89 C4 running problem

[john walker's workshop]

i have an 89 C4 with a running problem. hopefully someone on the board can save me some diagnosis time.
the bosch hammer says "closed loop lean not present".
obviously running leaner than it should.
it has a rough idle after warming up a minute or so, and sometimes, when it's hot, it won't rev up when given throttle, but it will idle. if the key is turned off and engine restarted, it will rev ok until it returns to idle. the throttle microswitches are good.
it has a new oxygen sensor, head temp sensor, airflow meter and DME relay. the compression is good, and it has decent tuneup parts and a fresh valve adjust. there are no vacuum leaks. i swapped in a known good fuel pump. i haven't tried a computer yet, but that's about the only thing left. i did pull it out and checked the contacts and it doesn't appear to have a chip, as the tabs are untouched. i checked for broken wires in the airflow meter loom, and it's plug is clean and tight.
the rough idle and no acceleration problem goes away with the oxygen sensor unplugged, and it drives well that way. obviously i need to find the problem so the car can have the oxygen sensor in the system. any ideas would be appreciated.

[Bill Wagner]

Hi John:

I've only owned my 964 for over a year, and I'm NOT an expert on it's electronics yet, so a good deal of this is speculative based on my experiences with other cars.

I'm assuming that "closed loop lean not reached" means that the O2 sensor can't drop down to what it sees as a lean condition, or the ECU is misinterpretting the data it's getting from the O2 sensor. As I'm sure you're aware, the O2 sensor puts out signal levels based on oxygen content in the exhaust. Typically these are programmed in the ECU in an almost binary fashion...at a low extreme the ECU says "I need more gas", at the high extreme it says "I need less gas", and then it's supposed to adjust accordingly. The same applies pretty much for air in the systems since air and fuel make up correct fuel to air ratio.

Obviously you're faced with an imbalance. The only things that I can think of are the following:

1. An injector is stuck open (or sticking) and either putting too much fuel into the system all the time, or putting too much fuel into the system when open, hence the "lean" level the O2 sensor wants to see is never present. My guess would be an injector is stuck open, otherwise the problem would be erratic. To test this you could probably do an emissions test and see if the output is too rich. If it is, I would disconnect the electrical leads to each injector one at a time, and ONLY one at a time. Of course you will be creating more problems, but the car MAY react differently on one or two injectors, indicating the problem boys.

2. Make sure the air intake mechanisms are operating properly....all the "flaps" are opening properly, etc.

3. Use another O2 sensor. Things are never perfect, and you COULD have a bad one, but it's unlikely.

4. Look for problems with the cables and harnesses themselves. This will likely be the WORST problem to troubleshoot since it would amount to sequentially replacing all cables/connectors with new units until the problem is isolated.

Hope this helps,

Bill Wagner

[john walker's workshop]

thanks bill. well, i just swapped in a borrowed computer, and same thing. i can drive all day with the oxy sensor disconnected, and other than not idling cold for the first minute, it runs fine. so to me that would eliminate wire loom and basic injection system problems, from the computer to the engine. it just runs too lean as soon as the oxy sensor kicks in. it did it with the old oxy sensor too. i think a continuity test of the loom between the oxy sensor and the computer is next. any more ideas? anyone? i can still remember the good old days. carbs, no smog crap,......

[Speedster94]

hallo
i had a similar problem and it was one of the Cableconnectors was broken inside the plugin from the Harness to the Sensor.the one under the Blowermotor on the driverside .
harald

[TRE Cup]

Hi John
964s can be a pain in diagnosis at times: Check all the plug connectors on both ends- they can cause you a ton of grief looking for the logical solution. Check the wires too.

Clarification- I was referring to the SPARK PLUG wires and connectors. We've had a few that gave us "other than ignition related" problems and the spark plug wires were at fault!

[Wayne 962]

I doubt the continuity test would yield anything, since you are getting results when plugging in the sensor. Still, if there is a short in the wire, I suppose...

-Wayne