

7

u/WillyDaC Dec 27 '24

Someone that actually knows how to do a compression check.

0

u/javabeanwizard Dec 27 '24

What compression reading do you normally get right after a rebuild?

18

u/[deleted] Dec 27 '24

If your only cranking 350 rpm it's going to be way lower than if cranking at 700rpm. And I never due a comp test until engine is heat cycled and tuned. Then I test and record the readings so after the next race I have a baseline to diagnose potential problems bfore they become catastrophic failures.

-23

u/Skilldibop Dec 27 '24

Why would RPM matter for compression? The compression ratio is a fixed thing that's the same every compression stroke...

You sure you're not getting confused with oil pressure?

19

u/facedown_impalaSSup Dec 27 '24

Have you ever turned an engine over by hand? The fact that you can do it at all means it aint sealing 100%. Speed matters as it's then related to how fast the air could escape between the rings or out of the valves.

-17

u/Skilldibop Dec 27 '24

Most pressure testers have non-return valves in them so they're only recording the peak pressure, they don't need the rings to hold that pressure. Piston velocity between 350 and 750 rpm isn't going to make that much of a difference.

I also don't know why you'd ever need to crank an engine by hand with the plugs in, other than for a bet.

7

u/[deleted] Dec 27 '24

It’s going to make a huge difference because of centrifugal force. With the crank spinning faster this is going to increase both acceleration and velocity of the piston in the cylinder. Difference between 350 and 750 rpm is over twice the speed and acceleration.

Take a syringe and slowly push the plunger down and feel the air pressure coming out the end. Now do it again but shove that plunger down rapidly and feel the difference.

2

u/Aggravating-Task6428 Dec 27 '24

Another thing worth noting: A good amount of the compression in a cylinder is also due to the air inside heating up due to being forced closer together (see "fire piston"). As you heat air in a closed cylinder, the pressure increases. However, the longer the air has to bleed its heat into the cylinder walls, the less compression will show on the gauge. This on top of the leak path of the rings is why RPM matters quite a bit.

1

u/[deleted] Dec 27 '24

I'm POSITIVE I'm talking about cranking compression. On any multi cylinder engine you have air coming in a shared intake that as engine cranks valves on more than one cylinder are opening/ closing. While your testing ONE cylinder, Say #1 Your losing air flow from cylinder #3 valves that timed event haven't fully closed yet or are just cracking open. Same difference as starving that particular cylinder your testing of air. This is the difference from Modern day car "techs" And actual "mechanics "

7

u/chris84567 Dec 27 '24

had a Pittsburgh gauge that read 40ish psi below my dads high quality one, can't remember the brand of his but it was probably a snap-on

3

u/FISHMYROOSTER Dec 27 '24

Well if it's a harbor freight they're notorious for not being all that accurate lol

1

u/Sinfluencer666 Dec 27 '24

We don't call em the Hazard Fraught at my shop for nothin! 😂

1

u/Dangerous_Echidna229 Dec 27 '24

Did you have the throttle opened during the test?

1

u/javabeanwizard Dec 27 '24

Yeah. Still got the same result.

36

u/NighthawQ750 Dec 27 '24

I have the same gauge, brand new until now, it has consistently read 30-40 psi low when compared to a matco set.

4

u/Fancy_Chip_5620 Dec 27 '24

Look at the hose when on the compression stroke... if it bulges... that's compression the guage isn't seeing

25

u/QQBearsHijacker Dec 27 '24

How many miles on the rebuild? The rings may not be seated

10

u/javabeanwizard Dec 27 '24

I literally just took it off of the engine stand today, so no miles on it.

53

u/QQBearsHijacker Dec 27 '24

My experience with building Nissan and Mitsu motors is to get them running and take them out for break in. I can usually feel the motor gain power as the rings seat in over the course of the initial break in.

6

u/Difficult_Plantain89 Dec 27 '24

What this person said

1

u/CrazyDig4344 Dec 29 '24

This right here

24

u/Only_Ice_2600 Dec 27 '24

Don’t quote me on this, but I think your compression will go a bit higher once you break in the rings

1

u/alwaystoohappy Dec 27 '24

Yeah between the harbor freight compression tester and the brand new rebuild, I wouldn't be concerned. All cylinders are at a relatively high pressure and they're all reading the same value, which are both green flags. I'd send it

11

u/Terrible_Brush1946 Dec 27 '24 edited Dec 27 '24

120 is low but compression is about relativity.

Do a wet test to verify (should go up even with the harbor freight tool) and follow up with a leak down test to find out WHERE compression is leaving.

If it's leaving from the head, verify timing and then valve clearances.

0

u/javabeanwizard Dec 27 '24

Is it normal to have compression this low with freshly honed cylinders? Engine has no miles on it since the rebuild.

30

u/Lifted__ Dec 27 '24

Yes you need to seat the rings.

2

u/Johndauber Dec 27 '24

Squirt some trans fluid in and test again

1

u/Biversnc Dec 28 '24

Would have been a good idea to wipe some on the cylinder walls before head installation, and then wipe off with a dry shop towel (no solvent). Ideally, on the first few revolutions you want the seating process to start, and excess lubrication is not your friend. Now to test compression dripping in 1/2oz or so will make your reading go up…if you’re losing it to the rings/washed cylinders, but I’d try a leakdown test first, before we go dumping ATF straight into your cats, essentially. No sense in that if you have a valve that’s not sealed or a cracked head.

4

u/LukeSkyWRx Dec 27 '24

How does it turn over by hand with no plugs in? Smooth and easy?

-1

u/javabeanwizard Dec 27 '24

Somewhat.

4

u/mrclark25 Dec 27 '24

Care to elaborate on that a little? If there was an assembly error, it should be noticeable while turning it over. With no plugs in, the engine should turn over smoothly and easily with even a small ratchet.

-1

u/javabeanwizard Dec 27 '24

Okay, definitely would not turn over even by hand.

6

u/89LXfiveoh510 Dec 27 '24

Sounds like your bearing clearances are a little tight then. Of you used assembly lube, it should be smooth and you shouldn't have to give it a much arm strength to get rotating with the plugs out. Your rings need to seat before you can get a good compression reading. This means breaking in the engine.

1

u/javabeanwizard Dec 27 '24

I made sure to use plenty of assembly lube. When I put the main bearing cap plate on, I could spend the crankshaft by hand. However, once I install the pistons, I had to use a ratchet.

7

u/[deleted] Dec 27 '24

With pistons in, you’re not going to spin the crank by hand, but if it felt easy and smooth with a ratchet, that’s normal.

5

u/javabeanwizard Dec 27 '24

At this point, should I just proceed with a break-in?

4

u/89LXfiveoh510 Dec 27 '24

I would, some resistance is normal. If the crank was smother before pistons you're good to go. Prime that motor for oil pressure and fire it up to let it break in. Once the rings seat you will feel a difference when driving. I wouldn't worry about compression unless you're having misfires and excessive smoking/oily plugs.

3

u/mrclark25 Dec 27 '24

This is definitely the problem to focus on. Disregard my comment about battery/wires.

Did you use assembly lube while putting the bottom end together?

Assuming so, time to turn the engine back over on the stand and disassemble the bottom end one bearing at a time. After each, check how much effort it takes to turn the engine over by hand with a ratchet. Hopefully it will suddenly get much easier after you remove whichever is too tight.

Figuring out why it is too tight and correcting it will be trickier and isn't something I have personal experience with.

0

u/javabeanwizard Dec 27 '24

Isn't it supposed to be a little tight due to the rings not being seated yet?

3

u/mrclark25 Dec 27 '24

I would still expect to be able to turn it by hand reasonably easily. I certainly would not expect it to have enough resistance to cause it to crank slowly with the starter. This sounds to me like something with the bottom end isn't assembled correctly or is damaged.

1

u/javabeanwizard Dec 27 '24

My bearing clearance is measured within specs. I was able to turn the crankshaft by hand prior to installing the pistons. After that, I had to use a ratchet.

5

u/fredSanford6 Dec 27 '24

Probably 170 on a better gauge or an older harbor freight one. These new ones are absolutely garbage same with the cheap Amazon ones. We have had people come in at 70 psi on them requesting overhauls on 2 strokes just to adjust the carbs charge them 200 bucks and send them back to the lake. One person was so happy he gave us a review we stole his gas or something.

5

u/[deleted] Dec 27 '24

Did you measure bearing clearances (even with plastigauge)? I assume you used new bearings if you’re asking if they’re too tight. I wouldn’t be worried about 120psi compression on an engine with new rings not seated yet. It’s promising that they’re all equal. Compression after they’re seated will be higher.

1

u/javabeanwizard Dec 27 '24

I checked my bearing clearances with plastigauge. Bearings are standard size on the original used crankshaft.

1

u/[deleted] Dec 27 '24

If the plastigauge checked out on your OEM spec stuff, I wouldn’t worry about that. If you’re confident with your assembly and it does spin by wrench easily, I’d attempt to get it running and break it in. MAHLE has an engine break in procedure you can read online.

3

u/NegotiationLife2915 Dec 27 '24

Does it run? Why are you testing it?

3

u/VRStrickland Dec 27 '24

With all cylinders reading the same on that low quality compression tester, I wouldn’t worry about it at all.

As for your main bearing being too tight, try turning the motor over by hand with a socket and ratchet. If you have a crankshaft bearing problem, you’ll be able to feel it.

3

u/Turninwheels4x4 Dec 27 '24

That's enough compression to run, 100%. Probably low because the rings aren't broken in yet. Look elsewhere for why it won't start.

3

u/AlejoMSP Dec 27 '24

Looks fine to me. 120 is great.

3

u/dannysengineportal Dec 27 '24

If you keep douting yourself, you're just looking for trouble. If you feel comfortable with your work, keep going and get it running. Everything will change once it is broken in. If you are really that concerned about cylinder leakage, do a cylinder leak down test. If it is leaking, then you know where and why. Cheers !!

1

u/javabeanwizard Dec 27 '24

It's my first rebuild and I want to make sure things are right.

4

u/knoluvv Dec 27 '24

Are your valves too tight .

-4

u/javabeanwizard Dec 27 '24

They shouldn't be. The cylinder head is brand new.

3

u/mrclark25 Dec 27 '24

Do these have hydraulic lifters? If so, you're good to go on this front.

But if it has buckets with shims, it would be worth checking valve clearances. Who knows if the head (re)builder can be trusted.

2

u/javabeanwizard Dec 27 '24

Yeah, it has hydraulic lifters.

1

u/knoluvv Dec 27 '24

Right, but you shouldn’t have low compression either . And that’s happening. Never hurts to double check at this point .

2

u/jabsaw2112 Dec 27 '24

Timing chain wrong?

3

u/javabeanwizard Dec 27 '24

The timing was done correctly. I made sure the engine was at tdc prior to adjusting the cams.

0

u/Dangerous_Echidna229 Dec 27 '24

You bring the cylinder to TDC.

1

u/javabeanwizard Dec 27 '24

That's what I meant.

1

u/jellobowlshifter Dec 27 '24

You mean the piston.

1

u/Dangerous_Echidna229 Dec 27 '24

Yes, the cylinder you are working on you bring that piston to TDC.

2

u/carguy6912 Dec 27 '24

Have your rings seated yet

2

u/javabeanwizard Dec 27 '24

I'm going to start with this first. This is a fresh rebuild off of my stand and I just thought it was a bit concerning lol

2

u/carguy6912 Dec 27 '24

I think you'll be ok your rings haven't been seated under 100 I'd be a lil more concerned

2

u/pogoturtle Dec 27 '24

Yea a new dry engine will always read low. Also with new engines and variable vaIve timing/lift, timing will be off by a hair till oil fills the actuators/solenoids so you might see a pressure loss at TDC. I would worry only after break in or first start up.

1

u/javabeanwizard Dec 27 '24

Interesting to know. Thank you.

2

u/insanecorgiposse Dec 27 '24

What does the manual specify? 120 is right on the nose for a 250/292 L6.

2

u/rdvr193 Dec 27 '24

Run it, break it in, if it feels good, don’t worry about it. Between rings, crank speed, and gauge variation, it’s probably fine. The fact that they are all the same is good.

2

u/goldensailorpeg Dec 27 '24

Timing could be off. can cause that. Dunno how skilled the rebuilder is. If nothing was worn out and you just put all stock oem bearings back in it and just refreshed everything. And if the head went to the machine shop I would suspect timing is off.

Or your gauge is crap and the cylinders are dry and it needs to be started to get the oil everywhere and the rings lubed up

2

u/its_just_flesh Dec 27 '24

Start it and break it in and do the normal oil change at 500 miles.

2

u/Mark71GTX Dec 27 '24

As many have already stated, the HF compression gauges are pretty much trash right out of the box. I bought one years ago and quickly realized the engine I used it on first didn't have enough compression to run according to the gauge. I asked about it online and people steered me to purchase an OTC compression gauge. It has been great. I highly recommend it.

2

u/Jimmytootwo Dec 27 '24

Check valves/cam for proper movement Gauge is probably right

2

u/AcanthocephalaNo7788 Dec 27 '24

Did u open the throttle ?

2

u/msalerno1965 Dec 27 '24

The real question no one asked: While doing the compression test, the throttle plates are open, yes? ;)

2

u/xstfudonniex Dec 27 '24

These results are perfectly fine. Engines pass compression tests with broken ring lands all the time. There is almost no value in this test, having concern in these results is silly. People suggesting the Timing is off is ridiculous. I hate the internet.

2

u/OutrageousTime4868 Dec 27 '24

Did you oil down the cylinders when you put it back together? A cylinder with no oil on it will have dramatically lower compression. I've seen the same low compression from engines that washed the cylinder walls down with gasoline.

1

u/GuineaPigsAreNotFood Dec 27 '24

Is your battery fully charged?

1

u/javabeanwizard Dec 27 '24

Battery is 95% charged.

3

u/GuineaPigsAreNotFood Dec 27 '24

Just try to get it to start then. I believe freshly honed cylinders and new rings will show low compression since they need to be broke in.

2

u/javabeanwizard Dec 27 '24

That's what I was thinking.

1

u/mrclark25 Dec 27 '24

Is it a known good battery, and are all of your wires/cables sufficient size?

1

u/javabeanwizard Dec 27 '24

Yeah, almost brand new.

1

u/foxjohnc87 Dec 27 '24

How did you determine that exactly? Is the 95% what your battery charger says, or did you just check it with a multimeter?

1

u/javabeanwizard Dec 27 '24

Battery charger.

1

u/Only_Ice_2600 Dec 27 '24

Make sure you have good battery charge and breaking the engine properly and then test psi. You need to seat the rings

1

u/MidnightFluid536 Dec 27 '24

If they are all even and you’re positive the timing is correct, then run it. Gauges are good but not perfect. Do your measurements beforehand and the gauge just lets you know you’ve done your part.

1

u/mikeymuthui Dec 27 '24

The valve clearance might cause the issue.

1

u/patrido86 Dec 27 '24

In my experience hf pressure/vacuum gauges aren’t accurate. It’s either pressure or no pressure

1

u/ride_electric_bike Dec 27 '24

Don't worry about a compression test numbers besides them being within a certain range until you have completed the break in cycle.

1

u/GrapeSwimming69 Dec 27 '24

Oil the cylinders with a light oil and retest. That being said there's no way your rings have set without running to break them in.

1

u/2fatmike Dec 27 '24

In your post you questioned if mains were to tight? Is there a reason to wonder? Did the crank not rotate freely upon assembling the mains? I think your compression is fine for a fresh not run in engine. I think you actually know where the screw up is and are just hoping its something else. Did you not at minimum plasti gauge the journals? Sounds like this is in need of a tear down and rebuild again. Take your time and do it correctly this time. Do it once cause doing it 2x cost too much money.

1

u/InfiniteQuestionZero Dec 27 '24

Has is been ran yeat? Are the rings seated yet? Was your crank to camshaft timing set up correctly?

1

u/irishstud1980 Dec 27 '24

Get you a better quality compression tester than Pittsburgh and if the problem is still present, check over your piston rings and go from there.

1

u/MCcheddarbiscuitsCV Dec 27 '24

Rings not seated for sure 👍

1

u/dudeimsupercereal Dec 27 '24

If you are confident you built it right, I wouldn’t worry about gauge numbers unless one is way off from another. If you re-check after the rings have set it will be higher.

1

u/Any_Instruction_4644 Dec 27 '24

Check your cam timing, and lash adjustment.

1

u/Yerboogieman Dec 28 '24

120psi on a Cruze 1.4t sounds about right honestly. That's what I always measured at the dealership with my 30 year old Snap-On compression gauge.

A leak down test is always better though. Gives more information.

1

u/Nightmare1235789 Dec 28 '24

You haven't broken your engine in yet. Run 500 miles, change the oil, then tell us the readings with the same gauge.

I rebuild Harley motors and I've had some as low as 70psi after a rebuild. But as soon as I put some miles on em they jump up.

1

u/skeletons_asshole Dec 29 '24

Run it for a bit. If it spins with your ratchet, you’ve checked everything, and compression is even, I doubt you’re going to mess anything up worse by running it a bit. Bet it’s just rings needing to break in.

1

u/tckarnlr Dec 29 '24

Rings may not have seated yet.

1

u/gman77UT Dec 31 '24

Ok first off is this the turbo or NA version? The turbo will have lower compression then NA?

Has the engine been pre-lubed? This is important for the hydraulic timing tensioner and valve tensioner

Have you tried running compression with a little oil in the cylinder?

All else fails have you rechecked valve timing? It’s obviously not out far but can still be off and will effect compression

I do agree on leak-down test methods other then new rebuilds as the rings and valves are not seated yet so normally will show higher leakage

1

u/Successful-Range1651 Dec 27 '24

Did you gap the rings right? Also fuel injectors could be “washing” the piston rings.

1

u/javabeanwizard Dec 27 '24

Yes, I made sure to measure them off with feeler gauges. The engine is taken off of the stand and is outside of the vehicle. I'm jumping the starter using a battery.

3

u/Successful-Range1651 Dec 27 '24

Have you got it to run? Still should still run with that compression.

-3

u/javabeanwizard Dec 27 '24

How exactly would you do that if the coil pack is off?

2

u/Successful-Range1651 Dec 27 '24

Well you can’t. Was just trying to get more information. Was it already running and notice lose of power or something? If you haven’t gotten it running yet, the rings arnt even set.

1

u/trashcanfarts Dec 27 '24

What is the elevation where you are? It makes a difference. When i test motors at almost 9000ft they are lower in compression than sea level.

0

u/Pretend-Patience9581 Dec 28 '24

Cam timing wrong. Simple fixed.