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Well,
I've disassembled, cleaned, checked, and re-assembled the entire pre-manifold intake (PCV, IAC, TB, MAF, EGR).
I found significant crud (not a blockage though) on the EGR entry point to the inlet manifold.
I found heavy gunk in the IAC valve, soaked it in carb cleaner, cleaned it out, then soaked it in oil, then drained it overnight before re-assembling.
I found evidence of heavy oil content in the blow-by: standing oil in the bellow pipe, oil in the inlet manifold, lots of gunk on the throttle body, etc. I also found that the throttle position switch was full of oil, so much so that I thought it may be broken. I took it off of the throttle body, cleaned with carb cleaner, let it dry overnight and re-assembled.
I cleaned all vac hose connectors, inspected all vac hose connectors and other parts.
I cleaned all hoses out with carb cleaner and rags.
The EGR valve itself was quite clean. Side note: why does this car have two EGR systems: one metal running around the rear and entering just after the throttle body, and one rubber running in the front and entering at the filter box? This is my first EGR car.
The PCV oil trap was not clogged! I replaced the o-ring and the flame trap (old one was broken and fixed with epoxy).
I re-set the stop screw on the throttle body and re-installed the (now clean and working) throttle switch. Side question: it has three contacts, but only two of them seem to do anything: the middle and lower contact are closed when idle, and open when accelerating. The top contact does not seem to have any function. Is this correct?
Throttle body gasket and EGR gaskets have been replaced, and anti-seize put on the EGR valve threads.
I checked the fuel pressure regulator: it's relatively new (still shiny!), and no leaks.
I drained the oil and changed the oil filter: when I got the car I had done an oil+filter change and put in conventional oil, and suspect that may have contributed to increased oil in the intake, so I switched back to fully synthetic. As an added precaution, I "rinsed" the inside of the engine with a quart and a half of clean oil and drained that as well, before the filter change.
I also did a coolant flush (well, I didn't mean to: after putting everything back together and starting the engine, I wondered where the puddle of water had come from, eventually traced the water hose above the PCV oil separator... well, long story short, it's all together now).
Result?
No luck.
Symptoms:
Upon startup, it runs fine, revs around 1200 for a bit, then drops to normal idling speed (around 800). Still running smoothly. After a minute or so (not really long enough for the engine to warm up) the revs start to seek: rising to 1000, falling to 700, then rising to 1100ish, falling to 650ish, and continuing like that, in widening amplitude, until it stalls.
For about a minute I can stop it from dropping the revs by pressing the accelerator, and it will rev steadily per my input. When I let off, it will return to the seeking pattern, or it will just drop all the way down to zero.
Sometimes, it will rev up when I press the accelerator, and then drop slightly to around 1200 and run rough: for all the world it seems like it's missing, but it's not. I have tried pulling each plug boot in turn, no change, and disconnecting each injector in turn, also no change.
Next in line: change the fuel filter (it's due anyhow, and I have a filter lying around for it). Oxygen sensors costing around $80 for the Bosch universal, I have to try everything else first.
One other issue: I have to $5 multimeters, neither of which state that they have the required internal impedance (1M-Ohm) to avoid damaging the electronics... so maybe I have to get a new multimeter? Or just get a new O2 sensor? I'm pretty sure it has never been changed.
My backup car: a Ford ranger running on three cylinders and two brakes, just popped the clutch master cylinder on my way home from work: I had an interesting time bypassing the clutch switch with a paperclip just so I could turn it on again and get home. But now I really need my volvo back...
All input gratefully considered! Thanks,
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