Engine surging

Factual Questions
1

I have a 1986 Lincoln Mark VII LSC, 5.0 High Output and when I start the car, after about 10-20 minutes of idling in Park, the engine will surge, now the car already idles higher than spec, 1000mos where it idles, and it will drop to about 700, then back up to 1000, then back down, then up, and repeat the process and eventually the car will stall out and die. Now I think it could be my EGR Valve or MAF sensor, but I am not sure, I checked all my vacuum lines, all of em are good. Anyone have an idea od what’s up with the car? FYI, it was sitting for 4 years, has a fresh tune up

2

Check the PCV hose(s) carefully. They tend to rot and develop splits that cause vacuum leaks, with symptoms often as described.

3

You could unplug the MAF and see if it smooths out. I think that would indicate a faulty one.

4

This may be of help:

http://www.stangnet.com/mustang-forums/threads/surging-idle-checklist.626583/

5

Same engine prob as on a previous Mustang 5.0.
You have an air bypass valve that most likely needs cleaning. It’s a little cylindrical thingee on the side of the intake manifold (throttle body) that let’s air bypass for idle. Sounds like it may be sticking or dirty.

6

I’m actually pretty sure this car doesn’t have one. I think Ford went over to a mass flow EFI system sometime in the later 80’s/early 90’s so this car would be speed-density system with a MAP and IAT sensor.

The older speed-density systems can cause weird surging issues if one of the sensors is just marginal, but I would definitely check and recheck for a vacuum leak or issue with the EGR valve before really tearing into that.

Also, OP, you say the car sat for 4 years? How old is the gas in the car? When resurrecting a vehicle that’s been sitting for a while I’d definitely run a full tank or two of fresh gas through before I start worrying about minor running issues like this.

7

Thanks GreasyJack, I was looking through my repair manual for the last 2 days and saw that the EGR is probably stuck, cause the diaphragm is against the housing on one end but isn’t on the other, I’m not used to fuel injected engines, I have worked on carbureted engine my whole life hahaha again, thank you.