RPM drop-off when coasting (30-40 mph)

1997 with 155k - Other than this issue the performance is GREAT; no starting, shifting or other drivability issues. -

I have an RPM drop-off (almost to stall) when coasting between 25-40 mph (comes right back up and is not "hunting"). SES light code comfirmed MAF sensor (high) - Checked all vacuum lines and intake for leaks. Checked ALL connectors.

Took the chance and replaced MAF for short $$ and 15 min; ok , maybe 20 min. Reset - 30 seconds in run - key off- remove key. (thanks guys)

Fine for a week and then same problem - The SES light comes on from time to time for an hour or a day with no change in performance aside from the RPM drop-off that is there with or without the SES light - just pesty and only to the point that I know something is not right.

Fairly new parts installed (within last 6 months):
Plugs and wires
air filter (re-charge)
PCV Valve
fuel pump and sendy (ouch)
FPR !!!
Fuel filter
radiator and thermostat

Any thoughts? - without having a dealer hook it up?

The situation you described sounds fairly normal when coasting in drive. Is it struggling to run at low RPM or is it still smooth? What happens in 3rd or 4th?

Never did this b4 - RPM drops to about 200 or less in 3rd or 4th but not every time while coasting at that speed; like the fuel is cut off for a second - maybe 50% of the time.

I was thinking maybe the fuel filter got some crap in it when i did the pump but WOT is not restricted.

I would replace the filter and run a 20oz Chevron Techron Concentrate through it and go from there.

My '86 Mustang would periodically and randomly drop to 1000 RPM on the hywy and at 30-40mph also. One time it went to zero! Put in carb cleaner. Shell brand gas. OK for awhile. Happened again a few times. Finally died on the way to work. Now, won't start. Any ideas? Ignition system? points, plugs, distributor, electrical system, carbuerator?

Where to begin?

tlotzer

I am having the same problem. I just had the MAF sensor changed and the car was fine for about 2 weeks. Then, one night when I started the car, it idled rough again. Now it stalls out if coasting or decellerating quickly.
These are the same symptoms that I had before. I just find it odd to get them again so quickly.
I also noticed that my lights and power accessories lag with pulses, almost as if I have an electrical problem causing my sensor malfunctions.

I fixed mine although I am not quite sure how. I expect that it may come back again.
I unplugged all the sensors on the throttle body, let the car run a bit untill normal temperature and then plugged all the sensors back in.
Car ran fine after that. Disconnected the battery to reset the SES light and it drove fine today.

Something else I noticed with mine, the headlights would come on as soon as I put the car in gear, even though it was bright out and I had the sentinels turned off and the manual switch on steering column off. It doesn't do it now that the car is acting "normal"

I had these same issues with my '89 Grand Prix I used to have. I fixed after a couple of fruitless trips to the mechanic by generally cleaning out the throttle body. Basically there was a ton of carbon built up around the intake area and it was gunking up the idle air control motor in particular.

I ended up getting a new IAC and using a toothbrush and a can of throttle body cleaner to get the worst of the carbon off the back of the butterfly valve and the other sensors and things in there. One word of caution though- make SURE you get throttle body cleaner for fuel injected cars and not the highly flammable carburetor cleaner. It, uh, kinda blows up your air filter box when you go to start the car after you're done (my buddy's car that had similar troubles no less... oops. His fault for having an old car of carb cleaner in his garage)