Man this has been a little annoying. I had a thread open some time ago with little to no help regardimg my specific problem and everything ive found seems to be closed out with no answers. So, im hoping that this thread will bring some ideas or help the next person with similar issues.
How i got here: Bought the truck with the intention of spending some money on getting it to run smoothly. I was aware of the misfire and thought it to be something simple,not the case.
Quick Notes: 99 Tahoe,182k,5.7L Vortec. Vehicle had exhaust leak at Cat flange to manifold when i bought it, no biggie,heavy heavy misfire when it first warmed up but ran fine cold or could barely feel a misfire if there was one. Have thrown some money at it to no avail just yet.
The long story:
When I was purchasing the truck, i showed up to the kids house ,it started right up. No issues. No hesitation. Literally,turn key and split second to start. Ran smooth with a slight backfire (intermittent,figured it was from the exhaust leak,ecm thinking it was lean and dumping fuel).as i was letting it run and walking around the vehicle, it's like a switch went off and started misfiring..badly. to the point where it almost stalled... backfiring .."pop ,pop,pop" and a cloud of black smoke and then again,switched back to run fine,revs came up, smoothed out. Screw it. Gave the kid some money and grabbed the keys. Whats the worst that can happen?
So, i get it home. Ran fine for over an hour on the highway with a barely noticeable miss. If any. I say barely noticeable because i could feel something but at the time it was so loud and i couldnt tell if i was feeling a miss or the leak. HA.
Anyways,it would only start to run like crap when i would slow down and idle for a few seconds. But then itd kick itself out of its mood and run right again. Only when i got home and spent some time looking around and finally putting a scanner on it did i realize what it was doing.
Quick Diag notes:
Starts right up, smooth quiet motor during OPEN LOOP (coldstart) ,quiet besides the exhaust leak at this point.Once it warmed up,it switches to closed loop and thats exactly when the misfire starts. Bad. Lasting maybee 1 minute max. Then ,something weird happens. It kicked itself back into open loop? What gives? Started reading O2s ,voltage on primary and secondary were too low,(lazy) i had put up some pictures of the data readings i got on my other thread. It had a P0171,P0174,P0300 code. Fuel trims were ridiculously high. 25% to 50% percent positive!! Truck seemed to run better when at a steady RPM (ex:my highway drive home)
Well, at this point the truck wasnt registered,so i spent a little time over a few weekends doing the usual. Spark plugs,oil change, air filter,clean MAF,IAT,removed MAP sensor and cleaned out some gunk in it,didnt touch the wires or cap since previous owner had just replaced it in hopes of fixing the issue. Checked for vac leaks, checked PCV vac, checked the wire orientation..etc. Everytime i did something , i checked for the issue again,and everytime.. The problem was still there. Though, unless i was a little crazy, after cleaning the hell out of the MAF it seemed to run great for about 30min. Thought i fixed it, woke up and started the next morning and same problem came up.
As of today..Registered the truck, took it to an exhaust shop. They fixed the leak at the flange and i had them replace all 4 o2 sensors. I brought the truck home last night. Its quiet now, still starts up great,misfire is easier felt now at any temp, now it stays in closed loop. Set a code for P0302 earlier..
Im debating on what to do next, i hate throwing parts at cars to fix problems and id like to figure out the true cause before doing so. Any ideas? If not my plan of attack is:
.Change MAF Sensor and MAP sensor
.Check EGR Valve,clean and reinstall?
.Coolant temp sensor? Though now i feel as if this cant be it since it stays in closed loop.
.doubt its fuel pressure since i have zero problems starting or at WOT
.
Ideas? Suggestions.
How i got here: Bought the truck with the intention of spending some money on getting it to run smoothly. I was aware of the misfire and thought it to be something simple,not the case.
Quick Notes: 99 Tahoe,182k,5.7L Vortec. Vehicle had exhaust leak at Cat flange to manifold when i bought it, no biggie,heavy heavy misfire when it first warmed up but ran fine cold or could barely feel a misfire if there was one. Have thrown some money at it to no avail just yet.
The long story:
When I was purchasing the truck, i showed up to the kids house ,it started right up. No issues. No hesitation. Literally,turn key and split second to start. Ran smooth with a slight backfire (intermittent,figured it was from the exhaust leak,ecm thinking it was lean and dumping fuel).as i was letting it run and walking around the vehicle, it's like a switch went off and started misfiring..badly. to the point where it almost stalled... backfiring .."pop ,pop,pop" and a cloud of black smoke and then again,switched back to run fine,revs came up, smoothed out. Screw it. Gave the kid some money and grabbed the keys. Whats the worst that can happen?
So, i get it home. Ran fine for over an hour on the highway with a barely noticeable miss. If any. I say barely noticeable because i could feel something but at the time it was so loud and i couldnt tell if i was feeling a miss or the leak. HA.
Anyways,it would only start to run like crap when i would slow down and idle for a few seconds. But then itd kick itself out of its mood and run right again. Only when i got home and spent some time looking around and finally putting a scanner on it did i realize what it was doing.
Quick Diag notes:
Starts right up, smooth quiet motor during OPEN LOOP (coldstart) ,quiet besides the exhaust leak at this point.Once it warmed up,it switches to closed loop and thats exactly when the misfire starts. Bad. Lasting maybee 1 minute max. Then ,something weird happens. It kicked itself back into open loop? What gives? Started reading O2s ,voltage on primary and secondary were too low,(lazy) i had put up some pictures of the data readings i got on my other thread. It had a P0171,P0174,P0300 code. Fuel trims were ridiculously high. 25% to 50% percent positive!! Truck seemed to run better when at a steady RPM (ex:my highway drive home)
Well, at this point the truck wasnt registered,so i spent a little time over a few weekends doing the usual. Spark plugs,oil change, air filter,clean MAF,IAT,removed MAP sensor and cleaned out some gunk in it,didnt touch the wires or cap since previous owner had just replaced it in hopes of fixing the issue. Checked for vac leaks, checked PCV vac, checked the wire orientation..etc. Everytime i did something , i checked for the issue again,and everytime.. The problem was still there. Though, unless i was a little crazy, after cleaning the hell out of the MAF it seemed to run great for about 30min. Thought i fixed it, woke up and started the next morning and same problem came up.
As of today..Registered the truck, took it to an exhaust shop. They fixed the leak at the flange and i had them replace all 4 o2 sensors. I brought the truck home last night. Its quiet now, still starts up great,misfire is easier felt now at any temp, now it stays in closed loop. Set a code for P0302 earlier..
Im debating on what to do next, i hate throwing parts at cars to fix problems and id like to figure out the true cause before doing so. Any ideas? If not my plan of attack is:
.Change MAF Sensor and MAP sensor
.Check EGR Valve,clean and reinstall?
.Coolant temp sensor? Though now i feel as if this cant be it since it stays in closed loop.
.doubt its fuel pressure since i have zero problems starting or at WOT
.
Ideas? Suggestions.
