gas level and intermittent check engine light

[oneway]

so I've been going to war with the check engine codes 98 and 54 for the last few months.

According to the manual:
Code 54: ACT indicated -40F / open circuit.
Code 98 simply says: Hard fault

I get the codes during the KOEO test and KOER tests.

So I replaced the ACT sensor. No change in code. I did everything to try and erase the code using the jumper wire and then pulling it out during the code process. No luck.

Now the fun part: EVERYTIME I get more than a quarter tank of gas (just below half tank or MORE) the check engine light will go off after about 5 minutes of driving and remain off and I'll get over drive back while driving and the RPMs will go back to normal and the truck will have better throttle pedal response. It will not shift into over drive while driving with the check engine light on. When the gas level falls below 1/4 tank - the check engine light will remain on showing a code of 98 then 54.

If I get gas but not enough to get up to just below half a tank - the check engine light will remain on.

So - it's not a broken wire or anything....sooooo I'm completely stumped.

The check engine light came on randomly one day when all I did was slam the door getting in the vehicle. I turned it on and boom - check engine light. It was never present at any other time.

[oneway]

so I have -9 volts at the ACT and 4 volts at the ECT.

KOEO test.
both gave tone when checked for continuity.

Shouldn't I get tone when I touch the positive wire from the voltmeter to the female end of the harness and the negative prong from the voltmeter to the male end of the sensor?

Either way - the check engine light goes off after a few minutes driving with more than a 1/4 tank of gas. This is what stumps me as it can't be an issue of a broken or shorted wire or it wouldn't go off at all....but it does. Once below a 1/4 tank of gas the MIL light will stay on.

Could it be the MAP sensor? If so - why is it giving me a code for the ACT??

[oneway]

so through my testing, I've gotten:

12 volts on the ground wire for the ACT - this is correct
-9 volts on the power wire at the ACT sensor
13 volts on the power wire at the ACT sensor

It's getting a jolt of power from somewhere. It should only be receiving 5 volts.

tested the ECM pins and the ACT sensor for continuity and everything checked out.

tested the ground from the ACT to the TPS and got continuity
tested the ground from the ACT to the ECT and got continuity

Was unable to locate the MAP sensor to test that...only thing that sucks is that STUPID Haynes manual doesn't show where the MAP sensor is

The ACT, TPS, ECT, and MAP all share the same ground.

The PCM thinks the temp is -40 so it will not shift into OD while driving unless I have over 1/4 tank of gas.

The MIL light will go out after driving a bit as long as I have more than 1/4 tank of gas....that's why I'm thinking this is a MAP problem...but why am I getting the ACT code 54??!!! Could it be a temp sending unit problem?

I'm soooooooooooooo fucking stumped

[oneway]

still doin it. The MIL light will actually flash somewhat continuously now at idle during a cold start - causing the truck to "pulse" then remain on when i give it gas....then eventually shut off as I drive.

or sometimes it just stays on regardless of the amount of fuel. It stayed on when I had almost a full tank and I did a lot of driving. Just the other day I put in enough for just below a half tank and it went off on my stop and go commute to work.

LOL - I'm think I'm going crazy. Do I take it to the shop...do I not take it to the shop...do I take it to the shop...do I not take it to the shop.

[oneway]

changed trans fluid (about a month ago)
new battery (about 2 weeks ago)
new spark plugs (last week)
new spark plug wires (last week)
new air filter (about a month ago)
new ACT sensor (code from the MIL light - but didn't fix it when I replaced it 2 months ago)

and today was the first day (since 3 months ago) the truck started without the MIL light coming on immediately and the idle dropping. The MIL light stayed off the entire stop and go commute to work.

And I have less than 1/4 tank of gas and the MIL light stays off.

It felt good to have OD back and drive at less than 2k rpms instead of always being at or above 3. Thanks for everyone's help.

hope it stays this way *crosses fingers*

will be changing the distributor cap and rotor and ignition wire and coil in the next couple weeks.

[oneway]

well that was short-lived. The MIL light came on when I left work.....but went off 10 minutes later down the road in stop and go traffic.



However - I started it this morning and no MIL light - didn't even flicker at all all the way to work. So the 1/4 tank of gas thing is no longer prevelant. It used to only go off while driving if I have more than 1/4 tank of gas.

I did spray some carb cleaner into the air intake vaccum lines after we did the spark plug wires...it stalled. I later read the can and it said not to use that stuff on FI vehicles....to use the air intake cleaner instead.