Hello guys.
I own a 92 F150 with the following overhauls done on it within the last year.

NEW
remand 5.0l HO
fuel injectors
Ignition coil
idle control valve
throttle position sensor
spark plugs and wires.


Heres my problem. once in a rare case my truck stalls in reverse. i find that when it doesn't, my truck does idle way down low, then picks up its revs past a idle, only to drop them again, this time not to a stall or low idle but where it should be idling.

I also just recently found out when i was driving in the city for the weekend, near the end when i was leaving every red light my truck would idle up then wind back down, repeatedly each rev lasting seconds. repeating within seconds. very fluid like in action. I did get the truck home about 40 minute drive. by the time i was home the problem is no more.

To me im wondering about computer, timing, distributor magnetic electronic PCB card. Mainly wondering about that piece of electronic inside the distributor body. The Hows, Whats and Whys, could it even be the problem?

 

Is it MAF or SD? And have you checked for codes?

 

Unless the OP did a MAF swap the truck would have a Speed Density powertrain control system.

Rarely does a rolling idle like this trigger a code, but it is possible. The most common cause is a vacuum leak or a bad/sticking IAC valve. Even if the IAC is new, it can be bad out of the box. There is a reason why Motorcraft brand costs $$$.

 

sorry not the best at all the acronyms.

From what i understand i do not have a Mass Air Flow Device only a MAP wasn't until 95 or 96 for the MAF. i believe MAP stands for Mass Air Pressure Sensor. and i will have to check this.

No codes are present.

The IAC is from NAPA but no not motocraff.

Vacum lines have been cut and replaced with rubber vacuum lines as the urethane plastic ones were very brittle. each line was done one by one very carefully. But i feel i should check for a suction or blow out of each.

Speaking of the Vacuum lines. i have never replaced the Vacuum control valves, beside the ignition coil.

Ill do some test tonight after work... Thank you for the quick replies.

Thanx Mark Leppington

 

Speed Density engine control uses a Manifold Absolute Pressure sensor to calculate engine load and other factors, which what your truck should have.

Please define "no codes present". There can be a stored code or other issue in the Key On Engine Off (KOEO) tests without having a Check Engine Light.

Did you ever test the vacuum reservoirs when you changed out the vacuum lines?

For reference: Fuel Injection Technical Library » Manifold Absolute Pressure Sensor (MAP)

 

Well problem solved if what you mean by the MAP being the tube after the y pipes going back up to the engine heads with a 2 vacuum hosed switch controlling/ reading it. as that pipe is currently... well not fully intact i need to get the truck into the mufflerman or something soon, as im am almost out of ideas on how to keep this pipe on the y pipe.

I have no check engine light which is why i have not checked the codes... but i surely will now.

The two soup cans, er i mean Vacuum reservoirs as far as i know are air tight. but i will do a soapy check tomorrow night.

Thanx again for the quick response.

 

No, the MAP sensor looks like this:


It really has nothing to do with the Thermactor system, but a leak in that sub-system can cause issues as you describe.

 

not looking under the hood but would this be near/ beside the AC/heater blower (passenger side near fire wall) directly connected to the upper intake?

 

I'm going to be researching more into the vacuum lines on my truck in what each line connects to and what's the function of it all.

Recent KOEO test, Had a throttle position sensor below minimum voltage. witch before i installed the sensor i remember having a code similar but about the EGR below minimum voltage... could this all be coincidence or is there something more along the lines of alternator or wiring harness problem. As i do not have the EGR error code anymore, it is now saying the TP sensor is below the minimum.

Also after checking both soup cans the lower can by the rad, driver side. has a rust hole in it. going to either patch or see what is takes to replace... but either way its getting fixed..

 

You are correct: