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Discussion Starter #1
I got myself a cable and downloaded Unidiag and connected to the ECU on my 156 2.0 Twinnie. The car has sporadic hesitation issues on light throttle at around 2000-2500 rpm. I set up Unidiag to realtime graph the voltage output from the exhaust oxygen sensor, started the car and went for a short drive. The engine was warm but had been sitting off for about 15 minutes.

The first thing that became apparent was that the O² sensor stayed 'cold' for about 30-40 seconds, which from what I have read is way too long. It's reckoned 8 seconds to get the sensor to 500º C, so I think the heater in the sensor is kaput or has a broken wire or some such.

What is weird though is once warm the voltage started to oscillate nice and quickly, but wave amplitude got really big, peaking at 900+ mV and less than 50 mV, occasionally going off the reservation and dropping below 0 mV and bringing up a momentary error message in the status line to that effect. The voltage swing stayed in this wide range for the entire 8 minute drive.

Unidiag throws up an unknown DTC 0x01173 with the explanation of a short circuit to ground (where else would it go :rolleyes:) and makes mention of the minimum signal going out of range - which I asume refers to 'drop past' 0 V. This message comes back when cleared

I also noted a DTC P0170 Fuel Trim Malfunction Bank 1, which I assume is also related and doesn't return when cleared and the engine is warm. But has returned at some stage during the day.

From what I have read, and I'm a complete newby to Bosch Motronic, being for many years a Bosch Mechanical (in my 72' W108 280SE) and CIS-E (late lamented 190E 2.3-16) guy- that dud lambda sensors have slower 'cross-counts' and smaller voltage swings (sluggishness).... which i don't seem to be suffering.

I'm starting to think air leak downstream drom the MAF. Any ideas?
 

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I don't have anything to add that may be useful, other than to say that I get stuttery and seemingly restricted performance when cold and I also get that same voltage drop (reports open loop) from my lambda sensor.

What makes you think air leak?
 

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Discussion Starter #4
Low voltage (apparently) indicates lean mix. If the voltage is dropping right off, possibly the mix leaning out past the bottom range of the sensor? The other thing I've thought, is that I have a rattly variator (new one in the box ready to go in in the next week or so) and possibly any valve timing inaccuracy sub-3200 rpm caused by this may effect the accuracy of the mix off idle.... but this doesn't explain why the exaggerated wave form would persist at rpm above 3200.... unless the variators not operating properly at all...... hmmmmm.
 

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First step would be to investigate the heater circuit I think. Replace the fuse & relay that feed the heater and check the terminals in both fuse & relay socket base, plus check the wiring to the probe. If it's had an 'aftermarket' replacement, it may not have been done well so you could just be looking at a poor connection.


It's all too easy to have similar symptoms and blame the same problem, but I had a light load/low rev hesitation on my v6 155 for a very long time. Looked into everything...and I mean everything. Turned out it was an intermittent heater fuse. Annoying, as that was the first thing I'd checked. It was cracked (not blown) so kept making & breaking.

HTH
 

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Discussion Starter #7
First step would be to investigate the heater circuit I think. Replace the fuse & relay that feed the heater and check the terminals in both fuse & relay socket base, plus check the wiring to the probe. If it's had an 'aftermarket' replacement, it may not have been done well so you could just be looking at a poor connection.


It's all too easy to have similar symptoms and blame the same problem, but I had a light load/low rev hesitation on my v6 155 for a very long time. Looked into everything...and I mean everything. Turned out it was an intermittent heater fuse. Annoying, as that was the first thing I'd checked. It was cracked (not blown) so kept making & breaking.

HTH
Thanks Chris, I look at this today. :thumbs:
 

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Discussion Starter #8
Checked fuses, they seem ok. Unplugged the connector and there appeared to be wet residue from the contact cleaner I used.. eeek! Cleaned this out, replugged and restarted, went for a drive and it appears that the hesitation is gone. Pulls cleanly and strongly from idle.

Plugged in Unidiag and checked the errors, the same ones were there, so I cleared them and went for a drive. Still got the occassional drop but much fewer and far between. I've done a screen shot and added it as an attachment.
 

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There are a couple of drop outs still in that plot aren't there. I think that the wide swing is normal, as long as it works out to a 50% duty cycle, that suggests it's working as it should - it's the drops to zero that are worrying.

For the sake of it, I'd still swap out the fuse and relay just to be certain, but it does sound like it's improved.
 

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Discussion Starter #10
Thanks Chris. Yep its better, although the intermittant slight flat spot off idle has re-appeared. Bar the dropouts, its good to know she's operating in closed loop.
 

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I know I've got a bit of a bee in my bonnet about the lambda heater....but since I spent probably 2 years off & on trying to diagnose a VERY similar sounding problem on my 155 and that was the cause, I do tend to bang on about it! Look for relay base and fuse holder contacts that have pushed down inside the shell, same goes for the AMP 2 pin jobs that are inline, replace the fuse, even if you buzzed it out and it looked OK (the culprit on my 155 was a cracked fuse that would re-make from time to time....typically, every time I tested it with a meter.....). Others have had relay problems and socket problems & wiring problems that caused the same issues. Without the heater working properly, the low speed light throttle scenario allows the probe to cool below it's operating temperature and the result is a trouble light, a hesitation/flatspot and a code stored in the ECU. I used to get 1224 codes all the time.

I'll try and find the picture I took of the fuse I found....
 

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In the good old days(!), the fuel and igniton was so iffy the slightest problem would stop the things...then they were easy to fix! Darn things keep on going no matter what these days - makes it hard to diagnose at times.
 
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