8.1 Very Rough Idle At Low Rpms

[MEGA14]

Months ago , one of the rear O2 sensors triggered an engine code .

 

Mechanic cleared the code and told me so .

 

The engine light came on , went off , came on , went off and stayed off .

 

Never replaced the O2 sensor .

 

Recently , the engine began to stall and I made it home , barely .

 

Mechanic came over and replaced the 7th cylinder coil and plug as it appeared on engine scan as misfiring in the 10's of thousands .

 

The others misfired at low numbers .

 

Got it to the mechanic and changed the remaining 7 spark plugs and they were very ugly .

 

He wanted to frame a couple of them .

 

The truck still ran rough but somewhat better .

 

It is still idling very rough at low idle .

 

What's next ?

[yukon25008.1]

Months ago , one of the rear O2 sensors triggered an engine code .

 

Mechanic cleared the code and told me so .

 

The engine light came on , went off , came on , went off and stayed off .

 

Never replaced the O2 sensor .

 

Recently , the engine began to stall and I made it home , barely .

 

Mechanic came over and replaced the 7th cylinder coil and plug as it appeared on engine scan as misfiring in the 10's of thousands .

 

The others misfired at low numbers .

 

Got it to the mechanic and changed the remaining 7 spark plugs and they were very ugly .

 

He wanted to frame a couple of them .

 

The truck still ran rough but somewhat better .

 

It is still idling very rough at low idle .

 

What's next ?

 

Not sure whats next but i figured i would just let you know i just changed my plugs 04 8.1 66,000 and they looked real bad, yellowish/gray powder buildup and they looked really lean so i called the dealer and they said thats normal due to additives in the fuel.

[MEGA14]

Yet on the box it says there's a 100,000 mile guarantee for these plugs and I didn't come close to 50 K but to be fair I do idle ALOT .

 

Based on the searches I've done here and my truck's symptoms , I'll be swapping camshaft position sensors on Monday .

[markw8]

What about the O2 sensor that wasn't changed ?

 

Mark.

[MEGA14]

What about the O2 sensor that wasn't changed ?

 

Mark.

 

That's what I want to know .

 

I was told it wasn't the cause of these problems but would make my fuel economy suffer .

 

That there are 4 of these on the truck is nuts to me .

 

I like that some of the companies that provide tunes for our trucks are able to disable the 2 rears as I understand it .

 

I might just get a tune .

 

Also this week , the oil cooler line and the rad both started to leak .

 

It seems like 2 things after the other with these trucks .

 

No wonder they went bankrupt .

[MEGA14]

What about the O2 sensor that wasn't changed ?

 

Mark.

 

That's what I want to know .

 

I was told it wasn't the cause of these problems but would make my fuel economy suffer .

 

That there are 4 of these on the truck is nuts to me .

 

I like that some of the companies that provide tunes for our trucks are able to disable the 2 rears as I understand it .

 

I might just get a tune .

 

Also this week , the oil cooler line and the rad both started to leak .

 

It seems like 2 things after the other with these trucks .

 

No wonder they went bankrupt .

[markw8]

The only thing the rears do is monitor the cats. They don't affect driveabilty.

 

Mark.

[MEGA14]

It turned out to be the EGR valve stuck open by a piece of carbon .

 

What a PITA that was .

 

Codes that were brought up were :

 

P0300 - Random misfire

P0404 - Exhaust Gas Recirc.

P0300 - Random Multiple Misfire

P0404 - Exhaust Gas Recirc. ( again )

P1404 - EGR Valve

 

 

Afterward , no codes came up . Not even the O2 sensor which I never replaced .

 

Strange .

 

Maybe my mechanics are even stranger .

[Haulin' 8.1]

Most likely the o2 code was telling you there was a problem, i.e. low catayst efficiency due to crappy condition spark plugs overworking the convertors. If the code meant the o2 wasn't functioning properly it wouldn't fix itself. There's more than a few codes that can be set by any single o2 sensor, some mean the o2 is malfunctioning the others are something the o2 monitors is malfunctioning. The PCM knows the difference. One of the biggest mistakes I see my DIYers is replacing a perfectly good o2 sensor because "it" set a code that's telling you something else is wrong.

 

The excessive carbon blowing through the engine would have been also related to poor plugs building up carbon then good ones burning it off. So it's all related.

 

Vernon

[MEGA14]

Most likely the o2 code was telling you there was a problem, i.e. low catayst efficiency due to crappy condition spark plugs overworking the convertors. If the code meant the o2 wasn't functioning properly it wouldn't fix itself. There's more than a few codes that can be set by any single o2 sensor, some mean the o2 is malfunctioning the others are something the o2 monitors is malfunctioning. The PCM knows the difference. One of the biggest mistakes I see my DIYers is replacing a perfectly good o2 sensor because "it" set a code that's telling you something else is wrong.

 

The excessive carbon blowing through the engine would have been also related to poor plugs building up carbon then good ones burning it off. So it's all related.

 

Vernon

 

It all makes sense the way you just explained it .

 

The first mechanic was ready to just change the sensor and I wasn't that happy with his labor charges especially after he came recommended by a close friend so I went to the guy I trust who has less knowledge .

 

I think next time I might try your spark plug strategy and see how I like it .

 

Cheers .