1993 California version Explorer. Factory AC and EGR sytems. When I accelerate normally the motor lags until I mash the throttle to the floor and then it takes off normally.

First I thought it was the AC compressor ******* and the WOT signal to the PCM was killing the AC compressor and then it takes off...but I have never seen such a dramatic change with just the WOT effect. (Controls system speaking.) And i tested without the AC on and the issue is still there.

There are 3 things I know of that the WOT signal from the throttle position sensor affect - 1) AC off, 2) Fuel evapaorator canistor off and 3) EGR off.

So I am thinking that if my DPFE is not reading the real, true exhaust flow into the intake manifold, then the PCM is not leaning the mixture, then it is actually running rich.

I get a 173 code which is right side rich...but the plugs look good. Not sure why I don't get both sides rich if my theory is correct.

I changed the right side O2 but had no effect. These have one right and one left O2 and usually, a side only code only happens with a bad "side" O2. Maybe the right side is a little more sensatived...or my left O2 is toast anyways...

On the scanner, it must average the two sensors as there is only 1 O2 reading. Seems to be high, above .4 V typically which means rich...but it does switch...but not too often or too fast.

This is a wierd one so give it to me!

 

To add to your list of things that happen at WOT: 4) O2 sensor is ignored.

Your scanner can only "see" one O2 sensor?? Are you certain your '93 has 2 O2 sensors? As I understand it '93 was the transition year, and some had one O2 sensor and others had 2. I thought it was supposed to be Fed vs. Ca emissions, but I'm not sure. 173 would apply to the single O2 sensor if it only has one.

If it really does have two sensors, I might try to figure out why my scanner can only see the one. Perhaps you need a different scanner?

One thing I like to suggest when there are two O2 sensors is to switch the two and see if the DTC follows the sensor. If you switch them and the DTC shows up for the left side, then you know the problem is in the sensor.

Another first thing I would do with a rich code would be to check the fuel pressure.

 

Yeah MR Shorty - it is a Cal truck and has 2 O2's. The scanner can give all the codes, right and left independent, but the real time monitoring only shows one O2 reading...so they must avg the 2 sensors or something. It is an old, now obsolete, AutoXray scanner. I had it updated to test OBDII for my Durango and it does show multiple O2 readings, real time, on other OBDII cars...just not the OBDI in the Ex. I also ran a KOER test with the paper clip jumper and the CEL flashed the same 173 code.

OK I forgot, WOT also cancels closed loop operation and ignores the O2's...which again is part of the issue here. The truck runs OK when it ignores all the miscellaneous inputs and only goes off of the fixed WOT tables for A/F and spark curve.

What I am trying to understand is what is causing the right O2 to read rich? I already put a new sensor on that side and no change. There is no point in swapping the leads as the left one would then get the code. There are no reasons for just one side to read rich or lean other than the O2 sensor itself...or a serious problem like a head gasket or other leak on one side. But the basic air and fuel delivery circuits (MAF, TPS, Coolant and air temp, fuel pressure regulator) affect both sides at the same time.

I checked the EGR today realtime driving to work...tried to be safe about it and keep one eye on the road.

At idle and WOT, the EGR go open was at 0%. So the PCM is telling the EGR to stay closed at the right time. DPFE feedback was .5 to .6 volts at idle/WOT. When crusing, the % open would be between 30% and 50% and the DPFE feedbacl was 1.4 to 1.7 volts.

If the 2 are matched and linear, a 50% open should give a 2.5V return right? That is 1/2 the 0 to 5 volt reference signal.

The only refernce I found for the DPFE signal lists vacuum and pressure, not flow, to the 0 to 5 volt signal. Zero pressure is 3.25 volts and the higher the pressure, the higher the voltage. The readings I saw would indicate a vacuum at the DPFE...which can only come from the intake manifold meaning the EGR valve was open.

If the EGR valve is open, the the DPFE is reading it correctly, then the PCM should reduce fuel flow and the O2's should read OK...but obviously, the code says right side rich.

Other thing is that at idle or WOT, the DPFE reads .4 V which again is "vacuum". should it not be full "pressure" from the EGR closed and the exhaust pressure build up?

 

I'm not familiar with the 4.0 EGR systems. I know later ones used a DPFE (differential PFE) vs. a regular PFE. I played around once with the PFE sensor on my BII 2.9 (not DPFE). ~3.25 V was 0 pressure relative to ambient, 5 V was the maximum exhaust backpressure it could read. Do you know for sure if your '93 uses a DPFE vs. a PFE?

Unless you get into my business, pressures are almost always read relative to something. PFE's measure pressure relative to ambient pressure. (3.25 V means that exhaust pressure is the same as ambient pressure, 3.25-5V means exhaust pressure is higher than ambient, 0-3.25 V means exhaust pressure is less than ambient aka vacuum).

I expect a DPFE would have one input on the exhaust side of the EGR valve and one input on the intake side of the EGR valve. 3.25 V means exhaust pressure is the same as intake pressure. I would expect from your explanation that 3.25-5V means exhaust pressure is higher than intake pressure. 0-3.25 V would then mean that exhaust pressure is less than intake pressure.

Will your scanner work in a KOEO mode? Meaning, can you turn the key on and use a vacuum pump on the PFE/DPFE inputs to see if the sensor is responding properly? Is it possible the inputs to the DPFE have been crossed or are plugged? I can't think of any scenario that would really induce a vacuum on the exhaust side of the EGR valve, so it might indicate a bad sensor. Of course, it could also indicate that we don't really know how the sensor is supposed to work.

Like fuel pressure, MAF, etc. wouldn't the EGR system effect both sides equally?

I really wish I could help more. My '92 doesn't have an EGR system, so I can't go out and tinker with it to give you any other numbers to reference to.

 

Yes it is a differential pressure sensor. EGR tube runs from header to the EGR valve. Orafice in the middle and 2 rubber tubes attach on either side of the orafice and go to the DPFE. The lines did not fall off, did not get crossed (can't 2 different sizes) but might be clogged...I didn't check them.

My chart shows 3.25V also at zero pressure. Since I only saw a max of 1.7V, if this were a true reading, then the EGR valve must be full open and the EGR tube must be physically block allowing the intake manifold to draw a vacuum. That's like a 1/2" diameter tube so I can't see how it could get blocked.

Again, from a logic perspective, if the PCM sees a vacuum at the DPFE then that must mean to it that the EGR is open and then it would lean out the mixture. Why am I getting a rich code?

It sure feels lean. There is no black smoke out the back like the last time I had an O2 go out. Then again, mashing the accelerator down, it has all the HP it ever did.

 

Well the DPFE wasn't it. I need to look at some other things. I might try swaping in the stock MAF (has a granatelli one in it now) and maybe swapping out the PCM with my spare one. Maybe I'll disconnect the manual switch to the electirc fan as it appears that circuit of the fan needs a "special" switch per the fan's instructions.

 

OK. Backin 72 is going to kill me on this one. It was a bad PCM.

I'm just not sure what killed the PCM though. The new electric fan I installed last winter ties into the PCM via the AC clutch wiring as a "fan go on" input. And there is another manual switch for the fan that some how kept the engine running if it was switched on and I pulled the key out of the ignition. I still need to go back and figure out if the fan circuit somehow tweaked the PCM.

New DPFE, EGR valve and fuel pressure regulator...no change. I was OK buying the new parts since they all had 187,000 original miles on them - go OEM Ford!

My scanner interface with the PCM got worse. First it didn't read KOEO tests, then it didn't do a KOER test. But I could still read the DCL values real time. I could see the injector pulses hang at about 15 MS with increased throttle untill I hit the WOT response from the TPS and it jumped to 25 ms. The PCM was running it way lean for no reason. When I watched the O2 readout, it hung at .41 to .45 V, no real switching and barely moved.

With the new PCM, the O2 is switching very fast and from .7 to .2 volts...a good up and down response. And my KOEO/KOER tests now work off the scanner and the CEL flash codes match the scanner codes.

Life is good once the computer is happy.