Sudden loss of power and backfire at idle

[454cid]

The distributor hold down bolt is hard to reach, but not that hard.

I wonder where your CMP offset is now. The spec is 0, +/-2. Mine was at -7 and ran fine.

 

[JackE]

Wow! $2280, plus the rental, and how many trips back and forth? My wife would have told me to junk it and go buy something else to get us home! The lack of integrity is one reason I quit wrenching in shops. I wasn't making the shop owner enough money because I wouldn't throw parts at the customers. And I definitely would not lie to the customer! I had more than one boss tell me I had to lie to cover for him. The day I was asked to lie was the day I packed my tools and walked out. Sucks quitting your job when you have a family to support, but I did it twice because I was told to lie to the customer.

 

[east302]

I wonder where your CMP offset is now.

Lol, call me pessimistic but I’d put money on it being out of spec.

OP- does your scanner have a PID for CMP Retard?






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[JavaMan]

My scanner does not show CMP Retard. There is a PID called Timing Advance. Seems to hover -30 to -28 while idling, -30 to -24 while driving, and -26 to -16 while accelerating with the -16 occurring near WOT.

I seem to remember these being positive values when I looked months ago. So is this thing still way off, but the ECU is able to compensate?

 

[454cid]

I have not read this thread yet, but it's talks about something that will read the CMP Offset

https://www.gmt400.com/threads/car-gauge-pro-can-read-cmp.35275/

 

[Chuck_13]

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.

I guess I'm missing something here;You said the Burb downshifted and caused the timing to jump a tooth?How is that even possible without destroying the cam or dizzy?And NONE of the shops threw a timing light on it?Or a scan tool,I suppose being as the ECM controls engine timing.I'm just glad it wasn't my truck,cause I'd a been mighty pissed off.
Sucky mechanics suck.

 

[JavaMan]

Yes the problem started just after a downshift at high throttle going up a steep long hill pulling a trailer. I actually used a cheap bluetooth scan tool prior to taking to the first shop, but there were no DTC codes or pending codes.

Both shops supposedly used their expensive scan tools. The first one said the fuel trims are way off which indicates failing fuel injectors and once one has gone bad the others will fail soon, so I had to replace all of them. That didn't fix the issue so they were going to throw more parts at it and I argued paid and left. Even though they had not fixed anything, they said I had to pay them to take the truck. The problem symptoms had not changed at all, and it was evident they were just throwing in parts to get my money, no way was I sticking around.

I got some readings from my scanner, and posted those on the forum.

Since I could not come up with any ideas to change when researching on the internet, I took it to the second shop. The quickly decided the MAF was the issue and replaced it. When that didn't change the symptoms, they tried more troubleshooting. At some point they put the timing light on and discovered the distributor being off a tooth. They said the CMP Retard was maxed out at -24. Since the distributor had not been touched in 20k miles and they said it could not have been off a tooth without causing any issue, it must have changed recently somehow.

After the money I spent at the first shop, when the second was going to get it working for $750, I just let them spend hours and hours trying to get the timing right.

While the second shop charged for a MAF that did not need replacement, they did not charge me labor for removing the intake to get the distributor back in the right tooth, and spent like 6 hours trying to get the timing right. They did not charge me for all the labor working on the distributor. I figured I would have spent nearly the same for all the distributor labor as I did for that MAF replacement.

At this point, I don't know what really was the problem, but the issue is finally fixed. I just hope I don't breakdown hours from home again. I do not want to deal with mechanics.

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[454cid]

At some point they put the timing light on and discovered the distributor being off a tooth. They said the CMP Retard was maxed out at -24. Since the distributor had not been touched in 20k miles and they said it could not have been off a tooth without causing any issue, it must have changed recently somehow.

I'm not sure it's possible for a distributor to skip a tooth, unless maybe the gear is very well worn. I thought you meant the distributor body rotated a bit when the engine down shifted... I could see that happening if the clamp wasn't tight enough. There is only so much room for it to turn back there, Though.