bad compression check? please help

[rvought]

i just did a compression check today and a vacume test. the test goes from left to right 135 125 115 115. from what ive read this isnt good. the vacume test showed a 19-20. i took that off of the throttle body. i didnt do the WOT, i just saw that now, should it matter that much, same with operating temps. the car is a 99 gst with only 64,000 miles on it.
could this also be the problem that is just hesitates and flutters when i reach about 5000 rpms, if i hold it down, there is alot of black smoke coming out of the exhaust until i let off, i cant seem to build boost and takes like 2 mins just to go 60 mph. i need to replace my o2 sensor after the cat. that will be here tuesday. any suggestions to what the problems may be would be greatly appreciated!
~rob

 

[kenamond]

Redo the compression test. Motor needs to be fully warmed up for the test or the rings won't be expanded and will leak past the gap more than when warmed up. And do it at WOT.

If the compression is still low, you may want to try a Seafoam treatment.

The stuttering and black smoke sound like you're running rich. When's the last time you replaced your plugs/wires. BPR6ES plugs gapped to 0.028". Also, do a boost leak test. Boost leaks can lead to a very rich condition.

I'd say do the boost leak test first and fix all leaks. If the plugs are old, they're like $2.50 each at NAPA. Wires are like $30 or something (forget) for Bosch wires (I use them and haven't had problems). The boost leak tester will cost a few bucks for a coupler, PVC end cap and a tire valve. If you don't have an air compressor or portable compressed air tank, that'd cost some more ($70 for my 11gal portable tank), but that's handy for topping off tire pressure and doing future boost leak tests.

That's my $0.02.

Good luck!

 

[1g_tsi_awd]

Put about a spoon full of oil in each cylinder, if it makes your compression go up you have bad rings, if not, it is probably the valves or something in head.

I am guessing it may be a head gasket.

And about the smoke, kenamond nailed exactly what I wanted to say right on the head.

 

[Dream On]

The o2 sensor after the cat isn't going to do anything for you, except maybe shut off a check engine light. As already stated, check your plugs and their gaps, redo the compression check, and maybe also check your cat (if you still have it) to see if it's clogged.

 

[rvought]

thanks for the advice guys! i will re-do the compressoin check today when its all warmed up and at wide open throttle. should I do the wet test when its warmed up also?
With the o2 sensor bad, doesnt that tell the computer to go into safe mode and dump more fuel in because it thinks Im running lean. And no i dont have a cat anymore replaced it with a test-pipe.
I dont think its a head gasket, theres no white smoke coming out the back, my oil and coolant all look fine.
I replaced the plugs about 2 summers ago, i checked them out and looks like i could use a new set.
I have a 255 walbro fuel pump in right now with a stock regulator, havent had the money to get an aftermarket one yet, that could be a reason of running rich, i have the safc-2 sitting in my room, just dont have the time to install it right now.
Again, thanks for the advice and will re-do the test and post up later!
thanks,
~rob

 

[kenamond]

thanks for the advice guys! i will re-do the compressoin check today when its all warmed up and at wide open throttle. should I do the wet test when its warmed up also?
With the o2 sensor bad, doesnt that tell the computer to go into safe mode and dump more fuel in because it thinks Im running lean. And no i dont have a cat anymore replaced it with a test-pipe.
I dont think its a head gasket, theres no white smoke coming out the back, my oil and coolant all look fine.
I replaced the plugs about 2 summers ago, i checked them out and looks like i could use a new set.
I have a 255 walbro fuel pump in right now with a stock regulator, havent had the money to get an aftermarket one yet, that could be a reason of running rich, i have the safc-2 sitting in my room, just dont have the time to install it right now.
Again, thanks for the advice and will re-do the test and post up later!
thanks,
~rob

The rear O2 sensor just senses when the cat is bad. The front O2 sensor mounts to the O2 housing (hence its name). It is used for AFR at partial throttle. At full throttle, the front O2 sensor is not used.

Make sure you properly gap your plugs to 0.028". Too big of a gap leads to missing. I'm not really sure what too small of a gap does.

You're probably overrunning the stock FPR with the Walbro 255, but some folks say it doesn't make a big difference while others say it's a problem. The AFPR will cost you about $200.

Be careful with the wet test. If you put too much oil into the cylinder, you can hydrolock. And you only really need to do the wet test if your compression is low. If the compression numbers change a lot (20psi higher in the wet test than the dry test), it tells you your rings are probably worn leading to increased blowby.

Edit: Here's a link for describing a compression test including a wet test, compression specs, etc.:
Link