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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
I have a 1986 Mustang GT, with a 302 HO S.E.F.I. (M) with a 5 speed tranny. It is all original, and has almost 400k miles on it, but I take such good care of it, you would never know it. The problem I'm having is that due to age, rain, heat, cold, etc.. it wants to run rough and stall when first started in the morning, my gas mileage is down and I believe it has something to do with the vacuum hoses between the different sensors, throttle body, etc, etc, etc. I've had to resort to turning up the idle speed screw in order to keep it running decent, but it constantly idles at about 1100 RPM, instead of the normal 750. I'm trying to figure out what hoses from my vacuum manifold goes to which sensor(s), etc, but I'm not having much luck figuring out what diagrams I can find, because almost every part named is in code, and it doesn't say if the line on the diagram is a electrical wire, a vacuum hose, etc. 90% of the problem I'm having are the mess of hoses located in the back of the engine near the firewall. Another issue I'm having is that what diagrams I can find are for the same engine, but for the years of 1988, 1989, etc, and that won't necessarily work for a 1986.
Is there anyone out there who has an original car like mine that still has the hoses intact that can send me some actual photos of their hose routings on their car, where I can look at it and say, OK, this vacuum hose goes from this sensor to the throttle body and/or has a split and also goes to the MAP sensor?
Any help would be appreciated!!
I've tried going to a Ford dealership, but all they want to do is either try to sell me a new car, or if I do get to speak to someone in service they end up scratching their head and say that they have no idea and they don't have diagrams from that far back.
Eventhough the vacuum hose diagram (shown at the bottom ) is labeled for a 88 model,its the same diagram used for the 86 & 87 model too,so this is the correct one you need to use.
A vacuum leak can indeed cause a rough idle,high idle,surging,excess fuel usage,a strong odor of raw fuel,etc,etc.Why?? When a vacuum leak is present,this unmetered air thats getting ingested by the engine gets metered in the exhaust by the o2 sensors & this reading triggers a lean condition.When the ECU sees this lean condition,it richens the fuel mixture in an attempt to bring the afr back to a stoich value,but since the engine didnt need the extra fuel to begin with (it just thought it needed fuel because of the lean o2 sensor reading) it causes too much fuel to be fed to the engine which then, subsequently,causes the rough idle & other vacuum leak symptoms.I would definitely replace all vacuum hoses or at least the ones that are leaking.The vacuum hoses at the charcoal canister & purge valve are notorious for dry rotting and leaking.Gaskets at the lower intake/egr valve/iac valve/throttle body and egr spacer & injector orings/tab/
tad/evr solenoids are other areas where leaks can be present.You can buy vacuum hose by the foot at the local part store.I believe Advance Auto even sells the really small nylon vacuum tubing like runs to your egr valve,evr solenoid,smog valves,etc.Most recommend against it,but if youve got a fire extinguisher handy and the engine is cold, you can use carb cleaner or propane to find leaks.Use a small handheld propane torch with a hose attached & run the hose along-
side all hoses,gaskets,etc,etc & if the idle surges when passing over a hose/gasket, youve found your leak.If you suspect a vacuum leak is present,its best if you find/ repair the leak before doing further trouble shooting.If you locate/repair the leak,but symptoms are still present,other leaks may still be present or numerous other parts could be the cause of your issues.A stuck open egr valve,dirty iac valve,defective tps, clogged pcv valve are just a few of the things that can also cause your symptoms.To check the egr,just remove the vacuum hose from it,plug the hose and see if symptoms disappear.If they do,trouble shoot the egr system.
** SPECIAL NOTE **
Is that a new/remanufactured iac valve I see mounted to your throttle body?? If yes,theres another thing that could be causing your idle issues.89+ Mustangs iac valves & all reman iac valves have a diode built into the iac valve.86-88 Mustangs iac valves do not have a diode built into them.A diode was spliced into the actual iac harness instead.If you replace the iac valve on a 86-88 model,with a new/reman iac valve that has a built in diode,you must remove the diode from your iac harness.If you dont,the diode inside the iac valve will compete with the diode thats on the iac harness and this can cause idle issues like you're going through.The harness diode is located a few inches from the plug that connects to the iac valve.To check your iac valve to see if it has a diode built in,just
look at the plastic cap area.The letter D will be embossed into the cap,like shown below.
Last edited by wbrockstar; 10-23-2022 at 10:48 AM.
If you haven't cleaned or replaced the iac valve recently,a dirty or defective iac valve is usually the #1 cause of an engine dying as soon as you start it,because the ecu opens the iac fully on a cold start and keeps it open fully until the engine reaches operating temp,so if the engine is dying the iac is suspect.I'll post a link below you can use to do a base idle reset procedure.Its imperative that you set the mechanical idle speed between 550-650rpm.If you set the idle speed above that value,its almost impossible for the ecu to control idle correctly because the ecu shoots for a target value of 672rpm and although the ecu can increase idle to/above the target value with zero issue,it has a very hard time decreasing idle to the target value if mechanical idle is set above the 650rpm value.It appears your Foxbody is stock,so theres absolutely no reason why it shouldn't idle correctly,other than a vacuum leak,faulty component or something out of adjustment.BTW- mechanical idle is the rpm value set using the throttle body set screw with the iac harness disconnected.Once the iac harness is reconnected,the ecu will take over idle control from the mechanical idle value set & up.You can also use the following info to test the iac valve to verify its working correctly.Disconnect the iac harness,set a multimeter to read ohms,touch one meter lead to one iac terminal and touch the other meter lead to the other iac terminal,take a reading.Now move/reverse the meter leads to the opposite terminal and take another reading.If youve got a 86-88 oem iac valve (without a diode built in) you should get a 7-13 ohms reading for both of your readings.If youve got a 89-93 oem or remanufactured iac valve (with a diode built in) you should get a 7-13 ohms reading one way and a very high/OL/infinite ohms reading with the meter leads reversed.Now touch one meter lead directly to the iac valve housing and touch the other meter lead to one iac terminal then move it to the other terminal.Both readings should be very high/OL/infinite ohms.If any of the tests above fail,the iac valve is bad.
Check voltage at the red iac harness wire with the key on too.It should read 10.5-12v.
Make sure your tps voltage readings are good too.The signal (dk green/lt green) wire should be .50-1.19 volts/closed throttle.Then open the throttle from closed to wot to closed/voltage should increase/decrease with no spikes or drop offs.WOT voltage should not go above 4.84 volts.The vref (orange/white) wire should be 5.0 volts.If the tps signal wire shows 5.0 volts at closed throttle,it will trigger the engine to die as soon as you start it.
One other thing to check is your ect sensor.The next time you attempt a startup,do this.Hold the gas pedal to the floor while you're cranking the engine.If the engine sounds as if its trying to start,release the gas pedal.If it starts and continues running using this method,this usually indicates the ect sensor is defective.
I have a 1986 Mustang GT, with a 302 HO S.E.F.I. (M) with a 5 speed tranny. It is all original, and has almost 400k miles on it, but I take such good care of it, you would never know it. The problem I'm having is that due to age, rain, heat, cold, etc.. it wants to run rough and stall when first started in the morning, my gas mileage is down and I believe it has something to do with the vacuum hoses between the different sensors, throttle body, etc, etc, etc. I've had to resort to turning up the idle speed screw in order to keep it running decent, but it constantly idles at about 1100 RPM, instead of the normal 750. I'm trying to figure out what hoses from my vacuum manifold goes to which sensor(s), etc, but I'm not having much luck figuring out what diagrams I can find, because almost every part named is in code, and it doesn't say if the line on the diagram is a electrical wire, a vacuum hose, etc. 90% of the problem I'm having are the mess of hoses located in the back of the engine near the firewall. Another issue I'm having is that what diagrams I can find are for the same engine, but for the years of 1988, 1989, etc, and that won't necessarily work for a 1986.
Is there anyone out there who has an original car like mine that still has the hoses intact that can send me some actual photos of their hose routings on their car, where I can look at it and say, OK, this vacuum hose goes from this sensor to the throttle body and/or has a split and also goes to the MAP sensor?
Any help would be appreciated!!
I've tried going to a Ford dealership, but all they want to do is either try to sell me a new car, or if I do get to speak to someone in service they end up scratching their head and say that they have no idea and they don't have diagrams from that far back.
Do you have a oem style neg battery cable or just a universal cable?? The oem cable came with a 12ga wire that ran alongside the 4ga cable & got screwed to the driver fender apron.Then the wire split at the apron,went a few inches then ended with a plug that you connected to a matching plug coming from the main ecu harness.Once connected, the two plugs resembled an inline fuse holder.If your cable doesn't have this wire,you have to run your own wire because this wire is imperative to an efi engine and its absence will cause many issues,including idle issues.
Its called the EEC ground & its the power ground for the computer.It runs to pin 40 & 60.This ground is tied into pin 46 also,inside the ecu,which is the signal return ground for most of your sensors.The picture below shows the oem cable
I hope you replaced yours too man.Thats one caked up pcv valve filter screen.I remove mine every few oil changes and clean it out thoroughly with gas,because any oil vapor that doesn't turn back into liquid oil and drain back into the lifter valley,will eventually keep building up until it cakes up like yours.
Last edited by wbrockstar; 02-16-2019 at 10:34 AM.
01-16-2019, 06:16 PM
vacuum hose routing
