Hi everyone, I have a problem with my 1985 F-250 4x4, and it's driving me nuts.
For the past two months or so, I've been experiencing erratic acceleration in third gear ( New Process four speed) right before I hit 25 MPH. It happens about 75% of the time if I'm accelerating quickly, and less than half the time when I'm slowly picking up speed. Occasionally, the truck will accelerate fine in third, but struggle to gain speed in fourth. As the truck warms up, the problem becomes less pronounced. The engine is the 300 ( 4.9l). No alterations have been done to the engine; she's bone stock in that department.
I have tried everything I can think of, including:

-completely ( to the last component) cleaned and rebuilt carburetor
-new MAP sensor
- new throttle positioning sensor
-replaced every single vacuum hose
-new fuel pump
-new fuel filter

I've used a code reader, but it only pointed to the MAP sensor and pcv valve as being issues.
All of my emissions components are in place and, as far as I can tell, working correctly ( although I'm not too clear on the function or testing procedures for the thermactors). The engine has been serviced regularly, and has excellent compression, doesn't use oil, and runs fine otherwise.

Any ideas what could I be missing?

Thanks,
-James Huston

 

When you put the new MAP sensor in place, did that get rid of the code? You can pull the battery cable for about 30 seconds and erase any codes. Then drive it and then pull the codes again.

 

Dave, I'll try that; thanks for the suggestion!
-James Huston

 

Huston, we have a problem?

Betcha never heard that one!

 

That's the first time today, at least.
Jeez, one distant relative gets a city named after him...

-James Huston

 

Well ,I cleared the computer, drove the truck around a bit, and read the codes again. The truck runs the same, even after I replaced the EGR valve ( which wasn't working correctly, either). In third gear, right before I hit twenty five , the truck loses acceleration, and I have to push the pedal down a bit more to keep the same rate of acceleration.
These are the codes, currently, but they don't make sense.
63 -throttle position sensor voltage too low ( I need to look over the wiring to it)
68- ?
31-evp circuit below minimum voltage
86-?

I'm starting to wonder if the computer itself is failing.

-James Huston

 

Here's a couple diagrams of the engine controls. In the bottom diagram you can see the throttle position sensor and the MAP or both fed by a orange/white wire. This wire is the 5v wire from the computer, I would check to make sure you have 5v on this wire. I can't find a EVP sensor in your engines diagrams. See what else a code 31 could be for your engine.

 

I have 5.08 volts on the orange wire leading to the TPS, and 5.07 to the MAP sensor. I'm going to clear the codes again, drive it a bit and see what develops . In a way , this issue has been a blessing in disguise , because it's made me fix all the little I'll-get-to-it-later items that never receive proper attention. Still, I'd like to get this corrected, because the truck runs so well in every other respect that the intermittent lack of acceleration set my teeth on edge.

-James Huston

 

I commend you for your persistence in this endeavor. I just bought a 1984 Bronco II with a 2.8 v6. Ford used the same system on it, with all the sensors, TFI distributor, computer, and the solenoid that controls the carb. I have messed with it and fixed all the obvious things, it doesn't run too bad except when it takes a fit and suddenly advances the timing. Then it hgh idles and misses. I pulled the codes on mine, I have so many it takes the little reader I have 5 minutes to spit them all out.

I have decided to ditch this system on mine. I have already ordered a distributor for a 1978 mustang with a 2.8, and a GM HEI ignition module. I am already running it with the carb disconnected as a experiment, and so far it seems to run ok. So I might be able to work with this carb that I have. I am going to miss the little dc motor that controlled the idle, it worked pretty neat and would keep the engine from stalling, especially when it was warming up. It will be a welcome sight to get rid of all those wires though.

 

It's been a while, but I've finally had time to look into this further, and the codes I have are:

13- RPM at idle out of range (low)
18- SPOUT circuit open or grounded, spark angle word circuit failure , loss of tach input/IDM circuit failure
63-throttle position fault ( below voltage)
69-idle tracking switch


I think I need to do some research, but at least I have a place to start.
-James Huston

 

The code 18 is going to seriously affect your performance. Have you found the spout connector and verified it's in place? On my 2.8 someone had taken it out and just used a wire nut instead. It worked, but would have thrown someone off if they didn't know what to look for. The spout connector is what you undo to set the timing with a timing light. It takes control away from the computer as far as timing. Then when you are done you plug it back in. Someone may have forgotten to plug it back in. If it has been lost, you may have to do the wire nut trick yourself.

 

Try driven it with the connector for the carb feedback solenoid disconnected, it will have the issues you have if it is energized while trying to accelerate.
I have been using a feedback carb without a computer for over 10 years, it runs fine without it.
Jim

 

I can verify this, I am running the feedback carb on my 2.8 with it disconnected and it runs fine also. I have been getting 17mpg though. I wonder if I can get any better by messing with the solenoid some. Thought about building or buying a cycle timer that I can adjust to cycle the solenoid on and off and see what affect it has on it.

 

So he is getting a bunch of different codes now no?
So if he ups the idle RPM, checks the SPROUT connection and clears the computer and see what he gets do you think the rest of the codes (63 / 69) would clear?
If he then gets different codes I would look to wiring and the computer.
Dave ----

 

Here is how I did it. I designed and built a variable frequency and pulse width generator which could replace the missing computer supplied voltage signal to the feedback solenoid on the carburetor. The idea was to be able to dial in the mixture at cruise based on the O2 sensor.

https://www.ford-trucks.com/forums/1...l#post14503806

https://www.ford-trucks.com/forums/1...ml#post9672141
good info in all the posts

For now, just connect a DVM to the O2 sensor. See if it goes lean at freeway speeds or during acceleration.