Engine stalling, rough idle, rpm fluctuation.

[abdullah_Zahra]

2010 Suburban. I don't know if the correct term to use here is "stalling" I will try and explain what happens.
Car's idle is a tiny bit rough, barely noticeable. When I am cruising at low speeds (1st +2nd gears) and after immediately letting go of the gas pedal, the car will start to kinda "jerk" you could say, giving me the impression as if someone was to hit me from the back. I'd be going back and forward in my seat while this happens
When I lightly press on the brakes during this episode, the car will slow down perfectly no issues whatsoever. Please note this is not a transmission issue. I don't have any problems in gears going up and down at all.
The problem will mostly occurs when I successfully go from 2nd to 1sr, at around 1200-900 RPM. During the episode, the RPM needle will not go down smoothly. As if the car is going through hiccups.
Not to mention that overall, I feel like the engine's power and responsivity aren't what they used to be.

No check engine light, once scanned, I found p0174, p0171 codes stating a lean condition. but those codes weren't pending, they only lasted a day or two and then they were gone.
I thought of cleaning MAF, throttle body, maybe injectors and changing coils and plugs. Anything else I should check?

 

[Marky Dissod]

You say this is NOT a transmission issue. Can you duplicate symptoms in neutral?
If so, seriously consider checking / replacing throttle body assembly and associated wiring.
Remember, your car has an electronic signal throttle (not a physical cable),
so the ecm can second guess or even countermand your foot's orders.

You can also try adding 2x normal fuel system cleaner to several gas tanks.
(If you run E85, don't double dose.)

How many miles on the O2 sensors?

Before firing the parts cannon, fire the parts derringer:
Replace the oldest cheapest and easiest parts first.

 

[donjetman]

If its throttle body to pedal miscommunicating it would throw a P2135 code.

 

[abdullah_Zahra]

You say this is NOT a transmission issue. Can you duplicate symptoms in neutral?
If so, seriously consider checking / replacing throttle body assembly and associated wiring.
Remember, your car has an electronic signal throttle (not a physical cable),
so the ecm can second guess or even countermand your foot's orders.

You can also try adding 2x normal fuel system cleaner to several gas tanks.
(If you run E85, don't double dose.)

How many miles on the O2 sensors?

Before firing the parts cannon, fire the parts derringer:
Replace the oldest cheapest and easiest parts first.

I say this is not a transmission issue because the transmission was completely rebuilt 6 months ago. When the issue started rising a couple of months back, I figured it had something to do with the tranny, so i took it to the same shop. He drove it around and told me it can't be the transmission as he changed every single thing brand new.
Regarding the o2 sensors, I have no idea as the car was purchased second hand (only drove it for 5-6k miles so far) it has about 102k miles now.

 

[Marky Dissod]

I say this is not a transmission issue because the transmission was completely rebuilt 6 months ago.
When the issue started rising a couple of months back, I figured it had something to do with the tranny, so i took it to the same shop.
He drove it around and told me it can't be the transmission as he changed every single thing brand new.

I remember saying the same as above about an LT1 Caprice wagon I once owned. 8 months after that rebuild, he re-rebuilt it.
Assuming you have better luck than I did, and you don't have to beat him with a rubber hose til he re-rebuilds it,
O2 sensors are supposed to be replaced every 100,000 miles or so.

 

[abdullah_Zahra]

I remember saying the same as above about an LT1 Caprice wagon I once owned. 8 months after that rebuild, he re-rebuilt it.
Assuming you have better luck than I did, and you don't have to beat him with a rubber hose til he re-rebuilds it,
O2 sensors are supposed to be replaced every 100,000 miles or so.

Good point! I am definitely going back to that guy.
Your point on the o2 sensors is valid, I've just seen this video that also describes my exact issue, solved by replacing the o2 sensors.

 

[opfor2]

Good point! I am definitely going back to that guy.
Your point on the o2 sensors is valid, I've just seen this video that also describes my exact issue, solved by replacing the o2 sensors.

Do yourself a favor and only install Genuine GM O2 sensors.

 

[rdezs]

In going for the least expensive possibility first.... Check for vacuum leaks. The intake from the mass air flow sensor, your PCV hose from the driver side valve cover to the intake. Very lightly spray starter fluid around the intake with the engine idling and listen for a change, indicating a vacuum leak. You also have an o-ring on the dipstick tube and oil filler cap. And of course the PCV fresh air hose from the passenger valve cover. These usually won't throw a code, unless it's large enough to show a lean condition, yet will be noticeable off idle.

Caution: if you use starter fluid, very small short sprays as it's quite flammable. Some people prefer to use something less flammable, such as WD-40.

 

[OBSandaNNBS]

In going for the least expensive possibility first.... Check for vacuum leaks. The intake from the mass air flow sensor, your PCV hose from the driver side valve cover to the intake. Very lightly spray starter fluid around the intake with the engine idling and listen for a change, indicating a vacuum leak. You also have an o-ring on the dipstick tube and oil filler cap. And of course the PCV fresh air hose from the passenger valve cover. These usually won't throw a code, unless it's large enough to show a lean condition, yet will be noticeable off idle.

Caution: if you use starter fluid, very small short sprays as it's quite flammable. Some people prefer to use something less flammable, such as WD-40.

I'm sorting through a similar problem!
Upon inspection for vacuum leaks with propane, I noticed the oil filler tube is wobbly. I thought maybe that could be an issue but dismissed it.

It's supposed to be pretty solid, right?
Although my 02s could use a replace too I'm thinking.

 

[rdezs]

Oil cap with a new o-ring fits a bit tight. It's not necessary to buy a new cap, stop by somewhere like a Napa Auto parts store and they'll match the o-ring.

 

[rdezs]

.... Same for the dipstick. If it lifts out with absolutely no resistance, time for a new o-ring.

 

[OBSandaNNBS]

.... Same for the dipstick. If it lifts out with absolutely no resistance, time for a new o-ring.

Dipstick is rather loose. No change in rpm when taking it off or plugging the hole.
I checked the oil filter cap too. I think it fits well and I'm almost sure there was a slight RPM change when taking it off. Tried a few times.

I was referring to the fill tube in my post. It was a wobbly codjer. I ended up taking it off, breaking the locking ring, with the engine running. Doh! Thank God it just fell right down and I was able to retrieve it after searching for 20 minutes for my long reach needlenose...it fits better now but no change in idle roughness.

Anyway, I'm not understanding how the oil orings are related to engine vacuum/idle. Maybe the filler makes sense, but not the dipstick? To me at least.

This is the first modern engine I've run, so hopefully I don't sound like too much of an idiot.

 

[rdezs]

Your PCV system pulls a vacuum on your engine crankcase. It also pulls some fresh air in through the connection on the passenger side valve cover. (The hose usually connects near the throttle body. Some applications directly into the intake tube.) This air is calculated through the mass air flow sensor, which is used in figuring out your fuel mixture along with the O2 sensors.

If the o-ring on the oil filler cap, or the oil filter tube or the dipstick is not sealing, you have additional air entering the crankcase from the outside and not having been measured by the mass air flow sensor. It will give you a slightly lean condition, a change in the idle, etc. That's why you'll notice on a brand new engine, if you just lift up the dipstick an inch it's suddenly acts like a vacuum leak and the idle picks up a little bit.

Now in a high mileage engine with a lot of blow by past the rings, as well as past the valve guides and seals.... It can actually overcome the vacuum from the PCV system. Sometimes this results in idle issues, fouling spark plugs, and the ECM struggling to keep the mixture where it wants it. It also builds up a lot of sludge in the engine. On a well maintained LS engine, that can be at 300,000 plus miles. Eventually the catalytic converters get fouled from too much oil mist from the blow by being burned.

You'll be surprised how much those o-rings really smooth out an idle....

 

[rdezs]

As far as your old small block Chevy with a four barrel carburetor, it's basically the same. There's a vacuum pulled through the PCV valve, and the opposite valve cover draws in fresh air. Only you get to adjust the mixture on the carburetor itself... easy enough to adjust to compensate as the engine wears and the internal blow by increases.

Without this PCV system pulling in fresh air, mixing with whatever blow-by exists, and getting sucked into the intake to be burned.... You would get condensation building up in the bottom of the oil pan, and acidic environment and a buildup of sludge. That's why it's always a good idea on a high mileage engine to periodically remove the PCV hose going to the intake and cleaning it to make sure it's not plugged. You'll note that it should have an insulated cover over it. That's to prevent the water vapor from condensing inside.

 

[rdezs]

..... Believe it or not, every part of that engine has had a lot of thought go into designing various functions. Right down to how many threads do we need on that bolt to hold that item. (And remember, their target for the item to function is as long as the warranty it seems. Think of the LS engine exhaust manifold bolts)

So after digesting all of that about your PCV system, you may ponder why is it necessary to have the amount of vacuum it has. It's because every engine always has some blow by past the piston rings, even brand new. And that amount increases at full throttle. Other than keeping the inside of your engine clean, the other very important aspect is it prevents the crankcase getting pressurized by the blow by gases. If it gets pressurized, it starts pushing those gases mixed with oil vapor out the weakest seals or gaskets. A plugged up PCV is usually spotted first by an oil leak.

 

[OBSandaNNBS]

Haha digesting is the exact word!!
Thanks for your informative replies! I'm saving this thread. It's impossible diagnose if you first don't understand function.
I didn't realize how technological and involved the 5.3 was compared to my old 350 when I bought it. I really do enjoy it, driving it and the engineering aspect.
And I'm getting new orings and fill tube with my 02 sensors for sure!

 

[rdezs]

If you want to go overboard and be a perfectionist.... I actually have an LS filler cap that I drilled and tapped threads to mount a 4AN fitting. Then I have a 4AN female to a vacuum line hose barb and connect it to my vacuum gauge to check crankcase vacuum at idle and at 2,000 RPM. I like to see about 7 inches of vacuum at idle, which usually nets me about 4 inches at 2,000 RPM, and down around 1 to 2 at full throttle. If you can achieve that, you'll insure no oil leaks from an overpressurized crankcase.

So how do you fine tune it? The newer revised valve covers for LS engines have no PCV valve, but rather a very small orifice. Over 100,000 miles it's normal to have some more blow-by than the new engine.... Removing the valve cover and increasing the size of the orifice with the next size up drill bit, will increase the crankcase vacuum a bit.

 

[donjetman]

That uncontrolled leak at the oil filler cap is allowing UNFILTERED (dirty) air into the engine to wreak havoc on the oil, etc.

 

[OBSandaNNBS]

If you want to go overboard and be a perfectionist.... I actually have an LS filler cap that I drilled and tapped threads to mount a 4AN fitting. Then I have a 4AN female to a vacuum line hose barb and connect it to my vacuum gauge to check crankcase vacuum at idle and at 2,000 RPM. I like to see about 7 inches of vacuum at idle, which usually nets me about 4 inches at 2,000 RPM, and down around 1 to 2 at full throttle. If you can achieve that, you'll insure no oil leaks from an overpressurized crankcase.

So how do you fine tune it? The newer revised valve covers for LS engines have no PCV valve, but rather a very small orifice. Over 100,000 miles it's normal to have some more blow-by than the new engine.... Removing the valve cover and increasing the size of the orifice with the next size up drill bit, will increase the crankcase vacuum a bit.

I'm far from perfectionist due to lack of skill, but that sounds like something I would do. I do wonder how mine stacks up? How many miles on your truck? There are 156K on mine. I don't have a vacuum gauge...yet!

I had wondered about the PCV valve since NBS LS had them but NNBS don't. Any chance that orifice can get blocked/plugged?

 

[rdezs]

On a high mileage engine with a lot of blow by, yes that orifice can get plugged.