Technical Uno 70sxie rough running and stalling

[dodgem]

Hi all,
Ive been trying to fix a 1990 UNO 70 sxie (mono jetronic) that has a history of going well for a few months then suddenly playing up to the point it is undrivable or just cuts out at idle or when cruizing at low speeds. It also intermittently runs rough at idle. I have been lucky enough to aquire a Fiat /Lancia tester and the appropriate program module for this system and the ignition system.
I have now ben through the tests and the only thing it showed up was the o2 sensor so i went through the test proceedure and it sujested to check connections which are all ok. I ran the test again even with the car running rough and no faults found.
On my last attempt the car slowly peterd out on the side of the road i went throught the list of itmes i could check operation and i could see the throttle was opening gradulally wider which i guess was the stepper motor trying to keep the engine running.
I have also watched the lambda system operating and it swiches from closed loop to open loop and also shows lean, intermediate and Fat (Rich?).
I suspect there is a problem with the 02 sensor and it has a second one fitted already but about a year ago.
For some reason i dont know the tester often freezes and it did this twice when trying to monitor the o2 system of the ecu. Ignition system is fine checked all ok and the idle stepper motor and cold start are working perfectly.
Im wondering if i should replace the o2 sensor with a new one?
Cheers
Mike

 

[dodgem]

Hi All,
I have now got the car to fault on a regular basis and after some diagnosis with digital voltmeter and a tailpipe mounted o2 sensor discovered the closed loop o2 circuit in the ECU is faulty and jams on the FAT (rich) mixture setting causing the ecu to reduce injector timing and lean off the fuel until the car basically wont run!
I had a second ecu fitted to the car about a year ago also, and i can now confirm it has the same defect! I have just recieved anothe tested replacement ecu and it works great. I have been driving the car around and watching the output from the o2 circuit and it hasnt faultered since.
There is obviously some weakness in the mono jetronic ecu that causes the o2 loop circuit to lock up in rich setting even though the o2 sensor signal is telling the ecu the mixture is lean.
Maybe another member may find this information helpful as the same ecu is also fitted to Tipo 1.4ie and some other fiat and lancia cars of the era.
Cheers
Mike

 

[alexGS]

Hi Mike,
Thanks for posting the update, even though seemingly no-one was awake for the first message (I didn't see it!)

I do find this interesting. Was the output from the o2 sensor actually switching? They're supposed to cycle around 1V, going alternately high and low. I suppose it's a chicken-and-egg situation though, because if the ECU provides the wrong fuelling, then the o2 sensor will not give correct signals, and then the ECU provides the wrong fuelling...

Also I was under the impression that the o2 sensor was not consulted by the ECU for all running; only light throttle.

But now that we know the ECU was faulty, that's useful information.

Cheers,
-Alex

 

[dodgem]

Hi Alex,
When the ecu locks up in rich mode the o2 sensor basically goes down to 0.2v and stays there this was the main clue to the fact the ecu was jaming up. In fact i found it difficult with a good ECU to get the 02 system to go open loop! The three main conditions that switch the o2 sensor to open loop condition are:
1/ hard aceleration or sudden movement of the throttle.
2/ Engine over run/ coasting where of course the fuel is cut to for economy/emmisions reasons down to idle rpm. No point having the o2 sensor telling the ecu to go rich when the point is to cut fuel.
3/ Warm up phase till aprox 50deg

Under normal running the o2 sensor cycles quickly possibly more quickly than the tester can sample. With a digital test meter hooked to the sensor and the test gear it was easy to diagnose the fault and prove the sensor was operating correctly. However its good to note also the test gear never did recognise the fault with the ECU. It seems it can only cope with a sensor or other external system error.
Cheers
Mike

 

[SteveNZ]

I have used the Fiat/Lancia tested a bit and found it interested but not overly usefull. They dont have much of a test procedure. It would be hard to diagnose this fault without it though.

Im surprised to find a faulty ecu, let alone 2. They are usually very reliable.