Recently changed my bank 1 (bulkhead) O2 sensor, it cleared the fault codes but the car developed a missfire/stutter on part throttle part load, initialy I thought that I might have damaged the vacuum pipes, so did a smoke test and they seem fine. This afternoon I disconnected the sensor and low and behold the missfire/stutter is now gone. Can I assume that the new sensor was faulty even though it didn't throw up any fault codes or is it the case that now the ECU is receiving data it is playing up, btw the new sensor was £50 ebay UK firm, and the other sensor that I purchased from them for bank2 is working fine? I suppose that I could just swap the sensors over and see if the fault follows the sensor, but that seems like a lot of work to confirm what I think I already know.

 

Andy, it is possible that you got a bad sensor. So, the question that I see is whether you want to buy a new sensor with the knowledge that the sensor is most likely bad or to take the time and try to prove the sensor bad (ie, swap the sensors). Either way I think is going to give you good information. It is just a matter of whether you want to spend money to keep things possibly more simple or to spend a little time to confirm the problem.

 

Sound advice as always from you Thermo, i have been in touch with the vendor, who queried if I had reset the ECU? Would a reset be necessary because I didn't have to reset for the other sensor, as it is I've taken the offending new one out and put the old one back in, JoJo,is a happy purring big cat again with an annoying warning light. Btw I took your advice about adjustable rear suspension arms, managed to source a pair of Volvo rear arms, huge difference and both rear wheels are now pointing the right way, vertically of course. If a reset in necessary i assume that it's the normal disconnect battery and leave for a few hours.

 

Andy, is a reset necessary, NO!!!!! Is it a good idea? I don't see any harm in doing it. All you need to do is loosen the negative battery terminal, pull it off the battery, wait a minute or so, then reattach it (you can wait longer, but you are not gaining anything). Please note that I mention the negative battery cable. Most people don't see a difference as to what battery terminal that you disconnect. And, for the most part, there isn't a difference. But, I recommend the negative battery cable as I have seen what happens when you are removing the positive battery cable and your wrench accidentally touches the body of the car. In the case of my friend, the wrench hit his metal expanding band watch. In short, welded the watch to his wrist.

 

So the replacement sensor arrived this morning and I installed it, the same issues arose on light to medium throttle 2000-2500 revs slight to medium load or slightly uphill. I then changed the plugs only 51000 miles old but they were the original plugs from new. Same problems still! I read on here about the inlet manifold valves which control (even out ) flow in-between the banks and wondered if mine might be either leaking or not operating properly, as it is I've ordered the sealing rings for both. Is there any way of testing these valves without swapping for a known good one? The only other alternative I can think of is to buy yet another sensor from a different distributer. It does seem strange that the car runs silky smooth with a faulty sensor and slightly rough with an operational sensor? Whatever ever is happening it is only on the bulkhead bank because it occurs only when that sensor is changed. Looking at the old plugs they didn't seem too bad, dark tan not white or black. I think anything fuel related can be discounted, as can the throttle body and MAF, I'm now at my wit's end with this problem so any ideas would be very welcome.

 

What code(s) are you getting? Knowing that would help to make sure the right thing is being looked at.

 

The car isn't showing any codes, it was showing 1646/7 which was when the whole sensor saga began, I changed the front upstream sensor which cured one code but when I changed the rear upstream sensor the slightly rough running started, the car runs better with cheap rear sensors which obviously cause an engine warning light to come on, I have an Elm 327 which shows that the fuel trims are very erratic with the latest sensor and much more stable with the old sensor. This makes me wonder if now that the ecu is receiving accurate information it's highlighting another problem elsewhere. I suppose that I could fit the good sensor from the front bank into the rear bank and see if I get the same result, at least that would confirm where the problem lies or confirm that my new sensor is again faulty in some way.

 

Having thought about it further, this morning on the drive into work I had the opportunity to try hard acceleration uphill 3rd and 4th gear hesitation still there and more pronounced under load. This evening on the drive home I will disconnect the front sensor to see if that makes any difference or not, if it does alter things then I'll know that the problem isn't confined either to the latest sensor or the rear bank. The only other thing that I can think of is to swap the sensors from back to front and see if it's happier like that. Is it possible for the sensors to give conflicting information so that the ecu gets confused?.

 

If you have no codes could the hesitation possibly be from a partially clogged fuel filter?

 

Fuel pressures are all ok and it only occurs when both sensors are active, it has certainly got me puzzled.

 

So I disconnected the front upstream sensor before leaving work this evening, got no ewl on the dashboard the car pulled perfectly in every gear and idled smoothly, stopped after about 3 miles turned off waited a minute then turned back on then got a ewl on the dashboard, so I suppose now I'll have to try and run it with both sensors disconnected to try and see if that causes any changes. What I find really puzzling is that the irregular running hesitation only occurs when both sensors are connected, is it possible that one sensor is reacting to the other and it leads to some kind of feedback loop?

 

Don't - you'll guarantee code(s) and probably run OL (open loop) which will start to destroy the cat(s).

Only with a bizarre miswiring or some such (i.e., 99% no).

Something's badly wrong, go back to OBD etc basics - check trims, sensors, fuel pressure, etc. Also maybe the cat(s) is/are already badly damaged.

 

With both sensors connected and fuel trims varying from -7 to +7 I took the car out for a drive after having followed the ECU reset procedure in the Haynes manual, it ran like an absolute dog at 70mph, lots of hesitation etc. I did gingerly remove one of flow control valves on the intake manifold to check which o ring seal was present, it was one of the yellow brittle ones so I have new ones on order, I've tried to check for leaking around them but I can't hear any hissing and when I sprayed them with break cleaner i got no change in engine note or revs. So my plan is now to put the old sensor back in and then change the orings when they arrive and then start all over again! I've cleaned all the wiring connectors they all looked nice and shiny, I've inspected the throttle body plate it looks fairly clean to me. All in all from my years of experience it does feel like it's a weak mixture problem, if the new orings don't achieve anything I'll have to give up and get a garage to fix it.

 

Stft


So here is my current data, after reading around a bit it does seem that I have a leak somewhere, i fear l was trying to cure the symptoms and not the cause. After these measurements were taken I increased the revs to 1500 and then back down to idle, that gave 25 degrees of short term fuel trim both negative and positive so the adjustment was probably maxed out. Just another thought is it worth changing both the upper and lower manifold seals, the engine only has 51000 miles on it and to the best of my knowledge the manifold has only been off once.