Bad Compression/Turbo

[Rabid]

So here's whats been going on. My 99 GST ran well, then started to lose power, then wanted to die at idle, then started to die at idle, and now won't start. My compression is 100 50 100 50psi and had a valve job done about 5k miles ago. the turbo is old and i'm sure needs replacing (guessing has about 160K on it). Could the turbo alone create bad compression? Good timing gone bad, if so how? Any other comments are greatly appreciated

 

[adamc95tsi]

sounds to me like you need a new shortblock .

 

[jwahl16]

A bad turbo will not cause bad compression. Something is messed up with your head gasket or your block, did this start happening immediately after the valve job?

And do a leak down test as well

 

[TuRBoTaLooN]

X2 above

Obviously compression is low, but make sure you crank the same number of times for each cylinder and that the battery has enough juice to crank the motor. Low voltage to the starter will yield low compression results.

Also, a leakdown test will tell you where you are leaking. But a quick test you can do while doing a compression test is to put a teaspoon of oil directly into the cylinder you are testing and then do another test. If your compression goes up, it would indicate your piston rings are not sealing and you need a rebuild. Otherwise, a leakdown test will indicate if your headgasket is leaking, intake or exhaust valves, or piston rings.

Good luck!

 

[Black95TSIawd]

The capfull of oil trick is a little misleading as a healthy engine will still show an increase of compression when oil is poured into the combustion chamber.

 

[Rabid]

No this did not happen right after the valve job, it started couple months later. This leak down test, drop a cap full of oil down the spark plug hole and do another compression test, if the psi goes up then piston rings need to be replaced... if copression stays about the same then its something in my head? Also, could a bad turbo burn my valves? I head this was true but I don't know for sure.

Thanks for the quick input too

 

[ramsack]

I think the cap full of oil trick is just the volume in the cylinder shrinking, not sealing the rings. Think of how much volume the dish in the piston has and how much oil you would need to pour in there to spill over the piston to get to all parts of the ring. That and you are pouring it right down a hole, so it's not going to have any chance of going anywhere buy straight down. In an engine with tilted pistons (V's, slanted inlines, boxers) then you are just oiling one small part of the ring.

 

[TuRBoTaLooN]

Good points about the cap full of oil trick. But either way, a leakdown test is definitely the way to go to find out where you are leaking. You can rent them from most auto parts stores.

 

[ramsack]

I consider leakdown tests more realistic than compression tests.

 

[BLINKY619]

piston ring time... maybe the stems but damn they would have to be shredded to cause 50psi of compression

 

[Rabid]

Ok so I've done a little more research and realized that if my rings were bad my car would emit smoke... well there has never been smoke coming from my tail pipe... so this leans me more towards cam timing, ideas?

If it is cam timing, I have HKS cams and would need to know how to put them back in time (and is it different than timing for stock cams?)

 

[ramsack]

They should be timed the same as stock cams, if you have stock cam gears. If you have adjustable ones then they should be degreed, but that's mainly to properly squeeze every bit of power out of cams as a mod.

 

[Nvisabl]

a leak down test is your only way true way to know for sure what is going on. Yes a bad turbo seal could lead to burnt valves all the excess oil in cylinder can cause a burnt valve. My guess is a burnt valve or something in the valve train when the rings go usually they go you wont have any compression. Good Luck