legacycentral bbs

Heres my battle. I have a 91 Turbo Legacy engine in a 85 wagon that has been running super great, then a couple weeks ago I started to notice a detonation like sound starting once I get around atmospheric pressure. I though that was weird so I checked

fuel pressure
wiring harness(ECU and engine)
injectors(resistance I cant check flow, of spray pattern)
cleaned MAF
checked TPS
changed plugs
then changed again to one step colder
changed wires
changed both fuel filters
checked and changed knock sensors(first one was good)
checked plugs for little salt and pepper flakes
tried different fuel
tried 98 octane fuel for a bit

And nothing, basically I dont really know were to go next. I was thinking watching the pressure sensor voltage, MAF voltage and O2 voltage to see if those sensors are dying, maybe build a test bench to check the injectors? So at this point I am second guessing if it is detonation, so I am about ready to go for a drive again and record the sound for later contiplating(really to go get ice cream). So what do you all think I should try next?

Matt

If you added 98 octane, and you still had it, it was not detonation. Maybe you should check your timing though.

WHich timing belt tensioner do you have on their? Is it the original style with the seperate pulley and tensioner or the one piece unit? I know I replace a few of the new style ones each weak here because they start to leak and smack against the block. Sounds like a good engine knock. Its a long shot but thought I would throw it out there

So I checked the pressure sensor, and it seemed to be operating properly. Then onto the oxygen sensor, hooked up the volt meter and off for a drive. Then the weird started happening, the o2 sensor was reporting -0.380 volts to approx. 0.440 volts. Since thats how o2 sensors dont operate I started to poke around the grounding system of the engine, since the voltage would always drop approx. 0.200 volts when the fan would turn on.

Now that I am done rambling, the problem was in the grounding system from the battery to the chassis, and the abnormally high resistance from the O2 sensor to the chassis (64ish ohms). So a quick wire from the battery to a couple points on the chassis, and then hose clamped to the O2 sensor and now the car runs like a champ. And dosen't have any hesitation like before.

Matt

I'm glad you know how to not throw parts at acar.

good diagnosis....