91' Blazer(Jimmy) o2 sensor question

[Fusionross]

I have a full size 91 Jimmy with the 5.7 in it. I recently switched out the transmission to a th400. To got the trans out by cutting both sides of the exhaust just below where they bolt on. I also removed the cat, muffler and tubing since it was rusted out anyways. I had the truck running and it was running fine for about 2 months. Driving it around town and besides being super loud seemed to run fine.

Approximately a month ago I attempted to start it when it was 25 degrees out and struggled. I began to smell fuel after a few times attempting to start but eventually it did. Almost as if I flooded the throttle body. It struggled and kept sounding like it was about to stall but eventually went back to normal. It did throw a check engine light that cuts on and off at that time. Now when its running it idles up real high then bogs down as if it wants to stall while idling. When driving it, it stalls but fires right up.

My question is the only o2 sensor I see is the one coming off the drivers side exhaust manifold. I checked the old exhaust and there were no downstream O2 sensor on it. Is this the only location for the o2 sensor? Since the exhaust is cut could the o2 sensor be making the engine run rich? The exhaust was cut prior to where it crossed over below the trans on both sides. Any suggestions would be helpful.

This is exactly what my truck is sounding like.

 

[SpeedlabDan]

Supposedly the o2s need to be further inboard than 18" from the end. Other 5hen that I'd check the o2 resistance

 

[6872xtc]

I don't think that it really likes having the exhaust so short, I personally would fix the exhaust and plan on a new O2 sensor as they are fairly cheap for these and yours could be tired anyway from lots of years. I would also check vacuum lines, even the new one to the vacuum modulator on the TH400.
The ECM could possibly have learned something from the exhaust being so short that has turned into an issue.

 

[dyeager535]

O2 has zero bearing on startup. If it's having trouble starting and immediately smells of fuel, something else is going on.

Coolant temp sensor, ignition, etc.

Entirely possible messing with exhaust DOWNSTREAM of the O2 can affect how it works at times, but IME that had no appreciable difference on how mine ran. It's not a good variable to introduce, but what you are talking about doesn't sound like an O2 issue regardless.

 

[6872xtc]

O2 has zero bearing on startup. If it's having trouble starting and immediately smells of fuel, something else is going on.

Coolant temp sensor, ignition, etc.

Entirely possible messing with exhaust DOWNSTREAM of the O2 can affect how it works at times, but IME that had no appreciable difference on how mine ran. It's not a good variable to introduce, but what you are talking about doesn't sound like an O2 issue regardless.

But his post reads to me that he has issues even after it has been running, that it will die and he has to restart it. While I do agree that there is a good chance that something else is going on, do you think that there could be some influence due to the O2 being off? I know that the ECM will compensate for altitude, so if it was reading off because of poor information from the O2, I would guess that it could contribute to the erratic idle and other things.

 

[rampage]

You need to check the code that it’s throwing. Can do that with a paper clip. It stores the code for 50 starts or until the battery is disconnected.

Here’s how:
https://www.obd-codes.com/faq/read-gm-2-digit-obd-codes-free.php

 

[dyeager535]

But his post reads to me that he has issues even after it has been running, that it will die and he has to restart it. While I do agree that there is a good chance that something else is going on, do you think that there could be some influence due to the O2 being off? I know that the ECM will compensate for altitude, so if it was reading off because of poor information from the O2, I would guess that it could contribute to the erratic idle and other things.

If it's got cold start issues, that's open loop, and something is wrong before the O2 comes into play. It's possible the O2 is also part of the issue(s) once it warms up, but if there is a cold start issue, that needs fixed to see if the other problems go away. O2 can't fix, and can rarely compensate for a leaky/lazy injector, vacuum leak, bad CTS, broken wire, etc.

I second the check engine light. Its not going to tell you what to replace, its going to tell you what component(s) is seeing issues. It's a diagnostic assistant, not a parts replacement reminder.

 

[6.2Blazer]

If it's got cold start issues, that's open loop, and something is wrong before the O2 comes into play. It's possible the O2 is also part of the issue(s) once it warms up, but if there is a cold start issue, that needs fixed to see if the other problems go away. O2 can't fix, and can rarely compensate for a leaky/lazy injector, vacuum leak, bad CTS, broken wire, etc.

I second the check engine light. Its not going to tell you what to replace, its going to tell you what component(s) is seeing issues. It's a diagnostic assistant, not a parts replacement reminder.

I was going to say the exact same thing as above. The cold start issue is completely unrelated to the O2 sensor.....and whatever is causing the cold-start issue is likely also causing the other driveablity issues. There is always the chance that two completely separate problems popped up at the same time, with one being the cold-start issue and one being the driveablity issue but I would say that is unlikely. In any case the O2 sensor would not be my first concern.

Also as mentioned below, the code does not necessarily mean that is the part that needs replaced. I've seen so many people get an O2 sensor related code and immediately change the sensor only to have the code pop back up. Most O2 sensor codes are telling you that the sensor is detecting a rich or lean condition that caused by something else.