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Discussion Starter #1
I recently changed my catalytic converter, but I didn't change the o2 sensors. Now my truck seems to be hesitant to kick down when climbing uphill or entering the hiway. The shifting seems out of sync. Also it's hesitant to start after it's warmed up. I'm thinking it's a fueling issue. Could o2 sensors cause all these issues?
 

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Yes, it can. Is the check engine light on?

You are going to get god awful fuel economy. Because the air/fuel mix is not right. It will run either too rich or too lean. It seems like it is running lean. That information is going into the computer, which may cause abnormal shifting. Change the sensor.
 

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Failed Oxygen sensors will not cause poor starting - the engine ecu will not run closed loop off the Oxy sensors for at least 10 - 15 secs.It fires up on info from the crank and cam sensors,coolant/air and fuel temp etc.When it gets to 500 rpm it will start to look for a Maf reading too.A session on testbook will show up most likely causes of the poor running.
 

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Discussion Starter #4
Roverrad95 said:
Yes, it can. Is the check engine light on?

You are going to get god awful fuel economy. Because the air/fuel mix is not right. It will run either too rich or too lean. It seems like it is running lean. That information is going into the computer, which may cause abnormal shifting. Change the sensor.
Check engine light is on and a new code will log in once I come to a stop with the truck in gear, if I come to a stop and let it idle in neutral, then no fault will log in the ecu.
allyv8 said:
Failed Oxygen sensors will not cause poor starting - the engine ecu will not run closed loop off the Oxy sensors for at least 10 - 15 secs.It fires up on info from the crank and cam sensors,coolant/air and fuel temp etc.When it gets to 500 rpm it will start to look for a Maf reading too.A session on testbook will show up most likely causes of the poor running.
My guess with the poor starting when it's warmed up would be due to the truck running lean? Just a guess. The messed up thing is I have rovacom, but no laptop to run it on. I dropped it and the screen is kaput. I'll give it a once over once I get me hands on someones laptop. I have it in my mind I'm going to need new o2 sensors. Thanks for the replies guys!
 

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You dont need the Rovacom to check if the Oxy sensors can switch,a volt meter is good enough to see if they switch down to 0v - rich from 5v.Hook up to the black wire on the sensor and to earth.If the oxy sensors fail to switch at all it will run very rich.
 

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Discussion Starter #7
kmagnuss said:
Normally when O2 sensors fail, it makes the engine run uber rich...thus not allowing the engine to grenade itself.
Rich making it smell? Well that's what it does at idle. Maybe I have some other air/fuel mixture problem? Grenade itself? I'm not sure what you mean by that?
 

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I didn't see the starting issue.

I still am going to disagree, while it is a closed loop at start up, if the air mixture is still not right, then it can cause abnormal start up.
 

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Roverrad95 said:
I didn't see the starting issue.

I still am going to disagree, while it is a closed loop at start up, if the air mixture is still not right, then it can cause abnormal start up.
But its not closed loop on starting,its a fixed map from mostly coolant temp.For the mixture to be too lean at cranking there would have to be low fuel pressure or similar - but nothing to do with closed loop control from Oxygen sensors.
 

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Discussion Starter #10
allyv8 said:
Roverrad95 said:
I didn't see the starting issue.

I still am going to disagree, while it is a closed loop at start up, if the air mixture is still not right, then it can cause abnormal start up.
But its not closed loop on starting,its a fixed map from mostly coolant temp.For the mixture to be too lean at cranking there would have to be low fuel pressure or similar - but nothing to do with closed loop control from Oxygen sensors.
In my case it turns right over when the engine is cold. If I turn it off and try to start it when the engine is warm I have to crank it longer than usual/normal prior to changing the converter. Is it possible due to have a lean start if the engine is still warm and the 02 sensors are still warm?
 

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You know, I agree. But I'm proposing something new.

Could be a relay, but could also be fuel pressure regulator. The diaphragm maybe sticking open too long after you shut it off resulting in no/poor fuel pressure to start. but you say it starts fine at first.... weird.
 

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Discussion Starter #12
I was thinking if unplugging the 02 sensors all together would make a difference. Causing a default setting. I know what allyV8 suggested but it's cold and snowing here now and I'm not for crawling under my truck with a volt meter in a cold garage on a cold garage floor.
 

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How did you 'change your cats'? Which bits did of the exhaust system did you move/remove? Which exhaust gaskets did you replace whilst doing so? Are all the gaskets mating/sealing well?

An exhaust leak upstream of the cats could/would, cause bad values from the O2 sensors and effect (closed loop) fuelling.
 

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How many miles have the O2 sensors been on your truck? I was told by my 'man that knows' that the O2 sensors are good for about 100k as they get clogged up with soot etc and are hard to clean.
I was having lots of rough running isuess with my 96 4.2 at 120k miles and it drank lots of fuel, changed the O2's and all is well. Here in the UK, a set of O2's costs about £100, thats for two sensors, but I think you may have four on yours, ie pre and post cat. One thought might be to swap the pre and post sensors round as the post sensors should have been exposed to less soot and I think the pre sensors are the main ones that determine how the engine runs, but I may be wrong..........

SID.
 

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You need to forget anything to do with the Oxy sensors in terms of starting issues,they simply are not hot enough to switch with out at least 10 secs worth of heating - even after restarting immediately,and even if they did work the ecu would not be looking for their input till it decides to swop to closed loop.
Look elsewhere,fuel pressure,pump relay,crank sensor,fusebox etc,etc.
 

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Discussion Starter #16
paul.adshead said:
How did you 'change your cats'? Which bits did of the exhaust system did you move/remove? Which exhaust gaskets did you replace whilst doing so? Are all the gaskets mating/sealing well?

An exhaust leak upstream of the cats could/would, cause bad values from the O2 sensors and effect (closed loop) fuelling.
I removed the crossmemeber, unbolt the down pipes from the manifold then unbolted the rear portion of the cat from the silencer and lowered it down. I put the manifold gaskets back, and torqued everything back as per the workshop manual.
SID said:
How many miles have the O2 sensors been on your truck? I was told by my 'man that knows' that the O2 sensors are good for about 100k as they get clogged up with soot etc and are hard to clean.
I was having lots of rough running isuess with my 96 4.2 at 120k miles and it drank lots of fuel, changed the O2's and all is well. Here in the UK, a set of O2's costs about £100, thats for two sensors, but I think you may have four on yours, ie pre and post cat. One thought might be to swap the pre and post sensors round as the post sensors should have been exposed to less soot and I think the pre sensors are the main ones that determine how the engine runs, but I may be wrong..........

SID.
Miles on the sensors? I have no clue, they are due for a change Im sure.
allyv8 said:
You need to forget anything to do with the Oxy sensors in terms of starting issues,they simply are not hot enough to switch with out at least 10 secs worth of heating - even after restarting immediately,and even if they did work the ecu would not be looking for their input till it decides to swop to closed loop.
Look elsewhere,fuel pressure,pump relay,crank sensor,fusebox etc,etc.
Only reason Im lookin' at the 02 sensors is because I didn't have the power loss, erratic/hesitant shifting, as well as the prolonged cranking to start the car prior to changing the cats.
SID said:
How many miles have the O2 sensors been on your truck? I was told by my 'man that knows' that the O2 sensors are good for about 100k as they get clogged up with soot etc and are hard to clean.
I was having lots of rough running isuess with my 96 4.2 at 120k miles and it drank lots of fuel, changed the O2's and all is well. Here in the UK, a set of O2's costs about £100, thats for two sensors, but I think you may have four on yours, ie pre and post cat. One thought might be to swap the pre and post sensors round as the post sensors should have been exposed to less soot and I think the pre sensors are the main ones that determine how the engine runs, but I may be wrong..........

SID.
I was just thinking of doing the swap thing. Being that I put the 02 sensors in new cats in the opposite positions compared to where they were in the told cats. (Precat 02 sensor in post cat position) and vice versa.
 

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Where did the Cats come from ? Since the fault has only been there after they were fitted I'd try taking out the upstream sensors,leave them plugged in and hanging down.Then see how well it starts,I'm thinking there could be too much back pressure - if it fires straight up you have your answer.
Here in the UK aftermarket Cats have been causing allsorts of problems.
 

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Discussion Starter #18
allyv8 said:
Where did the Cats come from ? Since the fault has only been there after they were fitted I'd try taking out the upstream sensors,leave them plugged in and hanging down.Then see how well it starts,I'm thinking there could be too much back pressure - if it fires straight up you have your answer.
Here in the UK aftermarket Cats have been causing allsorts of problems.
I bought the cats from atlantic british. Are the upstream ones before or after the cats?
 
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