Engine idles perfectly but misfires with any gas
 

Hi, I've prowled the internet trying to solve this problem without success. I have a 2000 4.3L and I have had a reputable mechanic fail to fix this issue after a month and now I'm just trying to do it myself to save the shop labor rate. In July my Blazer's gas pedal went to the floor going up a hill and it died and didn't start back up till it was home.

It had no trouble codes, I let it idle for a long time and no trouble codes registered.

I towed it home and immediately tested fuel pressure, I got 60 psi at idle and within spec at all gas pedal locations.

I cleaned my K&N filter and replaced it for good measure before I decided to have a professional diagnose this and avoid throwing parts at it.

I took it to a shop where they replaced my timing chain, cap, rotor, plugs, wires and distributor (along with fixing some unrelated oil leaks) which did not fix it and apparently he was able to do some "adjusting" and it ran fine for ~600 miles and now I'm back to square 1 with the exact same symptoms.

Since then I talked myself into replacing the MAF, coil, and TPS and also got no results

Long story short: if you have any idea how my car can run perfectly at idle and be undrivable as soon as I touch the gas pedal I would be very grateful to any suggestions

possible cat plugged up? Have seen them do some funny things. Remove the 02 sensor in front of the cat and see how it runs.

There's no such thing as professional looking at it.
Mechanics are not professionals especially the ones that ripped you off and didnt fix it. It's Consumer FRAUD to blindly replace parts and not fix it.

Mechanics are skilled trades workers, not professionals..

Guessing has done no good.

STEP NUMBER ONE IS ALWAYS " LOOK AT THE SPARK PLUGS"

Always. Few do it.

Plugs are the key to knowing what's happening inside the cylinders.

Pull all of them and photograph them and post the pictures.

I pulled the O2 sensors last night and it ran pretty much the same so it would see the cat is good. I also tried to see if it was rattling and had no luck

I'll pull spark plugs at lunch and post the pictures

https://cimg1.ibsrv.net/gimg/blazerf...b59be0c7bd.jpg
https://cimg6.ibsrv.net/gimg/blazerf...d185b28e13.jpg
https://cimg6.ibsrv.net/gimg/blazerf...751a15ea4e.jpg
my first impression is there's no way that mechanic actually changed these, my next is that the gaps look a bit large, should I replace them?

Those plugs don't look terrible, but they definitely haven't been replaced recently... Can tell just by how dirty the ring around where the boot slipped over them is that they've been there for a while. That and the surface rust on the metal portion...

You say you replaced the coil, what about the ignition module that's next to it on the same bracket?

No use throwing a ton of money at new parts, see what a local junk yard would let you have one for. Bolt it on and see if anything changes. If it runs better, great, you found the problem. If you're satisfied with installing and running used parts, then roll with it. But if not, then invest in a new one from somewhere and use it.

I would say that the next thing I would try would be the module that's inside the distributor, under the rotor. But if you had the distributor itself completely replaced, that should rule that out. Unless it got changed the same way those plugs did...

I did the ICM too with no luck, at the time it was an easy replacement and I didn't have any way to test it

So I also changed the plugs and got no improvement (I figured they had 35,000 miles minimum anyways). I have a scanner that will show me trim, MAF, etc if that helps but I don't know what to look for

Easiest way I've found to test out most sensors is to just unplug them. For MOST sensors, simply unplugging it will put the computer into open loop and it will ignore the "no input" from the sensor and fall back to a set of parameters by default. Typically running off these set values, it'll run better than having a bad sensor reading. I've had vehicles (not the Blazer) run like hot garbage with a bad AFM plugged in. Simply unplugged it and ran like that for a week or so until I installed a new one. Didn't run GREAT with it unplugged, but it ran better than it did with the faulty AFM plugged in...

This works with most digital sensors like 02 sensors temp sensors, possibly MAP sensor and TPS, but will not usually work with analog sensors like a crank or cam sensors as those are typically REQUIRED to give a reading in order for the engine to fire, so if those sensors give out they engine wouldn't start at all...

I am sorry but this has been eating at me since i read it. I am a licensed mechanic...yes by trade. I also consider myself a professional in my field. I have been under cars and trucks since i was 7. I will agree that there are some out there that like to guess and they shouldn't have a license in my eyes. As a professional a proper diagnosis is key. Track the symptoms, narrow down the cause and repair as needed. As i said i am sorry to say it but you need to have some respect for those of us that put our hearts and souls into this trade as a profession. There are some of us that deserve the credit and we are the ones that love our jobs and do it for a reason. Remember it is all of us trades people that keep you on the road. Just putting that out there.

I'm assuming this is not in response to me but another in the thread. If you have any advice in telling the good mechanic from those who want to guess and replace (or insight to my specific problem) I'd love to hear it