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I wanted to start a new thread fresh about the problems of hesitation and backfiring through the intake. There were a couple threads going and things seemed to get a bit confusing. My hope with this thread is to help anyone who encounters this in the future.
So a bit of background, I have a 98 Ram 1500 with a 318, auto 2x4. Recently during cold starts and after about 2 minutes of running, I would experience engine hesitation/bogging followed by backfiring through the intake. Upon my initial investigating I noticed that IAC would stay open and suck air so I assumed it was bad and replaced it.
Well that wasn't it so I turned to this board for help. I was advised it may have been the dowstream O2 so I replaced it, still the problem existed.I had the truck scanned and O2's tested and was told they were okay. I then began to believe from the responses and reading that it was a bad/clogged cat so I replaced it with a MagnaFlow catted y-pipe, no dice the problem persisted.
Finally after almost $500 in parts and scans I had enough so I sat myself down and began to really think about it.

This is where some may get confused if they are unfamiliar with pcm programming.

I took what I knew of the problem and began to dissect it. First the truck only had the problem on a cold engine and it didn't occur until 2 minutes after start up. Well I know the truck starts in a closed loop which means it takes its fueling from a table until the O2's can heat themselves from say 5V and get down to 100mV at which time it goes into open loop and begins to reference fueling off the O2 reading, hence the 2 minute delay for the bogging and popping. Next to overcome the bogging putting it to the floor got the truck going again. This is because at WOT the truck goes back into closed loop and into PE (power enrichment) mode.
Okay so you ask how does the O2 cause the bogging/popping. Simply put once the pcm enters open loop it begins to reference the O2 and assumes it has reached its predetermined voltage (I don't know what it is but I am guessing around 100-120mV) But because the O2 is "dead" it still reads 5V, or whatever, and the pcm freaks out thinking the truck is pig rich and starts pulling a ton of fuel causing a lean condition which causes the power loss and backfiring throw the intake. Also remember that IAC I spoke of earlier, well it opens and sucks air for a reason. The pcm is trying to compensate for what it thinks is too much fuel. This just leans out the truck more.
Eventually the truck will begin to funtion normally because the exhaust gases which get extremely hot under lean conditions, heat the O2 to give it the proper reading.
So easy fix just replace the upstream O2 you say, not really. Yes replace the O2 but the problem is Chrysler f'd up. Every parts catalog lists our trucks as having 2 upstream O2's, even the dealership parts guy kept telling me this but I know it only had 1. What I ended up doing was taking the Bosch 15705 which they listed as correct for the LH side. It is the right O2 except that the wires are to short to reach the harness above the trans so all I did was extend the wires about 7" by cutting the wires and souldering in extra peices.
Also I wanted to add that the reason the O2 checked out okay at the garage was because the truck was already hot.
I won't say this is everyones problem but it was for me and I hope it can save someone the headache and expense of replacing a buch of parts for nothing.
 

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You have your open loop and closed loop backwards. Closed loop looks at the 02 sensor. If you saw 5 volts on the 02 sensor data on a 98 with a narrow band 02 sensor. The sensor itself was either open to the pcm or shorted to the built in heater. Which would drive the system lean. The sensor also starts off with low voltage say 100 mv and is able to produce higher voltage ( 900mv) as it warms up. Once it reaches somewhere a round 700mv it will go into closed loop. If the rest of the programmed criteria has been met. ( engine temp , run time , ect )
 

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Thanks for the clarification. I have done the same thing in the past when we have tuning discussions on another board for my other car.
Also I didn't have any scanning info myself, I pulled those numbers out of the air as an example.

Does the pcm definitely go into closed loop once it reads 700ish mV? My guess was it was a predetermined amount of time for 2 reasons, 1 the truck always acted up after the same amount of run time and 2 the O2 is disregarded during open loop for fueling so why would it monitor it for proper voltage to enter closed loop.
 

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Yes as long the pcm is happy with everything else . The pcm is looking for the 700ish mv . It is monitoring it to make sure it is producing the voltage to know that it is warmed up and ready to give the needed signal . The reason for the heater was to get it in closed loop fast for emissions. If it used it before it was warmed up it might see 200 mv and try to richen it up when the mixture was already rich due temps sensor.The sensor has to reach about 500 degrees to be accurate . At lower temps it can not produce the higher voltage above 450 mv.Not sure of how good of explanation this is . On a side note the downstream 02 sensor is only used to monitor the cat. The pcm does not use it for fuel adjust or trim. Just for clarification above 450 mv is rich and below is lean. Hope this makes sense.
 

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Excellent explanation, Chris The tie in between the temp sensor info and the 02 info going to the pcm in conflict when one is bad is the cause of cold start probs more than most realize. If replacing the 02 on the basis of an 02 code being set doesn't cure the prob consider replacing the temp sensor next. BTW the temp sensor may not set a code and still contribute to the cold start issue.
 
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