| 92 manual TT with 110k miles. Just cleaned throttle body and replaced PCV valves and hoses. Cylinder #3 is now misfiring. When it is cold, it works fine. But when the car warms up, it misfires. It seems as if the misfire gets worse with higher temperature. When slightly warm (although dash gauge already indicate the usual mid-point operating temperature), the misfire is evident at idle, but if I give it gas, it seem to smooth out. However, as the car gets warmer, misfire happens even at higher RPM. The misfire was diagnosed with balance test, and I have confirmed that with an inline ignition tester (bulb lights up when there is a spark). Here's what I have done so far: x ECU code 55. x I have changed spark plug and coil with a known good cylinder and the problem stays with cylinder #3. x PTU connector has been cleaned and recleaned and recleaned. Sanded, Caig Deoxit'ed and Progold'ed, so that's about the best I could do with that. x Harness connectivity from PTU to coil has been verified (cold and when misfiring). x Injector checks out OK, 12 ohm at the injector and 13 ohm at the ECU. But since there is no spark, it is probably not the problem anyway. Also dumped in Redline fuel system cleaner more in hope than anything else (and I used Chevron gas almost exclusively). x Ground and battery voltage at coil connector verified (when misfiring). x PTU diode/impedence test checks out OK, when cold and when misfiring. I think I have the new style PTU although I don't remember doing the recall (bought the car in 1997). I followed the helpful numbers here. x I have tried to cool down the PTU by spraying electrical contact cleaner (safe for plastic, non-conductive, evaporates fast for quick cooling) on the metallic backside but that had no effect. x Also followed all of the hesitation FAQ except (a) change coolant, (b) induce intake turbulence, (c) change spark plug (NGK, changed 5K miles ago, and looks in great shape and color), and (d) clean intake valves and manifold. Right now, I am suspecting the PTU. Unfortunately, I don't have access to a oscilloscope to test the signal and don't have a spare PTU to swap-test with. What I have in mind is to rewire the connections to the PTU so that I swap pins on cylinders #1 and #3, on both sides of the PTU, of course. If the mifiring goes to #1, then I know it is the PTU. My question is: Can I do that? A better question would be: What really can I test that would make more sense. (I guess the PTU rewiring would be a little unusual.) Uh, besides checking harness connectivity from ECU to PTU (just thought of this). Another question: Can the CAS be the cause? I have already cleaned the CAS connector, of course. Does the ECU fire each cylinder by each signal from the CAS? I thought the ECU only gets the 1 deg and 120 deg signals and calculates everything else. Any insight from the collective wisdom would be appreciated by this n00b here. :-)
Cheers,
e.
P.S. My idle is high at about 1k, and has been like that for a long time. PCV change and throttle body clean were my first steps in preparation to take the idle down to spec (misfiring started soon after). I don't have a timing gun yet so I don't know what my timing is at the moment. Just in case any of these might be relevant.
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23:37:43 03/16/05
