Engine Idles Unevenly and Almost Stalls When Cold

movietvet said:

You can do it yourself without a scanner/code reader. Just pulling codes that is. Look at you tube but all you need is a paper clip and pen/paper. Look at your connector, under the dash. Put the clip in the top two spots on the top row right. A and B circuit. KOEO=Key On Engine Off. Watch Service Engine Soon light. Should start to get flashes and should start with code 12. One flash and then 2 flashes after. It does that three times and then codes start. Count the number of flashes and each code will do it 3 times. Then code 12 to finish it out and tell you it is done.

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I just found the ALDL physical layout/electrical pinout info in the '94 FSM Electrical & Diagnostic manual.
This illustrates what movievet was accurately describing:



Searching Tip:

I opened up my local copy of this manual, did the old school 'Ctrl F' (Find function)
typed in 'ALDL', and got *nothing*. Looks like this acronym was never used by
the Tech Writers in this particular manual? Instead, they used DLC, for Diagnostic
Link Connector.

But I didn't know the DLC acronym. But what I did do was read around a little bit,
and electrical circuit "451" was identified as the one used for 'diagnostic test'. So
I searched the entire manual for '451', and in seconds it found this number in 35 different
locations. And on the 2nd instance I ended up with this.

I share all this with you coming from the perspective of owning my own full set of paper
manuals for my '99 chore truck. And I enjoy reading from them. But when I need to find
something quickly, these .pdf files on a fast laptop is such an unfair advantage.

If you are reading this, and as yet haven't downloaded the electronic versions of the FSMs
that cover your machine, please do so at your earliest convenience. It's a game changer.

OP from your problem description, when my 95 had the same symptoms, it turned out to be the 02 sensor.

how old is yours?

You know, I'm not sure. If I had to guess, I'd say they're probably original.

I bought the truck used in 2017, so I'm not totally sure of its maintenance record or repair history before that, so I have no idea if they were ever replaced or not.

c

1997 said:

OP from your problem description, when my 95 had the same symptoms, it turned out to be the 02 sensor.

how old is yours?

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I had the same problem with my 2005 Z71 but that was only when it was warm, in closed loop. Was a bad oxygen sensor. I replaced all 4.

Unlike virtually all the other sensors except the MAF, an O2 sensor has a finite lifetime.

They can be kinda-sorta spot-checked by looking at the crosscounts or voltage swings on a scan tool. They can be proven "bad" but not proven "good". In the end, O2 sensors can have issues that defy diagnostics.

They--and MAF sensors--are the only ones that sometimes need to be replaced "just because" they're old--and MAF sensors can sometimes be cleaned and restored to usefulness.

What makes this worse, is that the longer an O2 sensor is in place, the harder it can be to remove. "O2 sensor wrenches/sockets" are great for installing them, less than optimum for removal. Most O2 sensor tools have a split down the side for the wire(s) to pass through. The split opens up with enough torque--which then allows the socket to slip on the O2 sensor's wrenching surface, rounding-off the corners.

Sometimes the only way to get an O2 sensor out is to cut the wires, and use a non-split socket, or perhaps a 6-point box-end wrench.

And then, you may need to restore the threads in the exhaust system that got damaged by the seized sensor.

Schurkey said:

Unlike virtually all the other sensors except the MAF, an O2 sensor has a finite lifetime.

They can be kinda-sorta spot-checked by looking at the crosscounts or voltage swings on a scan tool. They can be proven "bad" but not proven "good". In the end, O2 sensors can have issues that defy diagnostics.

They--and MAF sensors--are the only ones that sometimes need to be replaced "just because" they're old--and MAF sensors can sometimes be cleaned and restored to usefulness.

What makes this worse, is that the longer an O2 sensor is in place, the harder it can be to remove. "O2 sensor wrenches/sockets" are great for installing them, less than optimum for removal. Most O2 sensor tools have a split down the side for the wire(s) to pass through. The split opens up with enough torque--which then allows the socket to slip on the O2 sensor's wrenching surface, rounding-off the corners.

Sometimes the only way to get an O2 sensor out is to cut the wires, and use a non-split socket, or perhaps a 6-point box-end wrench.

And then, you may need to restore the threads in the exhaust system that got damaged by the seized sensor.

Click to expand...

A 7/8" box end wrench is my preferred tool to install and remove O2 sensors...if there's enough room to swing it. If not, I use a socket or a special crowfoot wrench that's made similar to a line wrench. That's gotten them loose on my Burbs; haven't changed any on Rawhide yet. But there is a good bit more room on him... that's one thing I looked at, soon after buying that truck!

HotWheelsBurban said:

A 7/8" box end wrench is my preferred tool to install and remove O2 sensors...if there's enough room to swing it. If not, I use a socket or a special crowfoot wrench that's made similar to a line wrench. That's gotten them loose on my Burbs; haven't changed any on Rawhide yet. But there is a good bit more room on him... that's one thing I looked at, soon after buying that truck!

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Especially when I lived in rust country of KC, Mo., any time I crawled under my rigs, I would shoot some penetrant on the sensors threads. Even if I was looking at something else. Thankfully, here in the PNW, that is rarely a problem.

movietvet said:

any time I crawled under my rigs, I would shoot some penetrant on the sensors threads.

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The O2 sensor is sealed to the part it screws into using the same sort of gasket that used to be on gasketed spark plugs.

It's a gas-tight seal. If hot exhaust gas can't get out...liquid can't get in.

Penetrant can't get to the threads.

Schurkey said:

The O2 sensor is sealed to the part it screws into using the same sort of gasket that used to be on gasketed spark plugs.

It's a gas-tight seal. If hot exhaust gas can't get out...liquid can't get in.

Penetrant can't get to the threads.

Click to expand...

We have conversed about this before. It makes me feel better to spray them.

IF (big IF) the penetrant could get past the gasket (and I am not conceding this point)...

...it would wash-out the anti-seize that's supposed to be in between the male and female threads.