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760 LH 2.2 Functionality- Partial O2 Sensor Failure?

brickborg

Active member
Joined
Mar 1, 2007
Location
Nomi, MN
Car is an 85 760 Turbo. Trying to be as detailed as possible, so this might be a long one. When I bought this car, it refused to idle. Replaced the IAC, then it wouldn't drive AT ALL till it got up to temperature. This is the current state of play:

Symptoms:
- When started from cold, the car will start right up and idle fine for several seconds- rock solid at 800RPM.

- About 50% of the time, the car will idle well until it is put in gear, at which point it starts running rich enough for me to smell unburnt fuel, and the idle goes to hell.

- The other 50% it will start running rich on its own, even if it isn't put into gear.

- When the idle changes, the motor pulls very low vacuum (ie. almost zero), which I assume is a symptom of the bad mixture.

- If you let the car idle for a few minutes (or drive the car even a block or two), it will warm up enough to run just fine, and the idle goes back to normal.

- You can then drive the car as normal, with decent power. However, even when it's running ok, the fuel mileage is very poor (I'm averaging 15mpg city, which would make it by far the worst B230ft I've ever owned).

- If you attach the LED tool for setting "base idle" settings to the test point on the passenger fender, it is impossible to get it to blink on and off and regular intervals. It is much MORE regular when the car has been warmed up slightly, however.

- Even when the car is warm, the coolant temp never exceeds the halfway mark. It will get to the halfway mark, the thermostat opens, then the temp drops back to the 1/4 mark and stays there during normal operation.

Diagnosis so far:

- Replaced timing belt and re-timed engine- The car when I bought it had been timed 2 teeth retarded when the timing belt was replaced. The previous owner drove it this way until he sold it to me. With the engine timing having been so bad for so long, this may have had an adverse effect on any number of things. Re-timing the motor helped substantially with the car running well at all temperatures, obviously.

- Smoke tested the intake tract- replaced exhaust and intake manifold gaskets due to massive leaks. This improved things substantially. You can now drive the car cold, it just idles badly and has low power until it warms up per above. It used to die entirely if the car was driven cold.

- Reset base idle- I adjusted the throttle body linkage, the base idle screw, and the MAF mixture screw per the factory procedure. The car runs better than before, but the symptoms persist.

- Re-tested for vacuum leaks- there is still a small leak in the ACC climate control actuators somewhere, but otherwise I saw no additional smoke after replacing a few sundry vacuum lines. This small leak in the climate control should not have such a great effect on running, unless I'm mistaken.

- Checked the FPR- No fuel or fuel smell in vacuum line when removed

- Replaced distributor and re-timed ignition to factory specs- Small improvement, but overall about the same as before.

- Replaced spark plugs- They were due anyway, but overall looked ok when removed. New NGK installed, no improvement but things didn't get worse either.

- Tested the 02 sensor by backprobing with a multimeter- this showed that from cold, the voltages are fluctuating from rich/lean at random intervals. This would be consistent with what I saw on the test LED, although I've been told that this test doesn't really tell you anything unless you have a high-dollar scope and can get precise output voltages.


So with all that said, I'm wondering if it's possible to have a partially failing 02 sensor, or if that could be the cause of my issues. I'm thinking that because the car runs well if you get even the tiniest bit of heat in it (ie. get it into boost once or twice), the self-heating function that the sensor is supposed to have is either no longer working, or doesn't work well enough to make the sensor function properly until the EGT's get a little higher.

Am I crazy to think that the 02 sensor could be failing in this way? Is there something else that happens with Lh 2.2 on the transition from cold-start to warmup that I'm unaware of?
 
The o2 sensor in your car should be a heated sensor. They warm up a lot faster so your car can go into closed loop operation sooner. A sensor on a warmed up engine that randomly switches is bad. The sensor should be switching at a bit faster than a change per second. As they age the switching slows down until they stop. But a slow one is bad.

I would also chack the engine temp with a temp gun. Make sure the fully warmed up temp is in a zone that the stock thermostat operates. You may have too cold or a failed thermostat which can make the engine think it's not warmed up and stay in rich open loop operation.
 
The o2 sensor in your car should be a heated sensor. They warm up a lot faster so your car can go into closed loop operation sooner. A sensor on a warmed up engine that randomly switches is bad. The sensor should be switching at a bit faster than a change per second. As they age the switching slows down until they stop. But a slow one is bad.

I would also chack the engine temp with a temp gun. Make sure the fully warmed up temp is in a zone that the stock thermostat operates. You may have too cold or a failed thermostat which can make the engine think it's not warmed up and stay in rich open loop operation.

That's kind of my thought process as well. I could watch the sensor blink RICH RICH LEAN RICH LEAN LEAN LEAN RICH RICH LEAN etc. while the LED was plugged in, so I'm thinking that the sensor is on its way out.

The thermostat was also replaced with OEM parts about a year ago, so I'll take another look at that- very possible the wrong temp range was used. My boss has a swanky IR thermometer that I'll point at the upper rad hose when it opens and see what's up. I also suspect the fan clutch might be tired, as it's original to the car. I have a new tropical version that I'll try and see if it makes any difference.
 
Thanks for the correction, I'll use the thermostat housing for the coolant temp.

I did notice that the seals for the injectors are rock hard, so I bought some and haven't installed yet. They aren't bad enough for the smoke I was using to escape, but it's probably not a bad idea to replace them anyway.

The MAF was new two years ago when i bought it and it's a legit Bosch part, so hopefully that's still good, but I'll run through the diag procedure on Brickboard and try it out.

The brake booster did have a cracked valve, but that was replaced and a new gates hose installed as part of the vacuum leak diag. I'll double check that my work is still good and look for a way to test it. It's never lost power braking no matter how long I sit with my foot on the pedal, but I'll investigate.

Thanks for all the advice, it's much appreciated.
 
...seals for the injectors are rock hard

Older vehicles need replacement of these o-rings...in colder weather, hard starts would not be uncommon.

.
>starts running rich

If your ECU's temp sensor is defective, richness will happen...I would check it via ohm meter, and check to see if it is connected to temp sending unit firmly. Hence, is sending unit good, and is the wire connected to it fit for duty.
 
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