Sudden loss of power and backfire at idle

[7thdarkangel]

Did you by chance check your timing? Possible chain jump?

 

[DerekTheGreat]

It's always best to put them on the spot before you authorize the repair. Make them explain exactly what troubleshooting steps they took and the results that lead them to this conclusion! I always agree that techs should be paid for their diagnostic work... but I have always been on your side of the argument when they replace something that doesn't fix the issue. But doesn't that always happen in life? You pay the Dr. even if his/her diagnosis and treatment is completely wrong...

I think you can take them to court over malpractice and such. Firing the parts cannon isn't diagnosis work, that's child's play and sounds like how I usually fix stuff. I sure as hell wouldn't pay a guy like me to do any work haha. I watch South Main Auto all the time and Eric O. is a great example of a hard working, honest tech. The guy will test things sometimes three times before making the call as to if the car needs a certain part. I'd gladly pay him his labor rate for the amount of time it took him to reach that conclusion. That dude is a wizard.

 

[ram1220]

Distributor cap. Especially since the OP cleaned the engine and said he/she got moisture under the cap.

 

[JavaMan]

I hooked up the Bluetooth OBD adapter and got some measurements. There's some correlation between the RPM and the long term fuel trims. When testing I would throttle to get an RPM and hold it there for about 20 seconds prior to taking screenshots.

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[JavaMan]

I took it to another shop. They diagnosed that the MAF was way out of spec, reading nearly double what it should be, 9 grams at idle rather than 4 grams. So they swapped it out. $480 for that MAF part. Plus it needed fuel system intake cleaning.
Still had the backfiring. Turns out the distributor was a tooth off, and because the previous shop that worked on the distributor stripped out the screws that hold the cap, so they could not align the distributor correctly. They say the distributor is fine, other than those stripped screws. So they spent like three hours pulling the distributor turning it slightly putting it back in, checking the timing trying to get it right. Since I had been there from them opening at 8am till 4pm, I decided to just leave it and get back home 3 hours away.

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[454cid]

I took it to another shop. They diagnosed that the MAF was way out of spec, reading nearly double what it should be, 9 grams at idle rather than 4 grams. So they swapped it out. $480 for that MAF part. Plus it needed fuel system intake cleaning.

4 is low... mine is 8 or 9. The manual says 7.05 g/s as a typical value for a truck idling at operating temp in closed loop. I went through that early this year as mine was only 2 g/s. I got a salvage yard MAF.

Make sure to request your old parts back.

Fuel system intake cleaning sounds dubious to me.

Still had the backfiring. Turns out the distributor was a tooth off, and because the previous shop that worked on the distributor stripped out the screws that hold the cap, so they could not align the distributor correctly.

The screw holes can strip. I'm not sure how that relates to aligning the distributor.

 

[454cid]

I was going to run the scanner on my truck and show you numbers from my truck, but for some reason my scanner isn't working right. Reading my old thread on another board, mine is probably closer to 7.

I hope my scanner isn't bad. It's been sitting in the garage, and normally I keep it in the house.

 

[ozawkieng]

I have a 2000 GMC Yukon Denali with the 5.7 and it was stalling at low speed such as when you would start at a stop sign. My wife (it is her vehicle) would say that at times it would backfire. Last week it stalled out and quit; my mechanic said it was an electrical problem such as the fuel relay. I disagreed and put in a new fuel pump. Once back together it fired right up and last night I drove about 150 miles with a couple of stops where I got to turn it off and restart it and it performed as good as ever. It never set a code but had very poor reaction for quite some time leading up to this. My guess is that the fuel pump was getting weak and would not keep the fuel pressure up leading to the stalling and or backfire. Ozawkieng

 

[Jared Jackson]

I took it to another shop. They diagnosed that the MAF was way out of spec, reading nearly double what it should be, 9 grams at idle rather than 4 grams. So they swapped it out. $480 for that MAF part. Plus it needed fuel system intake cleaning.
Still had the backfiring. Turns out the distributor was a tooth off, and because the previous shop that worked on the distributor stripped out the screws that hold the cap, so they could not align the distributor correctly. They say the distributor is fine, other than those stripped screws. So they spent like three hours pulling the distributor turning it slightly putting it back in, checking the timing trying to get it right. Since I had been there from them opening at 8am till 4pm, I decided to just leave it and get back home 3 hours away.

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480 for a MAF?!?! Rock Auto has the AC Delco MAF at 112 + shipping and it's two hose clamps to install it!!
I can't stand shops sometimes...

The "fuel system cleaning" is usually an overpriced joke as well...

 

[JavaMan]

Finally got the Suburban back. The shop took hours and hours trying to get the timing set right. They eventually needed to remove the upper intake to get the distributor aligned correctly. Now it runs good again.
Yep, $480 for a MAF sucks, but the shop owner was unwilling to give me a discount because his shop can't take a loss on selling parts, and people should never buy from RockAuto or any other online retailers because they are full of knock off parts and you can't compare their prices to quality parts.

According to the measurements I get from my scanner, while idling and driving the airflow in CFM is reading about the same as it did prior to this issue. So I don't think the fuel injectors or MAF were needed at all. The entire problem was the timing somehow got way out, and the ECU even at max of -24 degrees was still not enough. Yet mechanics complain that people don't want to bring their vehicles in for repairs, $2280 worth of unnecessary repairs for one damn issue is why I hate mechanic shops. If it wasn't that the timing is supposedly computer controlled and nothing I can set, I would have checked it myself.

I'm guessing that the ignition timing got out when the trans downshifted from 3rd to 2nd when I was at near full throttle, and the sudden engine RPM change made the distributor turn. I've always hated those unexpected downshifts going up a long hill.