It actually started a couple weeks ago. The car started idling rough (with nothing changed) for a couple of days and then the problem went away.

Advance to today. It started again. This time I threw the scanner on it and noticed something very peculiar:

With the computer on the passenger side O2 sensor, no problems. 128-130 BLMs, and constant.

With the computer on the driver side O2 sensor, BLM was at 160. When the fans came on, the BLMs started to come back down to normal (about 130). SUre enough, after the fans turned off, the BLMs climbed back up to 160 and then declined back down to 130 when the fans turned off. I'm trying to digest this myself, but anyone have any ideas?

EDIT: 7730 ECM. I swapped O2s from side to side to verify, but got the same results.

 

Ignition problem (misfire), dead injector, dead cylinder, fouled plug. Anything to cause a misfire will pump O2 at the sensor, which will cause a "lean reading" O2 sensor. The computer will add fuel in a way, such that even though it may be getting crosscounts from rich cylinders and the misfiring lean cylinder(s), the frequency will play tricks on the closed loop (it's adding and subtracting fuel at a different frequency), and it'll increment slowly.

 

Well, from my battle against the misfire from last year, I had the heads checked out and ran a leak down test, so I know that's not the problem.

I checked the plugs and they're all the same color- a light tan, and very clean. I also started pulling injector plugs off one at a time- on each injector, the engine responded the same- a slight dip in the rpm and increased roughness.

I suppose I could ohm out the wires- intermittent open perhaps? Or could the ECM be "hiccuping"? Thing is, it doesn't feel like a misfire- at least not they way the previous misfire felt (like a dead miss). This is simply a rough idle.

 

Leaky header then, probably. Even a downstream leak will cause problems.

 

could be a bad ground on the fan.

 

I have noticed a big voltage drop on one of my fans. Apparently one of my fans takes over 40 amps. It also drops my volts down almost a full volt at idle.

Check your voltage, just for fun, when the fan(s) come on.

Was your Injector PW vs. BATT table changed at all?

Good luck.

 

Nope.

 

Thats what it sounds like to me (PW vs Voltage), Watch PW when fans come on...
Sounds like you alternator may net be up to par maybe.

 

I DON'T think it's fan related. I'll stick to my leaky exhaust or injector problem. The evidence is in that when he leaves the ECU connected to the passenger side header's O2, there's NO problem, fans or not. Driver side has a problem. The fans won't affect only one side, and a leak or injector problem can.

 

Ohh, I misunderstood then.
I thought that he had same problem on both sides..

You I would go with the Leak as well.
My collector gaskets are about 8" past o2 and mess up o2 when leaking.

 

while scanning, why not try unplugging the fans, while running off the *bad* side O2, and see if the air disturbance is causing the problem, or if its related to the ECM commanding the fans.

 

Oh yah. I see now. Maybe a bad ground on that side of the injectors. How are they grounded? Do they share a common ground with the fan?

Could it be a bad engine ground? But then I would expect it to affect both sides of the injectors.

 

Well I would probably guess you have a bad wire on that side, when a plug missfires 02 will read lean due to unburned 02 in the stream.
Maybe something as little as when fan come on it slightly pushes on wire enough to allow arch. Either way you know what side the problem is on, I am sure you have an old wire laying around.
You could always pull each off cap as car is running as you did injectors..
If you have a hand help pyromoter (sears $40) you can shoot the header tubes to see which one is cold.