I'm stumped! Help with high idle problem when purge valve open

[SonicWeaver]

Looking for some help with diagnosing a 1506 code on my '98 4.6 that appears to be caused when the purge valve is open.

When the purge valve opens idle rpm jumps several hundred from around 600 or so to 900 while in gear (800 to 1100 in park) and stays high for anywhere from a few second to 20 to 30 seconds and then returns to normal when the purge valve closes and continuously cycles in this way.

Other than this problem it runs smooth and strong.

I have smoke tested from the PCV hose and show no leaks anywhere. Have also smoked the purge line from the purge valve to the canister while closing the vent. Again no leaks.

Swapped or replaced: Purge valve, MAF, IAC valve and PCV.
O2 sensors look fine.
Short term trims are fine when purge valve is disconnected but show lean when purge circuit is operating.

It was idling higher (900-1400 in park) and I had a hard time getting it to staying at a specific RPM when checking trims. Say trying to have it stay at 2000. It would rapidly ramp up and overshoot then dive back down. Replacing the MAF changed this and made it less jumpy and lowered the idle to the 900 - 1000 range with purge control on.

When the purge valve is closed is idles fine (600-700). If I unplug it I no longer get a 1506 and it idles correctly but obviously it throws an evap code.

This thing is screaming vac leak but I'll be damned if I can find it.

Here's a missing peice of info I'm looking for help with. I know that the purge valve is pulse width modulated but I can't find what the duty cycle should be. The only thing that I've found says it modulates a couple of times a second which is not at all what I'm seeing. The amount of time (seconds to dozens of seconds) it is currently staying open appears to be way too long and would explain why the idle is so spastic.

Does anyone know what this open/close cycle should be?

I've swapped and tried 2 new valves and the symptoms don't change so I think the valve is good. It seems like the computer is send the wrong control signal or a purge valve drive device that is bad.

If the duty cycle is bad what could be causing the long open signal?

Appreciate the group's insight!

 

[Yupster Dog]

I've swapped and tried 2 new valves and the symptoms don't change

Check the wire harness.

Welcome to the forum

 

[SonicWeaver]

What wire harness...the purge valve wires?

The valve does turn on and off so I'm guessing it must be fine but I will check for power at the connector pins.

 

[Yupster Dog]

I will check for power at the connector pins.

Inspect the wire also and where it goes to make sure not near any hot stuff, no signs of rubbing....
Just suggestions that you can disavow and move on, or find something else totally while checking.

 

[SonicWeaver]

Can someone tell me what the proper purge cycle is?

After warm up and when idling in gear is turning on for ~15 second and then turning off for ~18 seconds. My RPMs go from around 600 to 900. The gas cap (which I have replaced with not change of symptoms) is making loud sucking noises in rhythm with the purge cycle.

I am up a creek without a paddle at this point and may not have any option except to junk it as it will not pass smog.

Is the a chance someone can take a look at theirs and tell me what the cycle time is?

Any help is appreciated!

 

[JVinOlathe]

How did you run the smoke test? Did you use a smoke test machine or just a water or starting fluid spray? Without proper equipment, smoke test might reveal the obvious and visible leak but will not show smaller or hidden leaks. I had a vacuum leak issue on my '99 5.4, and I could not find it. I took it to a shop where they run a proper smoke test and found the leak in the throttle body; once the leak was found, the repair was fairly simple. Also, check the elbow booth on the PCV valve, it frequently leaks. Somethi9ng to think about.

 

[SonicWeaver]

I used a smoke machine.

Where was your throttle body leak?

Replaced the PCV and double checked the elbow.

The odd thing here is that if I disconnect the purge valve it idles perfectly and does not throw the 1506 high code. With it functioning the idle runs 300-400 higher when it's turned on and then returns to normal when it turns off.

 

[SonicWeaver]

** FIXED!!**

The problem was the Dorman 911-231 Vapor Canister Purge Valve. I purchased it because reviewers on Amazon said it worked for their Expedition and the part is made in the USA.

I had replaced the original twice with this valve and assumed it was functioning properly after the 2nd swap because it behaved the same way. This sent me on a wild goose chase for a vac leak that didn't exist. When this valve turned on I got full manifold vacuum to the charcoal canister which I now know is not correct and the source of the phantom vac leak.

My last attempt to figure this out before dropping $300 at shop for a full diagnostic and smoke test to find the leak was to try a Motorcraft CX1597 Vapor Canister Purge Solenoid. It worked instantly and the truck passed smog yesterday!!!

The difference was that when the OEM valve opens there is only a fraction of full manifold vacuum to the charcoal tank. So the orifice must be smaller or the valve does not open completely. Regardless it fixed the problem.

Live and learn about using aftermarket parks. Some work and some don't but when they don't you can loose a LOT of heartbeats and hair trying to fix something that is being caused by the new "good" part.

 

[aircooled]

I had a similar problem with my 2003 F150 with the 5.4 engine. I had a P0443 code show up. However the engine would run fine. I cleared the code and it would reappear. I changed the purge valve with a Dorman 911-231. The first two times the truck went into the purge cycle the engine died. The next few times the engine would stumble but recover. It got better and better until it was virtually unnoticeable. I think the computer must have been learning what to set the fuel trim to when it the purge cycle but I'm guessing. I connected my scan tool and saw the short term fuel trim on both banks would jump from less than 5% to around 20%. I connected an LED to the purge valve control wires and hung it in front of the dash so I could see when this was happening. Sure enough the high trim levels corresponded with the purge energized. After reading this post about issues with the Dorman part I compared the internal size of vacuum ports to my Ford part. The internal size of the canister port on the Ford part is much smaller. Of course I didn't discover this until I had leak checked the system and removed the evap canister to inspect it with no problems found. With my newly acquired 3D printer I made a restrictor with a hole diameter of about 0.090 inches that would slip into the purge valve. l used PETG plastic which is fuel resistant. It's the blue part in the pic stuck in the port. With the restrictor installed short term fuel trim stay around 5 or less with purge on or off. Thanks SonicWeaver for your post. It set me on the path to a solution!

 

[jtoth32377toth]

Problems with junk new parts seems greater then ever. If you question a new part as I have offten, A visit to pull and pay is in order. Find a wrecked one, I.E. running good when hit, and go parts shopping. Will done.