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My husband replaced the 4.6L V8 engine and transmission in my '95 Cougar with another 4.6L V8 engine (with approx 46,000 miles) and transmission out of wrecked '95 Cougar. I know he replaced wires, plugs, and power pack. After all was back together he drove it and the check engine light came on. Had it checked to see what the code was.....P0402. After researching that code, we found several things regarding the EGR could be faulty. He first replaced the DPFE sensor, unhooked the battery to reset computer. I drove car to store (no light came on), after starting car to go home the light came back on. Had code checked again, still P0402. He then replaced the EGR valve (what a job that was for him), unhooked battery to reset computer. He drove car to town (no check engine light on) and when he started car to come home, light came on again. He stopped at a different auto parts store and had code checked and was told it was the PCV valve (didn't tell him the code). He then replaced the PCV valve. He also decided to change the computer thinking it might be the problem (the computer that was in the car was the computer with the original engine and he kept the computer that came with the engine from the wrecked Cougar). He started the car this morning to take it from his shop to the driveway (no check engine light came on). When I got in it and started it the check engine light came on. I will be stopping again on my way home to have the code checked. In the meantime, I am hoping someone has some suggestions of what the problem might be and what needs to be checked or replaced. Thank you for any help or suggestions!
 

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If he used the intake from the new engine and not the old "trouble-free" one, I would be suspicious that the EGR passages are clogged. I had the same problem on my 94 years ago and I saw a post by RichardM I believe and it showed the trouble spots on the intake where the clogging occurs. It is cake to clean this passage compared to changing that EGR valve. You basically remove the plenum/elbow whatever you want to call it that the throttle body attaches to. After you pick up the gasket, you will see the clogged passages. Good luck and let us know what happens. Perhaps someone else has some good info to offer.
 

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if there is excessive flow through the EGR valve and you are POSITIVE that the EGR valve itself is functioning, then the excessive flow can be traced back to two things: the DPFE sensor and the EGR vacuum regulator solenoid. The computer uses the solenoid to command the EGR valve to open (by using engine vacuum), and then it "looks" at the DPFE sensor to make sure that the correct EGR-% is being introduced into the engine.

I have a new-style DPFE sensor that was used for about 1.5 years (so its practically brand new), and a working EGR vacuum regulator solenoid for sale, if you'd like to rule those two things out

also, since we don't know the entire history of your car, it would be a good idea for you to trace each and every single vacuum line and replace them all. If they don't give you headaches now, they will later, guaranteed.
 

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If he used the intake from the new engine and not the old "trouble-free" one, I would be suspicious that the EGR passages are clogged. I had the same problem on my 94 years ago and I saw a post by RichardM I believe and it showed the trouble spots on the intake where the clogging occurs. It is cake to clean this passage compared to changing that EGR valve. You basically remove the plenum/elbow whatever you want to call it that the throttle body attaches to. After you pick up the gasket, you will see the clogged passages. Good luck and let us know what happens. Perhaps someone else has some good info to offer.
P0402: Excessive EGR flow

if the passages are clogged, the pcm would be "seeing" insufficient flow, via the DPFE sensor.....I don't think clogged EGR passages would result in a P0402, but I guess it could be possible
 

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Update: I didn't make it to get the code checked.....the engine died while stopped at a light but I was able to get it started again. Drove it about 1/2 mile and it started cutting out, made a clunking noise a couple of times, and I knew it was time to get off the road. My husband ended up coming to rescue me. It ended up that swapping the computer out yesterday wasn't the answer. He put the original computer back in (the one that came with my car when I purchased the car brand new). Evidentally, the computer out of the wrecked '95 Cougar wasn't any good. My husband disconnected and reconnected the battery cable to reset the computer. Good news......so far.....the check engine light has not come back on, even after making one stop on the way home. It drove great, had the power again. We will see how it does tomorrow, I plan on braving it and driving it to work again. After 15 years, still love my car, looks great out on the road and still getting compliments on it. I'm glad I found this forum, there is so much helpful information. Thank you Rob and guitar maestro for your responses.
 

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lol yeah clogged passages will not cause excessive flow. I was under the impression that p0402 was insufficient egr flow, but damn I guess I was wrong.
 
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