I recently had a valve job done on my 91 240SX. The engine ran OK before except for a tendency to die at idle. I went to change the plugs and discovered a stripped sparkplug hole. Since I had to remove the head to have that repaired, I thought I'd get a valve job at the same time. Upon reassembly the car ran poorly and had little power. I did a compression check today and had 90 psi on all cylinders. I rechecked my cam timing and it is just as the service manual outlines. What is going on? Could an incorrect valve job cause low compression? Did the rings go bad while the head was off?
HELP!!
low compression
-
QuinGarcia
- Posts: 119
- Joined: Fri Oct 18, 2002 11:43 am
The rings wouldn't just go bad while the head was off. The only way that could happen is if you let some foreign matter fall in the cylinder and gouged a cylinder wall, but then you would have quite a variance between cylinders on a compression test. Yes, a poorly done valve job can bleed off compression, but who knows if that's your problem. I don't know what amount of psi a healthy nissan motor is supposed to come up with on a compression test. But I do know that it should be consistent from cylinder to cylinder. However, 90 psi sounds a little low to me. Did you hold the throttle wide open while you did the compression test? If the throttle isnt open, you will receive low psi readings. How did the spark plugs look after running it after the valve job? Did you use the right plugs and gap em right, or did you change to a different type of plug. What does the exhaust look like when you run it now? Smoky like oil or extra steamy like water?
-
QuinGarcia
- Posts: 119
- Joined: Fri Oct 18, 2002 11:43 am
Compression can be checked on a cold engine but for best results it should be done hot and with the throttle wide open and ign. disabled to prevent fire from ign. spark. Standard comp. is 190 psi and min. is 140 psi. If the valve stem heigth was not set at the machine shop than the valves could be open slightly do to the stem length being to long.
update
Valve clearances are correct. Leakdown test showed 82% leakdown. Putting some oil in the cylinder brought leakdown up to 98% until the oil bubbled past the rings. I have had a few people tell me that putting a fresh head on will cause this problem. The explanation I was given is that the valves now seal like new and cause excessive blowby. Looks like a tuneup has turned into a complete rebuild.