Engine Idles Unevenly and Almost Stalls When Cold

[Road Trip]

I think I understand now!

So it was flashing fast for the first few minutes, then it slowed down, so that must mean it entered closed loop. It is at this point that the problem usually happens (though not always; it seems less common in warmer weather). The interesting thing I noticed is that it seemed to bounce between open and closed loop a few times (the light would flash slowly, then it would speed up for a bit, then slow down a bit). It seems to me that once it enters closed loop, it should stay in closed loop, yes?

Also, given that it seems to do the right things regarding warming up and switching to closed loop when it should, the temp sensor is probably fine, I would think?

cc333,

Sounds like you've got the hang of it. And your observation that when your computer transitions to Closed Loop and
the symptoms start to present is a pretty good hint that your O2 sensors are tired/no longer working
as they should.

NOTE: Elsewhere in the forum it's been discussed that even with live data, although we can prove
that an O2 sensor is bad, by the same token we can't always prove that they are good. In a
case like yours we have to rely upon other clues as to whether or not the O2 sensor(s) should be
replaced:

* Have you ever replaced them before? If so, what brand did you use?
* If you haven't replaced them, did the PO give you any receipts for the parts
he replaced prior to selling you the truck? If so, were the O2 sensor(s) replaced,
and how long ago?
* When you crawl underneath and look at the sensor(s), do they look like they
have been there since the truck rolled off of the Assembly Line? Note that most
owners were guilty of giving them a good leaving alone, as in the infamous
"If it ain't broke, don't fix it" mentality. :0)

The bottom line is that the O2 sensor lives in the harshest environment of all
(the business end in the hot exhaust stream, and other end underneath the
truck, with people fording high water, sandy beaches, mud bogs, etc. Meanwhile,
the sensor's job is to compare the quantity of oxygen in the exhaust stream against
the quantity of oxygen in the surrounding air.

And that they should be considered a wear item like a spark plug, for they never get
better with age & use. But we can't see what you see unless you post a photo.

But the good news is that single wire O2 sensors are very affordable, so unless
you are convinced that your O2 sensors are already fresh then you should seriously
consider swapping them out.

One last thing. You question about once the engine goes closed loop, it should stay in
closed loop? Ideally for your truck, yes. But owners of single wire O2 sensor trucks
that monitor the behavior have reported that their vehicles will fall out of Closed Loop
when the truck idles for a period of time. (Waiting at a long light, etc.)

On the other hand, in order to meet increasingly stringent emissions regulations,
newer versions of your truck upgraded from single wire O2 sensors to heated
three wire sensors, so that these trucks go closed loop sooner, and then
stay in Closed Loop for the duration of that trip.

Guess what? They retrofitted their trucks to the 3-wire sensors, and some
reported improved all-round driveability and improved fuel economy. Which
makes sense, for as mentioned earlier, with healthy O2 sensors your truck
will drive it's very best, give you the best possible MPG, and at the same time
pollute the least.

The bottom line is that they really are a tuneup item, but most owners wait until
the truck doesn't drive right, a normal tune up doesn't resolve the problem, and
only then are the O2 sensors given any thought.

If it was my truck and they look to be vintage, I'd swap them out in the interest
of science & see how much of an improvement they make.

Hope this helps.

Cheers --

 

[Sabinoerc]

@Schurkey and @Road Trip great responses and explanation. Thanks for taking the time to share, I learned some new info.
I would agree next step is to replace the O2. Even if not the culprit/only culprit it’s time to check it off the list. Easy enough to DIY, or at least see if it comes out ok before taking to a shop. My 93 is a single, single wire O2. @cc333 Let us know what you find!

 

[cc333]

@Schurkey OK, that makes sense.

* Have you ever replaced them before? If so, what brand did you use?

I've never touched them.

* If you haven't replaced them, did the PO give you any receipts for the parts
he replaced prior to selling you the truck? If so, were the O2 sensor(s) replaced,
and how long ago?

I don't know. The PO had died and I bought the truck from a local used car lot, so I never got to see much, if any of the prior repair record.

* When you crawl underneath and look at the sensor(s), do they look like they
have been there since the truck rolled off of the Assembly Line? Note that most
owners were guilty of giving them a good leaving alone, as in the infamous
"If it ain't broke, don't fix it" mentality. :0)

I don't know yet. I'll have to get under there and take a look.

Hope this helps.

It helps very much!

I would agree next step is to replace the O2.

OK. What tools do I need?

They retrofitted their trucks to the 3-wire sensors, and some
reported improved all-round driveability and improved fuel economy.

How do I do that? Is it as simple as routing a 3-wire harness up to the engine compartment, splicing in the original 1 wire lead and wiring the other two to a power source?

c

 

[Sabinoerc]

on mine it’s in the exhaust manifold on driver side and easy to reach from top. You’d need to check if yours has one or two on both sides. It takes a wrench which fits the hex on it, I don’t remember the size. A normal open ended wrench might round off the nut/hex on it if it’s hard to remove. A crescent wrench which you can adjust to be fit snugly would increase odds of not rounding over the nut. Best bet would be to get an O2 socket(I think that’s what they are called?) which is a socket with a slit to allow it to be put on the long sensor with a wire attached.

I‘d suggest going and getting the socket to minimize chance of rounding over the nut (as long as what you get seems tight on the nut - I bought one years ago which was crap quality from somewhere and no better than an open ended wrench).

I’ve only changed a few on personal cars and they were easy to get off but I’ve read they can be a bear. I don’t know if having engine hot will increase odds of removing it (while increasing odds of burning yourself) or no. Others here would have more experience than myself on suggestions.

 

[Road Trip]

OK. What tools do I need?

IIRC, you'll need a 7/8" oxygen sensor socket in order to do the job.

Do yourself a favor and invest in a name brand as opposed to a cheapie
that will deform before the sensor comes out. (Lisle is good, as are
the normal tool truck brands) (OXYGEN SENSOR googling)

Note: Sometimes when the O2 sensor fights being removed to the
point where even a good Oxygen sensor socket wants to spread
open, Plan B is to cut the wiring harness off of the old sensor, and
use a 7/8" deep 6-point socket (or better, thick wall Impact Socket) and
force the issue. Essentially, use the 6-pt deep socket for the removal,
and the sensor socket (with the slot cut out to accomodate the sensor
wiring) for the installation.

NOTE: Sometimes a 7/8" wrench will work best, but some aren't aware
that although the default is a 12-point box, there are 6-point box wrenches
out there. (It's what I use.)

****

As for the removal, cold extraction may or may not work. If it's an old soldier, some
have reported that running the engine for 5-10 minutes first in order
to get a good heat soak into the exhaust manifold allowed removal
where it wasn't happening when cold.

Using penetrating oil prior to a cold removal attempt is somewhat
controversial. Some claim success, while others question if this
capillary action is possible? Me? I live in the rust belt, and I think
I have at least one of every brand out there on the shelf, and even
if it's just a placebo effect I don't care. My current go-to is Kroil,
but YMMV.

****

That pretty much covers the tools. I'll dig around a bit for your 3-wire question
and share what I find in a bit.

Good luck. And if you can't get it out a good muffler shop should be
able to take their smoke wrench to it and persuade it to surrender.

Cheers --

 

[Schurkey]

Note: Tree'd by Road Trip!

Most "O2 sensor" sockets are great for installing sensors. May or may not be adequate for removing them.

A 7/8 box-end wrench is a viable alternative.

I bought a 7/8 deepwell, six-point, impact socket in 1/2" drive, with unique, deep broaching that allows the sensor body to fit inside. Gotta cut the wire(s) off the sensor, though. NO slit on the side, which makes the socket far stronger and less-likely to round the hex on the sensor.


Made by Wright, a family-owned, USA company.
www.amazon.com/Wright-Tool-4928-6-Point-Impact/dp/B002VKBRCC/ref=sr_1_1?

 

[Road Trip]

How do I do that? Is it as simple as routing a 3-wire harness up to the engine compartment, splicing in the original 1 wire lead and wiring the other two to a power source?

Per the '94 Electrical & Diagnostic GM Factory Service Manual, upgrading to
the 3-wire O2 sensor is uncomplicated. For whatever reason lost to time,
in 1994 the General elected to give the LB4 (4.3L V6) a heated 3-wire O2 sensor,
while your 454 got by on the unheated 1-wire sensor?

IF you wire your sensor in per the FSM and you have any issues, then
it will make it much easier for the remote troubleshooters to figure out
what's up. To give you a quick idea on how you would wire this in,
I have attached a couple of pages out of this manual. (See below.)

While we're on the subject, I don't remember if we've discussed this yet,
but taking advantage of all the info in GM's Service Manuals is a smart move.
The good news is that there's a cache of the online versions of these manuals
that are available free over HERE. If you haven't already done so, then give
yourself the gift of the repair procedures written by the ones who know best.

If you have any further questions, don't hesitate to ask. Otherwise, it
looks like ordering a 3-wire O2 sensor for a '94 LB4 motor & a little wiring
will give you closed loop operation 100% of the time after initial warmup.

Hope this is helpful.

Cheers

PS: @Schurkey, I think that the answer to the mystery of why would a '4WD' fuse
affect unrelated stuff on a 2WD vehicle has just been tripped over?

FWIW when the signal ground was added (4-wire O2 sensors) they
moved the O2 heater circuit over to the ENG 1 fuse? Essentially
a blown 4WD fuse making for a 'cold' O2 sensor was the '94 (& '95?)
2WD 4.3L V6 trucks? Kinda rare new, so not exactly something you would
happen across on any given day nowadays?

 

[termite]

Per the '94 Electrical & Diagnostic GM Factory Service Manual, upgrading to
the 3-wire O2 sensor is uncomplicated. For whatever reason lost to time,
in 1994 the General elected to give the LB4 (4.3L V6) a heated 3-wire O2 sensor,
while your 454 got by on the unheated 1-wire sensor?

IF you wire your sensor in per the FSM and you have any issues, then
it will make it much easier for the remote troubleshooters to figure out
what's up. To give you a quick idea on how you would wire this in,
I have attached a couple of pages out of this manual. (See below.)

While we're on the subject, I don't remember if we've discussed this yet,
but taking advantage of all the info in GM's Service Manuals is a smart move.
The good news is that there's a cache of the online versions of these manuals
that are available free over HERE. If you haven't already done so, then give
yourself the gift of the repair procedures written by the ones who know best.

If you have any further questions, don't hesitate to ask. Otherwise, it
looks like ordering a 3-wire O2 sensor for a '94 LB4 motor & a little wiring
will give you closed loop operation 100% of the time after initial warmup.

Hope this is helpful.

Cheers

PS: @Schurkey, I think that the answer to the mystery of why would a '4WD' fuse
affect unrelated stuff on a 2WD vehicle has just been tripped over?

FWIW when the signal ground was added (4-wire O2 sensors) they
moved the O2 heater circuit over to the ENG 1 fuse? Essentially
a blown 4WD fuse making for a 'cold' O2 sensor was the '94 (& '95?)
2WD 4.3L V6 trucks? Kinda rare new, so not exactly something you would
happen across on any given day nowadays?

Do you recall if the fuse block referred to if the dash mounted or underhood block? I'm planning to get around to dropping the 3-wire into my suburban soon and like the idea of mimicking the factory circuit better than adding a relay and signal wire.

 

[Road Trip]

Do you recall if the fuse block referred to if the dash mounted or underhood block? I'm planning to get around to dropping the 3-wire into my suburban soon and like the idea of mimicking the factory circuit better than adding a relay and signal wire.

It's the dash mounted fuse block. I found a decent picture of a '94 fuse block not
only showing the 4WD fuse, but also the 2 other fuses shown on that wiring diagram:

You must be registered for see images attach

Let us know how the upgrade goes!

Best of luck --

 

[termite]

It's the dash mounted fuse block. I found a decent picture of a '94 fuse block not
only showing the 4WD fuse, but also the 2 other fuses shown on that wiring diagram:

You must be registered for see images attach

Let us know how the upgrade goes!

Best of luck --

I'm off next week for the kid's spring break. Hopefully I'll be able to have some tinkering time of my own and not just honey-do projects.