GMC 3500 Problems...

Ive got the following truck
2000 GMC Sierra 3500 w/ 454/7.4 engine (regular cab, *Non*-Dually)
168,000 miles

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?


Discussion is locked
Reply to: GMC 3500 Problems...
PLEASE NOTE: Do not post advertisements, offensive materials, profanity, or personal attacks. Please remember to be considerate of other members. If you are new to the CNET Forums, please read our CNET Forums FAQ. All submitted content is subject to our Terms of Use.
Reporting: GMC 3500 Problems...
This post has been flagged and will be reviewed by our staff. Thank you for helping us maintain CNET's great community.
Sorry, there was a problem flagging this post. Please try again now or at a later time.
If you believe this post is offensive or violates the CNET Forums' Usage policies, you can report it below (this will not automatically remove the post). Once reported, our moderators will be notified and the post will be reviewed.
- Collapse -
Rough running

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.
Good luck.

- Collapse -
More info..

Accidentally marked this as "answered my question".
Thank You Loren for the good reply, I'm going to try to look into these.
The second shop claimed that they did a pressure test but I don't believe them lol .
But anyways, the *only* code its throwing is "po174", took a while for that to happen, and it happened shortly after I cleaned the MAF sensor, so I don't know if I triggered that or not. But the engine light doesnt stay on (it goes away but then comes back) and these rough running / popping symptoms happened for several days before the light finally did come on.
Its popping only out the exhaust, when im going slow, slowing down, but usually not when its idling in park.
And yeah the previous owner had the computer "flashed" and deleted the rear o2 sensors. I got it two years ago and it's ran wonderfully for 40,000 miles without that sensor.
This issue started all of a sudden when I was driving down the road one day about 65mph and it just started just popping and losing power all of a sudden, I pulled over and got out and there was something stuck to my exhaust burning and a piece of wood in between the cab and the bed was smoking, a cop seen me and and hit it with a fire extinguisher -- whatever the wood was laying on in between the cab and the bed had it smoldering. And from that point forward it's ran like this and popped out the exhaust..
I've also since had the muffler changed out and the people who did it looked at the exhaust and said it looked fine. Maybe the burning thing on my exhaust was a coincidence? I don't know...

- Collapse -
how did you clean MAF sensor?

Some can be quite touchy, easily damaged.

- Collapse -
Cleaned it with maf sensor cleaner..

Yeah, hit it with maf cleaner.

- Collapse -
Lot of work

As mentioned there will lot of work to do, especially addressing the burnt valve issue. If you don't have the knowledge and resources, better to let it go and get another reliable truck. It would be a waste if you junk it. Sure there we will someone who is willing to bring her back to glory.

- Collapse -
Had more work done...

Got a pressure test, everything was what its supposed to be.
Fuel pressure was a bit low, had fuel filter replaced. Didnt solve the popping.
Have it at a shop now, a supposedly very good one...and they cant seem to figure it out, either.
Dont wanna sell it, was an awesome truck...

- Collapse -
that popping won't happen unless...

...a spark plug is firing while a valve is open at least slightly. That can mean a damaged valve, or ignition timing is wrong, but at those miles, sounding more like a timing chain/belt that's slipped/jumped/stretched a tooth.

Post was last edited on August 6, 2019 8:38 PM PDT

- Collapse -
Shop says...

...that theyve checked *everything"-- according to them:, timing, ignition system, compression, maf, etc., and everything else are all good, and they cant find any vaccuum leaks...absolutely nothing wrong with it except, after changing the mucked up fuel filter, the fuel pressure is "10+ psi" too low, thereby causing a lean condition which, in turn, is causing the rough idle and exhaust popping.
As for whether or not it's causing the popping etc. I don't know, but I'm sure there's some truth to it having a faulty fuel pump because I always run it low on gas and actually ran out of gas a couple of times by accident, plus these fuel pumps on this truck are notorious for failing.
Your opinion on this?

- Collapse -
vacuum leak

I would check any hose that went to brake power assist, also the PVC valve area. A worse vacuum problem would be if the intake manifold, or "air horn" as some call it now, had a gasket leak somewhere, but with fuel injectors now used, that shouldn't have anything to do with an exhaust firing or popping. It would make me wonder what could cause both any large vacuum leak at the engine top end, and also a problem at the bottom exhaust end. You say the compression test is OK, so that should rule out any head leaks on the engine. No oil in the radiator? No antifreeze in the dipstick oil? One way to test dipstick oil is touch with finger, put a bit on tongue, see if there's a "sweet" taste, indicating antifreeze getting into the engine oil.

- Collapse -
They say...

I checked some og that, they checked all of that, so they say, and low pressure from the fuel pump is all they have found.
Im hoping they are correct as i just read an article that said a fuel pump can cause my exact symptoms.

- Collapse -
I still think it's stretched...

...timing change, and the vacuum problem is entirely unrelated to that and it's symptoms. Have someone who is a mechanic, take a breaker bar (key off!) and put a large socket on the crank at front of engine, (may need to remove spark plugs to make easier) and move the bar back and forth to check the "play" in the timing chain. There should be almost none. If there is some, then it's a stretched timing chain causing the problem. As for the vacuum leak, it's not related.

"How do you know if the timing chain is worn? Often, if the timing chain is worn the engine may feel sluggish, low on power or have troubles starting. This is because as the chain wears out, it stretches and the ignition and valve timing becomes more ********." (reeeeetarded)

Post was last edited on August 7, 2019 10:02 AM PDT

- Collapse -
Shop just called...

New fuel pump installed, they called and said the popping has disappeared but now there's a misfire...
At this point I'm wondering if I'm being taken for a they have a theory about the fuel injectors and want to check them. I'll report what happens with this. And yeah, myself, and several others, were thinking timing chain too... I guess not. I don't know.

- Collapse -
just replace it on faith

It's probably original, with 168,000 miles on it, and while some with proper oil & filter changes last to 200,000 miles, it's a good time for replacement. At least have some competent mechanic check the play in it, and I don't mean some "Pep Boys" or "Sears" or "Walmart" auto sales guy.

Post was last edited on August 7, 2019 10:28 AM PDT

- Collapse -
Timing chain

Hi, GMC; I hate to be the odd-ball here, but this doesn't sound like a timing chain issue to me. You can measure the slop in the chain. As James mentioned, start by lining the timing mark at Zero degrees, then carefully turn the crankshaft with the breaker bar until the 'play' is gone. If it has moved more than 6 degrees, yes, that's too much slop, but still shouldn't cause the symptoms you are experiencing. I've had vehicles with 10 or more degrees slop and still run okay. I could be wrong. It is not difficult to check the injectors. Easy check is to use a stethoscope to listen for the clicking at each injector. Then check resistance of each one. Use a Noid light to check for power getting to each injector. I would try cleaning the injectors before replacing if the other tests are good. Not the snake oil that they sell in cans. Find a shop that uses a Snap-On Motor Vac. We've rescued many injectors doing so.
Good luck.

- Collapse -
Fixed it...

Map sensor and distributor.... running great now.
Thanks though, appreciate all of the suggestions.

- Collapse -
This reminds me of some old advice.

Long ago a mentor shared a hunk of gold advice.

"If you can't figure out exactly what part is bad (or causing the issues),
replace the cheapest part first."

CNET Forums