1993 Ford F-150 extremely poor gas mileage

Tiny
505DANIELM
  • MEMBER
  • 1993 FORD F-150
  • V8
  • 2WD
  • AUTOMATIC
  • 122,000 MILES
My 93 F150 has a 5.0 with AOD. I bought it in 1995 and was getting 16-17 MPG consistently in town. Now I barely get 10 MPG city or hiway. I had to replace both fuel pumps because when I ran off the front tank fuel would return to rear tank and pressurize the rear tank. I've replaced plugs, wires, rotor, dist. Cap, PCM, fuel pressure reg, catalytic conv, all fuel injectors, O2 sensor, and more. It doesn't use any oil, if the motor was shot I would expect oil consumption. After replacing the O2 sensor, I can drive 200 miles and it will blacken it so it is running VERY rich. The exhaust system was very black. I live in a city with 5,000 ft elevation and pull 19 on a vac gage, so I don't think there's any vacuum leak. The computer doesn't set any codes. Base timing is normal. Trans. Is working normally, brakes are fine. Shops in town tell me 10 MPG is "normal". A 40% drop in gas mileage is NOT "normal". What could be wrong, faulty ignition?
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Friday, April 17th, 2009 AT 10:08 PM

19 Replies

Tiny
RACEFAN966
  • EXPERT
Ok what you need to do is check the coolant temp sensor near the thermostat housing on the intake. What you need to do is unplug it and ohm it (not the wiring harness). Get back to me with the ohms and the temp at the time of the test. Do the test when the engine is cold so you can give me air temp ok.
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Monday, April 20th, 2009 AT 11:44 AM
Tiny
505DANIELM
  • MEMBER
Temp sender near tstat is 42.9K @ 59 degrees F
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Tuesday, April 21st, 2009 AT 11:38 AM
Tiny
RACEFAN966
  • EXPERT
Sorry to put you through all these tests but as a mechanic I do not like to change parts unless they are actually bad. The coolant temp sensor looks like it is good. Ok now there is a air charge temp sensor in the manifold on the left (drivers) side of the engine looks similar to the temp sensor. I need you to do the same test if the temp is the same then the reading should match the coolant temp sensor. Let me know what you find if this seems good then we look into the throttle position sensor and map sensor. Get back to me with what you find.
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Wednesday, April 22nd, 2009 AT 10:33 AM
Tiny
505DANIELM
  • MEMBER
Air charge sensor was the same, I removed it to make sure it wasn't full of carbon, it was okay. I could see it drop in resistance when I had it in the sunlight as it was in a cold intake when I first measured it. TPS has been replaced. Something else I noticed is when I'm driving 30-35 MPH and step on the gas it just sits there. Doesn't accelerate unless you stick it to the floor and the tranny downshifts. It doesn't do it all the time. WIth as much gas that it's sucking I would expect it to fly, but for a 302 it's a real dog. I checked the TPS and it seems to change resistance evenly from closed to wide open throttle. I also had the EGR off to check, I had to replace it a while back because it was clogged up with carbon. It was okay
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Wednesday, April 22nd, 2009 AT 3:24 PM
Tiny
RACEFAN966
  • EXPERT
OK so you checked the TPS by back probing the green wire and then key on engine off you lifted the throttle and watched the voltage go from like.08 to 4.5 to 5 volts with no glitches or droppes in voltage? Now I would like to test the Oxygen sensor you will need a paper clip to back probe the wire and then you will start it and let it warm up first then with you volt meter you will need to first pull a vacuum line (any small one) voltage should decrease and do it quite fast now I would like you to put you hand over the throttle body or spray throttle body cleaner in it and the voltage should increase and do this quite quickly too. The wire you will back probe is gray with a light blue stripe on it. Let me know what you find if this is good then we will test the map sensor next.
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Thursday, April 23rd, 2009 AT 10:58 AM
Tiny
505DANIELM
  • MEMBER
TPS goes.98V to 4.5V closed to open throttle. I think it needs an exorcist; When checking the O2 sensor, I know I had a good ground and good contact on the connector. The voltage jumped all over from.08V to.9V when it was idling. This is the 4th O2 sensor in this POS. It was fully warmed up. The TPS voltage is exactly what it says in the Haynes manual. O2 voltage is not even close. There's 12V on the O2 sensor heater as it should be. I tried probing on the gray/blue wire many times and got the same result. I know the meter isn't flaky because I tried another with the same results. There was no difference in voltage when opening a vacuum line or spraying TB cleaner. The gray/red wire is common to most of the sensors, including the O2. I will check continuity from the O2 sensor connector back to the PCM tonight after work. Is the gray/red wire supposed to be chassis ground, or is it unique to the PCM? Should I measure the voltage across the O2 sensor (gray/blue HI to gray/red LO)? Or is that possibly why the voltage was so erratic because of using chassis ground as a reference?
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Thursday, April 23rd, 2009 AT 10:52 PM
Tiny
DROBB0151
  • MEMBER
"Something else I noticed is when I'm driving 30-35 MPH and step on the gas it just sits there. Doesn't accelerate unless you stick it to the floor and the tranny downshifts."

I had a mileage issue just recently and when I towed a boat tranny went out. Replaced seal, filter, and black fluid and no longer have proper shifting. I suspect the torque converter went out on mine. I'll keep adding to my question for interested parties.

Look to see if your rpms fluctuate while driving I believe that may contribute to slippage, whether TC, gears, or bands I have no clue and I am planning on doing my first rebuild, so you can understand my experience level, i.E. Novice.
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Friday, April 24th, 2009 AT 10:22 AM
Tiny
505DANIELM
  • MEMBER
Thanks for the input, drobb0151. The engine RPM doesn't change at all untill you put it all the way to the floor and it downshifts. The fact that the exhaust is black indicates there's something definitely wrong with combustion. At this point I'll look at anything that might be wrong. Fords are unreliable, nothing but trouble, and I don't think I will ever buy another. And they get PO'ed when Hyundai, Toyota and Honda takes their customers away.
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Friday, April 24th, 2009 AT 11:16 AM
Tiny
505DANIELM
  • MEMBER
I ended up backprobing pin 29 of the PCM (O2 sensor input). The engine was at normal operating temp. I started it up and it read about.35 to.40 V. I sprayed some carb cleaner in the TB and it jumped to a little over.90V and then returned back to.35-.40V or so. After a couple minutes it started jumping all over the place and I could hear the eng. RPMs change erratically. It was a high and very unstable reading, which indicates a very rich condition. It seems the O2 sensor works but something is causing the injectors to be "on" too long regardless of the O2 sensor input.
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Friday, April 24th, 2009 AT 11:02 PM
Tiny
RACEFAN966
  • EXPERT
I would like you to know that I am a die hard Ford man and tech by nature and never had problems with 95 F-150 with 275,000 miles and my 98 Mustang with 115,000. I don't buy other then American made cars either I will even run the vin to make sure as I feel that keeps the US strong, no need to debate just my feelings thats all.
Anyway we need to check out the map sensor ok. I need you to unplug the vacuum line to the map sensor once the truck is warmed up and let me know if the truck gets richer and dies or just runs the same. I am thinking this is our next item to test. Also what is your timing set at? Thanks.
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Saturday, April 25th, 2009 AT 9:17 AM
Tiny
505DANIELM
  • MEMBER
I have a new MAP sensor and it checks okay according to my Haynes manual. I pulled the SPOUT connector and base timing was 10 deg. B TDC (5 ticks to the RIGHT of 0 deg.). I reconnected the SPOUT and started the engine and at idle it was 2 deg AFTER TDC (1 tick to LEFT of 0 deg. Mark)and would go as far as 10 deg. AFTER TDC as speed increases. Whoah! Whats causing the timimg to retard so badly? The higher the RPM the more advance (a few more degrees BEFORE TDC) so you have spark before the piston is at the top of travel, right? If I'm seeing this right, I'm getting spark when the piston is on a downward stroke. Timing marks are on the harmonic balancer and indicate B or A 0 deg. I checked things twice, same result. Is there anything that could cause the PCM to retard the timing that much, or has the PCM lost its mind?
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Sunday, April 26th, 2009 AT 5:15 PM
Tiny
505DANIELM
  • MEMBER
I did some more checking, I checked the PS pressure switch, it read open. I started the truck and turned the wheel all the way right and the switch closed (which is what I would expect it to do.) I straightened the wheel and immediately connected the harness and I could hear a definite change in the RPMs, connecting and disconnecting would change RPMs. I suspect the PS switch sticks closed. Would this 1 input cause such a drastic timing change (see last message)? I will be getting a new switch, I will install and let you know what happens. Anthing else I should check in the meantime?
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Monday, April 27th, 2009 AT 2:44 PM
Tiny
RACEFAN966
  • EXPERT
Ok good after that if this dont change things then we will check the knock sensor ok. Get back to me.
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Monday, April 27th, 2009 AT 7:08 PM
Tiny
505DANIELM
  • MEMBER
I put in the new PS pressure switch and timing is normal now! I let it run a few minutes, turned the steering hard left and back, timing was >12 deg. Before TDC and would increase advance as RFPM increased. I don't have a tach, but it seems the idle is kinda high and fluctuates. It will idle fast when you start it up, even when it is warmed up. I have a fairly new idle speed control motor on it. Any ideas what to check?
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Monday, April 27th, 2009 AT 11:59 PM
Tiny
RACEFAN966
  • EXPERT
Ok when it starts up it should be about 1500 for about 30 seconds when cold. When it is warmed it should idle about 1000 rpm for about the same then drop to about 750 or so. With out a tach or a way to check you rpms its hard to say what to do. I would first check all vacuum lines and make sure there are no vacuum leaks around the plenum and intake. Let me kwow what you find here ok.
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Tuesday, April 28th, 2009 AT 10:22 AM
Tiny
505DANIELM
  • MEMBER
No difference at all. I took it to the Ford dealer and they checked the PCM and ALL the sensors. They found nothing wrong. I drove it with the new PS switch connected, no change. I drove it with PS switch disconnected, no change. Still got <10 MPG. Checked the timing after driving it today and the stupid thing is 2 deg AFTER TDC at idle(SPOUT connected). Ford said the PCM is fine. They checked the timing and it was okay THEN also. The only thing they found was the TPS was at.9V at idle and it should be about.67V. I seriously doubt the TPS is going to make that PCM retard the timing like that. I think my erratic idle is nothing more than the PCM going open loop all the time. What could be making this thing go open loop all the time?
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Wednesday, April 29th, 2009 AT 4:29 PM
Tiny
RACEFAN966
  • EXPERT
Ok they are correct this can cause strange stuff to happen when the voltage isn't correct. Where the voltage is at will cause it to run rich as the PCM thinks the throttle is open more then it really is. The TPS ties into the EGR circuit and the Knock sensor as well as the coolant temp sensor, it even tie into the transmission circuit to help with proper shift timing so this can have a MAJOR effect on the timing too. If you would like use a digital multi meter and a paper clip and test it.
Use the paper clip to back probe the gray wire with the white strip should be reference. So with meter hooked up you should see with the key on engine off.6-.8 volts then as you slowly lift the throttle is should increase to 4.5-5 volts at wide open throttle. It should have no drops or glitches in voltage as you lift the throttle. If by chance I am looking at the wiring diagram wrong and this wire has 5 volts then check the brown wire with the white stripe. Let me know what you find
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Thursday, April 30th, 2009 AT 10:36 AM
Tiny
505DANIELM
  • MEMBER
I bought a new TPS and installed it and have it set so it reads.709V at closed throttle and will go to 4.7V at wide open. (Gray/white wire).
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Thursday, April 30th, 2009 AT 10:29 PM
Tiny
RACEFAN966
  • EXPERT
Good that means it is good. I never assume that a new part is alway's good as you can get bad ones sometimes. So now I would like to test the MAP sensor. First unplug the vacuum line to it after you have it warmed up for about a 3 minutes and at idle if you unplug the vacuum line does it go rich and try to die? If it does great. Otherwise I need you to get you volt meter and you need to know about what you elevation is and check you signal return (refer to pic below) and get back to me with the elevation and voltage you found there with truck running ok. I also need to know if your truck has a mass air flow meter or not. They come both ways through 95. Get back to me and we will go from there.


http://www.2carpros.com/forum/automotive_pictures/249564_Graphic_81.jpg

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Friday, May 1st, 2009 AT 10:15 AM

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