97 GL tries to stall, engine oscillates at idle

[dimovsp]

My Taurus tries to stall and misses when unload the engine - switching from gear to neutral/parking or when changing gears. When change from gear to neutral or parking, sometimes the RPMs will oscillate between 300 and 900 and it will stabilize in few seconds or if I press the gas pedal. In the second case it hesitates a little. Same thing happens sometime when I stop on traffic light or when in traffic. The car runs almost perfect when warmed up (drive it for 10 - 15 minutes) or when accelerating. The engine has plenty of power when accelerating except some hesitation when changing gears.
The problem first appear around 3 years back but was so intermittent that every time I took it to a mechanic the car would run fine. Back then the crankshaft position sensor was broken but fixing it didn't improve anything.
Lately the problem deteriorated and happens all the time.
Last week changed the EGR valve and pipe (broke the pipe while taking out the valve) - the valve wouldn't hold vacuum. Runs better but still oscillates when changing from gear to neutral.
2 or 3 months ago got P0455 code but cleared it out and it didn't appear again. When I drive the car for a while and unscrew the gas cap I do not here hissing as on my truck Ranger 94.
There are no codes stored in the computer.
The car runs better if the computer is reset but in a while the problem appears again.

I am taking it to a mechanic but may be there is something I can try myself before that. Change the idle control valve, MAF sensor?

Thanks.

[shorod]

How long has it been since the car has had a tune up?

The first thing I would do is add a bottle of fuel injector cleaner to the fuel tank and drive it for about 100 miles. If that has no effect, then you should pull the air intake hose from the throttle body, open the throttle competely, and check the throttle plate for carbon/gunk. If the plate and/or throttle body if pretty nasty, buy about three cans of STP throttle body cleaner and spray the throttle body and intake out as well as you can. Be prepared to pull the spark plugs and crank the engine with plug wells covered to prevent hydro-locking the engine.

You may want to turn on the air conditioner and see if the idle speed increases. If it does not, your idle air control servo may not be working, and may be part of your issue.

Beyond that it would help to have a scan tool with datastream mode to determine more accurately where the problem lies (electrical, fuel, emissions, etc.).

-Rod

[dimovsp]

Thanks for the tips.
I have scan tool. Got it from http://obd-2.com. I can get datastream and it draws some digrams, but I do not know what parameters to monitor and how to interpret them. Could you tip me on those?

Last tune-up was in May last year but I'm not sure what exactly was done then. About a month ago my car stalled after starting from a red light, I think when it switched to second gear. The engine was still cold and it wouldn't start for a few minutes. I released the fuel pressure from the test port and the car started after a couple seconds but would run close to stalling on idle. I checked the spark plugs the next day and found 4 and 6 with gap 0.6, it should be 0.42-0.46 on my car. Those two plugs smelled like gasoline and looked wet. I cleaned all plugs and gapped them. I have used anti-seize on the plugs and dielectric on the wires before. My mechanic yelled at me for using anti-seize on this type of engine - aluminum block, although Haynes manual say do so.
The car ran perfectly for awhile, I guess until the computer got retrained. If I reset the computer the engine will run fine and within 20 miles it will get back to the same.
Yesterday I had 40 miles trip and after the 10 miles the car will run much better. Still has problem when driving down-hill with trottle almost close.

I did vacuum test yesterday. It holds fairly steady on 17 psi. It trembles a little but no more than quarter psi from 17. On quick press/release of the gas pedal the pressure jumps to zero then jumps to about 22 and then slowly in about three seconds back to 17. I think that's what it should do.

I also dosconnected the electrical connector of the idle contol valve and started the engine - big difference, so it must be working.

If I turn the AC on/off the RPMs go down/up with about 100.

Today I will change air filter, fuel filter, PCV valve and grommet (the PCV valve doesn't fit tight in the grommet), will do an oil change and put some injector cleaner. I may put some oil treatment too.

Next week will clean the throttle body if there is no improvement.

[shorod]

In the datastream mode I'd suggest monitoring the voltage readings of all the oxygen sensors, as well as the coolant temperature sensor, MAF sensor, and TPS sensor. If you can capture what those readings are just before, during, and just after a stall, that might point us in the appropriate direction.

If you can get this data and post back on this thread, we'll see what we can do. Others may have some additional suggestions as well.

-Rod

[dimovsp]

I think the engine got fixed. At least the last two days it ran fine but it was dry and lots of sunshine. Tomorrow we expect some rain, so if the car still runs good then I'll be more confident it's fixed.
Here is what I did chronologically that could have fixed it:
- I changed the EGR valve last week - it was leaking. This by itself didn't fix it because the car would still rock on idle for a week.
- in one of my previous posts I mantioned that I have used dielectric paste on the spark plug wires, so I cleaned them good with alcohol two days ago.
and at the same time I also cleaned the connectors of the fuses in the power distribution box.
- Have 50 miles trip. At the end the car was running much better and the next day it was fine.

I'm guessing that driving with improperly gapped pluds for a year and having leaky EGR valve got the injectors and valves dirty. I put a botle of injector cleaner one month ago but probably it was not enough plus I haven't fixed the EGR back then.
And may be the computer needed some 40 miles to adapt.

Although the car seems to be fine, I changed the air and fuel filter yesterday. I put another botle of injector cleaner.
I have not changed the PVC valve yet and haven't changed the oil, nor have cleaned the throttle body.

This saturday I'll read the datastream and post the numbers. If the car still runs fine they can be used as a baseline.