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I have been working on my cousins Ford POS, it's a 1991 Mercury Topaz. It has been stalling randomly, about 90% of the time it has been stalling while the car is stopped, like at a light or stop sign. The other 10% is when it is coasting or coming to a stop. My cousin was getting off the freeway yesterday and it stalled. While she was trying to get it started back up (it starts right back up after stalling), she almost gets rearended by someone who wasn't expecting her to be stopped.

So, I pulled trouble codes, I got a code 32, which pertains to the EGR and/or PFE (which this car has) is malfunctioning in some manner. I went through the whole EGR system, checked for vacuum leaks everywhere, replaced the EGR control solenoid (tested out kinda iffy, so I replaced it), and just today replaced the oxygen sensor (which, for some reason, was completely dead). So, all of this doesn't fix it, I pull codes again after clearing the old one, and its the same damn code. But, sometimes, it either gives a code 32 or a code 33, which is the EGR is not responding to the ECM.

So, the only thing left to replace is the PFE sensor. When I tested the PFE sensor, it did the exact opposite of what it should have done, ie, when there is vacuum present the voltage went up, when there is pressure present the voltage went down. My source for testing the PFE sensor said just the opposite. Now, is my source taking drugs or has the sensor had one too many?

The role of the PFE sensor is to tell the computer that the EGR valve is doing what it is supposed to. If this sensor is three sheets to the wind, the computer thinks the EGR isn't doing anything and turns on the MIL.

Now this is the only thing left that hasen't been changed or cleaned thoroughly. I cleaned and even pressure tested the EGR valve to make sure it didn't leak exhaust gas, so it is functioning fine. I applied vacuum to the EGR valve while the engine was running and it stumbled, like it is supposed to, when too much EGR is present.

So, question is, is it the PFE sensor wrecking havic on this poor old Ford product, and am I correct in the way I tested it? My cousin doesn't have a lot of money to put into this thing (hence me working on it), but I have a feeling this will be the last and final sensor replacement for this darn car to get it running well. Any help you can give will be greatly appreciated by myself and my cousin.
 

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Have you checked the IAC and throttle plate for coking and clean them. It's rare not to pull up a EGR code on a eec IV car.Also check fuel pressure.
 

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All god troubleshooting so far. On older cars, you can't rule out an intermittant, or premanent break in the wiring. Usually under the hood. So trace back the PFE wiring.
 

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i used to work for Linconln Mercury, back in the 90's, the topaz's were infamous for dirty throtle bodies, causing stalling at lights & coasting. pull the throttle body off the engine, clean the butterfly valve & reinstall, then start the car, & get a can of carb cleaner, put the nozzle into the bypass valve port & spray it in, you will have to raise the idle to keep the car running, it will stumble a bit but keep going, keep spraying the cleaner into the bypass vale & keep theidle high, when the idle smooths out, you are done, the valve is clean.
as for the test on the PFE, there are 2 different valves, one opens with voltage, one closes w/voltage. you wil need the calibration codes of the engine to get the correct sensor.
good luck
 

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Stang32 has an excellent point. Dont forget to remove and clean (with carb cleaner) the idle speed bypass valve (which is on the throttle body).

If you want to know if the sensor reading is affecting your problem, just run with the sensor disconnected. This will put the EEC-IV into 'open loop' mode for that input which will supply a static, nominal value and allow the car to run. THis may help you in your diagnosis.

jer
 
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