hello. i am new to forum. i have a 2003 Blazer LS, 2dr, 4WD with just under 132,000 miles. Problem is random misfire when in Drive and stopped at a red light or stop sign. car never stalls, but you can feel the misfire vibration/stumbling. Not apparent when shift to Neutral or Park. Also, when under load going over steep hill engine does not stall, but stumbles and struggles to get to top of hill - very prominent random misfiring going on. When at top of hill and starting decent it smoothes out and runs normally. No driving issues when on normal level roads and slight inclines, but when going over steep hill this problem presents itself. Due to age and length of time Service Engine Soon light has come on and stayed on over years the LED has burned out and since soldered into instrument panel cannot replace bulb, so only way i know if there are codes thrown is taking to local auto parts store and using their scanner to check which i have not done recently. Last time a mechanic scanned it the code (unknown) pointed at MAF sensor which has never been replaced to my knowledge (i purchased vehicle pre-owned). I have had the following done during past year to 2 years: New catalytic converter, new plugs (AC Delco), wires, complete distributor with rotor cap/button (after market), cam sensor, radiator, new BWD spider injectors with fuel pressure regulator, removed a throttle body spacer that was installed by previous owner, added a Spectre cold air intake with oiled filter within last 6-8 mos (filter factory oiled, not oiled by me), BWD brand TPS, intake gasket and new idle air control valve. I don't think i'm missing anything. I have not replaced the MAP or the MAF sensors, however i have regularly cleaned the MAF sensor with CRC MAF sensor spray and i clean the throttle body and butterfly on occasion with CRC throttle body cleaner. Never have had any valve work or lifter work done. I change oil/filter at 6,000 mile intervals and at last oil change i added "Restore" brand oil additive for V-6 engines. Due for another oil change within the month. Again, car runs smooth when driving on regular roads and i do not do any off-roading or harsh driving. I NEED HELP FIGURING OUT THIS MISFIRE/LOAD STUMBLING ISSUE. I have had to dealership and different mechanics and no one can figure it out. Do MAF and MAP sensors wear out over time? i think i will order a new Delphi MAP sensor and at least replace that sensor since it is cheaper than replacing the MAF at this point. If i need to replace the MAF sensor i am looking to purchase the Delphi brand. Recently, i get a raw fuel smell when filling up at gas station. Dealership said needed a new canister and evap solenoid which i purchased and had installed. Did not cure the problem. Does not happen at every fill-up, but when it does i have to drive around 5 miles or more before it disappears. Could a bad MAP sensor cause this? Another problem that has started within last several months is 4WD does not engage into 4 Hi when button pressed on dash. When pressing 4 Hi the LED for 4 Hi flashes about 7 times, but does not engage. I replaced 4 WD module in dash just over a year ago (GM brand). When pressing 4 Hi you can hear module click, but no engagement. On occasion the 4 Hi will engage, but for days at a time it does not. One time recently it went into 4 Hi when i was checking it, but then i could not get it to shift back to 2 Hi. I have never had encoder motor replaced or the diaphragm actuator (assuming my vehicle has one). Sorry for lengthy post, but i am rattled. Please help! All help/ideas appreciated. Thank you.

 

You say the SES light has been on for a long time. Usually it's emissions related things that set it off. After reading all the things you've done, I'm thinking you may have a partially blocked catalytic converter. You say it feels " normal" on the straightway but I bet if you drove a Blazer with no issues you'd feel a difference in the way it drove throughout the rpm range.

 

Thank you for reply. I did replace catalytic converter about a year or so ago with a new one. I forgot to include that in my long description.

 

Then my next step would be to check and replace vacuum hoses.

 

I wrote such a long post originally that i figured i would leave some things out. regarding the vacuum lines, i did replace all of the lines that i could easily get to, but if there are lines under engine or not easily accessible those did not get replaced. not being a mechanic i have no idea where all of the lines are. the ones i replaced are the lines at top of engine on driver's side that are close to where line connects into driver's side fender. also, i fueled up this morning and i got that raw fuel smell again which has lingered all day. As of this evening smell is still apparent. charcoal canister and evap solenoid replaced within last couple of mos with GM parts. Fuel cap was replaced 2 years or so ago. Not sure how often fuel cap should be replaced. There are no apparent leaks under fuel tank or elsewhere under vehicle, so not sure what is causing this. No rust on filler neck where cap goes. Baffled by this issue as well. I like my Blazer and it is like new inside. If i can get the misfire/stumbling issue, 4 WD and raw gas smell fixed I wil be happier. Regarding 4 WD, mine does not have the "Auto" button just the three buttons - 2 Hi, 4 Hi and 4 Lo. I replaced 4 WD module in dash (passenger side) last year, but after about a year of having that replaced (GM part) whenever i push 4 Hi the little yellow light flashes about 6 or 7 times, but does not engage. When pushing button you can hear the 4 WD module click, but no engagement. It will go into 4 Hi occasionally when button pressed, but only when it wants to. As an example it went into 4 Hi about 5 different times today, but twice it did not. There was about 3 or 4 days in a row that it would not engage at all. It's a hit and miss type of thing - no certain condition present when it works. I have never had encoder motor replaced or vacuum actuator (assuming it has one) and the switch itself with the 3 buttons has never been replaced. The other day i took it to transmission mechanic and the guy ran scan, but no codes were present other than a code that referenced MAF sensor (he didn't tell me the code #). As i stated in my original post my SES light (LED) has burned out, so i only know if codes are present if i take it to Advance Auto Parts and scan with their scanner which I have not done in quite a few months. I only knew about MAF sensor code when transmission guy told me. Any more thoughts? Thanks.