02 Grand Prix Constantly Misfires

**Chuppster** · 07-30-2015

I've had my GP for over a year now, and it's always run fine, until a few days ago. I started it, and it started, then immediately shut off. Tried again, same problem. I was able to use the throttle to hold it over 2k rpm's and it didn't shut off then. Driving the car was interesting, as it would have periods of running fine, then it would start misfiring, and occasionally just stop firing altogether. I'd have to put it in neutral and start it again.

The next day, drove it to work, and it ran with no issues. On the way home, the problem came up again. It's been happening on and off ever since. Here's what I know.

It began 4 miles into a fresh tank of gas.

The engine is throwing the following codes:
P0128 - Coolant temp is always low
P0440 - EVAP small leak/no flow condition
P1810 - Transmission Pressure (TFP) switch manifold fault
P0135 - H02S11 heater condition
P0420 - Catalyst Efficiency low - bank 1
P0300 - Cylinder Misfire detected - random cylinders
P1133 - Heated Oxygen Sensor - bank 1 sensor 1 insufficient switching

It's a 2002 GP GT with 140k miles.

Any ideas where I should start?

Edit:Title (03->02)

**J57ltr** · 07-30-2015

How old are the plugs and wires?

**Chuppster** · 07-30-2015

No idea. I haven't changed them.

Would this happen with plug wires? All pistons are misfiring, right?

**J57ltr** · 07-30-2015

I think it could cause a few of those codes. Change the plugs and wires first use iridium if you want them to last a long time and copper if you don't mind changing them every 10k or so. Use the house brand plug wires (autozone/advance/ o'rileys/ whatever). Start with that.

Jeff

**Chuppster** · 07-31-2015

Okay, I can change the plugs/wires tonight. I should mention that this doesn't seem to be a single piston or even two pistons misfiring, often times it feels like I'm running on only one or two pistons (Can't maintain 30 mph with pedal to floor). Sometimes it revs, cuts power, then kicks in again momentarily, sometimes it runs with no power, then sometimes it'll drive fine for 1/4 - 1/2 mile then start acting up again. I believe the most odd thing is the fact that I haven't had any issues with the car when first driving it in the morning (around 8:00). It drives like normal then, but it misfires at all other times. Another thing, I'm getting 12 MPG.

**J57ltr** · 07-31-2015

I think someone else should chime in, but if it runs IC cool then maybe the ICM's connections need to be cleaned. Could be a partially clogged cat. Check the wiring going to the O2 sensor it may be damaged causing the O2 codes. the 1810 I am not sure about, it's an 03 so I wouldn't figure it would be like a vacuum operated modulator, but I am not sure on that car. The misfire on random cylinder code is probably just bad plugs and wires.

Lets get the checking and plugs /wires out of the way first.

**BillBoost37** · 07-31-2015

I would personally fix the fueling issue (swap out front O2 sensor and coolant sensor). Those two are hosing your fueling horribly, likely causing the misfires.

**Chuppster** · 07-31-2015

Both sensors have under 5k miles on them, I just recently changed them. I cleared the codes when I installed them. Are there two coolant sensors? I only changed the one that I found (near the thermostat, which I also changed. Also, couldn't the O2 sensor be having issues due to the misfiring?

**BillBoost37** · 07-31-2015

There is only one coolant sensor.

If you changed both and the codes came back, check fuses to the O2 and wiring to the coolant sensor.

**Chuppster** · 07-31-2015

J57ltr, It's a 2002 GP, I get confused sometimes because my last car was a 2003 GA.

BillBoost, should the gauge work if the sensor doesn't? It reads the correct temp.

**BillBoost37** · 07-31-2015

There are two sensors built into the one item. One feeds the PCM (hence the code) and the other feeds the dash. This explains why you see the right temp, but the pcm is complaining.

**Chuppster** · 07-31-2015

While checking for vacuum leaks, I bumped the cord for the Mass Air Flow sensor. It began running smoothly, so I unplugged it. I'm getting a new MAF sensor tomorrow, hopefully that will clear things up.

**Scottydoggs** · 07-31-2015

check the wires to the maf plug. if you see cracks or bare wire showing the pig tail is bad and needs a new one.

if the wires are good. maf plugged in, car running, tap the top of the maf, see if it runs better, then bad again when tapped on.

**Scottydoggs** · 07-31-2015

oh, and dont get a aftermarket maf, they suck in our cars, you want a oem maf. aka junk yards are cheap. used on e bay too. 40 tops most of the time.

**EHB** · 07-31-2015

i believe amazon has hitachi mafs for about 80 bucks...don't hitachi make them for gm ?

**Sven the Sexman** · 07-31-2015

No one has mentioned this might be a plugged cat? Remove the front o2 sensor from the exhaust and try driving it. It'll be loud but if you have your power back, there's your issue.

**HighOctaneRacing** · 07-31-2015

Dude I put a brand new Delphi MAF in my 97 and it was fantastic.

Yes Hitachi was the OEM supplier but I wouldn't be surprised if their quality went down the sh!tree.

Sent from my LG G3

**Chuppster** · 08-01-2015

Okay, so a lot of differing opinions here. I don't have a ton of money (broke college student), but Napa carries them re-manufactured for $135. I can afford this, but this price tag is more appealing:
http://www.amazon.com/Hitachi-MAF004...ype=automotive
I don't want to half-a$$ this, what is my best bet? I got the current MAF from a junkyard about a year ago.

**Scottydoggs** · 08-01-2015

top of your maf is the part # search that # in ebay for a used oem maf. 40 bucks tops most of the time, or less.

or hit a junk yard near by and snag one there. im running a jy maf for the last 5 years now. best 40 bucks spent. the maf can come out of a na or a sc car. same part #.

you need to find a car thats a 01-03 iirc double check the part # before buying one.