Wisemen Needed, crazy cylinder leakdown results

[ChiTownHussla]

Ok guys I was at school as I train to be an auto tech, and we did compression test and leakdown tests on my gsx.
Compression was 174.25 average with low being 170 and highest being 177.

Now on to the leakdown test. I got the #1 cylinder (closest to T belt) to TDC using cams and verified also on crank.

When cylinder #1 was on compression, 4 should be on exhuast right? All my teachers agree that exhaust should be #4 if #1 is TDC. Now when we applied pressure while #1 was still at same position (TDC), #4 cylinder held all 120 PSI!!!!!

Obviously this means something is wrong , so we took the valve cover off to inspect, and saw that as #1 was at TDC, #4 had just closed the valves.

Can someone clue me in as to why this is and if it is correct? this is baffling to even my teachers who have 20+ yrs in field (albeit not on imports mainly)

Reason I ask is because I am running lean, with no power and crazy low o2 Readings. 20mv is what my scanlogger says. That means super super lean.
My teacher said maybe the exhaust is not escaping #4 and o2 cant pick up what is not there. Just looking to gather the right info to get my car running right and also to gain more knowledge and xp in the field. Thanks in advance guys

 

[bryanwheat]

Ok guys I was at school as I train to be an auto tech, and we did compression test and leakdown tests on my gsx.
Compression was 174.25 average with low being 170 and highest being 177.

Now on to the leakdown test. I got the #1 cylinder (closest to T belt) to TDC using cams and verified also on crank.

When cylinder #1 was on compression, 4 should be on exhuast right? All my teachers agree that exhaust should be #4 if #1 is TDC. Now when we applied pressure while #1 was still at same position (TDC), #4 cylinder held all 120 PSI!!!!!

Obviously this means something is wrong , so we took the valve cover off to inspect, and saw that as #1 was at TDC, #4 had just closed the valves.

Can someone clue me in as to why this is and if it is correct? this is baffling to even my teachers who have 20+ yrs in field (albeit not on imports mainly)

Reason I ask is because I am running lean, with no power and crazy low o2 Readings. 20mv is what my scanlogger says. That means super super lean.
My teacher said maybe the exhaust is not escaping #4 and o2 cant pick up what is not there. Just looking to gather the right info to get my car running right and also to gain more knowledge and xp in the field. Thanks in advance guys

IF the cylinder is reading good during a compression test than your valves aren't leaking much if any at all. I would suspect that there is a problem with the leakdown tester or that you just don't have all 4 valves closed if you can only get 120 psi on the cylinder. What did you have the compressor pumped up to? Your teacher's theory is kind of out there i would have to say. If the cylinder is making compression than the valves are operating properly.

 

[Rsawd95dsm]

IF the cylinder is reading good during a compression test than your valves aren't leaking much if any at all. I would suspect that there is a problem with the leakdown tester or that you just don't have all 4 valves closed if you can only get 120 psi on the cylinder. What did you have the compressor pumped up to? Your teacher's theory is kind of out there i would have to say. If the cylinder is making compression than the valves are operating properly.

He said wiseman! Oh wait why are you not one yet?!?!

 

[ChiTownHussla]

He said wiseman! Oh wait why are you not one yet?!?!

I mean this is open to all speculations and opinions of course. I mean there are plenty of master techs on here who dont have the WISEMAN in front of thier name ..

Although his theory does make sense also. if valves seal and allow compression .....they are working . IDK guys its just something that came up in class and I dont know why . I am trying to better my understanding along with my class.

 

[DSMTyson]

If the compression test results were even, then the valves are fine.

If you really want to get an accurate leak down test result, pull the cams out.

 

[ChiTownHussla]

I think you are right.... I will leave well enough alone I suppose

 

[JacksonAuto]

Getting good results from a compression test does not mean the valves are sealing. It just means that they're not horribly bent. That's why you do a leakdown test in conjunction with a compression test to get a real idea of whats going on.

The ideal method is to remove the timing belt and loosen the cam caps. This will ensure that all valves are fully closed. If this is not an option you can thread the hose into the chamber and while rotating the engine, place your finger over the end of the hose. You will feel the compression stroke by doing this and when you do, stop when the piston is at TDC then apply air pressure. I always adjust in the input side to 100 PSI just because it's easier to figure out the percentage of leakage you're experiencing. If you've got 100psi going in and 96 coming out then you've got 4% leakage.

 

[92AWDHX40]

Sometimes its nice to leave things alone, sometimes good luck.

 

[ChiTownHussla]

I am starting to agree.

 

[92AWDHX40]

You can also pm a wiseman too i like to do that when i have a specific question of things i dont know i know enough but, even some wisemen have questions, good luck.

 

[delta448]

OP, how are you determining -and then holding- TDC on #1? Got a long screwdriver?
Stock cams?

You can also pm a wiseman too

^Please don't do this.
If you have a question or problem, post a new thread or revive one that's less than a year old since the last post. The staff of this site believe in sharing all tech info with everyone publicly.

Also, just because I have a flashy red nametag doesn't mean I know everything. I'm just here to help out where and when I can and stay up on the new stuff. To my knowledge, we aren't actively looking for more WM at the moment, but I surely wouldn't be surprised if bryan is one of those who are considered when the time comes.

 

[92AWDHX40]

Thanks zac.

 

[GSTurbo1]

take the head off... take it to a machine shop, get it pressure checked. Most likely problems with valves

 

[DSMTyson]

take the head off... take it to a machine shop, get it pressure checked. Most likely problems with valves

Waste of money...

 

[GSTurbo1]

Well if he is this concerned, go get it done. Obviously he is trying to get it fixed so this would rule bad/bent valves.

 

[bryanwheat]

Well if he is this concerned, go get it done. Obviously he is trying to get it fixed so this would rule bad/bent valves.

Actually the correctly done leakdown test will determine if the valves are leaking without even taking the head off. Until the leakdown test is properly done and the valves are verified leaking than pulling the head is a complete waste. The problem with the car is that is running lean, not low compression. The original poster needs to look into his fuel system and check around the intake manifold and injectors for vacuum leaks.

 

[Lou98GSX]

i agree if you do a leak down test properly it will tell you if your rings and valves are good with out pulling the head

 

[ChiTownHussla]

I will boost leak check tomorrow. My new o2 is functioning properly Now and yes I was using long screwdriver and had someone else watch cams to see if they moved or not. I am fairly confident that I have boost leaks and that that is the problem now. Will update tomorrow

 

[Colt4G63 also]

Do you have the parts to switch back to the 2g MAF? If you don't have any other mods, the 3" GM MAF is overkill, and they are more of a pain to accurately tune. If your setup runs lean, I would be looking into that, and checking fuel pressure before ripping into the cylinder head.

Granted I'm no "wiseman", but I did stay at a Holiday Inn Express once before.

Ps. I also have owned and operated an auto repair shop by myself for the last 20 years, in case that gives me some sort of qualification.

Ps. ps. The GM MAF does not measure air temp and BARO, so every time the tempratue changes, you'll need to adjust your tune.

 

[ChiTownHussla]

My mistakes, my profile car is old. I since have gotten a 2g gsx mostly stock.

 

[YZFR1]

I don't see this mentioned a lot but one way i feel like i can get a decent leak down test numbers, at least with my cams, is to pull the valve cover off and visually make sure each cylinder's cam lobes are on the base circle. This way you know the valves are as closed as they can possibly be. And is certantly much easier than taking you timing belt of and such, even though that could possibly give you a more accurate reading.

Another thing i have personally noticed is there is a slight difference between taking readings at 50psi vs. 100psi. With the later being more accurate obviously, just depending on how accurate you want to get.

I just did it this way a couple of days ago on my 134 mile stroker build. The numbers went from 13% leak down to 5-8% in about a 100 miles.

Good luck