| I need your guys' opinion on something. I helped a friend try to narrow down the cause of rough idle in his stock 91 TT. He has about 68K miles on it at this point but says it has been idling a little rough for a while and I could feel it as well. Here is what we did and the results. -Power Balance Test
-Fuel Injector Ohm test
-Cleaned all connectors (PTU, Injectors, coil packs, AIV, MAF etc.)
-He replaced the TPS about 6 months ago
-Vacuum test (~19psi at idle)
-Fuel pressure within spec
-Replaced Spark Plugs about a year ago and all look idenitical with no signs of engine problems in any cylinder. We then did a compression test and this is what baffles me. The passenger side bank of cylinders read (From front to back):
166 psi
169 psi
170 psi The Drivers side bank read:
135 psi
135 psi
133 psi So basically the drivers side bank is reading around 30psi lower than the passenger side but are all within a few psi of eachother. At first I thought bad head gasket or valve/head issue. However, the fact that it burns no oil, is not leaking or burning antifreeze, has good power, plugs all look excellent after one year, and he says he is getting around 19 mpg with city/freeway driving leads me away from this diagnosis. The other possible cause I though of is the fact that he had his timing belt changed by the local nissan dealership right around 60k. Is it possible that the driver side cam sprockets could have been misaligned which could be throwing off the cam timing and not allowing this bank of cylinders to compress correctly? I read on a few other sites that this can be a possible cause of low compression in one bank and being that other than a rough idle it seems to be in great condition in every other way, I am starting to think this may be the problem. I know a leakdown test would help determine if the valves/rings are the problem, but is it easy to take the front timing covers off to check the alignment? Any opinions on this?
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