EGR code 34 really starting to bug me....

Victim: 1986 E-250 van w/302 V8 fuel injected 150k miles Recently had the TFI battle whereupon I changed the TFI module, Ignition coil, rotor, cap, pcv valve and breather filter. Vehicle now
starts where it was dead before. HOWEVER, after running my diagnostic tester I get a #34. Removed the electronic EGR/EVP assembly and found it plugged with carbon. Cleaned to the point where I can apply a vacuum and the plunger/popett will move smoothly up and down. While there I cleaned (as best I could the matching inlet hole in the manifold with carb cleaner, round wire brush and paper towels). Reinstalled. Ran the diagnostics (did a clr before to be sure NOT reading the same/leftover code again). Still get a #34. Following the vacuum routing diagram vacuum comes from the vacuum canister to the EGR vacuum control selenoids assembly(2 small selenoids side by side). It appears only one controls the vacuum to the EGR valve. I started the vehicle and measured the voltage on the two electrical leads 12 volts +/-. Ran a 'run' test and the voltage never disappeared (remained 12 volts - normal??) Upon completion still get a #34. Manually energized the egr selenoid valve and could hear a hissing noise and felt small amount of vacuum at the egr fitting. Started vehicle again, at idle attached a manual vacuum pump and when it got to about 10in vacuum the engine dropped rpm and started to stumble. This tells me the valve is working and while there may still be some residual carbon in the intake there is still enough open area to allow the EGR system to work. My questions then are: is it normal for 12 to be constantly ON at the controling egr vacuum control selenoid? if not what is keeping it on? While the egr selenoid valve was letting some vacuum through can/could the valve be blocking the vacuum needed to move the valve? Would a defective EVP valve cause these problems (selenoid energized, no stem movement)? AS you can see I am stumped by contradictions to evidence. I don't like NOT being able to solve a problem therefore ANY and ALL help and suggestions will be greatly appreciated....... Look forward to your knowledgeable advice.....Thanks...
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snipped-for-privacy@yahoo.com wrote:

The Definition for DTC 34 on your van is " EGR Valve Position/Pressure Feedback EGR Voltage Above Closed Limit" What this is telling you is that the PCM is "seeing" the EGR valve open when it isn't supposed to be.
At the EGR vacuum line, at idle, it is not uncommon to feel a slight vacuum. I believe it is usually 1-2"hg. At the EVR solenoid there should be 12 volts at all times, the PCM controls the "ground".
You can do a quick check of the EGR valve and solenoid by connecting a test light to Battery - and back probing the connector with the engine running. On one side the lamp should be bright. This is 12v (VBAT). Back probe the other (ground) side of the connector. This will slightly energize the solenoid and partially open the valve. This will tell you that the solenoid and valve are able to operate.
With code 34, like I said, the PCM thinks the egr valve is open when it shouldn't be. The valve may or may not actually be open. There are 3 things that come to mind that can cause this: 1. EGR valve has a layer of carbon keeping the pintle up slightly, fooling the EVP.(Or a chunk holding it open, along with poor idle). 2. EVR filter clogged allowing vacuum to the EGR valve at idle 3. EVP sensor shorted internally sending a false signal to the PCM There could also be a high resistance short in the wiring to the EVP sensor or other wiring concerns but these are uncommon. At 150K it could be any or all of the above.
First, check the filter in the EVR. It's under the round cap on the solenoid. Clean it with hot soapy water. You can just remove it for testing but be sure to clean and reinstall it. If this is not the cause of your code 32 then look at the EGR\EVP asy. You mentioned that you had to clean the valve due to carbon fouling. In my experience, although you can free up the valve so that it will move smoothly, it is difficult to de-carbon the pintle area so that it can fully seat. Even though the valve may seal, the pintle is up a few thousandths of an inch causing a false reading at the EVP sensor. Speaking of the EVP, these were a high failure item on EEC-IV vehicles. You can test it with an ohm meter if you care to, but after 19 years and 150K miles.........well, you get the idea. If this were my vehicle, I would just replace the EGR and EVP and call it done. For yours I would suggest at least a new EVP. If the code is still present, grind about 1/32" off of the EVP shaft so that it is sending the proper "EGR closed" signal to the PCM. I've had to do this a few times even with new parts due to manufacturing tolerances. If you happen to have the factory Emissions Manual (Volume H) for your truck, it will tell you what the feedback voltage from the EVP should be when the valve is fully closed and how to test the EVP resistance. You won't likely find it in Chilton or Haynes. The info is also available on Alldata for a fee.
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Fully concur with Tom's elaborate and accurate response. I would like to stress one point from my experience of many years maintaining two vehicles of that vintage (one is still in service). The EVP sensing potentiometer is designed very close to the limit of the tolerance. Unless the pintle is perfectly seated, it will throw the the 34 code. Many a time I was tempted to 'adjust' the EVP tolerance by grinding a bit off the shaft, like Tom suggested, but with a bit of patience always found it unnecessary. If you don't want to throw out the otherwise perfectly functional EGR valve each time this happens, take it off (no need to separate the EVP), keep it open with a handheld vacuum pump attached, and carefully scrape both the pintle and the seat. A curved scraper is ideal; a small screwdriver will do. When you can't see any more carbon falling off when you tap it, scrape some more anyway. And then a bit more... To test, just release the vacuum, temporarily reconnect the harness to the EVP and re-run KOEO test. If it does not throw the code, you are done. If it does, scrape some more. Patience is a virtue... I remember actually determining the exact tripping point for the code and figuring out the exact EVP resistance at which the code is set, then using an ohmmeter attached to the EVP as I was scraping the valve on the bench, instead of going through the slow KOEO routine. No, I don't remember the number - you are on your own with that one... And it's probably different from one valve to another anyway, due to manufacturing tolerances.
BTW: Rumor has it that at some point Ford came up with a screen-type filter that was supposed to be installed on the exhaust port of the EGR valve adapter, and that reduced the carbon deposits. I have never seen it.
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Happy Traveler wrote:

And I too agree with you. My only argument is that I've spent a lot of time cleaning egr valves as you've described. It can be done. I'm a little older now and prefer to spend the ~$60 rather than spend ~2 hours cleaning a 15 year old EGR valve (If on my personal vehicle, Murphy dictates that the diaphragm fill fail exactly 8 days, 4 hours and 12 minutes after reassembly). If the vehicle were newer and had 75K fewer miles, I'd do it your way for sure. You say tomato.... IIRC the code will set at .8-.9 volts on the signal return, it's been a while. I believe the filter you're talking about is called a carbon trap. It was designed for carb'd and CFI vehicles that had the EGR in the spacer plate. These had terrible carbon problems. It replaced the EGR cooler used on many early to mid 80s cars. It won't help with carbon buildup in the valve, it filtered out the boulders that formed in the spacer plate then lodged the EGR valve open. I've never heard of one for an EFI vehicle.
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<snip>

http://www.tomco-inc.com /
Second part down from the top...
An e-mail to Tomco will get you the name(s) of stocking dealers in your area.
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