I have a V6 that seems to be running rough and rich at idle. The engine has 150K miles on it. I need to get it smogged and don't believe it will pass as is. I get no codes set. I have checked and/or replaced the following:

Vacuum Hoses - Checked and replaced some.
Air Temp Sensor - New
O2 Sensor - New
Spark Plugs - New
Plug Wires - Checked OK
Ignition Module - Checked
Pickup Coil - Checked
Ignition Timing - Checked
PCV Valve - New
Cat Convertor - New
Cylinder Compression - 130-140 All Cylinders

I have a scanner which indicates the O2 Sensor voltage is around a constant 900mV during warm idle. IAC ranges between 50 and 90 steps. The idle RPM is dead on.

I only get o2 crossings at part throttle. It appears that the MAP sensor and the TPS are reading good.

Should I get O2 crossings at idle? Could my IAC be faulty? How can I check it?

try cleaning out your throttle body

take it right off to do so and then buy some intake cleaner, take an air line too (compressed air) and blow out all the vacuum channels through it, just to make sure they are clear and debris free.

i just did this to my v6 because i am havng the same problems, while mine was off i noticed that the throttle plate no longer closes all the way and it wobbles a bit. I am suspecting that the bushings in it are gone so im looking for a new one now.

check this,
hope it helps
if it does rate me

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Pontiac Firefly 87 1.0L
Pontiac Fiero 86 SE 2.8L
Buick Skyhawk 76 3.8L
GMC Jimmy 85 305
"It's not how fast you go, but how good you look goin"

<bump>

Hi, I'd like to take a shot a helping you, but I'm a little confused with your post. You said it is running rough at idle, but at the end you said the RPMs are right on. Can you clarify "rough at idle", and also...is this the only time said symptoms occur? Do you have any trouble with start up or while cruising/accelerating? I would be very suspect of the 900mV reading on the O2. Seems like that is at maximum. I would definately try to verify that with a volt meter. If you do fail the sniffer, post the results so we can get you fixed.

Just as an off the wall side note stab in the dark...I noticed you did not have engine coolant sensor on your list of items checked. Failure of this, or even a stuck open thermostat can make your computer think you are running cold....hence, bring on the injector pulses.

What Cliff D. said, regarding the coolant sensor.
Is your scan tool showing temps that sound correct?

The following isn't Fiero specific, but can apply to cars with non-heated O2 sensors (like ours).
I remember reading that, at idle, the O2 sensor can get cold enough that it doesn't function properly, even if the engine is fully warmed up. If the engine can't get reliable readings it will revert to "open loop" operation. This will make it run more from pre-programmed tables and inputs from the other sensors, instead of O2 sensor readings.
Short story, it may be *normal* to get a steady reading at idle. Not sure what the value is supposed to be, though.

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Raydar

88 3.4 coupe.

[This message has been edited by Raydar (edited 04-12-2003).]

Cliff, by rough idle I mean that there is much more vibration than there was in the past (six months ago). It is almost like a cylinder is not firing, but all have compression and spark with new plugs, cap and rotor. The idle is steady and does not hunt.

In general, the engine runs a little rough while accelerating, however, when cruising it runs very smooth.

Make sure the vacuum line to the MAP is not plugged--mine was--or cracked. This will make it run VERY rich--to the point of not running at all--so that's probably not it, but it's easy to check if you haven't already. But to eliminate any MAP related problems, see how the engine runs by:
1. Disconnecting the MAP. This forces engine to use PROM tables to map fuel, based on TPS position. Sets a Code 33 you'll have to clear later.
2. Jumper the diagnostic terminals with a 10K resistor. This forces the engine to ignore the O2 sensor and stay in 'open loop' mode. It will idle a little faster because a few degrees more advance are cranked in.

Ditto the coolant sensor idea. It's easy to check with an ohmmeter. On my car, the car would run, but it would load up with fuel. The idle would drop, the ECM would advance the timing to correct the idle which would clean the engine out. The idle would settle down, and the process would repeat, over and over. Easily observed on a good running car by simply removing the sensor connector.

There's one other problem you don't want to hear about--bad injectors. Short of removing them for testing, I don't think there's much you can tell about their condition. My car was massively abused, barely running when I bought it, but the injectors were junk at 69K (but 15 years!). If you got 150K out of yours, they don't owe you anything! I put new 17# ones from Summit Racing in it--$180. Can't vouch for passing the smog test, though. My car has no converter. I've got almost all new sensors, though, so I'm crossing my fingers.

Maybe observing fuel pressure will tell you something about the injectors, but not if one of them is not opening (which I also had), which sounds like the problem. This would cause the O2 sensor/ECM to sense a lean condition (too much O2), and add fuel to the cylinders that are already working right, making the engine run rich.

One of these might give you a lead on what's wrong.

Thought I would bump you up.
I think the above diagnosis ideas should solve your problem. I have to return the ditto on the fuel injector not firing. SOME cars allow you access to the injector connectors so you can just unplug and re-plug them one at a time until you get one that does not change the idle. Maybe you could sneak an automotive stethescope down to the injectors and see if one is simply not clicking.
After reading your first post again, I noticed that you asked about 02 crosscounts. Your computer, if I am not mistaken will ignore the 02 voltage at open-loop and WOT. If you could see the 02 signal on a lab scope, you would notice a dramatic change into oscillations on the 02 voltage upon closed-loop being achieved. Good luck.

I appreciate everyones help so far...

A couple of thins to clarify:

1) Engine coolant sensor indicates 180-190 degrees. (But the engine really does not get that warm in my opinion). I thought about the thermostat and the sensor. I will be checking the sensor first.

2) Scanner indicates that the O2 sensor is ready and ECM is operating in closed loop mode. So, commments regarding open-loop mode and O2 not being up to temp don't appear to apply.

3) MAP sensor readings on the scanner look good and no codes are set, so it also appears that the MAP is functioning OK.

It almost runs as if it is not firing one one cylinder and the raw gas gives it the rich reading on the scanner. However, all are getting spark and have compression.

<bump>

[This message has been edited by mshill (edited 04-15-2003).]

"It almost runs as if it is not firing one one cylinder and the raw gas gives it the rich reading on the scanner. However, all are getting spark and have compression."

And we all agree this isn't possible. I have my Celica in the shop as we speak for high idle hydrocarbons. They informed me it has a 'lean miss' at idle. Just what you'd have if an injector was doing half of its job.

Changing injectors isn't that bad a job, once you get started.

Confounded injectors! They add, what, 6x2+4=16 wires to the car, and at least 32 connections, plus the solenoids, plus the umpteen mechanical parts, software and backup software, computer and sensors to run the whole mess. Not to mention the difficulty in troubleshooting them, and the general mystery and possible flakiness surrounding anything electronic. No wonder TBI is more reliable.

Contrast that to my Celica carb, with 342K miles on it, never been worked on. The shop pryed off a metal seal to enrich the idle mixture. Almost passed smog, with the original converter.