Hey All/Scotty, Longtime fan. This is a little long so bear with me...
’05 Pontiac Grand Prix
About 210,000 Miles
code P0300
Background...
Car runs GREAT considering Age and Mileage. Regular maintenance done since owning about 2 yrs ago. Had resonator replaced with pipe to quiet the car down last spring. Last June/July car started choking after startup and low speeds and would die at stop signs until speed over 30 mph was maintained for about 15/20 secs then would smooth out. Could hear exhaust choking and smell strong exhaust fumes. Car ran through for about 2 weeks then switched to low power mode. Restarted car and reset battery and ran fine for another week. Took it to my mechanic and on the way switched to low power, completely lost all lights/power after arrival (restart). Mechanic was puzzled, managed to force the engine through and a huge cloud of black smoke came out the tailpipe, ran fine for about another week (no oil in gas). Problem happened again and traced to completely shattered MAP sensor and TPS. Replaced parts and has been running great until about 3 days ago. Same problems, died after parked for a while and is currently at mechanic’s house.
Replaced 3/6 spark plugs last year (ones on the back are a pain in the ass to get to). Note that when I got the car I had to give it a little gas to turn over. Improved greatly when I replaced PCV valve and air filter, non issue after TPS and MAP sensor replacement. Issue started when the weather here took a sharp turn from -20 wind chill to 50 degrees in about a day (that’s the Midwest I guess?) Help please?
It's interesting that the symptoms sound like a clogged catalytic converter and they started a couple of months after you replaced the resonator immediately downstream of the cat.
Why replace the Accelerator Pedal Position sensor?
Just pull the clamp off the intake hose, pull the hose off of the throttle body, Key On, have someone slowly press the accelerator pedal and watch the throttle plate move. If it's moving with the accelerator pedal there's no reason to replace it. You could do the same by watching your Throttle Plate angle on your scanner's Live Data setting. Press the accelerator and watch if the throttle plate angle moves accordingly.
You say you've already cleaned the throttle body. Was it really crudded up with carbon? Did it clean up good? If it cleaned up good, why consider replacing it? I have the original throttle body on my 2001 Ranger, Never cleaned it once. Works fine.
In your shoes I'd do 2 things first.
#1 put a fuel pressure gauge on it. . I'm pretty sure you have a schrader valve on your injector rail so the entire test won't take more than 10 minutes. Once you know that you aren't dealing with a fuel starvation issue....
#2 Do a back pressure test to evaluate the condition of your catalytic converter. You pull out the upstream O2 sensor, screw in the exhaust back pressure gauge adapter into the threads where the aforementioned O2 sensor was removed from, start the car and observe the pressure on the gauge. Here again, maybe 20 minutes to a half hour of your time (depending how easily that upstream O2 sensor comes out). After this test you'll know if your catalytic converter is clogged or not.
The thing is, if you aren't willing to replace the catalytic converter, then don't throw another dime into it until you know if your cat is clogged or not.
One more thing. Auto Zone, Advanced Auto, and the other auto parts chain stores have a program where you can buy these tools and return them within 45 days for a full refund so these tests won't cost you a dime.
Scotty did videos on both of these tests.
The 1st is the Exhaust Back Pressure Test
The 2nd is the Fuel Pressure Test
https://www.youtube.com/watch?v=qJTmddbOxMk
https://www.youtube.com/watch?v=JoWZz1ckRGw
Considered the pedal position sensor based on some research but the codes those people were getting don’t match mine so I’ve more or less scrapped that. Obviously this is keeping me up and I can’t get in to work tomorrow so I took a personal day to try to get this hashed out.
Throttle body was absolutely disgusting. Completely black on the inside. Cleaned up pretty well but I didn’t want to use a wire brush just in case (I’ve heard the door is kind of sensitive). The door moved much easier after the cleanup and I gave it about 45 minutes to air out after I hit it with a can of air (like Scotty suggested in his Throttle Body cleaning video).
Throttle plate WOULD move even if I’m not getting any play from hitting the accelerator in your test scenario?
Thanks so much for the video links! I’m a DIY person who doesn’t have a shop but I’ve done quite a bit in the past and I usually rely on my home based mechanic to diagnose stuff that I usually fix (proudest moment was a Heater Core replacement on an ‘01 Taurus haha).
I’ll run those suggested tests tomorrow (the Auto Zone near me is great and has knowledgeable employees who haven’t steered me wrong).
One more thing to add... in my research I found someone who had the exact same problem and tried to replace the throttle actuator control on the Throttle Body with a salvage part. 17 minute video later and it didn’t fix it. I’m not sure what codes he was getting but he solved it by disconnecting the battery, unhooking the throttle body wires, cleaned the connections, reconnected and the car never had a problem again.
Thanks again for the lengthy reply! Trying to hold this one together until peak season for one of my clients starts again and I can get a new car.
For those still following, one more update...
Tinkered around with it most all day today, cleaned the harnesses and still nothing. Cleared the code twice and gas pedal came back with rough idle, check engine light is currently off, however I still wanted to fix the problem.
Spoke to two seasoned mechanic friends for last opinions. They basically both said the exact same things which were:
1) Codes being given indicate failed MAF sensor.
2) (I didn’t know this) apparently GM (Pontiac) is EXTREMELY picky about replacement electronics. I also was not aware that almost all parts shops sell reman parts (even Duralast).
3) They had me run a test while the car was idling. Unplug MAF from the computer. Car immediately calmed down and stopped sputtering. Once plugged back in the car choked out and died.
4) Buy a brand new OE Certified part from AC/Delco. Only supplier of AC/Delco is a wholesaler so I went through a trusted local tire shop to get the part. They’re having it couriered tomorrow and I’m going to give that a shot. If what my two mechanics said is true then I should have a fully functioning car again tomorrow.
UPDATE....
Replaced computer off old junker and MAF sensor (3 times). Codes are as follows:
P0420
P0101
P0068
Car turns over with gas applied, then Reduce Engine Power light comes on and stepping on the gas pedal does nothing.
Note the car ran fine for about two days just a little rough between 0 and 10 mph, idled fine. Also replaced PCV valve and air filter just in case when I got Duralast MAF Sensor.
Interesting thought Jack, I hadn’t thought of that. Jsyk the only symptoms I had prior to the resonator replacement was a hard start where I had to give it gas. I’m nearing my $ limit on this thing so one more update....
Cleaned throttle body and reset computer (manually I don’t have a code reader that can clear codes) twice. Got slightly better results for a little while longer but rough idle and surging lights at start up. Ran for about 5 mins and I kept the accelerator at about 2k rpms. ABS light and check engine light came on and went to reduced power mode.
Gas pedal still was getting a very weak response which is a change. Here’s what I’ve dug up so far....
Replace gas pedal (I’ll do this, about $55 for part and 20 mins of my time)
Replace Throttle Body (I won’t do this, takes 20 minutes but the part is $300)
Replace resonator (I would do this but I don’t do exhaust but I have a guy).
Replace Cat (I won’t do this, interested to know if the problem is the resonator if the Cat went bad if the old stick a pipe in it trick would work in which case I would do it).
Any thoughts are appreciated! I’m about ready to call it quits.
Before I completely gave up on it, I'd be tempted to try the cat fix you mentioned, especially if you aren't in a state where you have to have inspection.
Thinking about trying the pipe in the cat before resonator replacement. If still running rough I’ll try Resonator + gas pedal if mechanic agrees with me. Going to have to somehow get him to the house tomorrow as I actually don’t know how to do the trick haha.
Nope on inspection! It’s one of the few good things about living in IL. Made a few out of state friends jealous after the resonator sub for pipe fix 😉
Probably final update as it’s mainly now out of my hands.
Replaced the MAF with certified part, cleared the codes twice, started the car after reading and clearing code P0132. Turn over and ran like brand new except at lower speeds sometimes. Would choke a little bit and died 3 times when going from high speed to low speed. Sat overnight, started in the morning, got 5 mins into my commute and went into limp mode. Forced it through to get to work and then the shop. Shop called and confirmed what I originally thought about 8 months ago (new shop, actual certified mechanic, the other guy is like me with 40 more years experience).
Firing on 4 cyls out of 6, called back today and said all mechanical parts check out (plugs, wires, harness, coil, distributer) and the problem was the computer is not communicating for those two cylinders to fire. Mind you we “flashed” a new computer off a scrap dealer. Apparently you can’t flash the computer in an ‘05 the old fashioned way like my other guy did so I don’t even know if the car has taken the new computer. Heading back to the yard tomorrow to pick up the old unit and mechanic has scheduled his computer guy to come have a look at it next week. At this stage I think it’s one of three things:
1) Car would run fine with old computer because MAF issue has been resolved but improperly flashed new computer has caused fault in communication with spark.
2) Old computer has fault in it that I thought all along and computer guy may be able to program around fault.
3) Old and new computers are shot and completely new blank computer needs installed. If 3 is the case I’m done with the car.
Man, I hope the old computer will work or the new can be programmed, good luck to you!
RESOLUTION POST
And follow up....
SO I took the car out to the dealer and they had it for about two weeks. They replaced and flashed the new computer (for all of about $930 including the diagnostic so not too bad). They couldn't get it to go over 1,200 RPMs and kept it for about a week after replacing the computer. After checking the codes which were pointing to a bad MAF/MAP correlation but knowing I replaced the MAF Sensor and the MAP sensor recently, they didn't want to throw parts at it. The computer was also telling them that there was a problem with the Throttle Body but when the tech saw I had recently cleaned it he ruled that out. After some further analysis they found that the low power mode was being triggered by a blown PCV Valve (incredibly annoying) and the check engine light was being triggered by two bad solenoids. Since my business partner was footing the bill I told them to just replace the parts (even though the PCV is five minutes of my time and $15 or so shop charged like $110). $1300 later and my car is running like a champ.
Now for the fun part....
I visited a gas station about 9:30 PM to pick up some smokes and an energy drink for the next day. I always leave the car running with the keys in and I've visited it hundreds of times over the past three years or so. Used to at least shut her off before I walked in but I got comfortable maybe two years ago. Left her right outside the front door and some kids hopped in and drove off with it.
Car was recovered but everything was gone and strewn about downtown. I lost quite a few things (all replaceable). Long story short after all my troubles with the car I'm in the market for a new one for psychological reasons so I appreciate all the input here but it seems that someone somewhere didn't want me in this car haha.
Man what a roller coaster ride..