dies at low idle speeds, when put into gear and when stopped.

[Ricko1966]

I have 1989 so I guess that makes it ALDL, I always believed OBD1 and ALDL were the same.

The flash codes are easy enough to access, but not always the most helpful because multiple things can cause the same final result (example: a lean run condition could be caused by a faulty MAP sensor, O2 sensor, thottle postion sensor, temp sensor - or an actual mechanical problem like vacuum leaks, exhaust problems or injector issues). It can tell you "something is wrong" but isn't particularly willing to tell you why. Without more data than an error code its a detective puzzle to figure it out.

I think Windows 3.x was out by 1990 or so, but I wouldn't swear to it in a courtroom. I was too hung up in alternate platforms at that point to pay much attention to what Microsoft was doing.

I hear what your saying,but obd2 drives me nuts in the wrong hands. Oh,it needs an oxygen sensor. How do you know? Because the scanner says so. No,the scanner has an oxygen sensor code. This could be a bad plug wire,an intake leak an exhaust leak,a cut wire,a bad connection or a number of other things. Yes some people can use the data as an aid to make an accurate diagnoses. Some can't.

 

[fast 99]

ALDL stands for assembly line diagnostic link.

Yes I agree most codes did not point to "this part needs to be replaced". Lean was a common code. Most sensors could be eliminated with data or sweep.

Although I don't fall into the replace parts and see if it fixes it category, to save customers diagnostic time I had a supply of known good used parts that could be swapped out. The original GM sensors were pretty durable it usually ended up being a combination of several problems.

When I bought this SB [avatar] really looking for a 91 up TBI 4x4 std cab 1 ton. Around here they didn't exist. My thinking was TBI for the most part is less reliant on maintenance.

 

[papakay]

That’s a sweet truck you’ve got there! Your guesses are all solid, especially if it’s been sitting a while. Along with the fuel, carb, and vacuum leaks, I’d also check the idle adjustment on the carb—if it’s set too low, it’ll stall out when you come to a stop. Dirty or clogged idle jets in the carb could be causing it too. If it’s running a stock mechanical fuel pump, that’s another thing to look at, especially with old fuel. Also, make sure the timing is set right and check the distributor advance weights and springs—they can get sticky over time. Keep us posted on what you find!

 

[Broken85]

OP, Look for air leaks between the carburetor and intake manifold at the gasket. It’s usually an easy starting place and could explain symptoms of vacuum leaks or mixture problems.

Idle RPMs should be around 900 (give or take 50 depending on your set up) I keep mine around 850. If idle rpms are good, don’t mess with the idle screw too much to fix your problem or you will end up with the motor diesel’ing and could end up with back fires when you shut it down. Idle screws are usually a fix, but not the cause of your symptoms.