I've been trying to figure out why my truck is getting such low gas mileage. I've replaced the plugs, distributor cap and rotor. I've ran seafoam through the brake booster line and it really hasn't changed much. I've got some 9mm plug wires on the way as I've seen some arching between two wires. A few weeks ago I changed my water pump and the coolant was basically black, not sure when the last time it was flushed. During the water pump I had the battery terminal unplugged to reset the computer. I got a chance today to check the codes and got the following (if I didn't make a mistake):

KOEO:
111
111
1
124
124

KOER:
412
121
411
538

Any ideas if these codes can cause low gas mileage? I'm getting about 10mpg around town and it is a 4x4 with 31's and 3.55 gears. I know for a fact I have some type of exhaust leak as I hear it ticking on the passenger side under load (not so bad at idle). I also see the inside of my hood on the passenger side is blackened. Any ideas would be greatly appreciated!!

 

Codes: 121 - Throttle Position Sensor, performance problems. 124 - TPS, intermittent. 411 - Secondary Air Injection (Smog Pump). 412 - SAI, circuit malfunction. 538 - Intake manifold runner control stuck open. Looks like you have an exhaust leak (as you mentioned), faulty TPS, & smog pump.

 

Correction: KOER Code 538 = Insufficient RPM change during KOER dynamic response test/ Operator error


Looks like the OP did not goose the throttle enough/correct time.

I agree with the TPS codes being a probable root cause. Measure it with a meter. Some swear you have to use analog meter but I have used a DVM many times. If it measures bad replace it with a Motorcraft. Do not cheap out on an aftermarket, they are notoriously short lived.

Closed throttle Key On Engine Off TPS value should be between 0.6 to 1.0 VDC.

 

Thanks for the info guys. I did see that I didn't run the test correctly upon hitting the gas when the CEL prompted me to do so. I'd like to note that I don't really have issues of bucking or anything like that but I have noticed the following:

1. Acceleration seems nil. It seems like the engine really has to try to get the truck up to speed. Albeit that is when I am driving like a normal person (whatever that may be . When I hit the gas hard it seems to accelerate okay, but I rarely ever do this as I can't imagine my gas mileage then.
2. When I'm cruising at lets say 40mph and let go of the throttle to come to a stop, it almost seems that the throttle is still a bit open - not enough to accelerate, but enough to slow the truck down slower than it would if there was no throttle at all.

Are these symptoms of a bad TPS? I'm really hoping the TPS is the cause of my bad gas mileage as it is driving me insane!!

 

Continuous Memory DTC 124 indicates TP value was higher than expected.
DTC 121 indicates the Throttle Position (TP) sensor rotational setting may be out of Self-Test range.
Possible causes:
-- Binding throttle linkage.
-- TP sensor may not be seated properly (tightened down).
-- Damaged TP sensor.
-- Damaged Powertrain Control Module (PCM).
I would say yes a bad TP sensor that is the root of your problem as you did not get any KOEO codes.

DTC 412 indicates that during Engine Running Self-Test, engine rpm could not be controlled within the Self-Test upper limit band.
Possible causes:
-- Open or shorted circuit.
-- Throttle linkage binding.
-- Improper idle airflow set.
-- IAC solenoid contamination.
-- Items external to Idle Air Control system that could affect engine rpm.
-- Damaged IAC solenoid.
-- Damaged Powertrain Control Module (PCM).

DTC 411 indicates that during Engine Running Self-Test, engine rpm could not be controlled within the Self-Test lower limit band.
Possible causes:
-- Improper idle air flow set.
-- Vacuum leaks.
-- Throttle linkage binding.
-- Throttle plates open.
-- Improper ignition timing (Distributor Ignition vehicles only).
-- IAC solenoid contamination.
-- IAC circuit short to ground.
-- Damaged IAC solenoid.
You may need to clean or replace the IAC valve.

 

I was wondering about that 412 code. Looks like that can be a wide range of stuff. I've got one of those digital readout code readers coming in the mail tomorrow along with some 9mm plug wires. I'm going to retest with that code reader so I can be sure of my codes. For the 412 code, do you guys see it being something common that can be fixed/replaced?

 

Yes you may need to clean or replace the IAC valve for 411 & 412 codes.

 

I've changed the TPS in the truck and rode around for an hour and I'm not getting any codes now!! I will report back after a bit longer driving the truck to see if any codes come up.

I've been doing more research on the 302 motor and I'm thinking of swapping in the 300ci i6 motor. My truck feels way to much like a slug when driving around town and I really don't do too much highway driving. I think what I'm really missing out on is torque. How complicated would it be to swap in the i6 if I can find one? I really don't do too much towing - I go about 250 miles north to go riding my ATV with mine on the bed and two others on a trailer. Pulling the trailer seemed like the truck was really struggling until I got onto the freeway and even then when I'd punch it it really wouldn't move anywhere. Only other load I've had would be 2k-3k of logs from a tree that I cut down. Let me know what you think!