RWD - Engine running badly / poor idle?

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Engine running badly / poor idle? 900 1995

Hi people!

This is actually a continuation of this thread: https://www.brickboard.com/RWD/index.htm?id=1641092&show_all=2

I'm making a new thread so as to present the case concisely. My engine is running very poorly, and I can't make heads or tails of it. Here's the history:

*** Current Symptoms:
Starts up OK, then after about 30 seconds, RPM starts to seek, oscillating from stumbling low to high, and then settling on a rough stumbling low speed around 500RPM, and shaking like it's on just three cylinders.

When driving on the freeway, I can feel it surging slightly, even with no change to my pedal input. Surges last between 3 and 10 seconds, and come seemingly randomly. Engine seems to purr normally.

When driving on back roads, it sometimes fails to give any power when I ask for it. Coming up a hill it was around 2000RPM, and I wanted to accelerate, so I floored it. Aside from sounding like the throttle was open, there was no change. It didn't develop any extra power.

When driving in a parking lot, it alternates between purring normally and sounding like it's on three cylinders - like all the "old" cars in the movies sound. Very embarrassing when a nice young lady driving a 940 comments on my car! Haven't seen her since :( Accelerating seems to send it into "three cylinder mode".

Engine has 250k miles on it. It has been well maintained throughout it's life, according to the records.

*** Work done as part of fixing this:
Oil flush, oil and filter change (Napa Gold filter)
Fuel Filter change
MAF swap with known good
Complete intake clean, reset, lubricate as necessary, including PCV, EGR, IAC, TPS
O2 sensor replace (it had the 02 code, changing the sensor got rid of the code and the CEL, and for a few miles, it seemed to perform well, so I thought the issues were gone).
Checked for vacuum leaks with a Mityvac - nothing
Checked FPR, no leaks, even with pump on and pulling on it with mityvac.
Visually inspected for vacuum leaks - nothing.

*** Possible Clues:
When I changed the O2 Sensor, the old one was clogged with white ash.
When seeking RPMs, the tachometer sometimes moves erratically, The erratic movement does not seem to be connected with poor performance.
I pulled all the spark plugs today to inspect. The plugs seem quite new; no corrosion or heavy buildup. What I saw:
#1: black carbon "dry" fouling.
#2-4: black carbon "wet" fouling, oil in plug threads, central electrode had white deposits on it.
I pulled the fuel rail off to look at the injectors. The flat tip of the injector was clean, but there was carbon build up around the sides of it.
When I cleaned the intake components, I found standing oil in the intake bellows, TPS was full of oil, PCV was clear and clean, EGR pipe where it entered the manifold was crudded up, but not clogged, IAC was dirty, but not clogged.
Since the oil change I've driven almost 100 miles with these symptoms, and the oil is still a beautiful clear gold color on the dip stick.
Probably unrelated: the rear muffler (I think the correct term is resonator) is somewhere on the roads; I don't know when it disappeared, but it was during those 100 miles. It sounds faster now!
Usually idling in neutral or park it will purr happily. Putting it in Drive or Reverse will knock it down to the three cylinder sound. THis is not a rule though; sometimes it sounds funny in park, sometimes it drives just fine (though this seems to be increasingly rare).
If I rev it up a bit (say to 3k or 4k), then let it drop, I'll hear some mild pops in the exhaust.

Your expert opinions welcome! At this point, I'm leaning towards the following:
Injectors blocked?
Fuel pump going bad slowly?
ECU going bad?
I'm not quite sure how to test those items.


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New Engine running badly / poor idle? [900][1995]
posted by  swiftarrow9  on Sat Feb 18 18:34 CST 2017 >

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