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jazzmaster, ASE Certified Tech
Category: Ford
Satisfied Customers: 2951
Experience:  ase certified--32 yrs experience
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Ford F150 Pickup 4x2: have a 1986 Ford F150,

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 I have a 1986 Ford F150, 302, automatic. I am trying to solve what sounds like a common problem with these trucks. My truck runs fine when it is cold but when it warms up , to normal running temp, sometimes it will idle rough and cut out. It runs great at any speed but if I stop sometimes it idles rough and cuts outs out. I have replaced the plugs,EGR valve, and throttle position sensor. I would greatly appreciate any input you could give me.

hi, my name is XXXXX XXXXX I will help you ...

there seems to be a common issue with the upper intake gasket on these -the gasket is thin and after time becomes brittle .

the effect of this is usually a rough idle or high idle ..

spray some carb cleaner at where the upper intake meets the lower intake manifold ,while its running at idle ,see if that changes the idle at all ,and let me know ...thank you ..jazzmaster ..

Customer: replied 3 years ago.

Hi Jazzmaster,


I recently put valve cover gaskets on and replaced that gasket when I did the valve cover gaskets.


that was before this problem ,so I would still check around the upper intake anyways ,it is hard to see in the back of the motor when putting the upper intake on and you may have torn/ripped the gasket or it moved a little .now you are having issues and being that gasket is so thin ,it doesnt take much for the manifold air to suck it in somewhat ,,please let me know ,thank you ...jazzmaster ..
Customer: replied 3 years ago.

Hi Jazzmaster,


I tried spraying carb cleaner around the upper intake like yo suggested but got no change. The idle is getting rougher. I used to be able to put it in neutral and it would smooth out but now it idles rough in neutral too.

Thank you,

is it running rough like a plug miss or just rough ?
Customer: replied 3 years ago.

I put new plugs in it a while back but it is just rough and will slow down and then surge like it is trying to keep running.

check the vacuum reading from one of the intake ports and see what the vacuum reads , also, make sure that there is no vacuum going to the egr at idle ,

let me know what the reading is

Customer: replied 3 years ago.

I will need to get a gauge. By intake ports are you referring to unhooking a vacuum line and checking the pressure?

yes, I am -it should read around 18-21 inches of vacuum ..
Customer: replied 3 years ago.

I unplugged a vacuum line from the intake port on the upper intake manifold. I put my tester right to the nipple on the manifold and got 19 in. of vacuum. I also tested the EGR valve vacuum and there was no vacuum.

19 is good was the needle on the gauge reading steady or was it moving around ?

does this rough idle feel like a bad plug ?

Customer: replied 3 years ago.

The gauge was steady. Not moving at all once it got to 19. The idle is rough and fluctuates. It will slow down and run rough like it is going to cut out and then it will speed up like the computer is trying to compensate.

have you checked the fuel filter ,I know it sounds crazy but fuel pressure can do the same thing if the filter is clogged ,if the filter is good then I would still think some sort of vacuum leak even though the gauge read steady .perhaps a map sensor vacuum issue ..

Customer: replied 3 years ago.

I have also replaced the fuel filter. I have thought the same thing about the vacuum leak. I think I will try replacing the fuel pressure regulator. I too the vacuum line off and it smells like it is sucking gas.

anytime that you can smell fuel or see wetness at the fuel regulator the regulator can be letting fuel past the diaphram
Customer: replied 3 years ago.

That would not only let extra fuel into the system but be considered a vacuum leak, correct?

not really a vacuum leak but extra fuel being leak into the intake system .

there can be so many problems for a rough idle /perhaps check the plugs to see if any are burning funny .

Customer: replied 3 years ago.

I will do that. I had to take the fuel rail off to put the new regulator on and found a broken injector. I got it all put back together and now it has a stutter when I accelerate. Kind of like driving un rumble strips in the road. Do you think that will go away after driving it a while? Maybe some dirt got into the injector port ?


I want to thank you for all your help. I am hoping that the regulator and injector will have solved my problem.

make sure that all the injectors are firing right with a noid light (this is a testing light for the injector circuits) perhaps there was a little dirt that got in there ...jazzmaster ...
jazzmaster and 4 other Ford Specialists are ready to help you
Customer: replied 3 years ago.

The next morning it ran fine. Nice and smooth with great throttle response. I drove it for two days and now it is idling rough again and has a hesitation when I press on the gas. It no longer feels or acts like it is going to cut out. This is really getting frustrating.

can you check the injector that you replaced ? see if its leaking o-ring?
Customer: replied 3 years ago.

No leaks. Is there a way to check the MAP sensor? Could that be a suspect?

you can use a hand held vacuum tester and see if the map sensor will hold with vacuum applied ,it is very possible that the map sensor can be bad ...let me know ...thank you ...jazz..
Customer: replied 3 years ago.

Hi Jazz,


My MAP sensor is holding vacuum but it is fried. I checked the ground and got 12 volts, on the green wire that communicates to the computer I got no reading at all and on the hot wire I got 4.88 volts. Everything I have read points to it being fried.

yeah that wire that comuicates to the pcm/ecm should change and show voltage ,depending on how much vacuum is being applied ..
Customer: replied 3 years ago.

The wire that communicates with the pcm/ecm did not show any voltage at all. I tried applying vacuum but it did not change.

in that case ,I would try a different map sensor
Customer: replied 3 years ago.

I put a new one on yesterday. I have not driven much, just a couple short trips, but the test will be later today.

ok, let me know , also, did you get different volt readings with the new one ?
Customer: replied 3 years ago.

I did not check the new one it never occurred to me. It did not solve my problem. It is back to cutting out on me. I can definitely tell a difference with the new MAP sensor installed but it did not solve my problem.

can you measure the vacuum reading of the hose at the map sensor with a vacuum gauge ..
Customer: replied 3 years ago.

I can not do it today but I will check it tomorrow and let you know what it is.

ok, thank you ..jazz..
Customer: replied 3 years ago.

Hi Jazz,


I checked the vacuum reading and got a bouncing 10 to 13 reading with the engine idling high when I first started it. I tried to get another reading when the motor slowed down to an idle but as soon as I unplugged the vacuum line from the MAP sensor the motor shut off. I did not have time to connect the vacuum guage. I also did some tests on the MAP sensor. With the key in the run position but the motor not running I got 5v. on the orange wire, 12 v. on the black wire and 2.5 v on the green. The green is supposed to have 5v correct? I also hooked my vacuum pump to the sensor while I had the test lead on the green and it did not move with the application of vacuum it just sat at 2.5v.

try this either t the vacuum gauge to that hose or even apply 20 inches of vacuum to the map sensor and see if it changes anything ,you should have 17-21 inches of vacuum at the map sensor ,if it isnt and is bouncing at a lower vacuum reading then I still think you may have a vacuum leak ..jazz..
Customer: replied 3 years ago.

I applied 20 inches of vacuum and it did not change. I will try t ing the vacuum guage and see what I have. Is it possible to have a vacuum leak around the injectors. If I were to spray carb cleaner around the seals of the injectors would it cause a noticeable change in the idle like it does around the vacuum lines? I would do this before the motor heats up.The vacuum leak could be any place there is a vacuum line correct? Would it be to my advantage to replace the vacuum lines and check them as I go? There is not that many and it would eliminate them as a potential problem. That is my thoughts anyway.

yes, any vacuum leak will show pretty much the same results when checking with carb cleaner ,it will either run faster/smooter or stumble /stall .

it is possible to have a small leaking injector o-ring but I suspect that in that case you can also smell gas ..

Customer: replied 3 years ago.

Hi Jazz,


I finally was able to check the vacuum at the MAP sensor. I have a steady 18 1/2 inches of vacuum.

ok, let me get this straight ,did you have this issue before you did the valve cover gaskets ?
Customer: replied 3 years ago.

It was running rough before that. I had replaced the throttle position sensor, IAC , plugs, rotor, and distributor thinking it just needed a tune up. I replaced the valve cover gaskets because it was leaking oil onto the plugs and I thought since the other things did not fix it then maybe the oil was the problem. The rough idle was not as bad as it is now. I am now experiencing a rough idle and sometimes it will lunge.When I experience the rough idle and am sitting still when I give it gas to move it acts like it does not want to go and it has a hesitation.

and did you clean out the throttlebody with some throttlebody cleaner or carb cleaner ?
Customer: replied 3 years ago.

Yes. I also cleaned out the upper intake.

Customer: replied 3 years ago.

Hi Jazz,


I think I just stumbled on something. A friend of mine suggested I check my fuel pressure when the motor is hot which is when the problem occurs. I checked it and I only have 30 psi. When I give it gas ( as in revving the motor or as if I was trying to move ) the gauge went to 31 and when I kept it at a higher idle the gauge started dropping slightly. When I let off the gas all together it dropped to 28 and then went back to 30. Then the thing that surprised me ( it might not surprise you, LOL ) I left the gauge on and the pressure actually rose to 39. That makes sense why when I would restart the motor, even if I shut it off and it was running rough, it would run smoothe again for a while. Does this train of thought make sense? What is my next step fuel pumps?

well that may explain the hesitation and lunging/surging.

you do have a low pressure pump in the tank and there should also be a high pressure one on the frame rail -drivers side near the drivers door .

Customer: replied 3 years ago.

Hi Jazz,


I didn't mean to just disappear on you. I had to order the fuel pumps and I plan on installing them this coming weekend. I will keep you posted on the outcome. Thanks

ok, thank you ..
Customer: replied 3 years ago.

Hi Jazz,


I forgot to mention to you that while I was checking my MAP sensor and vacuum going to it I pulled the vacuum line off the air diverter valve from the smog pump and it had no effect on the idle. Does this mean anything ? Would I be able to take the smog pump and diverter valve off? It is an 86 with antique tags so I do not have to have inspection. What are your thoughts?



you can remove that emissions if is not working and the vehicle will still run the same .sometimes they run better (lol)
Customer: replied 3 years ago.

With what I mentioned in the email is it the case that it is not working?

well does that vacuum to the divertor valve come from direct vacuum (it shouldnt)
Customer: replied 3 years ago.

I will check. If not then I can unhook it?


Customer: replied 3 years ago.

I checked the one vacuum line and it does not come direct. It runs through some type of valve or something first and tees off. There is a pipe that runs from the diverter valve down the passenger side of the motor and to the back of the motor that has another vacuum operated valve on it too. If neither one of them are direct can I unhook them? Do I just cap off the vacuum lines? Another thought I had was if I have the motor running good, which I hope I finally do , should I mess with the smog pump and valves connected to it? While we were working on the fuel pumps we decided to put new injectors in too. When I replaced the upper intake and EGR valve I used RTV sealant and made sure they had a better seal thinking if there was a vacuum leak that would fix it.


Thanks for your input.

yes, you can cap off or plug the vacuum lines/hoses .

you should not used rtv sealant ,that may be an issue ,I would double check for a vacuum leak in both spots ...

Customer: replied 3 years ago.

The stuff I used is a gasket maker. I thought that would make a better seal at the intake.

yeah, i wouldnt use any type of gasket maker or sealant on the intake gasket ,you may want to redo that ..

Customer: replied 3 years ago.

Redo it and use only the thin gasket for the upper intake? Any suggestions on preventing leaks?

yes ,just be sure the surfaces are clean ...I know that gasket is thin but thats how they make them

Customer: replied 3 years ago.

I drove it today. It did great for the most part but did start idling rough a little bit this afternoon on my way home. It did not act like it was going to cut off but it would idle rough and the next time I would stop it would be fine.

if you have access to a scanner .I would check the tps sensor and map readings .this may give a better clue to see what that rough idle is about ..
Customer: replied 3 years ago.

I don't have a scanner. I could rent one. Will these come up automatically or do I need to test them individually? Also am I correct that if I unplug a vacuum line from whatever it is hooked to and nothing with the motor changes, idle, runs rough or cuts out, that valve is bad or not working and could be a possible vacuum leak in the system?

you have to look in the data section-this is where the values-voltages will show .

I am more interested in the tps sensor -the tps sensor is like an electric accellerator diaphram found in the older carbed systems .this will also give hesitation issues

I am not sure of the wire colors at you tps .but with the key on ,one wire should show 5 volts and when you move the throttle to wide open(engine off ,key on) the voltage should rise from around .5 to 4.5 volts on another wire ..there are 3 wires on this sensor,one will also be a ground .

as far as taking a vacuum hose/line off -the map sensor will definately show a change or maybe even stall .otherwise the idle may go higher

Customer: replied 3 years ago.

I bought a code scanner today. I called around and could not find anyone who did the free testing on that old of a truck. I got one for $30. I figure it will pay for itself. I will do some checking on the tps. I replaced it but I guess it could be defective. I will scan the computer and let you know what I find out. I probably won't be able to do it until Saturday.

I was wondering about the vacuum and valves. I was going to disconnect the valves one at a time and see if I could maybe find a vacuum leak. I guess I will know more after I run the scan.


Customer: replied 3 years ago.

Hi Jazz,


I hope you had a great weekend. I ran the scan and it came back with a code for the EGR valve position sensor, which I have replaced, a code for the fuel pump, which I have replaced and a code for the engine coolant temp sensor. I replaced the ect sensor and cleared the codes. I will be driving it for a 30 mile trial run Monday morning. I will run the scan again after that and see if the codes come back.

if the code comes back for the coolant sensor ,you may need to monitor the data from when you first start it up ..ok let me know ....
Customer: replied 3 years ago.

Monitor it by using the scan tool while it is running? I will know more tomorrow.

yes, with the scan tool hooked up ..
Customer: replied 3 years ago.

The coolant sensor did not come back. I got the code for the EGR position sensor and tested it and found out it is not working. I also keep getting the code for primary fuel pump circuit failure. What do I need to do to check this?

on the drs side fender apron check the fuel pump relay and power relay for corrosion ...jazz..
Customer: replied 3 years ago.

I found the pump relay but not the power relay. I am also trying to find the inertia switch. I looked on the drivers side in the kick panel and it was not there. I am thinking if that was the problem then neither pump would work. I can hear the pump on the frame but not sure about the one in the tank. I am going to lay under the truck and listen for the pump while my dad turns the key.