Rough Idle at Operating Temp [Archive] - Forum


View Full Version : Rough Idle at Operating Temp

12-07-2004, 12:46 PM
I start the car and everything seems fine, oil pressure runs at 60, and engine runs at about 1300rpm then drops to about 1000rpm, again smooth. Well if I drive for say a trip to the store where the engine can get hot, the oil pressure reads about 40psi the engine tries to stall out but doesnt, and see idles about 700 or 750rpm just low enough to make the trans pump pop like popcorn. I have a 3300v6 and all the ones that I have idle at 1000rpm as in my 93 skylark factory manual has all specs set @1000rpm. I checked the IAC and the thing was alittle gunked and so I cleaned it up alittle so I cleaned it up as well as the thottle body. I did get a low voltage reading at the TPS sensor, but it was just in spec (not enough to throw a code) Ive changed the spark plugs and wires, ICM and coils. Funny thing to I had the car running and I unpluged the IAC it did not set a code. I then unmounted it and had my buddy turn the key to the on position and it didnt move for crap, should I start it to see if it moves? AHHH this is racking my brain, AHHHHHHHH someone please.....

manic mechanic
12-08-2004, 06:45 PM
Some possible culprits I can think of are your O2 sensor or your temperature sensor.

I believe the Haynes manual gives an outline on how to diagnose both of these items.

Remember that the O2 sensor (oxygen sensor) only comes into play when the engine is warm. If the sensor is dirty or lazy it may cause some strange idle characteristics.

As an O2 sensor ages, it doesnít react as quickly as it once did. The increased lag time makes the sensor sluggish and prevents the engine from keeping the air/fuel mixture in close balance. If the engine burns oil or develops an internal coolant leak, the sensor element may become contaminated causing the sensor to fail. Back when leaded gasoline was still available, a single tank full of leaded fuel would kill most O2 sensors in a few hundred miles. (Thatís a main reason why the government finally eliminated leaded fuel.)

Because the sensor reacts to oxygen in the exhaust and not fuel, any engine problem that allows unburned air to pass through the cylinders will also trick an O2 sensor into reading lean. A misfiring spark plug or a leaky exhaust valve - even a leak in the exhaust manifold gasket - may allow enough air into the exhaust to screw up the sensor readings. It wonít damage the sensor, but it will create a rich running condition that hurts emissions and fuel economy.

The way the computer maintains a balanced fuel mixture is by doing the opposite of what the O2 sensor reads. If the O2 sensor reads rich (too much fuel), the computer shortens the on-time of each injector pulse to reduce the amount of fuel being squirted into the engine. This makes the mixture go lean. As soon as the O2 sensor detects this and gives a lean reading (not enough fuel), the computer reacts and increases the on-time of each injector pulse to add more fuel. This back-and-forth balancing act creates an average mixture that is pretty close to ideal. This is the "fuel feedback control loop" that allows todayís vehicles to maintain extremely low emissions levels, and the O2 sensor is the key sensor in this loop.

The computer uses other sensor inputs, too, like those from the coolant sensor, throttle position sensor, manifold absolute pressure sensor, airflow sensor, etc. to further refine the air/fuel radio as needed to suit changing operating conditions. But the O2 sensor provides the main input that determines what happens to the fuel mixture. So if the O2 sensor isnít reading right, it screws up everything.

Typically, a bad O2 sensor will read low (lean), which causes the engine to run too rich, pollute too much and use too much gas. A low reading can be caused by several things: old age, contamination, a bad wiring connection, or an ignition or compression problem in the engine.

Now if the coolant temperature sensor is on it's way out it may be telling the computer that the engine is still in open loop mode (rich) and thus ignoring the output from the oxygen sensor thus sacrificing a smooth idle and engine performance.

Sorry for being so long winded.

12-09-2004, 07:12 AM
I well I changed out the O2 sensor, anf the car didnt seem to idle as bad, I let it idle for abou 5min and it kept steady idle. Drove it around came back and put it in park and it drop idle just alittle but corrected. Now Im wondering if is not a injector and or fuel filter, maybe both. the reason I say this is that if I unplug the injectors on at a time the engine should drop idle alittle, well I tried it on my other 3300 and it did what it should dropped idle. Well on the car at hand all but one create a drop and that injector is on the 2nd cyl. if you tap the gas the some hesitation, s today Im gonna try that and see what happens.. I just need an injector, I have some from a 89-91 3800v6 but I dont think it will work.

manic mechanic
12-09-2004, 08:52 AM
The fault could also be in the wiring harness. You may want to check the wiring harness to the fuel injector with a "noid" light.

A noid light can no doubt be rented from a parts supplier. It's a small LED light that reacts to the injector pulse when the engine is running.

It connects to the wiring harness and basically replaces the injector.

This will help eliminate the culprit.

Is it the injector or the electrical harness?

12-12-2004, 12:39 AM
Well I checked the harnest and it was pulsing, I changed the fuel filter just to get it out of the way, thought it helped alittle be not a fix, I pulled the plug on the #2 cyl and the plug was dry small traces of fuel. I think for my last attempt Im gonna get an injector to try..... warm weather car runs good until hot ...... very cold here in michigan runs like crap warms a bit runs ok then gets to temp and runs like crap. I also think that the guy that I got the car from put a 3800 MAF sensor on it, also couldnt the TPS have that effect without a code? I know when I pulled the IAC solinoid the computer did not code.

manic mechanic
12-12-2004, 07:46 AM
Before you go and try another injector, try and isolate the problem. It could also be a weak spark or spark plug.

Why not pick up a calibrated spark plug tester from the local parts guy, or rent one, and see if every wire is firing correctly. If not, it could be the coil packs or ignition module - we'll get to that if need be.

If the plug is wet, it's getting fuel. It may not be completely burning it is all. You may also want to try changing and gapping the plugs (as recommended on the sticker under the hood) with the factory replacement brand. The gap is also critical.

Try the easy stuff first.

12-16-2004, 07:52 AM
The plug was dry...(there was traces that there was fuel burn) as for the electrical system, I had changed Ignition Module, Spark plugs, plugs, IAC. Things I havent changed is TPS, Coolant Sensor, or MAF sensor.

manic mechanic
12-17-2004, 07:12 AM
Is there a spark?

12-18-2004, 02:13 PM
yup shes sparking, very strong.... Im gonna try the TPS and see if there is a difference. Not only that but the other day it got real cold and the PS was bad whining and the steering was awfull, I looked into the Pump and the fluid was frothing, so I was like great, the fluid is very nasty looking probably OEM. So I have to flush it, also it strains the motor really good when turing the wheel at low RPM's

01-05-2005, 10:24 AM
Well, I did alittle looking when I was replacing the vaccum hoses on the engine,I noticed that there was a crack on one of the Coil packs.(I started getting this stalling parob after you started it then it would run after a restart)Also I noticed that someone had tried to adjust the factory set idle screw, So I reset it to spec and well I have no more probs with the idle hunting after I drive for a bit. There is however a prob with when in gear you can feel a growl type sound and feel. Not very noticable to the other person but as the driver you can tell. Im wondering if a bad manifold gasket , isnt the bad guy here.