1988 Ford F150-302 V8-AC-PS-Auto Trans-Dual Tanks-Lariat

My apologies for the long post here...didn't want to leave anything out.

On 10/16/14, Drove truck for 6 miles, shut down to do yard work. Four hours later, started truck but it wouldn't stay running unless some throttle was applied. As the engine warmed up, I could carefully lower the rpm and keep the engine running at the extreme low idle. Also the Check Engine Light came on but would go out if I increased the rpm. As I cautiously drove the truck home and some the following day, I found it would start and run at the low idle rpm even when cold.

When pressing the gas pedal normally, there would be a slight hesitation, not excessive.

The vehicle has dash gauges... Engine temperature OK, Charging system OK, Oil pressure reading 5/8 of gauge OK.

My first thought was the IAC unit failed. Bought a new IAC and installed it only to find that the new IAC unit made matters worse... engine would start and shut down immediately and the Check Engine Light stayed on constantly. I could only start the engine if I applied some throttle while starting. I reinstalled the original IAC unit and found that the Check Engine Light still stayed on.

I have the "Haynes Manual" and have been studying it so as to avoid expensive mistakes. Installed a new Power Relay which made no improvement... reinstalled original relay.

I noticed a sensor at the front portion of the intake manifold or block that was falling apart. The wiring led to the bottom of the throttle body (Throttle Positioning Sensor?). I installed a new sensor but it made no improvement.

I've been having a problem understanding how to read the codes... 2 seconds here, 4 seconds there, I get confused. However I will be trying it again and eventually get the knack of it.

What I have accomplished to date is as follows:
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Ohm reading of IAC solenoid should be between 7 to 13 ohms....Original and new IAC solenoid read exactly 9 ohms. Presently using original IAC unit (ohms within limits).
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Voltage read at IAC solenoid should be approximately 10/5v....With harness connector attached to IAC solenoid and turning key ON, voltage jumps to 9.75v "BUT" stays there only 2.5 seconds, then drops to zero.

With harness NOT attached to IAC solenoid and turning key ON, voltage jumps to 10.5v "BUT" stays there only 2.5 seconds, then drops to zero.
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The above according to the "Haynes" book tells me (I think) that the voltage should be steady and constant, and the voltage drop to zero after 2.5 seconds indicates that I am not receiving the proper signal form the ignition module?.... Is this correct?

The ignition system on my truck is the TFI-IV setup and apparently has nothing that we would normally refer to as a powerpack, pulsepack, ignition module attached to the firewall or fender apron... BUT... rather has a small distributor ICM module mounted in the side of the distributor. This would be the brains of the setup, replacing a powerpack, pulsepack, remote ignition modiule,... is that correct? Am I looking at a failed unit within the distributor?

As it stands......
For those 2.5 seconds that the voltage is applied to the IAC solenoid, the starting idle rpm is high BUT as soon as the voltage drops to zero, so does the rpm, very low but fortunately the engine continues to run.

The Check Engine Light stays on, (Oil pressure is 5/8 of gauge).

A slight hesitation exists when applying throttle initially, that is after the first bit of hesitation from zero to say 30 mph, there is no further hesitation from 30 mph up.

The truck is usable. I made sure that my cell phone includes the number of AAA.

 

hello! I have a E250 1988, 5.8liter MPI. C6 auto.
I'm an old diagnostic guy, new to Fords. I have a simular issue but wondering fast Idle. I recently changed the IAC but wasn't convinced it was bad. It made a bit better idle but the wondering idle was after warm up or thermostat opened. Now it's anytime. I don't have the answer but I'm suspecting hight pressure fuel pump and or fuel regulator. I experimented with fuel pump relays. Hight pressure off , it would idle but not above. I removed low pressure relay, reinstalled hight pressure relay, It would not start. I'm parts changed ... I do to use symptoms to troubleshoot. I hope a bit of my experience might help you.
BilldarrAZ (Bill Tust, Tucson, AZ)

 

Hi Joe !
This may be unreliable reasoning, but this thought did cross my little mind.
If the computer responds to signals from the o2 sensor and increases or decreases injector pulse width to either add or cut back fuel based on what the o2 sensor says it sees, could there be a restriction in the air intake system that is causing the fuel delivered at idle to be low to correspond to low air flow into the engine ? When was the last time the air filter was checked or could a family have moved in there and made a home with their nesting material? A common problem these days.

 

Hanky... Glad to see you jump in here so quickly.

I have no idea where that o2 sensor could be, however... I ran the engine with the flexible tubes intake tubes removed, bypassing the air filter with no change. I don't understand why I have the proper voltage to the IAC solenoid only for 2.5 seconds. (With a cold engine)

An after thought: Would disconnecting the battery, waiting a reasonable amount of time, then reconnecting the battery reset anything?

In reading thru the "Haynes" manual about the o2 sensor you mentioned, it states what it looks like and where I can find it... sounds like a complex system.

 

Joe,
The vehicle computer supplies the correct operating (V ref)voltage to a string of sensors. When one of them is shorted it will pull down the voltage to all in that string of sensors . I will get a diagram and see if I can list them for you to check. All we do is to disconnect them and see which one pulls the voltage down when reconnected.

Since you are getting a check engine lite , try getting the code it will save a lot of time.

 

There is supposed to be an Orange/White wire that supplies the 5 volt reference voltage from the computer to the Map sensor, the Throttle position sensor and the EGR position sensor on top of the EGR valve. What you can try is to disconnect each of those 3 and check that the 5 volt reference is present on that wire. If any one of those are shorted the voltage will drop when it is reconnected. If all checks ok let us know.

 

I'm a controller Type designer (ECU) with Hot Rod background. That 2.5v issue for a few seconds reminds me of a boken sensor input line as 2.5 is a specific voltage the in a TTL input (on ECU) wih no path to ground or to 5v. So, check for a broken wire the the harness between ECU an one of the sensors described to Hanky!
I helped.
BilldarrAZ
PS TTL is defined a Transistor Transistor Logic, where 5v is ture and 0.7 is false, 2.5 is and open are ... not true or false but a fault.

 

Hanky... Spent a few hours at the doctors office (no big deal), plus chores elsewhere. However, I have located the three (3) sensors you speak of and the voltage supply orange/white wire. I'll do that test tomorrow (10/25/14) and will also do the code test as many times as it takes to get them right, making notes.

BilldarrAZ... I'll keep the broken wire scenario in mind. If it exists, hopefully I'll come across it accidently.

 

BilldarrAZ,
Thanks for introducing some real logic. Stick with us , Joe will let us know what he finds.

 

Joe,
there should be two wires connecting to the IAC solenoid, one a white and Lt blue and the other yellow with red . The yellow should provide the 10.5 volts with the key on engine not running. The white is the negative side. They both come from the ECA.
The yellow comes from pin #21 and the white comes from pin # 41. The engine should always start and run at high rpm and drop down quicker if already warm.

Coolant temp sensors affect how long hi idle conditions run and should be checked. If all systems check ok there is only the ECA left.
What code(s) were you able to get?