Car Dies When Coasting in Neutral

ford
thunderbird

#1

This problem has plagued my '85 Tbird Turbo Coupe for about a month, and I’m running out of ideas to fix it…



When I put my car in neutral while moving, the RPMs fall significantly below idle and then the engine usually catches itself and after about 5-10 seconds idles normally.



If I do this at high speed (say 40 mph +), the RPMs fall to the point where the engine stalls and dies. If I do it at 25-40 mph, it will go down about 200-500 RPMs and then catch itself. If I do it at 15-20 mph, it will be just barely noticeable, and anything slower than that I can’t notice it. It will also do this to some extent when I’m stopped if I rev the engine up really high and let it off, but it is far less noticeable.



This car has had extensive work done to it in the past 6 months. Most recently, I had the fuel injectors replaced, throttle body cleaned, IAC valve cleaned, and the vehicle searched for vacuum leaks. (most of that in search of the solution to this problem as well as to improve the general performance of the vehicle.



A week before that, I also had a new fuel tank, fuel pump, and fuel filter installed.



I’ve got a very small leak on the back of my muffler, no cat, and no other exhaust leaks I can find.

I am getting a lot of black carbon out of the exhaust, but I have been running some fuel system cleaner through it. Also, the car sat for like 8 years before I got it, and the original cat became clogged and really gunked stuff up. I had all of the upper engine gaskets replaced, as much as we could get to cleaned internally, and the cat cut off at that point.



Timing is very close to 10btdc. Could be 9.5, but just about right there.

I double checked, no vacuum leaks that I could find with a can of carb cleaner.



I tried checking for codes, but it’s not even throwing me a confirmation code or anything, which is odd. But, my connector was pretty gummed up (I did clean it as good as I could) and just about anything wiring is suspect. I used two engine code readers that had previously been used on the car.



Other than this, the car has plenty of power and runs fine down the road.



Thanks for the help!













Here’s the master list of what I’ve replaced in the past 6mo:

FUEL SYSTEM

Fuel Pump (twice)

Fuel Pressure Regulator

Fuel Tank

Fuel Injectors



ASSORTED FILTERS

Fuel Filter (twice)

Air Filter

Oil Filter (several times)

Oil Filler Cap

PCV Valve



HOSES/COOLANT SYSTEM

Nearly every Vacuum Hose

Every Coolant Hose

Heater Core (twice)



SENSORS AND COMPUTER STUFF

Engine Control Module (ECM)

Vane Air Meter (VAM)

Idle Air Control Valve (IAC)

Oxygen Sensor



STARTING SYSTEM/ELECTRIC/Misc.

Spark Plugs and Wires (Motorcraft)

Starter Solenoid

Coil Pack

Distributor

TFI Module

Distributor Cap and Rotor

Battery

Timing Belt

EEC Relay

Ignition Switch

Tires

Front Brakes

Brake Master Cylinder (had previously been damaged)

Front tie-bars

Every gasket from the head up


#2

I’m not sure you have any problem. Why put the car in neutral when you are moving? The car isn’t engineered to run along at 50 mph in neutral. Perhaps you are simply confusing the ECM and it is sending a signal to cut off the fuel which is what is supposed to happen when you let off the gas with the car in gear.


#3

I’ve never had this issue on another car, and have always driven sticks.

Why go into neutral? I agree, at high speeds it would be very rare to. I simply tested it over a range of speeds. But, it is very common to drop the stick into neutral when pulling up to a stoplight or traffic jam, for example. You can downshift to some extend, but this also depends on how quickly you need to slow down and how much you want to work the clutch.

And, on most cars the RPMs will drop to near where your idle should be, not dramatically below it.


#4

My first bet would be on a sluggish idle air control valve. You can start by just giving it a good cleaning & checking its wiring.


#5

Cigroller stole my line…first place to look is right there…


#6

As you said it wasn’t noticeable below 20 mph and the car is so old we’ll never know what normal for that car was back when it was new.

The mid '80’s motors had a lot of vacuum lines to make them run smoothly with the pollution controls. If there are a number of vacuum lines on this motor you may have one connected improperly, a collapsed line, or a leaking line with a cracked “T” connector. Just something to check over.


#7

Why not just leave it in drive? About 99% do. Would it still act up?


#8

Those engines(2300 turbo pinto engine) torque alot and can break or tear motor mounts. I’ve seen this crack the air inlet tube from the vane / mass air sensor. Of course, the tube could be cracked from age. Anyhow, I’ve seen it before on the TC’s and Escort SEFI engines. If any air gets into that engine without passing by the mass air sensor, including a missing dipstick, the engine will run rough and/or stall. A leaking power brake booster can also cause this.


#9

Thanks for the replies guys. The issue has gotten worse, so that even sitting at a stoplight the engine wants to die until I go again. It doesn’t matter if the clutch is in or not, so leaving it in gear isn’t an option. (Obviously, clutch out the engine dies in gear, also dies in gear stopped with clutch in).


#10

Yes, it does.

In fact, at stoplights if I don’t rev the engine the vehicle will often stall out.


#11

Good advice. I inspected the mounts and hoses, and they all look good. Also, no difference when I spray any carb cleaner near them, so I’m counting on them being well sealed.

That said…

  1. Cleaned the IAC (Idle Air Control) valve, which was relatively dirty inside.
  2. Checked voltage on the TPS (Throttle Position Sensor), was at .96, right about where it should be.
  3. Checked voltage on the TPS while moving throttle up and down, seemed very even, with no skips.
  4. Sprayed throttle body cleaner around different vacuum hoses and gaskets, couldn’t find any leaks. My shop couldn’t find any vacuum leaks the other week either.
  5. Timing is just a tiny fraction under 10tdc, like 9.8 degrees, with the SPOUT (Timing Advance) cable unplugged.
  6. Still getting lots of carbon out the exhaust, but on a 25 year old car I expect that.
  7. Tried pulling codes, but the car won’t give me any, even a confirmation code, like normal. The connector is slightly suspect there, although I did try cleaning it. A couple possibilities… either the diagnostic wiring is bad, or something is pulling down the computer, possibly making it run in some sort of closed loop mode. My next step is to do some pinpoint testing and check the VREF at various sensors. (VAM (Vane Airflow Meter) is a possibility, since the problem started not too long after replacing that, but so much has been replaced on this car that it’s not the only option). In the past I have been able to pull codes and have used two different code scanners.

#12

Sounds like the idle control. There are other ways for it to go bad besides needing to be cleaned. This is a no-brainer.