Rough idle & 5 speed stalling fix.

[Dessertrunner]

This should have got everyone attentation but as suggested by others I just disconnected the O2 sensors to see if they were causing any of my current problems eg rough idle down low, high fuel consumption & stall when letting out the clutch. I can say about the fuel yet but the rest of the problems have gone away. At this point it could be that the computer has gone into default mode and when I install the new O2 sensors the problems will come back but I don't think so. The car now sits at 700 idle not 600 as before it pulls away smoth al the way through the 3000 that was a problem before. In addiation I had thought the engine was dieing with a bearing problem but now it apears it was over injecting on one side because it has lost a slight Knock it had before. When I get the new O2 fitted I will let you know the results.

[longassname]

There's not really a "default" mode that the ecu goes into when the oxygen sensors are dissconnected. Unplug one o2 sensor and the ecu ignores that input and uses the other one exclusively. Unplug both o2 sensors and the ecu ignores both inputs and stops o2 sensor based corrections. The ecu continues to run off of the same ignition and fuel tables and apply the all the same corrections as usual except for the o2 sensor based corrections.

Unplugging the o2 sensors generally does make the engine run a little better even if they aren't bad. The engine just runs better at the values indicated in the fuel table than it does at the stoichiometric ratio. If the improvement is great then there is likely to be another problem which isn't necessarily the o2 sensors. o2 sensor control involves oscilating around the stoichiometric ratio. If the engine is running correctly these are very small oscilations because the fuel table values at cruise are stoichiometric. If something is causing the calculated fuel injection quantity to be way off then the oscillations are large resulting in both poor engine operation and gas mileage. This could be caused by a bad coolant temperature sensor reading; the coolant temperature sensor controls most of the fuel enrichments. It's also a problem on vehicles where the wire to the ecu which tells it the ac is on has been wired to tell it it's on all the time (some people with manual transmissions or using the engine in another vehicle have done this in an attempt to overcome their stalling issue). You could also have a leaking fuel injector or fuel injector O ring. Talk about a knock. I forgot to put an oring on an injector once and it would fill the cyllinder with fuel and abrubptly stop it from turning over; that was a knock. You might have the same problem to a much lesser extent if you have a leaky injector or oring.

I would check your temperature sensor with a multimeter or just replace it if you want to.

If you can put a fuel pressure guage in your fuel line you should be able to tell if you have a leaking injector or oring. Turn the key on and the fuel pressure should go up to 34-38 psi. Turn it off and the fuel pressure should stay at that pressure for a long time.

[Dessertrunner]

Thanks for that I might plug one sensor at a time to see if one is bad while I am waiting for the new ones to arrive. I will check all your suggestions and see if I can find the problem clearly from what you say the rough low revs and the fuel problem are not the same. Makes sense as I didn't get any improvment in fuel ecomony since disconnecting them engine just runs better.
Tony

[Dessertrunner]

Mike, I have been thinking about this problem would I be correct in saying that if I disconnect both O2 sensors the ECU stops looking at other sensors and just functions on the tables you mentioned. Which leads to two questions,
1 - What sensors are used to decide on the air fuel mixture.
2 - In deciding which table is the TCU using the TPS.
3 - If the O2 are disconnected does the TCU still use air flow.

Interesting thing is that the idle has also improved when I lift my foot off the throttle it drops to 700 rpm in stead of dropping to 1,500 then slowly moving back. I think we may be onto something here as the car is running heaps better.
Tony

[longassname]

Hey Anthony,

The ECU doesn't stop looking at the other sensors if the o2 sensors are disconnected. The only sensors it will ignore because of disconnecting the 02 sensors are the 02 sensors. The ecu determines the amount of fuel to inject first by the mass air flow meter. This tells it the base fuel injection quantity. To this it then adds a great # of incremements and decrements based upon the other senors and other inputs. The most used sensor input the coolant temperature sensor. Coolant temperature is used for a couple of large enrichments based soley on coolant temperature (one is only on first start the other is at all times). Coolant temperature also makes up the x axis of many other enrichment tables. One such table is the enrichment which is added if the ac is on. This is a very large enrichment, I'm sure you've noticed the difference in fuel economy with and without ac. The throttle postion sensor is used for several increments and decrements to make up for the time lag in the mass air meter registering air flows. The function of these increments/decrements is very complex and interwoven and not something I discuss. The only time the ecu doesn't use the air flow meter voltage is if it determines the meter is bad/disconnected. When this happens the car defaults to a safe mode to try to keep it running. This is the true limp home safety mode. The car runs very poorly without the mass air meter.

Originally Posted by Dessertrunner

Mike, I have been thinking about this problem would I be correct in saying that if I disconnect both O2 sensors the ECU stops looking at other sensors and just functions on the tables you mentioned. Which leads to two questions,
1 - What sensors are used to decide on the air fuel mixture.
2 - In deciding which table is the TCU using the TPS.
3 - If the O2 are disconnected does the TCU still use air flow.

Interesting thing is that the idle has also improved when I lift my foot off the throttle it drops to 700 rpm in stead of dropping to 1,500 then slowly moving back. I think we may be onto something here as the car is running heaps better.
Tony

[Trevor]

Originally Posted by longassname

Hey Anthony,

One such table is the enrichment which is added if the ac is on. This is a very large enrichment, I'm sure you've noticed the difference in fuel economy with and without ac.

As a result of genuine interest and not as a matter of doubt -

Why is the mixture enriched, rather than simply increased in volume? I would have thought that if the idle speed was pulled down by the AC, this would be taken care of via the bypass air control solenoid valve and the mixture would be held to optimum right through the rev. range.

[Dessertrunner]

Mike,
I think you have given me the answer I was looking for, we all seem to be going round in circles trying to get the different problems sorted out with the idle, stalling, rough running ETC ETC. What we all need to do is try to development a test precedure that we can go throught to find out what is at fault. I may not be explaining myself properly but I disconnected the o2 sensors and the rough low revs went away but it didn't fix the problem on SVXcuseme car who must have a different problem. I think Trevor mentioned in a different thread that we need to do sensiable testing not stabbing in the dark and guessing. I don't know if this works but it might be as simple as this.

Step 1 - Disconnect both O2 sensors see if the problem goes away. If not reconnect sensors and move to step 2.
Step 2 - Disconnect MAF sensor and then drive to check. If not reconnect sensor and move to step 3.
Step 3 _ etc etc.

This could also involve blocking the air on the various other vales. Any way you guys get were I am coming from so feel free to start while I am away see you in 5 weeks.
Tony