I would highly suggest finding a diagnostic shop and not a general repair shop to diagnose this…a general repair shop that does not use a “scope” for diagnostic testing is one who will just shotgun parts and that will cost you a lot of money !!

 

ask for test results before buying new cat(s). Also, ask for old parts to be returned. Usually only a single cat fails that way.

Also, if you have a "point and shoot" infrared thermometer, you can check exhaust manifold temps. The hot one has the clogged cat. If it's as bad as you say, it might even be hot enough to have a dull red glow at night upstream from the blockage

 

Yes awesome advise I will let them know I am not going to be playing that game of replace this and replace that and oh it still runs like crap we dont know what it is .... WRONG!! I am not going to let that happen in fact I will be letting this shop know that the troubleshooting of this issue is key!! I want to know who what where when and how.... I do not play that game of it could be this or it could be that... ?
This is what the dealership said. " tech. scanned for codes found p0300-eng misfire detectd on all cylinders randomly misfire. Engine runs as if ir is low on power possibly due to back pressure from cat converters becoming clogged.
customer states a grinding noise upon rapid deceleration
.. tech found multiple locations of possible noise, radiator support has impact damage . Harmonic balancer is wobbling, multiple rattles in dash...

This is what I was charged 150.00 for ? No better off then before I rolled into the dealership .. ? So this is why I do not like dealerships..

 

Like why didnt the tech do a pressure test on the exhaust to see if the cats were clogged he just said "possibly due to clogged cats" and then he says possible noise coming from multiple locations ..." ?? cmon man what is it ? bullshit dealership and I want to say something but everyone here in this area thinks that this place is the best...? I do not !

 

What would anyone out there do if you were me ?

 

Ok so I went out today and my first stop was a muffler shop. I asked them if they could tell me if my cats are clogged ... long story short the answer was NO .. but they did refer me to the Smog shack down the way a couple doors and said this guy will do a complete troubleshoot diagnostic test and tell me what exactly is wrong with my car and why the random misfires that nobody wants to touch or get involved with or tell me why I have random misfires... It is like an unwritten law out there or something .. If I come in and ask you about my car and why is it running so sluggish and can we fix it ? The shop says sure what is it doing ? I tell them it has random misfires and it is running sluggish oh and the cats might be clogged . /??? Not sure actually nobody is really sure about that yet .. So they call me in three hours and say come get your car ok so I get there and they tell me it has random misfires and the cats are not clogged up and it might have a semi clogged fuel injector... ?? So everything else checks out good . So they say they put some cleaner in the fuel to try and clean out the injector but with no success ... They told me to let it get down to 1/4 tank and add some cleaner like techron to it and see if that helps.. So I say ok and they say that will be 143.00 So after two places, A dealership and a smog diagnostic shop and a muffler shop , I still have no answer or no clue as to why my car is running so sluggish... ?? What is wrong with this picture.?? ******* bullshit because shops dealers or anyone else out there can not do a real trouble shoot or diagnostic anymore/ .. They just want to guess at it and charge me 150 a pop. Where are all the mechanics out there .. anyone can read a machine and replace part after part after part until the problem goes away .. **** Where can i get my car repaired at now of days ?? Not the dealerships not the shops ? WHEre ? I can go around all day to shops paying them 150 each to do nothing and nobody blinks an eye .. WHY not that is robbery I am taking my car back to both places after I get it fixed to show them how poor there service so called service really is .. It is a joke why cant someone tell me what is causing the misfires nobody wants to get to the problem they just want to charge and replace charge and replace Frick that

 

Well dealerships would rather not do diagnostics because they don’t want to be tied up with something that does not generate a lot of income vs time spent !!…in a 2 hour diagnostic they can do a few brake jobs or as they say the “bread and butter” work and make 3 times as much money…you’ll just have to find yourself a competent shop with one who’ll put a scope and scan tool on it and see what’s wrong…in one hour with a scope they should be able to do a “relative compression” test in less than 5 minutes (no removal of spark plugs) to see if there is anything mechanically wrong with the engine and if there is do an incylinder pressure analysis which may 30 to 60 minutes checking at the intake manifold and exhaust, “current ramp” all 8 injectors and coil packs in less than 10 minutes, “fuel injector balance” test to see if any injectors are clogged…less than 10 minutes using a scope instead of a fuel pressure gauge…a “volumetric efficiency “ test to see if the engine is “breathing” properly….take the car for a quick spin down the street “under load”…15-20 minutes using a program where you plug some numbers into a program like temp, BARO, your max MAF reading in grams/second so my friend in less than one hour they should be able to do all those test !!…I’d call some Corvette clubs in your area and see who they recommend for a job like yours…good luck !!

 

Yeah well I found out that the cats are just fine no problem with them at all. So thanks for the guess dealership in Rio Vista but your 150 guess was off ... (best mechanic) lolol

 

So since they were wrong with their guess of the issue being the cats clogged ,, I called them and made another appt. for feb 14th and told them I would like to bring it in again and have the mechanic take another guess at what it could be is that ok ? ... The service guy was a little miffed but het thats the way you have to be with these guys let them know that you do not stand for guessing the problem especially if you are a dealership and you should know the product you sell inside and out!! I would if I owned a company and serviced my product ... Anyways they can take a look at it again for 150 bucks and no im not paying them this time for their time they already got paid for nothing ... I told them im bringing it back since the cats are not clogged...

 

Are you capable of doing the work yourself?

I don't mean that to be insulting. I'm just asking if you have tools and work space to do the work and basic skills.

Was this a random misfire or a misfire on a specific cylinder.

At the very least you should invest in an OBD-II scan tool or scan "dongle" and an app like Torque Pro (there are others). If you go the app direction, the OBD-Link MX Plus is one of the best bluetooth adapters for bang for the buck. It's reliable and quick and works with most cars up through the C6 era, and can also talk to many newer vehicles. Personally, I like Torque Pro for Android (I have Android devices).

With a decent OBD-II set up, you can do your own code reading, and read and/or capture some real time diagnostic and performance data. That will help narrow down the cause.

For some reason (I should go back and read the whole thread, but I'm lazy today), I seem to remember this was a P0301 error. If that's the case, and the injector(s) is/are suspect, you could swap the #1 injector and like #4 (not the same cylinder you swapped coils and plugs with) and see if the error changes and "follows" the specific injector. If it follows the injector, then the injector itself is the problem. If it stays in the same specific cylinder, it's a wiring issue of some sort, either the ignition wiring or the injection wiring.

Techron concentrate plus, AC Delco 10-3012 and GM Part # 88865585 are the only "cleaners" I'd recommend for any modern GM vehicle. A bottle of that with a half full tank would help a lot of potential issues and a bottle in a full tank about every 3 months is great preventive maintenance for fuel system issues. Not necessary (or good) in every tank, but about every 3 months for a regularly driven vehicle is good.

 

Remove upstream O2 sensor for cat(s) then take if for a drive. Yes, car will get pissed and throw code. If car runs better, cat is clogged.

EDIT: do not plug the hole. That's why you removed to O2, to let the gases have an escape path.

 

Probably not a good way to do it !!…trying to remember when I changed both my upstream O2’s what would be in the path of those HOT exhaust gases without the sensors installed..you don’t want to burn up any wiring !!..OP said his cats aren’t clogged anyway.

 

Right well thats what the smog diagnostic shop told me so I can not trust them I would feel better doing the test myself but It is difficult to just jack up these vettes never mind trying to remove a o2 sensor which is probably really hard to get to and I really do not have a place to work on it ... So yeah I am at the mercy of shops that do not care about what is causing the issue they like to all stand around and point to the codes and the engine light .... with their thumbs up their asses.. then say how they fixed it .. idiots! Thanks guys I will try to pass on the advise to my mechanics, I will !!

 

I also did get some more fuel cleaner from techron and added it to my tank last night and we will see if it will clean out the bad tank of gas i got according to the smog shop and when that gas is gone I will be driving my *** right over to the smog shop and saying well it still is acting like it was before the cleaner so will you take another guess or maybe you can actually do your job and diagnose the problem here are some helpful hints I got on line from my corvette forum ... I dont think that they will have a problem with that right?

 

Ok it could be cats but also could be ignition related if fueling is fine (confirmed by scanning the fuel trims).

Addressing the cats, you'd have to remove the o2 sensor closest to the engine on whichever bank you're testing. Insert pressure tester hose and observe.
Not just at idle either, that's probably the only place it was checked if it was checked by any of those shops.

I've seen this done on a chassis dyno where the pressure hose doesn't have to be very long at all, while on the street you'd want it long enough to peek into the interior for viewing.

However checking the fuel trims on each bank will tell you right away if something is affecting one bank. Cats usually don't fail both at once so one of the banks will hint at a problem if the fuel trims are far + or -

It could be as simple as the spark plugs being worn out with a resulting large plug gap.

 

I changed the spark plugs and the wires already and I am putting my stock air intake back on tomorrow. I am not sold on ruling the cats out yet either just because some tech said he doesnt think they are ..? I am taking it back to the smog shop after I have the harmonic balancer replaced along with the lower radiator cradle and upper radiator cover As I saw that lower drivers side has a little of the radiator cradle skid plate. Thats the noise i am getting upon hard deceleration as the whole left side of the radiator and all the other mounted items like condenser and fans all that is moving forward when I brake hard and the fan is hitting something as it is moved slightly when I brake hard everything kinda comes forward and because it is loose down by the radiator support it is in need of repair from body shop now as well ..

 

Does it have an oiled media filter on it now? Like a K&N or similar? If it does, it would be a really good idea™ to clean the MAF sensor when installing the stock intake system. Use the cleaner specifically for MAF sensors. I like the CRC brand, but that's the easiest one to find where I'm at. Follow the instructions on the product you use exactly.

Even if you can't do the actual work, some diagnostic tools would be a good thing to have. The OBD-II setup I mentioned in the previous post would let you read DTCs and read/log performance data to help with diagnosing problems like this. You wouldn't be as reliant on shops to do the diagnostic work, and you'd keep them honest if you can read that stuff yourself. Another handy diagnostic tool is the point-and-shoot infrared thermometer.

 

It did have one on it but it had no oil in it .. Now the MAF is the flat rectangular piece that is held in by two screws on the side of the air intake?

 

Your totally right about these shops ..? How are they still in business when all they do is rip people off and hardly lift a finger to do any real mechanical work at all ..? I just want to get into fixing mrs the correct way the way the service repair manuals and the experienced diagnostic mechanic which is really not that hard to perfect .. It is like a chart of yes and no .. Do this , what happened did it do this ..go right did it do this go left ... Now perform this and if this happened go up if this happened go down ... Now perform this test and if this does that go right if it does not go left .... Ok it is either this or that DONE!! what is hard about that ? The tracking down the issues and doing the test procedures ? If that is hard for anyone working on cars then they choose the wrong field to work in... I used to take out transmissions to replace clutches on my old camaros in my garage on my back with no help at all... Yes I did do that so tell me about it blah blah blah anyone can follow directions and that is pretty much all diagnostics are... And so is Tuning tuners do the same thing kinda just work off of a laptop squeezing HP out of every inch and while keeping the mpg down as well .. It is simply using what the manufacture has given us and dialing it in ... to our own specs ...

 

Rereading your initial post and seeing your balancer may be wobbling that can be the cause of the misfires…watch the video below from about the 8 minute mark…since the crankshaft position sensor is the engines “misfire monitor” and the reluctor wheel is at the other end of the crank from the balancer the PCM may be seeing this as a misfire because when the crank slows down slightly due to the HB wobble the PCM thinks the car is misfiring but also the PCM may be getting confused not knowing when to fire the injectors and ignition…result is low on power too since fuel and spark is not happening at the right time…I’ll reach out to some of my Facebook groups to see if someone may be able to look at the car…I believe you’re a little east of Frisco !!