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94 Cougar V8 - intermittent flats and loss of power

  #1  
Old 09-29-2011, 10:58 AM
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Unhappy 94 Cougar V8 - intermittent flats and loss of power

Hi everybody,

I've a 94 Cougar (4.6L, 38k) that purred like a kitten until a few months ago. After being parked for a week in damp weather (Newfoundland), it started having intermittent flats (as if the spark was missing on certain cylinders), most of them being rhythmic (it looked like one cylinder went off for a few seconds, then started running again). This evolved quickly into a rough running at any rpm, random (no pattern) and intermittent (the engine would run just fine for 2 minutes, then would start sputtering - with obvious loss of power - for 5 minutes, then come back to normal run for a bit.

The ECM never put out any codes except for a misfire - during one more severe episode of sputtering. I've been driving the car with a ScanGauge attached at all times, to keep an eye on what's going on. Here's what I did so far, to no avail:

- replaced spark plugs, ignition coils, ignition control module. I did not replace the cables because there was no repetitive sputter after the first week of rough running, which would have happened if one or more spark plugs would have had no spark due to a bad cable

- disconnected injectors (one at a time) to identify one that's not working. Found a dead one (probably plugged, it wasn't clicking), replaced them all with freshly cleaned remans, no change

- checked fuel pressure while the car was parked - no problem

- engine coolant temp and air intake temp are read correctly

- TPS, RPM reported OK

- disconnected MAF sensor and drove the car w/o it, the stutter improved a bit,but never went away. ECM put out the right code, and it cleared it once I reconnected the MAF.

- the oil pressure is a bit low (it even went to the lowest point of the "normal" range on the gauge for a few minutes during a hot summer day, but then bounced back to about 1/3 of the normal range.

Now here comes the weird part: during stutter episodes, the ScanGauge reports some incredible fuel economy (2.5 - 3 l/100 km while driving at 40-60 km/h) which is unheard of from a 4.6L engine. The instant fuel economy jumps back to what I'm used to (8-10 l/100 km) when the stutter stops. When I had the MAF sensor off, the reported fuel economy was shown under 1 l/100 km (sometimes it went straight to zero while driving on a highway...) The engine also knocks louder than I ever heard it - when everything was fine I hardly ever heard it knocking, unlike now. All this makes me believe that the ECM is either getting wrong info from the sensors (CPS, O2) or cuts off the fuel way too early, leaving the engine running with a very lean mixture. However, nobody could confirm this is the case, and I don't want to start replacing parts on the engine before I have a good idea what can be wrong.

I wanted to replace the MAF, but if this sensor were the problem, running without it should have eliminated the flats - which is not the case. The stutter continued, randomly, so apparently caused by something else.

I was also about to replace the o2 sensors on the assumption that a false reading of too little oxygen in the exhaust would cause ECM to reduce the amount of fuel injected, to balance the mixture. Nobody could confirm this assumption though ...

Please let me know what you think. This drives me crazy, I love my car dearly and I hate to see it suffering like this. Any ideas are more than welcome..

Thank you kindly,
cougar_fan
 
  #2  
Old 10-01-2011, 06:40 PM
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Dear friends,

Here's a fresh finding, maybe this would help us figure out what my problem can be: I programmed the ScanGauge with four XGauges (O2 sensors output, both banks, upstream and downstream from the catalytic converters). I visually confirmed that my car has four sensors, to make sure I'm not reading crap from the scanner. To my surprise, the LH side (bank 2) front sensor (1) would only go between 0 and 5 (open or close loop). The corresponding sensor on bank 1 would go between 0 and 90 (as it should) in closed loop, and stay at 85-90 while in open loop mode. I thought the front LH sensor was toasted so I replaced it today with a brand new one - NO CHANGE! I then left it disconnected to confirm I'm reading what I should, and the ScanGauge shown zero output - so the ECU sees too much oxygen coming out of the four left cylinders.

Weirdly enough, the ECU did not put out a code for the disconnected sensor in some 3 minutes of running w/o it (maybe I should have waited a bit longer).

At this point I suspect that my ECU is acting up. It either misreads the O2 sensor, or fails to properly drive the LH injectors. A good ECU should have tried to enrich the fuel mix (because it sees it lean) which doesn't happen (my car actually increased its fuel mileage with about 30% since this problem started, so it's not only what ScanGauge claims).

Again, please let me know what you think. Any idea, no matter how off the topic may seem, could be the way out of this.

Thank you,
cougar_fan
 
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