1994, 3.9, 2WD, LB

 

Yep. That’s worth investing in.

 

I let it idle for 10 min, and still only have the 12 and 55 codes.
Later, I let it idle for 15 min, and still only have the 12 and 55 codes.
Next I will let it idle for 30 min, and see if a code will register.

Anyone know how long it should take the ECU to throw a code? Should I rev up rpms? Should I put in gear a few times and let it stall?

Thanks for all your help!

 

Yea I'm pretty sure you have to drive it a few miles for it to reset any codes it might have had. I don't think you had any or the some combo of the flashes you saw would add up to one.

 

I let it idle for 30 min, and still no codes. Battery tests good, and alternator is putting out good.
I am not sure what to try next. This all started after I replaced the thermostat-- so would replacing the coolant temp sensor make sense?
It ides a little rough, and when I hold pedal down steady, the rpms fluctuate a little bit.
Should I remove and clean any sensors, EGR?
Any other tests I can do?

I am stumped. I put it in any gear, and it just stalls out.

I know it must be something simple................... but I am stumped.....

 

I get that it is frustrating not to be able to ask questions as they come up and that makes you hesitate to do things. If you think changing the sensor isn't the right move IMO the only other solution you will find is by reading on here or elsewhere. I suggest you read the entire FAQ page even if it doesn't all apply to this problem. Then while logged in use the search function on this site. Your truck looks like a carbon copy of mine but in better shape with 30K fewer miles than mine when I bought it. When I got mine the mpg was like 150 miles per 22 gallon tank and it wasn't great on the road. I ended up doing the plenum with a Hughes Engines kit and while I had that apart I changed the timing chain, water pump and added the guide that was added in a TSB. I also changed all of the sensors on the intake, except the IAC. I replaced the cam sensor in the distributor. Afterwards I ended up changing the crank sensor too. It's an old truck and most of those were still original.You can look at my earlier posts, I learned a lot of what I know about this truck on this site. It is the first motor I ever took apart that far.
So IDK if you can afford it all right away but you have a nice looking truck if most of it was kept up like that appearance and you like it it will be worth it to put new parts that should get replaced over time anyway, in it.
There are other possible causes but barring you getting the factory service manual (Bishko has it on cd for about $40) and going through whatever diagnostic procedures it has for your specific problem, reading this site is your best bet.

 

I concur with everything you said.

I don't need my arm twisted to change out parts. Critical ones may still work but they have seen most of their useful life after 29 years. So if I replace something and it doesn't fix my problem I'm not too upset about it, not when the truck is in otherwise excellent shape. And when the left one goes out I change the right one at the same time. Just cuz. Another factor is age. Not the truck, but mine. My days of laying on my back on a garage floor with the undercarriage stuffed in my face are about done. Working from the side and above are soon to follow. So if I want to do any work on my truck it's gonna have to get done fairly soon. So I'm hitting wear parts that are in the twilight of their lives while I can. Wont be long and I'll have to pony up the cash to have it done and I don't like that thought.

 

Thanks Guys.. Do you think if I got a scan tool, it may pull older codes from the ECU? Even if I had disconnected battery recently?

 

No. Once the computer is reset by disconnecting the battery for X amount of time, they are gone. IDK the amount of time it needs to be disconnected to cause a reset, 5 minutes pops up in my mind. If the problem is still there the code will come back once you start driving it for a bit. Again IDK how many miles you have to drive for the computer to relearn your system.
If you invest you want a tool that will read OBDI. There is a "set sync" function you may need eventually. The timing is done by the computer not by you setting it with a light and turning the distributor. But the distributor is used by turning it to synchronize when the injectors fire in the cycle. It can be done manually and it will run but you won't know exactly what it is set at without having a scan tool to know.

 

I just always used the on-board reader. I don’t even know where the OBD1 connector is.