Resolved 2g - 30 PSI lost in Compression on 2 cylinders

[Gabriel Pena]

Hey guys , so I will be doing a slight refresh’s on my 98 GSX due to me about to hit 85k miles and I ran a compression test just to see where I’m at before I do the refresh and my numbers came out a weird , 150,150,180,180 . There’s nothing wrong with the dsm, pulls hard , no oil consumption issues , really smooth , it’s just that the numbers are off . I already have a new HG , ARP Head studs , oem timing belt with all the new pullys , all balance shaft components (yes I’m keeping them ) , water pump , all accessory belts that are ready to be put in . I’m leaning that the HG is starting to fail in between cyl 1&2 but if you guys can give me your own predicament that would be great , thanks in advance

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[1990TSIAWDTALON]

Do a leakdown test.
If its running ok and you have all the rebuild parts, pull it NOW and freshen her up and you'll be fine. Have the head and block decked for perfect flatness, clean the dead end journals on the crank, hone your cylinder walls, have a valve job done and clean the rest all up and go back together. Easy peasy.

 

[AWD-Tony]

Leak down test

 

[motomattx]

My guess is that your not letting the gauge max out during cranking, I see no reason to take that engine apart.

 

[Gabriel Pena]

Do a leakdown test.
If its running ok and you have all the rebuild parts, pull it NOW and freshen her up and you'll be fine. Have the head and block decked for perfect flatness, clean the dead end journals on the crank, hone your cylinder walls, have a valve job done and clean the rest all up and go back together. Easy peasy.

Hmmm , i will definitely look into that , can’t find a shop that would actually preform the test near me tho , guess gotta keep on looking

My guess is that your not letting the gauge max out during cranking, I see no reason to take that engine apart.

Im already taken it apart to put in the HG and Head studs so if anything I will send the head to the machine shop to look over it

 

[Boomdeeze]

Im already taken it apart to put in the HG and Head studs so if anything I will send the head to the machine shop to look over it

Do the leak down test before you tear it apart, it will help narrow down the problem. Pour a little oil in the lower cylinders to see how much it raises the compression, will help rule out bad rings.

 

[DSMPT]

I’m leaning that the HG is starting to fail in between cyl 1&2 but if you guys can give me your own predicament that would be great

Usually when the head gasket between cylinders is blown, the car doesn't start/run very well. The symptom would be something more obvious and would affect performance more.

 

[waltah]

Im already taken it apart to put in the HG and Head studs so if anything I will send the head to the machine shop to look over it

Your guess that the head gasket is failing would also have been mine: It is true that HG failures usually cause a pronounced miss and rough part-throttle performance but they can start small and 30 PSI variation is only just more than spec, isn't it? So maybe not quite a perfect idle but everything else would be okay.

Were you able to get that part of the HG off in condition to allow a good inspection? When they're bad there will be a piece completely gone, usually at the point where the cylinders are closest together but at the start there might be just a crack or a charred area on one side.

You had a reasonable diagnosis but without a visible bad spot you can't be 100% sure your new head gasket will actually fix the problem.

The other way to get proof would have been the oil in each cylinder and then redo the compression test trick. If that 'fixes' a cylinder (temporarily) the leakage was past the rings -- If there's a head gasket hole the oil gets blown through it and makes no real difference in those readings.

Head gasket failures are not normal in cars that are driven as designed -- as a 'sporty' passenger car if stock, but if raced (etc.) then the engine is built accordingly. Probably the commonest cause is incorrect torque of the head nuts: Follow the shop manual procedure exactly or if someone else does the work, be sure he knows that's what you want. I've heard 'shortcuts' that would curl your hair and I've made mistakes that cost me a redo, too.

These are all 'torque to yield' cars, are they not? When you just torque a bolt (say 110 lb-ft on your wrench) part of that force goes into friction of the threads and the other part into pulling the head down. How much goes each place? Well ... you're always told to thoroughly clean and lube the threads to reduce friction there to the minimum but how accurate is that?

Torque to yield is "pull the head down until the bolt stretches beyond the point where it's permanently deformed" -- the metal is actually flowing rather than stretching like a spring. All the bolts are the same (best not to mix bolts from different sets) so that's exactly the same force on each stud. Furthermore in this 'plastic' region the exact amount of stretch doesn't much change the downward force so turning each nut precisely the same amount isn't as critical as in the older method.

Why does 'exact same force on every stud' matter? Because the forces are so large that you are bending both the head and the block and if it's done unevenly the gasket likely won't fix the problem -- it will seal okay at the start but lack support somewhere and blow out into the next cylinder under power.

Next to that a warped head, damage to the gasket (crease or nick), or foreign material. Putting on and torquing a head is the only job I ask my wife to "Stay inside for the next hour, please, Honey" because my concentration's not that great and it has to be done perfectly.

'Course the last time I did it the FedEx guy showed up halfway through the last round of tightening ... "You want this on the porch, Sir?"

 

[Gabriel Pena]

2/1/22 Update ! After doing a leak down test the results came out to my HG leaking in-between cylinders 1 &2, just as i suspected .

 

[motomattx]

How did you do a leak down test after you said you tore it apart?

 

[Gabriel Pena]

How did you do a leak down test after you said you tore it apart?

"i will be taken it apart" i still haven't yet . Wanted to know my problem first before i dive into it blind