Rough Idle HELP!

[02YukDenXL]

The oxygen sensors could be failing and still not have damaged the cats. They need to be monitored during a cold start to warm up and driving under load. Yes, the vacuum leak is also a source that could be the cause. Smoke test of engine is needed.

Monitor the O2 sensors during cold start to warm and under load, making sure they're within proper voltage limits?

Smoke test at a shop with proper equipment looks like it takes no time at all. DIY smoke tests look like a PITA.

 

[OR VietVet]

The tech will monitor the sensors and see if they are switching voltages like they should. The front sensors will work differently than the rear sensors. The front is monitoring the exhaust mix before cats and the rears after going thru cats.


Kind of like this:

Both sensor 1's (pre-cat) should fluctuate, alternating high and low, typically between 080mv (lean mixture) and 950mv (rich mixture).

Both sensor 2's (post-cat) voltage should be far more constant 700mv - 800mv.
A lower or fluctuating voltage indicates catalyst inefficiency (assuming sensor output is correct).

 

[02YukDenXL]

So upstream sensors make sense. They're fluctuating. Downstream sensors don't make sense. At idle they're staying down at .10-.15. Under load they spike to .70-.80+, cruising at constant speed they flucuate (.1, .4, .7, randomly) but other times at idle they don't read the same. Bank 1 may be .70 while bank 2 may be .1.

All of those are in volts not mV

 

[OR VietVet]

Sounds like the rear sensors are reading a problem with the cats. They may just be bad and the cats are good. If they are all 4 old sensors then I recommend replacing all four with AC Delco sensors. Or at least the rear 2 and get the readings again. You may also have cat problems that will need replaced. I have seen the sensors cause a rough idle. Not too often but some times. Let us know how it goes.


Rock Auto will have the sensors at a decent price.

 

[02YukDenXL]

I agree. If the sensors fix the problem that'd be great. Still gonna check for vac leaks. I don't wanna randomly replace cats on an educated hunch right now. Sensors are easy too. Think they're prob original. I know my exhaust is rusted up and the cats would be a PITA to replace myself. Thanks Raiderron. I'll let you know how it goes.

---------- Post added at 06:16 PM ---------- Previous post was at 05:52 PM ----------

Rock Auto will have the sensors at a decent price.

Any difference between AC Delco OEM and AC Delco Pro?

 

[OR VietVet]

I checked Rock Auto and see what you mean. I would bet the Pro series is just like what you get handed at the parts counter at the dealer and the other is an AC Delco part that is boxed differently and could be an aftermarket brand of AC Delco. I would go with the OE Pro stuff but hell it isn't my money. New sensors and then monitor and then can depend on the readings to see if the cats are efficient. I have even used Delphi sensors with no problems. Bosch can be ok too. They make a lot of sensors for other parts distributors.

If the exhaust is rusted I would spray penetrant on the sensor threads for a few days before trying to remove. Best penetrant is called Aero Kroil. Kind of expensive but is absolutely the best there is. I would make sure you either use a 6 point tight fit socket to remove or a box end tight fit wrench. No open end wrench. You can use PB Blaster in a pinch if needed. Just spray for a few days and let it soak in.

 

[02YukDenXL]

What a week. Stripped the threads in the bung for the O2 sensor. Was able to get an O2 thread chaser in there (18mm x 1.5 btw for future reference) and was able to replace the sensor on the passenger side. Makes me not wanna mess with the drivers side yet.

I also pulled each spark plug and did a compression test on each cylinder. All cylinders were about 150psi. On a few longer cranks the pressure got up towards 170psi. Each cylinder held pressure. So that makes me feel better. Hoping to button it all up in the next day or so and see how the two downstream sensors read with one new and one old.

Hard to get anything done with three kids and a wife and the weekends are slowly filling up with weddings, parties, etc.

 

[OR VietVet]

Sounds like you didn't use a penetrant and let soak for a couple days as advised. Or did you and you didn't mention that.

 

[02YukDenXL]

I don't think I soaked it enough. I used PB blaster. From what I've read the aero kroil is awesome stuff but you can't buy it in a store. I'm gonna order some just to have around.

 

[humer101]

So I'm going absolutely nuts over this. I have a 2002 GMC Yukon XL Denali (6.0L LQ4) with about 160,000 miles. No mods. Started having a rough idle a couple months ago. I have never had a CEL. No stored codes. Nothing. It has been getting progressively worse. I'll list everything that I've done from the start in the order that I did them in:

Plugs
Wires
Air Filter
Cleaned MAF
Fuel Filter
Cleaned Throttle Body (It was gross)
Seafoam
Replaced intake manifold gaskets
(Replaced knock sensors at the same time)
New transmission fluid & filter

So at this point I still had a rough idle (Or what I thought was a rough idle). After I did the tranny fluid, it almost felt worse. So then I thought that I had a shudder in my transmission that felt like a rough idle. Brought it to a transmission shop and they had a Tech II scan tool and I had random misfires (When I was with the tech he showed me on the Tech II cylinders 7, 2, 6, & 1). So after looking at it, he told me it was probably my ignition coils. I just replaced all 8 last night and the "rough idle" is still there!!!!! Power wise and shifting, overall everything feels smooth and strong. No loss of power.

I'm ready to bring it to a dealer to figure out wtf is going on. I tow a trailer when we camp in the summer and I want this thing running like a top while I'm towing. Thoughts? Comments? Thanks in advance!

I'm not mechanic but in one of my working trucks i have the same problem and was one valve was leaking very slow and cause the rough idle, if worth you have to open the engine good luck my friend.