Stearperson

I have a 1985 5.0 Mustang GT.
While driving along normally the car suddenly dies.
Turning the Ignition off and restarting the engine will fix the problem every time.
Changing gas pedal position will have no effect when car stalls.
I have driven with a timing light installed and the ignition continues to fire while car is stalling.
This is a first year injection car and does not have a MAF hot wire system.
Parts recently installed: Sparkplugs and wires, fuel press regulator, fuel pump and filter,throttle position sensor, fuel pump relay, EEC relay,new ignition switch and all new vacuum lines under the intake manifold.I have opened up the computer and all looks good with no burned components or obvious bad joints.
The car is also hard to get running normally until warmed up.
On initial start it runs at 1500 RPM for about 3 seconds then gradually loses RPM and dies. It will do this until warmed up then runs and idles well until the stalling happens.
The fuel pump can be heard running when key is turned on.
Any ideas from the cognoscenti?
Thanks, Stearperson

Stearperson

Sorry for the confusion this is a 1986 fuel injected 5.0 Stang.
Someone must have some ideas.

wbrockstar

Alot of times, its caused by a faulty tps,but you replaced it already. Everytime you turn the key off then back on,it allows the car to start because the tps resets to the minimum value read everytime.However, a new/reman part off the shelf is no guarantee these days,of whether the part is good or not.Just to be on the safe side,use the following link to check it.
http://sbftech.com/index.php/topic,29736.0.html

A faulty iac valve is normally to blame for stalling that occurs within seconds after startup.Its supposed to remain open fully until the engine warms up.You can test it.
Keep the following in mind if you wind up buying a new one. ** 1986-1988 Mustangs came with a diode wired inline with the iac harness.Therefore a diode wasn't built into the 86-88 iac's from the factory. All new/remanufactured iac's,as well as the original 89-93 iac's, came with a diode built into the iac valve itself.Because of this,if you buy a new iac and it doesn't solve the issue or creates more issues,the first thing youll need to do is cut the diode out of your iac harness and splices the wires back together. The existing diode will normally cause the new iac to act up because the diode's are fighting each other.I'll try to locate the info that shows how to remove it & where to splice.A picture of the diode is shown below.
To test your iac:
Disconnect the iac harness, touch a red multimeter lead to the iac red wire terminal,touch the black lead to the other terminal with the meter set on ohms.The meter should read 6-13 ohms.Leave the red lead on the red wire terminal, touch the black lead to the iac body,take a reading then move the red lead to the other terminal,take another reading.Both readings should be 10,000+ ohms or infinite. Test the iac harness red wire with the key on and make sure it reads 12volts.If any test above fails,replace the iac.If the tests pass & the iac is dirty,disassemble it and clean it thoroughly with carb cleaner. If the engine still stalls after cleaning and other components have been ruled out as the cause, replace the iac.

Another thing you should check with the meter is the ect sensor.Its signal/coolant temp is used by the ecu to control the iac valve. The ecu opens it fully at cold start and while the engine is warming up then begins closing it as coolant/engine temp rises.If the ect sensor is giving false readings,it will directly affect iac function and idle.To test the ect,disconnect its harness & touch the meter leads to the sensor terminals,with the meter set on ohms. Test it cold then warm the engine up & test it hot.The readings should match the values listed in the chart below. If not,the ect is bad.

If the tps,iac & ect all pass,the pip sensor or tfi module are likely culprits.The pip & tfi typically go bad due to heat soak and if either is at fault, the engine will normally stall after it has reached operating temp.If the tfi is at fault, a new one(especially a generic brand module) might continue going bad unless you do something to remove the heat soak issue,like buying a remote mount tfi conversion kit at Fox Restoration or another source. It relocates the module to a cooler location. You can also build your own kit if you're willing to spend time at the Pull-A-Part and a couple other places.

Check the EEC ground thoroughly too.Its the small gauge wire that runs from the neg battery post to the fender apron.It forks off here and dead ends into a black tubular plug which looks like a large inline fuse holder.This plugs into another plug coming off the main harness. Make sure this ground is tight/corrosion free at the fender and disconnect the plug to make sure its terminals are clean.It'll cause stalling if its loosening up while driving.

If you run a code scan,a code might be stored due to the stalling issue,which may lead you directly to the faulty component causing the issue.The check engine light doesn't have to be on in order for codes to be present.A scanner isnt needed.Just a short jumper wire,test light and the following link.The jumper wire will get connected between the single STI terminal and the Signal Return terminal at the self test connector.The test light goes between the pos battery post and the STO terminal at the self test connector. You cant use the cel because the 86-88 models didnt come equipped with one.Youll count the flashes of the test light to get the codes.
http://sbftech.com/index.php/topic,2471.0.html


ECT Chart
86-88 IAC Harness Diode

Stearperson

Thanks Brockstar, this gives me some more to go on.
I am leaning toward a bad PIP right now. Just wondering if the ignition will still show its firing with the timing light while car stalls due to fuel injection issues caused by the pip?
Will post if I get this solved.

Buck Sergeant

Stearperson, Don't know if this will solve your problem, but my nasty 308 had the same problem. This solved it.The fuel injected Mustang uses a small motor/valve assembly that allows a specific amount of air to enter the engine to control it's idle. This valve is commonly called the idle motor, the IAC (idle air control) or the IAB (Idle air bypass). When your car is new the IAC works remarkably well. The problem arises when the car gets some miles on it (usually 75K+) and carbon fouling takes its toll.
What happens is dirt, excess air filter oil, and most notably carbon gunks up the IAC valve and doesn't allow it to either open or close properly. This can cause an really high idle, a lumpy/surging idle or no idle at all. The solution is to either replace or clean the IAC. If this doesn't work, I am going to yell at my crew chief.

Stearperson

Replaced the PIP. Actually the whole distributor,PIP, cap and rotor and ignition module.
Car still dies while driving down the road at highway speeds.
Will test the IAC electrics now. Have already pulled it off and cleaned it a couple of times.
I was under the impression that the IAC does not have enough control to cause the engine to completely die at cruise speeds. Is this wrong?
Thanks, Stearperson

Buck Sergeant

Stearperson, i'm gonna murder my crew chief for coming up with all these bright ideas. Check out these words of wisdom. Sudden stalling under cruise conditions is usually electrical in nature. Air and fuel failures will generally cause the vehicle to surge and drop off in power before stalling; electrical or sensor failures will kill an engine outright, even if it's already at cruise speed. Two often-overlooked causes are bad engine ground connections and overheating ignition coils. A failing coil will get very hot under consistent use. The hotter it gets, the more it resists electricity, and then the hotter it gets. If your vehicle runs fine for a few minutes, and then starts acting up while you're going down the highway, check the ignition coil or coils. They shouldn't sizzle when you put a drop of water on them. Fuel pressure failures will also cause stalling under cruise, but it's usually more gradual -- like running out of gas. If you get sudden, random stalls under cruise, check the electrical system first.

wbrockstar

The ignition switch is the next item to look at then.

http://www.mustangandfords.com/how-to/engine/5657/

Stearperson

I did replace the ignition switch a couple months ago but am ready to look into the replacement malfunctioning. I was able to bend the tabs back on the original switch and it seems in good order. I may put it back in the car and see what happens.
I have ruled out the coil since I drove the car with a timing light and it continued to fire while the engine is stalling.
The ECT does show 100% resistance during a continuity test at 100 degrees temp so I have ordered a new one and that hopefully will help the dying while warming up problem.
I don't see how the ECT could cause the engine to stall at cruise speeds though.
Will get back to working on the car next week.
Any more ideas are welcome.
Really need to get this resolved.

wbrockstar

Does your 86 have cruise control?? If yes,it should have a vss (vehicle speed sensor) at the transmission end of the speedometer cable.If the vss is faulty, it can cause the engine to stall when slowing to a stop or when decelerating above 1500rpm with the throttle closed and the shifter in neutral,on T5 equipped cars.86-88 models with cruise & 88 California models are the only ​​​​ones that came with a vss,within this year range.
A vss can be added to 86-88 models w/o cruise by using a 89-93 speedo cable & vss then running the wires to ecm pin 3 & 6.

The iac valve is also supposed to open during deceleration,to prevent stalling due to the throttle blade slamming closed,so stalling can occur if its not opening during this time.

If theres an issue with the neutral gear switch on the transmission and/or a code 67 is present during a koeo test,stalling can also occur.

If you wanna test the ignition switch,backprobe its harness terminals at the following wire colors.Touch the black multimeter lead to ground and touch the red lead to the following colors,with the key turned to the position listed.
(Yellow) 12volts
key off & key on
(White/Pink) 12volts
key in start
(Red/Green) 12volts
key on & key in start

Yellow wire= power to the ignition switch from the starter solenoid.
White/Pink wire= power from the ignition switch to the starter trigger terminal at
the solenoid.
Red/Green wire= power to the coil,tfi,eec relay,alternator