94 XLT Engine Revving

[!KenRamsussen!]

The past day or so, my Explorer engine has been revving randomly. It occurs regardless of whether it is in gear or neutral, idling or driving down the interstate.

Any thoughts?

 

[Iron Weasel]

5-speed or auto?

CEL ever come on?

Have you checked for any codes even if the CEL light isn't on?

 

[!KenRamsussen!]

It is an auto and no CEL light is on.

 

[Iron Weasel]

Follow the information in this thread and see if there are any codes present. Even if the CEL isn't on, there could still be codes stored in the PCM, so you need to check and make sure. If you get three single flashes (111), that indicates a "System Pass" which means the PCM doesn't see a problem.

Off the top of my head, I can think of a few things that could cause it to randomly rev:

Fuel pressure too high / faulty fuel pressure regulator
Faulty Throttle Position Sensor (TPS)
Faulty Idle Air Control (IAC) motor / valve.

 

[!KenRamsussen!]

I also noticed that while it is idling, with the hood up, I can hear a soft rattle or popping/puffing noise occurring on the passenger side of the engine compartment when the engine speed changes.

 

[Iron Weasel]

Probably a misfire on one of those cylinders that's causing the popping / puffing.

I'd still like to know if there are any stored codes in the PCM though. Just because the light isn't on doesn't mean there isn't a code and if there is one, it'll make troubleshooting a lot easier. Otherwise, we're just going off theoretical situations and "what if's".

On a slightly related note, I just replaced the PCM in our 94 as well and it was exhibiting similar symptoms. I had the popping in the exhaust, loping idle, O2 sensor codes, etc. Did some diagnostic work on it and traced it down to the PCM being bad. Not saying that you have a bad PCM in yours, but the more information there is to go on, the more accurate the diagnosis.

 

[Roadrunner777]

I agree, and the first thing in my head was also a fuel regulator. You might pull the vacuum hose on it and see if you get gas. If so, it's shotski.

Pull codes and get fuel pressure, pressure will be tricky because you will have to catch it when it is failing.

 

[!KenRamsussen!]

Stopped by the local O'Rileys on the way home from work to have them pull the codes. Plugged the code reader into the EEC plug and there is no power to the code reader. Check the fuse and it is fine. I had them use the code reader on a different vehicle and it is working.

What does this now mean?

 

[Iron Weasel]

The DLC gets power from the PCM, so if you're able to drive the truck, you obviously have power to the PCM. The only basic check you can really do is to swap the PCM relay in the Power Distribution box with another one and then try the paperclip method of pulling codes I linked to earlier. If you still don't have power to the DLC, then you have a wiring problem between the PCM and the DLC.

However, I would try the paperclip method first just because a lot of the parts counter monkeys tend to try and use a OBD-II scanner on every vehicle, OBD-II or not...and the first gen Explorers were EEC-IV, not OBD-II.

 

[Roadrunner777]

There are two connections they needed to make, the big connector (4 or 5 tabs) and a separate wire. They might have missed that. Or, wiring issue, or bad PCM. If you are positive they tested correctly, then the next step is to track back and find out were the voltage stops. Could be a bad PCM.

 

[!KenRamsussen!]

So, I followed the thread and obtained the following codes:

KOEO is 565
CM is 124, 416, and 543.

 

[Iron Weasel]

So, I followed the thread and obtained the following codes:

KOEO is 565
CM is 124, 416, and 543.

Diagnostic Trouble Code (DTC) 565 or 569 indicates a failure in the Canister Purge (CANP) solenoid circuit.

Continuous Memory Diagnostic Trouble Code (DTC) 124 indicates Throttle position sensor voltage higher than expected.

Continuous Memory Diagnostic Trouble Code (DTC) 416 indicates Idle Air Control system at upper adaptive learning limit.

DTC 543 as a continuous memory code by itself doesn't mean a whole lot unless it's accompanied by DTC 556 as well.

I'll post a more thorough rundown in the morning, but I will tell you that you're going to need to need to diagnose codes in the order receive with KOEO first, then CM. So 565 first, 124 second, 416 third, and won't really worry about 543 right now.

 

[Iron Weasel]

DTC 565.

Find the canister purge solenoid. It is located in the vapor line between the fuel vapor canister and throttle body and has two vacuum lines on it and a 2-wire electrical connector. Disconnect the wiring to it and then measure the resistance across the two terminals on the solenoid itself. Resistance should be between 30 - 90 ohms. If it isn't, replace the solenoid.

If the resistance is OK, turn the key on and check for voltage on the wiring harness you disconnected. There should be at least 10.5v on the red wire. If there isn't then you have an open circuit and you need to find & repair that. If you have at least 10.5v on the circuit, then you need to check continuity on the CANP circuit.

To check the continuity, turn the key off, disconnect the PCM which is located behind the kick panel on the front passenger side. Test continuity between pin 31 at the PCM and the GY/Y wire at the CANP harness. If there's continuity, check the resistance also. Should be less than 5 ohms. If not, you have an open circuit on that line. If you're OK so far, then you go to the next step.

Now, you need to measure resistance between the following pins at the PCM harness.
31 & 40
31 & 46
31 & 60

Resistance should be greater than 10k ohms. If not, the circuit that isn't has a short to ground and needs to be repaired. If they all have 10k+ ohms, you need to check the following pins:
31 & 37
31 & 57

If the resistance on either set of pins is greater than 10k ohms, you have a bad PCM. If the resistance on either set is less than 10k ohms, you have short to power that needs repaired. After testing is complete, plug all applicable wiring back into the component you removed it from.

Once you get this DTC taken care of, let me know and we'll hit the next DTC which is 124.

 

[!KenRamsussen!]

Finished checking the steps outlined for the 565 code.
Everything checked out within working ranges. I hope that is not a problem.?.?

 

[Iron Weasel]

Finished checking the steps outlined for the 565 code.
Everything checked out within working ranges. I hope that is not a problem.?.?

Everything shouldn't have checked out or the DTC wouldn't have been set. :

1) Canister Purge Solenoid resistance between 30 - 90ohms?
Yes / No

2) At least 10.5v on the red wire at the CANP harness?
Yes / No

3) Resistance between pin 31 at the PCM and the GY/Y wire at the CANP harness less than 5 ohms?
Yes / No

4) Resistance between pins 31 & 40, 31 & 46, and 31 & 60 greater than 10k ohms?
Yes / No

5) Resistance between pins 31 & 37 and 31 & 57 greater than 10k ohms?
Yes / No

 

[!KenRamsussen!]

Each one of those items would be Yes. Additionally I replaced the Canister Purge Solenoid just for giggles.

 

[Iron Weasel]

Each one of those items would be Yes. Additionally I replaced the Canister Purge Solenoid just for giggles.

OK, then going down the list and answering "Yes" to each step on the diagnostic leads you to the last step which states:

KD10 - Check CANP Circuit For Short to Power

Key "OFF".
Canister Purge (CANP) solenoid disconnected.
Breakout box installed, Powertrain Control Module (PCM) disconnected.
Scan Tool disconnected from Data Link connector.
Measure resistance between test pin 11, 15, 31 or 54 (refer to Testing Schematic) and test pins 37 and 57 at the breakout box. See: KD - Testing Schematic
Is each resistance greater than 10,000 ohms?

Yes -- Replace PCM. Remove breakout box and Scan Tool. Reconnect all components. Rerun quick test. See: Reading and Clearing Diagnostic Trouble Codes\Reading Diagnostic Trouble Codes\Quick Test Appendix (Detailed Testing Instructions)
No -- Service short to power. Remove breakout box and Scan Tool. Reconnect all components. Rerun quick test. See: Reading and Clearing Diagnostic Trouble Codes\Reading Diagnostic Trouble Codes\Quick Test Appendix (Detailed Testing Instructions)

So if you answer yes to this one also that you have MORE than 10k ohms resistance between pins 31 & 37 as well as 31 & 57 with the CANP harness and PCM disconnected, then you have a bad PCM.