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2004 Grand Prix GTP misfiring/OBD2 tester won't read codes

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dbest1197

New member
Ok so I have a 2004 Grand Prix GTP it doesn't have very high miles (to me at least) I'm not sure exactly how many but I beleive it's around 160,000 miles well when I got the car everything was fine perfect and we kind of had a few races with my brother in law in his 04 Mustang GT and yes running right we still lost but he's pushing right at 330 HP with a good few mods and programmer believe it or not we spun WAAAY worse than he did for starters lol, anyways about a week after that the car started very very slightly misfiring and it was only uphill every rare now and then so we would just kick it down one gear, and all was good, well I figured that it was needing plugs bc thats one of the most obvious misfire causes, well we kept driving it and kept driving it and now here about a month since then it has started sputtering pretty bad at idle like it's running out of gas, usually you have to give it gas when it cranks because it will start and then die if you don't, hills are awful shaking and jerking but I can still kick down a gear and it stops but horsepower on that car has been completely robbed where it should have 260 it feels like MAYBE 200 probably less like it drives fine if you are just going normal but I won't even suck you back in the seat anymore, and for about a week maybe the Check Engine Light has been on, well, me and my brother in law decided to swap a few plugs over from his broke down 08 Silverado 4.3 because the plugs were the same size and sure we DID only swap the 3 front plugs but the car seems to be sputtering just as bad and even when i reset the engine light when it goes in gear it comes back on but strangely the car seems to have most of its power back maybe 85% well we plugged the OBD2 tester into the port for the first time the night before last and regardless of how you try it (Running, Key Completely Off, Switch Turned On) it will say it's reading and always get to a certain thing with lots of numbers and the only letters I can remember were PWM where it always stops and there are flat dashes up top and when the scanner stops there the first dash turns into a circle (like I pictured below) and then the scanner will go back to the screen saying to SCAN or SETTINGS this is the dashes I'll put below
------------------------------------------------------------------------
|- - - - - - - - - - - - - -| what it always does
|o - - - - - - - - - - - -| what it does when it crashes.
------------------------------------------------------------------------
I mean I'm pretty positive that it's the other plugs considering the first 3 made a power and mpg difference (it was literally about half of normal on mpgs it was awful) but a big concern I have is why isn't the 2 scanners we have (both are exactly the same brand, etc.) Working to scan our codes other than the misfire the car is good well aside from the whole car being rusted underneath because dude who had it either lived or was always in Canada but it's bad under there well and one last thing the rear sway bar linkage broke on the left side so my brother in law MIG welded it and it stayed for around a week or so and broke again so the rear end kinda goes side to side with the slightest wheel turn at a good speed on the interstate but if anyone can verify for me that it's the plugs and give the reason if you can as to why the car won't give us codes the obd port looks perfect it's not damaged or anything so idk honestly lol thanks in advance!!!!
 


if ithat 260 hp statment was even close to true, try 180 ish at the wheels, yup. its 240 at the crank.

and im not really sure whats wrong here, your post is all over and hurting my eyes lol
 
Way too much ramble, not enough about the core issue.

To address scotty, 260 is the correct crank HP for a 04+ but 210 to the wheel is reasonable to guess at.

Why would you not just buy new spark plugs instead of swapping in the wrong plugs??

Yes the plugs from a 4.3 fit but they aren't the correct number and the gap on the plugs is wrong from the get go. Do a compression test to rule out a bad cylinder, install new spark plugs (41-101 AC delco's), inspect the wires, clean the coil terminals, clean the MAF sensor and then see how it's running.

But if the compression test comes back bad, then you have bigger problems.
 
Ok guys sorry for the rambling by the way I'm really bad about that well basically what I'm trying to say is that I'm pretty positive it's just plugs that are causing the misfire because those 3 helped greatly to restore power the main concern to me really is why won't the car let me scan the engine codes. If any of you can help me out on that id appreciate it we haven't had any electrical issues or anything out of it at all.
 


Check for power at the ODB2 port, I don't know which wire/terminal is the power wire.

Check for a blown cig lighter fuse, they are on the same circuit.
 
This might be off a bit, but I was having a similar problem just after I did a compression test and put in new plugs. Under load, uphill or just WOT I was getting a bucking misfire. It happened a few times before the check engine light flashed and stayed on. Codes said Cyl 6 misfire.

From my compression test, Cyl 6 was doing great, so I could rule compression out.

I had brand new plugs, installed properly, with dielectric grease and cleaned out boots, so I was pretty sure I could rule those out,

I already replaced the fuel pump and injectors a few weeks ago (bought it not running), so I didn't think fueling was the issue.

On a whim, I pulled the boots off the posts at the coils and bingo: Number 6 was nasty rusty. They were all pretty bad, but 6 was definitely the worst. I wire brushed the posts till they shined, cleaned out the boots with sensor cleaner and compressed air, put on a thin coat of dielectric grease, and re-installed. Now it runs great. So check your posts!
 
Ok thanks I'll definitely check those the car is from way up north and EVERYTHING is bad rusty so that could be very possible!
 
On 2004 that power sorse is with the on star fuse had that problem with mine

Sent from my SAMSUNG-SM-J321AZ using Tapatalk
 


Alright thanks I'll definitely be checking that out like I said it powers the scanner on and reads most of the way through but then crashes the scanner everytime at the exact same spot so I really hope this helps I'll update of this was issue as well
 
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