Dear Car Talk:
My son, who is going to college, purchased a 1997 Nissan Altima for $5. The car will stall, and then you can restart it and it's fine -- it will start right back up. But this can be a problem, because people waiting behind him tend to be impatient. It does this especially in cooler weather. We had the distributor replaced. Someone advised us that it was the EGR valve, but we replaced that, and it seemed to make things worse. So we put the old one back in, and it now runs better, but it still will stall. Now when it stalls, if you put the car in park and floor the gas, it seems to make the engine run better after that. Do you have any suggestions as to what it might be?
Sounds like your son overpaid, Sarah. I think you may have been on the right track with the EGR valve. Here's a possible scenario: Let's say your existing EGR is gummed up and doesn't open all the way. The EGR shouldn't be doing anything when the car is at idle. If the EGR opens up when the car is at idle, it'll make the car stall.
So let's say your EGR valve IS getting an errant vacuum signal. It's opening up as much as it can, causing the car to stall. And when you put in a new, working EGR valve, it opens up all the way at idle (i.e., it actually works), and makes things even worse.
So one possibility is that there's vacuum going to the EGR when there shouldn't be. That could be the result of something like a faulty EGR solenoid, or vacuum hoses that are improperly routed. So you can experiment by temporarily taking the EGR valve out of the equation. Disconnect the vacuum line that goes to it, and try driving the car without it. If the car doesn't stall, that'll be a huge hint.
Then you'll have to track down the cause and fix it. You don't want to leave the EGR disconnected, because the EPA fines for that cost slightly more than a four-year college education.