Sudden loss of power and backfire at idle

[JavaMan]

Driving on the freeway late evening in my Suburban 2500 4x4 7.4L, engine suddenly changes pitch a little, and slows down. I adjusted the throttle a bit and kept my speed. It was enough of a difference that it was downshifting on the small hills, which it normally does not. Later, getting off the freeway, I notice it's backfiring while idling, then stutters as I accelerate again. We got to our destination, I played with the Suburban the next day. It backfires mostly at lower RPM. Couple backfires per second at 700, fewer at 1200, no backfire once I get to 1700 RPM. When accelerating, takes more throttle, and backfires till the RPMs get up over 1700. No CEL, no active codes or pending codes.

I took it into the shop near there. We were visiting my parents, three hours from home. The shop says it's a bad fuel injector, only way to replace them is all at the same time. Didn't fix it. They replaced the downstream O2 sensors also. So, $1400 later, still not fixed. They tell me the catalytic convertors are clogged, they want $1200 to replace those. I tell them no way, took it to a muffler shop, they had both replaced in 30 minutes for $350. Still backfiring, no changes. Shop now wants to replace the fuel pressure regulator for $450.

Our vacation time was over, so we got a rental car to go home, left the Suburban at my parents house for now.

The Suburban is three hours from my house, what should I check for next? These mechanics nowadays are lost if there fancy scanner doesn't tell them exactly what part is broken, and they just start swapping parts at my expense and say, oops, that wasn't it. So I'm hoping that someone here, knowing these vehicles, has experienced something similar.

 

[Jared Jackson]

Driving on the freeway late evening in my Suburban 2500 4x4 7.4L, engine suddenly changes pitch a little, and slows down. I adjusted the throttle a bit and kept my speed. It was enough of a difference that it was downshifting on the small hills, which it normally does not. Later, getting off the freeway, I notice it's backfiring while idling, then stutters as I accelerate again. We got to our destination, I played with the Suburban the next day. It backfires mostly at lower RPM. Couple backfires per second at 700, fewer at 1200, no backfire once I get to 1700 RPM. When accelerating, takes more throttle, and backfires till the RPMs get up over 1700. No CEL, no active codes or pending codes.

I took it into the shop near there. We were visiting my parents, three hours from home. The shop says it's a bad fuel injector, only way to replace them is all at the same time. Didn't fix it. They replaced the downstream O2 sensors also. So, $1400 later, still not fixed. They tell me the catalytic convertors are clogged, they want $1200 to replace those. I tell them no way, took it to a muffler shop, they had both replaced in 30 minutes for $350. Still backfiring, no changes. Shop now wants to replace the fuel pressure regulator for $450.

Our vacation time was over, so we got a rental car to go home, left the Suburban at my parents house for now.

The Suburban is three hours from my house, what should I check for next? These mechanics nowadays are lost if there fancy scanner doesn't tell them exactly what part is broken, and they just start swapping parts at my expense and say, oops, that wasn't it. So I'm hoping that someone here, knowing these vehicles, has experienced something similar.

I want to punch your mechanics in the face for being incompetent and taking your money in the process...

We all make mistakes, but cats can be checked by disconnecting the pipes and running it to see if you have an increase in performance. Any mechanic worth his salt knows that a rear o2 sensor would not cause ANY of the issues that you experienced.

Saying you have to replace ALL the fuel injectors is also a joke.

Since it is hard to diagnose newer engines without a scan tool, I would start by checking the cap/rotor/plugs...
I also think I recall a bad EGR valve can cause those types of symptoms... Ignition would be the easiest to check by yourself! Start with the small easy stuff and work up. Are you up to date on all of your tune up / filters?

 

[Jared Jackson]

@454cid have any suggestions?

 

[454cid]

I took it into the shop near there. We were visiting my parents, three hours from home. The shop says it's a bad fuel injector, only way to replace them is all at the same time. Didn't fix it. They replaced the downstream O2 sensors also. So, $1400 later, still not fixed. They tell me the catalytic convertors are clogged, they want $1200 to replace those. I tell them no way, took it to a muffler shop, they had both replaced in 30 minutes for $350. Still backfiring, no changes. Shop now wants to replace the fuel pressure regulator for $450

They have no clue what they're doing... or they do and they just want to take all your money. I don't see any reason why injectors could not be replaced individually, however I think most people probably replace them all... for a lot less than $1400.

My understanding is that the post cat O2 sensors don't even effect how the engine runs. GM even decided to leave them off the 2000 454 trucks.

The fuel pressure regulator should have been done when they did the injectors. They would have to undo some of their prior work. Did they even check the fuel pressure?

Did they give you the old injectors, so they you know they actually changed them?

The Suburban is three hours from my house, what should I check for next?

I would check the fuel pressure. If you have a gauge, it's easy to do since the Vortec trucks have a schrader valve. It's right near the upper radiator hose and comes from the under the upper intake manifold. My guage is a $14 Harbor Freight special, but I did have to make a gasket to keep it from leaking where the hose screws on. You should be able to find a better one at the parts store.

Do you have a shop that you trust where you live? If you can't work on it yourself, I'd think a tow bill would be less than the bill for more guessing at the shop where your parents live.

Unfortunately, I don't have any concrete advice on your trucks problem. I've not really had probalems with the way it runs, just a few things that have prevented it from running altogether... ICM, fuel pumps, and then brakes and suspension.

 

[JavaMan]

According to the previous owner's logs, the cap and distributor were changed at 71k; new distributor, cap, rotor, plugs, wires at 104k; upper intake gasket at 44k, 68k, 104k, upper and lower intake at 109k.
When I got the vehicle at 114k I replaced the fuel lines from tank to filter, fuel pump, and fuel level sender, drained and refilled the trans twice new filter, changed oil, changed air filter, cleaned out the front and rear AC, flushed the coolant switched from the green to Prestone long life, upgraded the cables from alternator to battery (got rid of cheap 10ga wire on a 145amp alternator), upgraded grounds battery to frame and engine

The fuel pressure regulator should have been done when they did the injectors. They would have to undo some of their prior work.

That's not too surprising. I don't know for sure what the mechanic did, but he's got a fancy snap on scanner.

I have a fuel pressure gauge, not much better quality than Harbor Freight though. I also have a basic scanner, I've scanned for codes multiple time with this, showed no pending codes. It does have ability to show live data from a number of sensors.
I have a tune up kit (plugs, wires, cap and rotor) that I bought when I was purchasing my other parts. Once I found the maintenance list, and saw that the dist/cap/rotor/wires/plugs were replaced only 10k miles before I decided to just hold onto those. So I'll take that down and swap those out.

 

[454cid]

Did they tell you that all the injectors needed to be changed together before or after they did the job? Suddenly, I realized why they likely said that... they're thinking of the spider injector system of the small block. A lot of people assume the 454 uses the same system or is only TBI. In this case, it rellly makes me wonder if they actually changed anything on your truck. Did you take a look at the new O2 sensors? They're stainless so old ones won't be rusty, but there should still be clues to having been changed.

You could have cap/rotor issues this soon if they're not OEM. The cheaper parts store stuff can go bad, as the conductors inside the cap cross.

Also the fuel pump... any idea what brand it was?

Any idea why your upper intake gaskets have been redone so much? 4 times, including the time when the lower was done. The lower can leak, but thankfully it's not as bad as the small block.

 

[JavaMan]

They said that one or more injectors were leaking, causing the backfire, and when one fuel injector starts leaking, the others are soon behind, and with so much cost being labor and parts around the injectors it only makes sense to do them as a set. I didn't look at the O2 sensors. On the engine, I keep things usually clean anyway, but it did seem a bit cleaner around the intake after they worked on it.

No idea what brand cap/rotor were used, nor why the PO changed them. Thinking back, last month I did a cleaning of the engine compartment and under it, there's a power steering leak that makes things messy over time, so I degrease everything occasionally. It's possible some moisture got under the cap back then and got stuck. This isn't our daily driver, it's the "we're driving for hours keep the kids separate" vehicle.

Fuel pump is Bosch.

I see some postings about the cap on these failing due to crossfire. If the new cap/rotor fixes the issue I'll be happy but pissed at the same time, having wasting so much money.

 

[DerekTheGreat]

I don't have anything of technical value to say, just that I'm blown away they expected you to pay them for NOT fixing your vehicle. I would have flat out refused to give them a dime. Why charge you for parts that didn't need to be replaced? I mean it's one thing when us peeps fire the parts cannon in our own driveway but it's another when a professional shop does it. F that noise and F that shop.

 

[Jared Jackson]

I don't have anything of technical value to say, just that I'm blown away they expected you to pay them for NOT fixing your vehicle. I would have flat out refused to give them a dime. Why charge you for parts that didn't need to be replaced? I mean it's one thing when us peeps fire the parts cannon in our own driveway but it's another when a professional shop does it. F that noise and F that shop.

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...

 

[454cid]

They said that one or more injectors were leaking, causing the backfire, and when one fuel injector starts leaking, the others are soon behind, and with so much cost being labor and parts around the injectors it only makes sense to do them as a set.

Well it does make sense to do them as a set, but that's not the same thing as "have to". Speaking of things that make sense to change while they are there... FPR!

I would think it was a fuel pressure issue, with one exception... if low fuel pressure I would think it would run better at idle than at higher rpm, and you are reporting the opposite. Out trucks (at least mine) will idle with 10psi, but probably not run down the road.

Fuel pump is Bosch.

That's probably ok, although I have not heard much about their fuel pumps one way or another... they are the biggest automotive part supplier in the world.

I see some postings about the cap on these failing due to crossfire. If the new cap/rotor fixes the issue I'll be happy but pissed at the same time, having wasting so much money.

I have not had problems there, but I've always used AC Delco parts for the cap and rotor. My father certainly had issues with his 96 350... it would refuse to start, and several times a cap and rotor change would fix it. He was often paying to have the work done and the shop was likely using non-oem parts. It wasn't too bad since they did last quite awhile.