87 F150 Sharp rpm fluctuations and stalling

1987 F150 V8 302 automatic. 130k miles.

New iridium plugs (gap checked, all correct), wires (tested for resistance with fluke dmm), ignition coil good (tested with dmm), motorcraft distributor cap, rotor, newer injectors, new fuel pump relay, new computer relay, fuel pressure is where it should be (when cranked during crank/ no start condition), newer fuel pressure regulator, new motorcraft fuel filter, pip sensor in distributor changed a few years ago, fuel pump changed 2 years ago, battery good, compression test read between 165 and 180 on all 8 cylinders, newer motorcraft TPS, new motorcraft IAC valve, alternator tests good, ignition module relocated and module itself tested properly. All parts have less than 10k miles on them. It was driving alright with a slight miss, been trying to find it. Today it started like always, idled down to about 750rpm, fluctuated a bunch of times and stalled. It wouldn’t start back up. Today, it would idle smoother for a while, then out of nowhere it will fluctuate like and stall. It’s as if I’m quickly and randomly revving the engine. When it stalls, the key is still on and the fuel pump relay primes a few times.

Recently, I was trying to find possible causes for poor gas mileage (10 mpg) and researched the factory idle reset. It had been idling about 1000rpm and the book says 650rpm, so I got it to 650-700 (followed directions, unplugged IAC, set the screw, etc). Also set timing properly (removed spout as directed). I vacuum tested one of the ports on top of the intake manifold and it registered and held (with minimal movement) in the green/ normal range. Tested 3 injectors that were quickly and easily accessible and they were fine, but I haven’t checked the other 5 yet. Started, RPM shot up, stalled. Repeated 3 times just the same.

How is the EGR valve? I would guess it is leaking or frozen up or both.

How is tne thermostat? Warming up quickly?

The EGR valve was replaced about 2 years ago and the thermostat and housing were replaced within two months. It’s warming up just fine and within a reasonable time frame. Radiator was also replaced about 3 years ago along with the water pump

Try unplugging the MAP sensor to see if the idle stabilizes.


I will give that a shot when I get out of work. I didn’t think of that

Good ideas above. I have an older Ford truck, but no experience w/Ford’s fuel injection or electronic ignition. Doesn’t stop me from taking a guess though :slight_smile: As I read your description my thoughts about the cause were

  • a vacuum leak is allowing unmetered air into the engine somewhere
  • the fuel pressure isn’t holding at the correct psi & steady
  • evap system fault
  • exhaust is semi-clogged
  • IAC is not working correctly

Does your 87 sport diagnostic codes? If so, that’s the first place to check. The EGR comment above is a good idea too. It may be fairly easy to do a temporary mod so the EGR valve won’t actuate to prove/disprove. You may be able to manually actuate the EGR valve with a hand-held vacuum pump to see if it stalls an idling engine, which it should if it is working correctly. Google how to find vacuum leaks, easy enough to do provided you have a hand-held vacuum pump, but can be somewhat time consuming. Check the fuel pressure regulator’s vacuum hose, any gasoline in there? If so, the FPR is bad. Make sure all the evap system hoses are in good condition, not split, not plugged. When the problem is occurring, try to figure out a way to temporarily bypass the cat, see if it has any effect. Sometimes its as simple as just removing the pre-cat o2 sensor. Might be worthwhile to take a chance on a dose of fuel injector cleaner in the gas tank.

Most likely, either a vacuum leak or faulty IAC. Both can produce similar symptoms. When my Corolla had similar problems, turned out it was a faulty IAC.

Was the test on the TFI module done on a cold module? Those things can test positive and fail when they heat up good.

Distributor shaft snug in the bushings? Just wondering if the shaft is bouncing around sideways.

Does this model use the EGR spacer plate between the throttle body and intake manifold? If so, I have seen those plate gaskets disentegrate and cause problems.

I unplugged the map sensor and I didn’t make a difference. Started, ran for about a minute, stalled.

Did hook up a spark tester to the wire from the ignition coil (adjustable tester and bulb style tester). When cranked, there was only one initial spark and nothing more