1986 5.0 EFI with 2 separate idle issues

[MBC1980]

Hey guys, my dad has a 1986 F150 5.0 4x4 that he ordered in Sept of 1985. It was the first EFI 4x4 off the line in Norfolk Va. Since it was new it has always had an issue with cutting off, especially on cold start. When it idles high, it will drop down and sometime stall or it will catch and stay running. Put in gear and it will stall typically. It is notorious for cutting off and not staying running and it has had that issue since it was new. No one could ever figure it out.

A while back it developed a skip in the idle. When taking off and climbing hills it runs good but in overdrive or sitting idle it has a skip and that affects how the transmission downshifts and it shutters when slowing down. When accelerating even mildly its smoother and the skip goes away.

So far here's some things that have been done recently.

Intake and valve cover gaskets

Idle air control valve

Thermostat

PCV valve

Throttle position sensor

MAP sensor

Fuel pressure regulator

Swapped out a computer from junk yard

Harmonic balancer

New belts and hoses

Spark plugs and wires

02 sensor

Fuel relay

Oil sending unit

Water pump

Fuel filter is relatively new

Solenoid

Starter

Motor was rebuilt 20 years ago and has low miles on it since rebuilt.

No vacuum leaks

Fuel pressure is good

Not sure about compression

The temp gauge stays very low and oil pressures reads very high.

It has been garage kept and never been a daily driver. We've had a specialist from a Ford dealership look at it and cant figure it out. Was this a common issue with this new EFI system at the time?

Thanks in advance for any help that could be provided.

[old55pete]

There was a recall on the IAC valve that pretty much the same symptoms as what you are talking about. To find out for sure if that recall has been done, there should be an emishion recall sticker on the header panel in front of the radiator.

Another thing I found out was that the grounds needed to be cleaned up. There is a ground connection in the stock ground cable between the batterie and the engine block. It attaches to the frame by the right engine mount. If it is not hooked up, it will cause something similar. There is also a ground cable that goes from a stud on the firewall by the wiper motor to a stud on the lower, left side, by the inside rear corner of the valve cover. The stud is one of the lower intake manifold mounting bolts. If ether end is dirty, it will cause similar problems.

In your shotgun list of parts replacement, I dident see that the TFI module had been changed. Has the TFI module been changed in the last few years? They have a bad habit of failing with similar symptoms also. The strange thing about the TFI is that it will over heat and fail and once it cools off it will work again

[myrl883]

I'm following this one... My '86 does the same thing on cold starts. It idles very high for a few seconds, then comes down to a very slow, lopey idle for a few seconds, then settles to a perfect idle speed. Usually it will catch itself and come back up to the normal idle speed, but occasionally it will die.

When put in gear cold, it will overcompensate the idle speed for a second, then settle to a perfect idle speed.

There is nothing that I can find "wrong" with it.

[MBC1980]

I'm following this one... My '86 does the same thing on cold starts. It idles very high for a few seconds, then comes down to a very slow, lopey idle for a few seconds, then settles to a perfect idle speed. Usually it will catch itself and come back up to the normal idle speed, but occasionally it will die.

When put in gear cold, it will overcompensate the idle speed for a second, then settle to a perfect idle speed.

There is nothing that I can find "wrong" with it.

well hopefully ours is related and someone can help both of us figure out this mystery.

[old55pete]

I'm following this one... My '86 does the same thing on cold starts. It idles very high for a few seconds, then comes down to a very slow, lopey idle for a few seconds, then settles to a perfect idle speed. Usually it will catch itself and come back up to the normal idle speed, but occasionally it will die.

When put in gear cold, it will overcompensate the idle speed for a second, then settle to a perfect idle speed.

There is nothing that I can find "wrong" with it.

MYRL883 yours sounds more like the ECM ground. It is grounded to a stud on the firewall behind the radio. Not hard to find once the radio is out there are several wires hooked to it. As a matter of fact just about everything in the pickup is grounded there in one way or another.

[MBC1980]

There was a recall on the IAC valve that pretty much the same symptoms as what you are talking about. To find out for sure if that recall has been done, there should be an emishion recall sticker on the header panel in front of the radiator.

Another thing I found out was that the grounds needed to be cleaned up. There is a ground connection in the stock ground cable between the batterie and the engine block. It attaches to the frame by the right engine mount. If it is not hooked up, it will cause something similar. There is also a ground cable that goes from a stud on the firewall by the wiper motor to a stud on the lower, left side, by the inside rear corner of the valve cover. The stud is one of the lower intake manifold mounting bolts. If ether end is dirty, it will cause similar problems.

In your shotgun list of parts replacement, I dident see that the TFI module had been changed. Has the TFI module been changed in the last few years? They have a bad habit of failing with similar symptoms also. The strange thing about the TFI is that it will over heat and fail and once it cools off it will work again

I'm not having a problem with it not running good once it heats up. I havent changed the TFI module out but will try it. If it needed the recall will it still fix it by replacing with a new TFI?I replaced the IAC but no change. The one that was on it was the same as the 90s model iac and not what came on it but it was really old looking. Must have been changed years ago at some point.

[old55pete]

I'm not having a problem with it not running good once it heats up. I havent changed the TFI module out but will try it. If it needed the recall will it still fix it by replacing with a new TFI?I replaced the IAC but no change. The one that was on it was the same as the 90s model iac and not what came on it but it was really old looking. Must have been changed years ago at some point.

The TFI and the IAC were both recalled at one point. I had to have the IAC recall done while I was stationed in Germany at a German Ford dealer.

I have sense had to change my IAC for a different reason. The IAC motor/valve is what controls the engine idle along with the throttle position sensor( TPS ). If the TPS voltage it to low( needs to read 1 volt, key on, engine off ) with the throttle in the idle position, then it may be the problem.

With what has already been done ( the long shotgun parts list ), it is hard to step back and say it is this one thing, change this part and it will fix it. It could be something as stupid as a bad ground.

Just as a heads up, I have taken new parts outta the box and they were just as bad or worse then the old ones.

[MBC1980]

The TFI and the IAC were both recalled at one point. I had to have the IAC recall done while I was stationed in Germany at a German Ford dealer.

I have sense had to change my IAC for a different reason. The IAC motor/valve is what controls the engine idle along with the throttle position sensor( TPS ). If the TPS voltage it to low( needs to read 1 volt, key on, engine off ) with the throttle in the idle position, then it may be the problem.

With what has already been done ( the long shotgun parts list ), it is hard to step back and say it is this one thing, change this part and it will fix it. It could be something as stupid as a bad ground.

Just as a heads up, I have taken new parts outta the box and they were just as bad or worse then the old ones.

I'll look at the ground and check the voltage and see what I come up with. Thanks for the info.