Sorry for the long post but there is a lot to the story.

My sisters 1994 3.8L 5speed mustang has an idle issue, When cold it will barely idle on it's own (you have to give it throttle else i drops below 400rpm stumbles for a little bit then stalls). It used to have really bad issues a while ago, the engine wouldn't even run above about 2700 RPM it would just stumble like it was running out of gas, it also stumbled when trying to accelerate and had bad throttle lag (you could have the accelerator all the way to the floor and back up long before the engine responded) Codes thrown are "System too lean bank 1" and "System too lean bank 2". We Replaced: Fuel filter, Upstream O2 sensors, TPS, Fuel Injectors (suggested by Ford Technition), and MAF sensor which solved the previous issues. However the cold idle issue remains along with one of the codes (I don't remember which one), Fed up with it we dropped the car off at the Ford dealer for diagnostic and $300 later they are stumped. This is what they told us upon picking it up:

"Both 02 sensors wack (yes they said wack) out at 3.5 Volts intermittently. Checked circuits nothing obvious showing. "dirty" O2 sensor signal as if electrical interference. Possible PCM or Harness but both pieces are discontinued and unavailable from Ford"

I took apart the lower O2 sensor harness (goes from the O2 sensors to the main harness) inspected it and wrapped it with foil to shield it from interference and no change. The next part of the harness requires removal of the Upper Intake Manifold and I would really like to avoid removing that again. A reman PCM is between $115 and $150, so I would like to make sure that is the problem first.

Is there a way to test the PCM? Any other suggestions what may be wrong? My sister loves this car but we can't really live with restarting the engine every time you stop (it stalls if the clutch is pushed down or it is put in neutral for more than couple seconds) until it warms up, it just not reasonable. I'm at a loss so any help is very much appreciated. Thanks in advance.
-Brad

 

hi, i have read and re-read the issues you are experiencing and its very difficult to explain why the sensors would read so high so here,s my guess , a big vacuum leak could cause mayhem !
i have no experience with the 3,8 engine but if it has the vacuum operated imrc actuators it may pay to check the vacuum pipes for leaks also to check the actuator linkages have not come apart. sorry could not help further, bokker.

 

Brad, I'm pretty much at a loss as well.

Any chance there is an air leak in the intake plenum after the MAF sensor?

And what bokker said. Vacuum leaks can cause all sorts of goofy things.

Does it have an IAC (Idle Air Control)? That device bleeds air to maintain idle. Maybe that is "wacked out".

There is a O2 sensor eliminator you can throw together and use to eliminate the O2 sensors. I doubt those are the problem, but it would be a way to try something.

The basics:
Proper air/fuel mix with adequate fuel pressure
Properly timed ignition -> good spark at plugs.
Valve train integrity

All the vacuum lines, and sensors, and ECM ,etc. etc. all ultimately control basic stuff. One way or another seems like you'll have to break it in half: Fuel/air or ignition and go from there.

I hope you get it figured out.

 

We replaced the ignition coil not too long ago so I doubt that is the issue, has new plu wires and the plugs looked good, so we moved onto timming, it has DIS so if it was off I couldn't correct it but timming looks good to me as well as compression on 3 cylinders (I didn't bother checking the other 3). We checked the fuel pressure a while back and it was good but i dont remember what it was. I looked for vacuum leaks and didn't find any but will continue looking. as for the IAC unplugging it causes the engine to stall so it is working, how can i determine if it is working properly? Dad decided to go ahead put a reman PCM in it. It didn't solve the problem directly but I told my sister to drive it for a while to see if it improved. A week later it has not however, I just checked the codes and it is throwing 6! The codes are: P1130, P1131, P1150, P1151, P0171, & P0174. I'm gunna call the Ford service department when I ca to see if they have any ideas but I thought I see if any one has any other Ideas here. thanks for looking
-Brad

 

Before you do anything. Get coolant tester pressure and pump to I think 16 psi max. It say on radiator cap MAX psi I think 13 or 16.


Our 02 Mustang 3.8L had exact issues you have. It only had 68K miles. It start downhill. Run rough at cold, Horrible mileage, run rich, egr keep clogged, and engine light come on everyday say EGR not flow or oxygen sensor read LEAN.

It turn out it had bad head gasket it suck coolant in cylinder and go to exhaust then Foul oxygen sensors. We never fix it got totaled in snowstorm 2 years ago. I was kinda happy it gone. It wasn't safe car for snow and it wasn't real reliable for long trip without break down.