1992 F-150 5.8 L High Idle Issue - Ford Truck Enthusiasts Forums

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1992 F-150 5.8 L High Idle Issue

 
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Old 01-14-2019, 05:46 PM
Boss_Hogg
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1992 F-150 5.8 L High Idle Issue

I'm having a high idle issue on my 1992 F-150, 5.8 L engine. When started cold, it has what I consider a normal high idle. Then as it warms up, it gets even higher. I don't have a tach but would suspect it's in the neighborhood of 2,000 to 2,500 RPM. It's high enough that the truck will run 35 to 45 mph on level highway. By the way, the truck only has 58K miles so it doesn't get driven much.

As far as troubleshooting goes, I've searched for leaks and probed most of the sensors. I looked for leaks via a length of rubber hose to the ear traveling along vacuum hoses. No leaks were found. I then probed all over the engine, including intake plenum, with propane. Again, no difference with engine speed noticed. I did disconnect the IAC valve and the engine went to (again, guessing) about 500 RPM. It would idle for 30 to 60 seconds before dying due to low idle. By the way, I've not messed with the mechanical idle screw. Both throttle plates appear completely closed. And I've cleaned the IAC valve. Again, no change. Not sure if this is related, but I hear a clicking on a solenoid valve near the EGR valve.

With regard to sensors, I've tested the following sensors and found all normal according the the (red) Haynes manual, MAP, TPS, IAC, and Intake Temp Sensor. I also checked the coolant temp sensor and it was normal although the instrument temp gauge quit working. I just finished a coolant flush and thermostat replacement due to the old thermostat sticking open. The only code I get is 121 but that may be self inflicted by unplugging the MAP sensor.

I'ts apparent that the IAC is the cause for the added idle RPMs. To my way of thinking it's got to be an air leak or a faulty sensor. Is the solenoid valve clicking near the EGR valve trying to tell me something? Otherwise, if it's an vacuum leak, I'll be darned if I can find it. And the only sensor that I've not checked, to my knowledge, is the O2 sensor. Any help would be appreciated.
 
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Old 01-14-2019, 08:41 PM
Will Joyce
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Originally Posted by Boss_Hogg View Post
I'm having a high idle issue on my 1992 F-150, 5.8 L engine. When started cold, it has what I consider a normal high idle. Then as it warms up, it gets even higher. I don't have a tach but would suspect it's in the neighborhood of 2,000 to 2,500 RPM. It's high enough that the truck will run 35 to 45 mph on level highway. By the way, the truck only has 58K miles so it doesn't get driven much.

As far as troubleshooting goes, I've searched for leaks and probed most of the sensors. I looked for leaks via a length of rubber hose to the ear traveling along vacuum hoses. No leaks were found. I then probed all over the engine, including intake plenum, with propane. Again, no difference with engine speed noticed. I did disconnect the IAC valve and the engine went to (again, guessing) about 500 RPM. It would idle for 30 to 60 seconds before dying due to low idle. By the way, I've not messed with the mechanical idle screw. Both throttle plates appear completely closed. And I've cleaned the IAC valve. Again, no change. Not sure if this is related, but I hear a clicking on a solenoid valve near the EGR valve.

With regard to sensors, I've tested the following sensors and found all normal according the the (red) Haynes manual, MAP, TPS, IAC, and Intake Temp Sensor. I also checked the coolant temp sensor and it was normal although the instrument temp gauge quit working. I just finished a coolant flush and thermostat replacement due to the old thermostat sticking open. The only code I get is 121 but that may be self inflicted by unplugging the MAP sensor.

I'ts apparent that the IAC is the cause for the added idle RPMs. To my way of thinking it's got to be an air leak or a faulty sensor. Is the solenoid valve clicking near the EGR valve trying to tell me something? Otherwise, if it's an vacuum leak, I'll be darned if I can find it. And the only sensor that I've not checked, to my knowledge, is the O2 sensor. Any help would be appreciated.
Been fighting the same problem with my 87 302 replaced the IAC, TPS,ICM, & ECM - just got a temp sensor & map sensor --will let you know if that fixes the high idle . maybe others can chime in about the O2 sensor -
 
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Old 01-15-2019, 08:45 AM
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My 92 bronco 5.8 does the same. No codes, no vacuum leaks. I unplug the iac when it's warm and or keep my foot on the gas yo warm it up
 
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Old 01-15-2019, 11:43 AM
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Boss Hogg;

What are you using to check your codes?
Are you getting 111 Code for either KOEO or KOER Self-Test ?
 
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Old 01-15-2019, 12:25 PM
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Originally Posted by Boss_Hogg View Post
I also checked the coolant temp sensor and it was normal although the instrument temp gauge quit working.
FYI;
You have a 2 wire Coolant Temp Sensor for the Computer, and a 1 wire Temp Sensor for you I/P Gauge. 2 different sensors.
 
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Old 01-15-2019, 06:08 PM
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Originally Posted by vjsimone View Post
Boss Hogg;

What are you using to check your codes?
Are you getting 111 Code for either KOEO or KOER Self-Test ?
After troubleshooting today, I reset the codes and then drove the vehicle for about 6 miles at 45 - 55 mph. I then re-checked the KOEO codes and got a 121 followed by 111. The check engine light never comes on.

Also, I think I can eliminate the vacuum leak as the source of the problem. I unplugged each line from the intake manifold and plugged it except the MAP sensor line. For the MAP, I replaced the line with spare vacuum line. No change on high idle speed for any of the vacuum lines. To test the EGR valve and intake manifolded base, I fogged them with first with distilled water, then propane, then carb cleaner. No change on any of the test. .
 
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Old 01-15-2019, 06:20 PM
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The temp sensor I was testing was the two wire one that feeds the computer. Not worried about the I/P gauge right now. As a matter of fact, it started working again today. I must have bumped it during my vac leak troubleshooting and re-established the connection.
 
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Old 01-15-2019, 09:43 PM
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Originally Posted by Boss_Hogg View Post
After troubleshooting today, I reset the codes and then drove the vehicle for about 6 miles at 45 - 55 mph. I then re-checked the KOEO codes and got a 121 followed by 111. The check engine light never comes on.

Also, I think I can eliminate the vacuum leak as the source of the problem. I unplugged each line from the intake manifold and plugged it except the MAP sensor line. For the MAP, I replaced the line with spare vacuum line. No change on high idle speed for any of the vacuum lines. To test the EGR valve and intake manifolded base, I fogged them with first with distilled water, then propane, then carb cleaner. No change on any of the test. .
Computer controls the Idle; Check the TPS output signal voltage to the computer.
You could smoke test vacuum system as well.
I like to block off the EGR-Valve or replace it during troubleshooting, in the case it is leaking.
Have you run the KOER Self-Test ?
 
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Old 01-15-2019, 10:29 PM
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Originally Posted by Boss_Hogg View Post
The only code I get is 121 but that may be self inflicted by unplugging the MAP sensor.

121 is a TPS Code...
 
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Old 01-16-2019, 06:22 PM
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Sorry for the late response. I have an Actron code reader I bought for my 1990 Bronco, 5.8 L engine years ago. This is actually my Father in Law's 1992 Ford truck. The same code reader works on both and both have the same engine.

I do get the code 111 after doing the KOEO test (First code 121 then 111). I've not had a chance to run the KOER test yet. I did however go back and re-test the MAP and TPS sensors. The MAP tested good again. However the TPS read 1.6 volt on the low end (supposed to be around 1.0 volt). Due to its location, it is hard to test and get good connections. I don't understand why it tested good the first time unless I didn't have a good ground on the black wire. I've ordered a TPS sensor and will replace that to see if it fixes the problem.

I fear I may have the problem still and the TPS was just collateral damage from my doing all the vacuum leak testing, i.e., spraying water and carb cleaner all over suspected leaking components. We'll see.
 
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Old 01-16-2019, 09:47 PM
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Originally Posted by Boss_Hogg View Post
Is the solenoid valve clicking near the EGR valve trying to tell me something?
“Not sure if this is related, but I hear a clicking on a solenoid valve near the EGR valve.”

TPS output @ 1.6 is not very high, but could be opening your EGR-Valve, you could disconnect the vacuum line from the EGR-V and plug it, or you can make a block off plate for the EGR-V.
You will need to prove the EGR-V is not leaking at some point.

Check the Throttle Body shaft for play….

For that high of an Idle, you possibly have Air leaking in somewhere or a failed computer.
At some point you may have to pop open your computer to see if there is any component failures.

The 111 is indication that there are no memory codes stored in the computer.
Have you cleared the codes recently ?

Did you check the frequency output of the MAP Sensor?
Did this high idle start all at once, or over time ?
 
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Old 01-16-2019, 11:52 PM
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High Idle

It has to be electrical in my opinion.

1- if it was a vacuum leak you would not have the truck stall out when you unplug the IAC. The IAC allows air to by pass the throttle body blades therefore controlling the idle speed. If you had a vacuum leak you would not see such a drop in idle speed because air would be bypassing the throttle body through the vacuum leak already.
2- The EGR would stall the engine if it was stuck open. It has no oxygen to feed the motor. The clicking is most likely coming from the canister purge valve. It could cause a vacuum leak if stuck open. You in my opinion ruled that out by the truck stalling with the IAC valve unplugged.

The EEC is being fed incorrect information from a falsely reading sensor therefore the truck is opening IAC. I think you are onto it with the Tps sensor.
 
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Old 01-17-2019, 02:39 AM
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The TPS showing 1.6 volts isn't on the low end, that's on the high end if it's with the engine idling OR with KOEO.

It shouldn't be higher than .95 And yes it will throw a code if it's above 1.0
Most of the TB's with TPS installed have never been higher than .96 And No lower than .87 volts.

Need to check the throttle stop screw to see of it has been tampered with. If it hasn't, then I would be inclined to say the TPS is at fault.
 
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Old 01-17-2019, 06:33 AM
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Originally Posted by Canadianice View Post
It has to be electrical in my opinion.1- if it was a vacuum leak you would not have the truck stall out when you unplug the IAC. If you had a vacuum leak you would not see such a drop in idle speed because air would be bypassing the throttle body through the vacuum leak already.
.
excellent point.
 
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Old 01-17-2019, 04:45 PM
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Problem solved, it was the TPS. Installed the new one today and everything is back to normal.

Just to recap in hopes of helping future troubleshooters for this era truck, my problem started out as an intermittent higher than normal idle. This progressed to a constant, very high idle. So much so that the truck would run 35 to 45 MPH with no additional feed from the throttle.

The key piece of evidence in my case was unplugging the IAC connector. This caused the IAC valve to close and the truck to idle off the mechanical stops on the butterfly valves. At this point I should have realized that the computer was receiving faulty info from one of the sensors and keeping the IAC open more than needed. However, I spent a couple of fruitless days chasing a nonexistent vacuum leak.

So my advice is this. If you have a higher than normal idle, do the easy thing first. Disconnect the IAC valve's connector. If the engine idles down, look for a sensor problem. If not, look for a vacuum leak.

Thanks to all who contributed to my troubleshooting efforts. It's nice to get a second opinion and have a second set of eyes looking at the problem.
 

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