Hi, GMC; There are so many reasons that can cause your symptoms. We've found that the GMC distributor caps can 'look' fine, but cause cross fires without showing a carbon track. I'd definitely want to do more thorough diagnosing before jumping into a valve job. Have they done a compression or leak-down test on the valves? If the 'popping' noise is coming from the intake during accel, yes, could be a valve issue. If the EGR is stuck in the open position, that would explain most all of the symptoms, including the 'hidden vacuum leak'. Can't remember, but is this throttle body or multi-point injection? The base gasket under the throttle body was a common vacuum leak issue. Without working O2's, anyone's guess is what the upstream one(s) are doing. Gas analyzer readings would help. I would expect to see high HCs due the lean condition and any misfires. Is the PCM showing any trouble codes? If so, please post them. Most auto parts stores will retrieve them and give you a print-out for free.
Ive got the following truck
2000 GMC Sierra 3500 w/ 454/7.4 engine (regular cab, *Non*-Dually)
Its running incredibly rough, low power, and the exhaust is popping, especially when slowing down and going slow.
I changed plugs,wires, K&N air filter, cleaned maf sensor, and even cut out the rear cat because one mechanic thought that it might be stopped up (previous owner had already deleted the rear O2 sensors, so i figured wth and deleted the rear cat).
Frustrated, i took it in for diagnosis at 3 shops:
2 shops say that i have a "burnt valve",
plus its running lean -- according to one of these shops I also have a "mystery vacuum leak somewhere " .
The burnt valve/valves will cost somewhere in the $1,500 -$2,20p range.
I genuinely love this truck as it comes in handy for hauling stuff, something I do multiple times a week, and to be honest I'm kind of attached to it.
Do you think it's worth fixing or should I junk it and get something else?
And do you agree with what the problem has been diagnosed as?
Initially I thought that maybe it was the timing but they're saying no, it's the valves.
Fix it, junk it? Agree with the diagnosis?