Rough Idle Cured When O2 Sensor Disconnected

[SacDoug]

I am posting this just to make sure that I diagnosed the problem correctly.

Situation:
My truck failed smog badly. It has a rough idle and even misses when idled at higher rpms. It also runs rough and has very little power when the engine is first started cold.

My catalytic converter is burned out. It was only 2 years old. I check the 3 plugs on the passenger side and they smelled of gasoline and had excessive carbon on them. I pressurized my intake and found no major vacuum leaks.

The test I did that points to the O2 sensor is with the engine at operating temperature and idling rough, I disconnected the O2 sensor. The engine immediately smoothed out and idled rock solid.


This to me does not make sense. Unplugging the O2 sensor should have dropped the reading to zero so the ECU should have either threw a code or thought that it was running extremely lean and richened the mixture up. Instead, there was no CEL and the idle smoothed out.

How can unplugging the O2 sensor fix the idle?

 

[SacDoug]

Re: Rough Idle Cured When O2 Sensor Disconnected

I just replaced the O2 sensor with a brand new one. The rough idle persists.

I still have no clue why it smoothed out with the O2 sensor disconnected.

 

[Davemc1963]

Re: Rough Idle Cured When O2 Sensor Disconnected

This might be better posted in the Chipping, Emmisions & Tuning Section.

 

[SacDoug]

Re: Rough Idle Cured When O2 Sensor Disconnected

This might be better posted in the Chipping, Emmisions & Tuning Section.

You are right. I can't find a way to delete this thread or move it.


Admins, if you see this, please delete this thread. I created a new one in Chipping and Emissions.

 

[turbodig]

Re: Rough Idle Cured When O2 Sensor Disconnected

This to me does not make sense. Unplugging the O2 sensor should have dropped the reading to zero so the ECU should have either threw a code or thought that it was running extremely lean and richened the mixture up. Instead, there was no CEL and the idle smoothed out.

You would have likely gotten a CEL eventually.

The voltage doesn't go to zero.... it actually goes to ~.450 when unplugged.

The sensor doesn't supply a voltage to anything. It's provided with a constant current, and the changing resistance of the element effectively produces a voltage.

I would check the o2 sensor ground on the back of the driver's side head.
Might also look at the fuel pressure regulator.

Without wide-band data, it's tough to know what's actually happening. I suspect that the o2 circuit is causing the ECM to believe it's too lean, which makes the ECM richen the fuel up. You'd see this as high BLM and integrator numbers.

Unplugging the sensor causes the ECM to go back to pre-programmed fuel... which is why everything smoothed out.

 

[SacDoug]

Re: Rough Idle Cured When O2 Sensor Disconnected

Thank you for the reply.

I will definately look into the ground for the O2 sensor. Where exactly is "the back of the driver's side head"? Does that mean the O2 sensor is grounded inside the cab?

I'm going to try to push a probe into the back of the O2 sensor to get an exact voltage reading. If it reads a constant high voltage then I know that the O2 reading is a symptom and not the cause.

 

[SuperStockChamp03]

Re: Rough Idle Cured When O2 Sensor Disconnected

Check the fuel pressure & hook up a scanner the can display the fuel trims ( block learn & intagrator).
Make sure your fuel pressure regulator diaphram is not bad, it will leak fuel into the vaccum line.

 

[wheels]

Re: Rough Idle Cured When O2 Sensor Disconnected

Thank you for the reply.

I will definately look into the ground for the O2 sensor. Where exactly is "the back of the driver's side head"? Does that mean the O2 sensor is grounded inside the cab?

The rear of the cylinder head. Not your head.

 

[SacDoug]

Re: Rough Idle Cured When O2 Sensor Disconnected

Thank you for the suggestion. I'll idle the truck a little then pull the vacuum line off of the FPR. If I smell gas, then I know the diaphram is bad.

I will have to research a scan tool. I would like to buy a laptop cable and run datalogging software that can pull codes and read real time data.

 

[SacDoug]

Re: Rough Idle Cured When O2 Sensor Disconnected

The rear of the cylinder head. Not your head.

Doh!

I feel silly now. :rotf:

 

[SacDoug]

Re: Rough Idle Cured When O2 Sensor Disconnected

I found something tonight that made me feel like a real dummy. My plugs were not sealing.

I bought irridium plugs nearly 4 years ago. The old plugs had no washer and neither did the new ones. So I just installed them as-is. At the same time I had removed the turbo and ported it. So when I first started the truck after the turbo port and plug replacement I just assumed the exhaust leak that I heard was coming from the turbo since there was no gasket. I was wrong. The leak was coming from the plugs!

I pulled a plug and looked at it closely and noticed that there was carbon on the plug boot and on the plug insulator. I decided to put an old washer on it from another plug and install it. When I started the truck, the exhaust leak sound was more quiet. I am going to the parts store tomorrow to buy more plug washers to put on all 6 plugs. I know at least one more is leaking.

I doubt this will solve my overall problem but it will make me feel better. If it does solve the problem, I will be a monkey's uncle.

 

[gkrcr882]

Re: Rough Idle Cured When O2 Sensor Disconnected

Makes sense, if you're getting outside air into the cylinder via a plug not sealing (or losing the air/fuel charge), it'll make all kinds of things go haywire. I had one back out on my S15, so it does happen. Scared the hell out of me when it did.

 

[sharkbait]

Re: Rough Idle Cured When O2 Sensor Disconnected

I found something tonight that made me feel like a real dummy. My plugs were not sealing.

I bought irridium plugs nearly 4 years ago. The old plugs had no washer and neither did the new ones. So I just installed them as-is. At the same time I had removed the turbo and ported it. So when I first started the truck after the turbo port and plug replacement I just assumed the exhaust leak that I heard was coming from the turbo since there was no gasket. I was wrong. The leak was coming from the plugs!

I pulled a plug and looked at it closely and noticed that there was carbon on the plug boot and on the plug insulator. I decided to put an old washer on it from another plug and install it. When I started the truck, the exhaust leak sound was more quiet. I am going to the parts store tomorrow to buy more plug washers to put on all 6 plugs. I know at least one more is leaking.

I doubt this will solve my overall problem but it will make me feel better. If it does solve the problem, I will be a monkey's uncle.

so the irridiums need plug washers? now i can't remember if mine had them:rant:

 

[SacDoug]

Re: Rough Idle Cured When O2 Sensor Disconnected

so the irridiums need plug washers? now i can't remember if mine had them:rant:

They are not supposed to have washers but for some reason mine were not sealing.


Update: I ran two cups of water through the engine and then Seafoam it to decarbon the engine. That smoked up the neighborhood but did not fix the irratic idle. When I pulled the plugs out, they were nice and clean. On a side note, if you run two cups of water through the engine, it will show you were any exhaust leaks are since the steam will exit through any gaps in the exhaust. I found a gap at the turbo and on the other side where the crossover pipe connects to the driver's exhaust manifold.

I bought a new set of AC Delco plugs. Pep Boys tried to give me the CR43TS plugs that go on the regular 4.3L. I asked to look in their parts manual and found that our trucks are supposed to use the CR42TS. I bought a set of them, put in in the truck. When I started it up the idle still wandered from 750-950 rpms which is a big improvement from 600-950 rpms. After I drove the truck to the smog station it was idling almost at a steady 950 rpms. It would dip every now and then to 850 rpms.

I don't know why my irridium plugs were causing the irratic idle. I will check the gaps on them. Maybe the gap expanded on a couple of them.

 

[SacDoug]

Re: Rough Idle Cured When O2 Sensor Disconnected

I found the problem. My Fuel Pressure Regulator was bad. I put a mechanical pressure guage on my fuel rail. When I first turned the key the fuel pressure would go to 40 psi then bleed away to zero in about 3 seconds. When I started the engine the fuel pressure pegged out over 100 psi. The guage only goes to 100 psi so I don't know what the actual pressure is.

I replaced the FPR last night and now my truck idles correctly. The fuel pressure go to 35 psi then bleeds off to 20 psi and stays at 20 psi. When I start the truck the fuel pressure is about 41 psi with the vacuum hose disconnected and 32 psi with it connected which is close enough for me. The truck is also idling correctly now.

 

[asmo]

Re: Rough Idle Cured When O2 Sensor Disconnected

Thanks for this Ive been having the same problems, my fuel pressure is at 100 psi idle too, no fuel in vacuum line. but runs rough and same thing all passenger side plugs soaked and major carbon build-up.

 

[turbo_tuna]

Re: Rough Idle Cured When O2 Sensor Disconnected

I have the same rough idle issue and cannot find any vacuum leaks. Were did you get your FPR. gmpartsgiant and gmpartsdirect show the item (25523720) as discontinued.

I found the problem. My Fuel Pressure Regulator was bad. I put a mechanical pressure guage on my fuel rail. When I first turned the key the fuel pressure would go to 40 psi then bleed away to zero in about 3 seconds. When I started the engine the fuel pressure pegged out over 100 psi. The guage only goes to 100 psi so I don't know what the actual pressure is.

I replaced the FPR last night and now my truck idles correctly. The fuel pressure go to 35 psi then bleeds off to 20 psi and stays at 20 psi. When I start the truck the fuel pressure is about 41 psi with the vacuum hose disconnected and 32 psi with it connected which is close enough for me. The truck is also idling correctly now.