Hello. I have an 88 F-150 with a 5.0 litre with EFI and a 5-spd manual trans. I recently changed out my weak fuel pump for a new one. Now I have an idle issue. Once my truck is warmed enough for the high idle to kick out and my truck is at low(curb) idle. It begins to fluctuate between 650/700 rpm and down to 300/500 rpm. it sometimes stalls, but will start back up again. Now...once in a while it also has another symptom. I will start driving and shift up through the gears, but when I go to shift my RPM will either drop or climb to about 1200 to 1400 rpm and sit there until I let the clutch out and the engine trans controls the rpm. Same when downshifting.

Now I think I have kind of narrowed it down but do not know which to do. I am thinking either the Oxygen sensor, idle/air bypass, or the throttle positioning motor. Has anyone any experience with these problems and able to possibly narrow this even more? Please help.

 

I have the exact same problem but mine started before I changed the fuel pumps and still does it. I have a C6 auto but if I put the trans in nuetral and tap the gas the rpm's will climb just like you described. I have had times where it wouldn't want to idle no matter what I do but for the most part as long as I give it gas it will keep running. After I drive my truck for a while it will run like it should.

My truck at one point wouldn't idle at all during a cold start ups. The rpm's would come down and then just stall. After I would restart it a couple of times it would run but search real bad for the correct idle like you describe. It had a period where it has done this twice and then gone away. Now the truck will start up and idle okay but stumble or search when I put it in gear. It will do this every morning I start it and after I drive it for a while it will be okay for 95% of the time after that cold start in the morning unless the truck sits for a long period then it starts all over.

For the most part I want to say I know when it will have trouble idling in gear by the way the motor sounds when it is idling in park. It sounds like there is a slight miss in the idling.

Heres what I have changed so far and still have an idling problem. New MAP, IAC, TP, TB, module on dizzy, Air temp sensor, coolant temp sensor, coil, distributor, cap, plugs wires, fuel pumps, a used computer, EGR valve and sensor and I think I listed them all.

I may try a new fuel pump relay and maybe one for the computer as well. I may also replace the ignation switch on the column since Ford had problems with them when new. I would hope this has been replaced since the bad switches where known to short and start fires.

I never got a code when this started. Then at one point I got a code 33 and now I get code 35 along with a check engine light that will flash when I am driving on the highway. The truck has also just completely stopped running on me and would not restart right away. Wait a while and it will start up and run like nothing happened. Its done this twice in about a month.

The injectors measured around 15.2 to 15.6. I can't find any vacuum leaks. Replaced the upper intake gasket. Checked the timing chain play and I only have about 5 degrees of back lash and maybe a little less with the new dizzy. I have measured the TP voltage and I am close to 1. The voltage on the EGR sensor is .55mV at idle and once the truck is warm I was able to get a reading of 3.?? something volts. If I remove the vacuum line from the EGR valve the voltage will not change when I rev it. Not sure what that means. All I know is code 35 says the voltage exceded 4.8 volts. Since I do not know how the rest of the system works I can not troble shoot it any more and no one has been able to tell me what else could cause a code 35.

I forgot to mention my truck is a 1989 with a 351W.

 

I have the same truck as you at work except it's a 1987. I had this problem last year and when I would go plowing it would stall all the time. So, in the summer I took of the IAC (Idle Air Control) valve and cleaned it out. Only use Throttle Body cleaner as opposed to carb cleaner. Look under the 3.0L forum and you will find some info there. It's is mostly for Rangers but applies to your truck as well. Good luck!

-Matt

EDIT: Here's a link I found for you. It basically applies to your situation:
https://www.ford-trucks.com/forums/s...2B%25252BValve

 

Any clues as to why mine stalls? I have tried two brand new IAC sensors?

 

i had a problem like that with my 88 5.0 and it ended up being the map sensor. it located on top of the heater box.

 

I'm not sure guys/gals if you have heard of an Idle Re-Learn. On our GM cars, we do an Idle Re-Learn everytime the battery wire is disconnected or the power sources to the computer is disconnected.

When the power source to the computer is disconnected or the IAC has been cleaned or replaced, you must do the Idle Re-Learn procedure.

A scantool can do it quick.

You can do an Idle Re-learn by disconnecting the battery cable for 5 minutes or more. Reconnect the battery cable. Turn the ignition switch to ON for 5 seconds and OFF and then Start the car, hold down the brake pedal, put it in Drive (automatic), put in Neutral (manual). Hold your brake pedal down for about 10-15 minutes and then put the gear in Park (automatic) and turn the ignition switch OFF . Wait 5 seconds and then start the car again. Hold down the brake pedal, put it in Drive (automatic), put in Neutral (manual) and hold your brake pedal down for about 10 minutes and turn the igntion switch off.

Subsequent driving should fully let the computer re-learn the IDLE again.

 

Well I have heard of what you speak of but never on a ford. I have driven the truck for 2 hours and when I slowed down after about an 1 1/2 of highway driving the truck stalled, restarted but wouldn't idle correctly. Even if driving one should restore the idle conditions this wouldn't explain how or why my truck will run fine one minute and then not the next.

I got some news tonight of someone having a problem like mine and they ended up having a bad relay. Go figure a $1,000 later and I may have a bad $12 part. I sure hope a relay fixes it.

I have changed the MAP sensor.

Any other ideas?

 

If your truck has a TPS (Throttle Position Sensor) then that would be the other sensor that you need to check on or replaced.

Make sure though that the TPS bolt is tight and not loose. A bad or loose TPS bolts would cause erratic readings and the computer would think that the throttle is moving and would have to compensate for its movement.

 

Which relay have they replaced that fixed this similar problem?

 

I didn't get the specifics and I even forgot what truck this was on. I know it was newer than mine. The owner of the truck said he had taken the truck to someone he knows and they messed with the relays and said go buy this and his problems went away. I will try to get more details today.

My guess is that in my case it will be the fuel pump relay since I have had a condition where my truck would stall and not start back until an hour later or so.

I will also be replacing the computer and fuel pump relays today but i will only change them one at a time. So when and if one fixes the problem I will know which one.

Besides the relays are the only thing left on my truck that haven't been changed.

 

Good luck and tell us if that fixes the problem

 

The relay for the fuel didn't fix my problem. I think I should give up and take in to see if a tech guy can find the problem.

 

RTM stated; "My guess is that in my case it will be the fuel pump relay since I have had a condition where my truck would stall and not start back until an hour later or so."


I had the above problem and it resulted in my changing the fuel pump. A 'learned & wizened, and FORD fanatic' mechanic friend of mine told me that this is a common problem with the ford in tank fuel pumps. It seems that the fuel pumps are lubricated by the fuel and if you run under half a tank of fuel too often the seals in the pump dry up too much and it weakens. Apparently FORD was aware of this problem and thusly advised keeping the fuel tanks over half full at all times.

 

This is a problem with ALL of the in-tank pumps. The fuel helps to cool the pump, so letting it run down makes a hot pump.

 

Hi guys! I want to thank you for all your help. My problem ended up being a faulty Fuel Flow Regulator. Instead of the 35 lbs pressure I was supposed to have I had 120 lbs +. I changed it out with a new one and all is well under the hood now. I even have a lot more power than I used to and my fuel mileage is awesome! The only other thing I had to do was retard the timing 5 degrees as it had been advanced to +15 degrees and is only suppose to be at 10 degrees advanced. But I do appreciate all the advice. it sis help me in negating certain possible problems and locating the correct fault. THANKS!