How is this possible???(Compression)

[MagickMan0730]

Okay I am going to try and explain this without going crazy because I am VERY angry....Ughhh so I put my car in the shop to get my head machined and to drill and tap a snapped exhaust manifold bolt out. Thats all. This mechanic that I "trust" has had my car for a MONTH and come to find out that he decided not to work on it after finding out that my 2nd cylinder is showing 40psi of compression!? So I'm like ??? The motor is a 4g63t 6-bolt with like 70k on it. How is this even possible??? Before I gave them the car I had no stuttering, stalling, kicking, boost problems, smoking, etc. AT ALL except for a little idle surge and running rich because I was VTA and had not yet recirc'd. I want to buy a compression tester and do it myself after reading up on it. I know my compression should be within 10-15psi of about 165 right? I just dont understand it. I just asked to get a new clutch installed and the head machined. A month???? He could of told me a little sooner. Any help please. Thanx in advance.......

 

[bmoha2]

Did you check the mileage on the odometer before you dropped it off? Did you drop it off in running condition? My buddy dropped his gf's Subaru etc off at a "trusted" shop and didn't see it for a month. When he called up he couldn't get a straight answer. Then he started receiving impound notices on the car. Turns out the shop owner had been arrested while street racing in the wrx and it got impounded. When they finally picked it up the tranny was toast. It's a lawsuit now. Oh yeah they also put approx 4500 miles on the car. Something about spring break.

 

[MagickMan0730]

Yea its only got like 7 extra miles on it. The car WAS definitely running and running hard when I gave it to them. I know I am almost near 400hp and I haven't had a problem with it. I would think with such a low reading it would be running like shit. Also I wanted them to do the head because of all the new gaskets they were gonna install. I was thinking I needed a new HG ### I kept overheating. You think since they did the test before they did the work that the HG was leaking and thats why it was so low. 40??? Like really? Lol smh

 

[BogusSVO]

with one at 40 you would have a miss or skip when running

there are only so many places the compression can go...

a) bent/tweeked valves... you would be able to find that by injecting shop air in the cylinder at TDC and listing at the TB(bent intake) or the tail pipe (bent exhaust)

b) past the rings.. you would hear it at the oil filler or dipstick

c) blown HG..
either to water, so you would "gyser" out the rad with the cap off

or

into the adjoing cylinder, if this was the case you would have 2 cylinders down on compression, for the reason the compression would swap back and forth between the 2 cylinders.


the bent valve is where I would look, you may have carbon build up on the valve stem that is locking up aginst the guide and keeping the valve from seating fully

 

[romeen]

a) bent/tweeked valves... you would be able to find that by injecting shop air in the cylinder at TDC and listing at the TB(bent intake) or the tail pipe (bent exhaust)

I recently did this and found out that in order for all the valves to be closed the cylinder had to be at TDC of the compression stroke. If I had it at TDC of the exhaust stroke then there was air going into both intake AND exhaust. Since this was the case for all cylinders I assumed it was normal(?). I have HKS 264's so I'm not sure if the valve overlap happens with stock cams.

 

[MagickMan0730]

Ok so I guess I would have to pull the head to check the valves. But just to clarify and kinda narrow things down: The motor has about 70k. I've had it for about 3 months(whole car) w/out any hesitations or bucking/stuttering. I dont know about the rings but I DO know that I overheated about 3 times and the car ran hot so I planned for a head gasket with the re-machined head. You dont think an exhaust leak would have anything to do with this right?

 

[romeen]

You might have warped the head when you overheated. As far as an exhaust leak are you talking about leaking exhaust valves or a typical exhaust leak somewhere after the head? The latter will not cause low compression.

 

[MagickMan0730]

Ok nvm its a typical exhaust leak. As for the head warping it could be. Idk. Its just that they did the test BEFORE they worked on it and I know that after they machined it and everything, new gaskets, etc it would probably be good. Its just funny how the car has beaten 2 MK3 Supras, 3 Subarus and tons of Hondas without a single stutter and it all of a sudden has 40psi of comp when I give it to him. I think I'll blow my car apart before I give it to another mechanic. Lol....

 

[dale187]

That's whyu you need to read tutorials and do everything yourself.

 

[jrohner]

I had a DSM with compression that low IN 2 CYLINDERS and it ran decent.

 

[Oldsid]

Ok nvm its a typical exhaust leak. As for the head warping it could be. Idk. Its just that they did the test BEFORE they worked on it and I know that after they machined it and everything, new gaskets, etc it would probably be good. Its just funny how the car has beaten 2 MK3 Supras, 3 Subarus and tons of Hondas without a single stutter and it all of a sudden has 40psi of comp when I give it to him. I think I'll blow my car apart before I give it to another mechanic. Lol....

Did you check the if the timing marks are aligned properly? A possible explanation to the 40psi in one cylinder is bent valves that were not there before! The mechanic may have not installed the timing belt properly after machining the head and he bent the valves during the test drive! If you can prove that the timing marks are off you can make pay for the damage.

 

[MagickMan0730]

That's whyu you need to read tutorials and do everything yourself.

Well said ! I usually do. And to OldSid-They didnt even do anything after a month of having the car. Thats why I was so baffled. But rather than trying to narrow the problem down I found one of my friends with a freshly rebuilt head. New everything. She should be running again in a week. All tests and checks are being done by me from now on. But Thanx alot everybody for the help...Really appreciate it

 

[mobythevan]

AT ALL except for a little idle surge

Same exact symptons on my friends car last year. Ran good, pulled hard, but idle would surge. Checked compression and had 3 good cylinders and one at 60psi. Pulled the head and found that a valve seat was burned/no longer sealing and causing the low compression. Had valve job done and put it back together, compression on all four good and no more surging at idle.

 

[MagickMan0730]

Wow...Come to think of it I did have idle surge. Everything else ran good. Maybe they're right. I dont know, but I guess it doesnt matter. I'm gettin a newly rebuilt head and just gonna work on my old one in case. Thanks for the help Moby....