Misfire killing my soul

[thegawd]

I didnt see mention of testing the tps... test the throttle position sensor. that's what was causing me a perceived misfire at 95KM/H. it wasnt really a misfire, just a failed tps that would read as if the throttle was closed when the throttle was 1/2 way open driving down the highway.

it never caused a misfire code and it took me a while to figure it out.

But your fuel pressure is also low.

 

[stutaeng]

Thanks for the response, My mistake. It’s a 5.7 vortec motor. I thought Tahoe’s and Yukons only came with those.

50psi At idle and it increases as u open the throttle. That was parked. I could try and log it under a load of the hose is long enough I guess.

I haven’t found anyone with a good scanner willing to just scan it for me, still working on that.

I looked online and didn’t see anything under $300 for a capable scanner or tech 2 Or clone, any ideas? I Found a great truck to buy but I don’t wana sell mine like this

I thought the Tech 2 clones from eBay were around $300? No?

I'd say this is going to be the best bang for your buck, even if they are slightly higher. Or, an older, full-featured Snap On, etc.

I'd say plan of attack is to figure out what cylinder (s) are misfiring as been suggested, then go from there. Do further diagnostics to see if you have a spark issue or faulty injector or something else.

Are your injectors original poppet or the MPFI? IIRC, the original poppet need higher pressure to start. Spec was like 60-62 psi, but both of my Vortec engines fire up with 55psi ...and you seemingly don't have a hard start issue.

I don't think fuel delivery is a problem since you say misfire occurs all the time, with or w/o load...

 

[Dnytro]

I couldn’t find the clones on eBay, maybe I’m not searching for the right stuff.

It has mpfi. I assumed it had poppets so I bought a new mpfi for it, once I took it apart and saw it already had it, I swapped it anyway just in case. No change.

So, I got impatient waiting for the scanner to arrive and found someone to read the codes today, there were 4.

P0306
Cyl 6 misfire
P0153
O2 sensor circuit slow response Bank 2 sensor 1
P1133
Insufficient switching O2 sensor Bank 1 sensor 1
P1153
Insufficient switching O2 sensor Bank2 sensor 1

And here is live scans I screenshot

 

[Dnytro]

And no it doesn’t have a hArd start issue.

So I plan to pull and replace #6 plug and wire, one at a time, and replace the pre cat o2 sensors.

With the new info, does the tps and fuel pump still remain possibilities?

 

[stutaeng]

I would double check your wiring of the O2 sensors before replacing them. There's a ground and heater circuit for each.

Can you graph the waveform if the O2? The upstream should have a sine-form, switching between close to 0.1V and 0.9V, or close. Flip the throttle or spray some carb cleaner to see if you can see a response.

Usually though, you'll get O2 sensor codes if you are running lean or rich. If you have a misfire for sure. Rookie mistake to change O2 sensors if you have any "O2 sensor codes." Been there done that.

 

[Schurkey]

I am known to be "big" on "testing before replacement" of suspected defective parts.

HOWEVER, when it comes to O2 sensors, better safe than sorry--they have a limited service life and they're almost universally ignored until their operation is so poor that they can't be overlooked any longer.

In this case...since it's not throwing "O2 heater" codes, it's throwing "O2 response" codes, I'd scrap at least the two upstream sensors.

In fact, I'd be very likely to scrap all four, especially if I was unsure of their age/mileage; or knew their age/mileage to be high (beyond 5 years/50k miles.)

A more-conservative approach would be to fix the misfire, clear all codes, see what, if any, codes come back. You run the risk of retaining O2 sensors that are slow to respond, but not so slow that they set codes. This could negatively affect both emissions levels and fuel economy.

 

[stutaeng]

RockAuto has the best prices on O2 sensors. Probably a bit less than $200 for all 4 sensors! Then there's a discount for GMT 400 member somewhere on here. They are almost $100 per sensor at most local parts stores!

I bought that Lisle tool that allows you to use your ratchet/breaker bar, as well as their thread chaser. Both highly recommended for doing this job.

 

[Dnytro]

Small update. Bout 300 miles later, still no codes after changing the plug and wire and cleaning the maf. I/m still says not ready bcuz the cats and egr and air pump aren’t ready, everything else says ready.

 

[SUBURBAN5]

I bought an autel for $400, so far reads everything I need on my 98 burb. Also I agree with the fellas. For a misfire I personally watch the motor run live. On my scanner I can watch o2sensors by graph, cylinders running or idling, and other mis sensors. I'm only saying that because with out it you could end up shooting a parts cannon which gets expensive. I had a misfires that drove me crazy til I ended up getting a tune and advancing the timing. I would of never guest that since gm did not require a tune after doing a stroker swap.....

 

[Schurkey]

I/m still says not ready bcuz the cats and egr and air pump aren’t ready, everything else says ready.

What does the service manual say about the requirements for getting those monitors to run? I'd expect you still have some faults that are preventing the computer from completing those monitors.