I have a 1988 F150 I-6 4.9L 2WD Automatic transmission. What's been happening recently is the check engine light has been coming on randomly and the engine sounds like it is experiencing a sudden loss of power each time the light comes on. The light will stay on for a random amount of time like 10 seconds or 5 minutes. When the light turns off, the engine sounds like it should.

I have run codes and the 2 most frequent that appeared were 12 RPM unable to reach upper test limit and 44 Thermactor air system inoperative--Right Side.

I have replaced a number of parts including the exhaust system, o2 sensor, a full tune up, IAC valve, air filter, fuel pump assembly in rear tank and the fuel selector valve.

Each time I start the truck for the first time of the day, it sounds like the RPMs surge high like I'm giving it gas and I'm not. The truck has also been dying out lately at a stop light or stop sign, especially when I put it in reverse and pull out of a parking space.

I'm seeking help and hoping that its something simple because I'm dreading taking it to ford and getting charged an arm and a leg.

What some auto parts stores have said is the computer might be bad which would mean the end of the truck for me.

Thanks

 

What were the other codes? Have you checked for vacuum leaks? Describe "full tuneup" please.

 

Another code that came up was 77 Operator Error. I was planning on checking codes again later today.

Full Tune Up includes plugs, wires, distributor cap, distributor rotor, ignition coil, ignition module and ignition switch.

No I have not checked for vacuum leaks.

 

I replaced the MAP sensor and Throttle Position Sensor and so far truck hasn't died out at least not yet. Check Engine light is still coming on tho.

Ran codes again and code 12 and code 44 did not come up this time.

Code 15 PCM keep alive memory test failed and Code 98 Hard fault present are the 2 codes that appeared.

Any suggestions?

 

Ouch! The Keep Alive Memory code and Hard Fault code do not sound good.
The K(eep) A(live) M(emory) should have to do with the computer or a problem with wiring.

That's why computers in cars make me frown, they can be 1/4 dead, 2/4 dead, 3/4 dead or completely dead, or perfectly fine and they tend to not give much warning.

 

Code 15 can be caused by removing power to the PCM when trying to clear the error codes. If you have pulled a battery lead, leave it disconnected for around 5-10 minutes before reconnecting. Code 98 is usually caused by a sensor being out of self-test range. There is typically another code or codes pointing to one or more sensors as a problem.

 

Code 21 ECT sensor out of self test-range came up

I replaced the ECT along with the thermostat about 3 months ago.

 

I took the truck to a mechanic and he ran codes and only thing he found was the ECT sensor was bad according to the diagnostic. Also said that my idle was low and that's what is causing the truck to stall out. I had him check for vacuum leaks and I'll find out all the details tomorrow.

Do ECT sensors go bad fast on these trucks because I replaced it 3 months ago?

What's the idle supposed to be set at for this kinda truck?

 

If truck is not brought up to temperature before running the tests a false code 21 can be generated by the user. If you know for sure the vehicle was brought up to temperature then the issue could be a faulty ECT sensor. I would verify the resistance of the ECT at different temperature as well as checking the feedback signal. The PCM does rely on accurate data from that sensor. Do they go bad frequently?....no

The idle is typically 600-750 RPM, give or take a few. The idle speed is set by the PCM based on several factors such air temperature, coolant temperature, and the feedback signal from the TPS.

 

Since the idle speed is controlled by the PCM, how would you adjust the idle?

 

From the archives and courtesy of Steve83: How to adjust idle speed (official Ford procedure)

In most cases you should not have to adjust the "idle stop screw" but sometimes a PO has effed with it, now you need to fix it...