it doesn’t seem like any of those would cause it to mess up the way it does though, complete sudden change and then back to normal. I don’t know about the plugs, but the plug wires say 1994 on them

 

You do you. I'm not telling you to do anything, I'm telling you how to fix it, correctly. There's lots and lots of these trucks running around that could be running better, but for whatever reason, people don't fix the broken manifolds.

 

Lol. Might be time for a tune up if wires are 27 years old.

 

Definitely time for new wires! And yes they can cause this problem due to intermittent condutivity. A plug can conduct most of the time as it is failing, then not conduct intermittently. I have watched this on an oscilloscope. I would say let's go ahead with a good tuneup before going any further. Just my opinion.

 

Interesting...I’m waiting for parts right now so not much I can do atm

 

question: I read that the o2 sensor doesn’t do anything until the engine is warmed up, so if it were reading lean (causing a rich condition), it would only do so when the engine is warmed up. Is that true or not?

 

Yes,
Assuming your ECT(tested) and your ACT(tested) are working correctly, while warming up, if they are out of range and PCM cant/wont/dont know, it can continue to be rich, using O2.

 

that makes sense. My truck will run rough as soon as it starts up sometimes, so that means it’s not the o2 sensor detecting the exhaust leak or a bad injector then, correct? I haven’t tested the ECT, ACT, or anything else

 

Yes on O2, havent proven good/bad on injectors yet...have you?, start with thermostat (195?) if not ECT may never sense warm enough, for O2 to take over.

 

no I have not, but I just changed the plugs and all of them were black from running rich, so that would seem to indicate all the injectors are working wouldn’t it, unless it’s an injector sticking open. I did get code 172, but can the o2 sensor throw that code even if the engine never reaches operating temp? also, wouldn’t it throw a code for the engine not reaching temp as well? sorry for all the questions

EDIT: if an injector is stuck open, then why would I get a lean code? basically, why am I getting a lean code and a running rich condition when the engine is cold and the o2 sensor doesn’t affect anything? this is hurting my head

 

youre lean code means at a time when conditions were met for the test to run the truck had to riches the AFR compared to what it should have been based on its inputs. This is based on the inputs from o2 sensor.

so if your inputs have issues your engine has unaccounted for variances to its ability to control the afr, or it literally can’t adjust because of its limits(clogged filters, blocked passages) for the load and power it was trying to create… yeah

Also a LEAK IN THE EXHAUST is a great way to have an O2 sensor read incorrectly especially BEFOREthe sensor such as most exhaust manifolds

stop asking questions you clearly don’t want the answer too lol you are correct the issue can be a vast array but take your logical brain and apply what you know, the fact a common af issue causes a exact situation and your truck is apart of the common group criteria, and you have an exhaust leak on one side of the engine(part of what you know) do diagnostics from there. With a soapy water you spray on a cold manifold and eh runnin a bit. Don’t let it make too much heat. Visually inspect. If you can seal the opening you could stick fog machine in oxygen sensor hole. Or just ignore that smoke.

you have options to exhaust (ha) before you should be typing back. Without seeing it we can’t tell you what it is just help uou understand what you share snd help direct your diagnosis

 

yeah I’ve been told the exhaust leak is a problem lol. I think it’s between the manifold and the head on cyl 4. But the point of my previous posts was, if the computer doesn’t read the input from the o2 sensor until the engine reaches operating temp, then while the engine is cold, it shouldn’t know if the engine is running lean or be affected by an exhaust leak, and run fine until it warms up. But my truck will mess up when it’s cold, so it couldn’t be input from the o2 sensor that’s causing it, see what I mean?

 

it doesn’t adjustfueling when in open loop but it’s important your engine has a certain level of back pressure,you would be surprised at how quickly a heated oxygen sensor can put it in closed loop esp in warm weather. You may have ecm programming that attempts to save the catalyst if it thinks you’re about to blow it up, you also may have leaky valves or worn parts that your exhaust leak is now causing a vacuum issue and maybe your fuel pressure regulator can’t give you proper pressure with a fluctuating vac etc like I would whip out a pesssure gauge but you only got a couple minutes on a cool morning before in closed loop

if you want to test the theory disconnect the upstream o2 sensor so it stays in closed loop

as soon as your engine looks at your o2 for fueling and you have an air leak it’s not gonna be able to be reliable at all especially if the leak size varies with driving. Also your..map? Maf? If your engine vac is being disrupted you are gonna screw with at least a map and that is part of important stuff

 

could a clogged cat cause this issue? sometimes it will run rough as soon as I start it up. I did check the fuel pressure, the engine was warm but it was running rough when I did it, and the pressure was good. I somewhat checked for vac leaks in the engine bay by spraying some carb cleaner around and didn’t find anything, but that doesn’t mean I don’t have one. I haven’t checked any of the sensors but I think I need to

 

A vacuum gauge hooked to an engine can be a good quick way to determine if it’s likely if I recall correctly a plugged cat would have certain needle behavior vs a clear one. If you want to test your theory,unscrew the oxygen sensor snd unplug it so that it will be in open period and the exhaust has an escape if there is a restriction if it runs the same crap it’s probably not.
also if you have a exhaust leak which I recall reading you prob domt have the issue if it’s big enough

alsssoooo

check for vacuum leaks but next time use a propane torch amd just leave it unlit. It will increase rpm but not likely to start a fire from heat like a solvent or make anything brittle

remember propane is heavier than air so hold above hoses unless you have it shooting out pressurized, which may make finding it precisely difficult but a held over should be picked up by a low leak I would think