Alright, i'm hopping on my diagnostic train here. Code 31 is a egr circut below min voltage.

Now, if memory serves me right, on the top of you EGR valve should be an EGR position sensor. (or do you have a DPFE sensor on yours?)

I know on my 85 F150, i had a code 31, and on top of the EGR valve was a sensor, the voltage was just a little below spec, and only because of some manufacturing differences. Some people removed the sensor and put some epoxy on the end, some people just dropped a penny down the hole where the sensor sat, and it was enough to "shim" the sensor so the voltage reading was correct.

I would imagine that your 88 is similar.

I'm sure someone on here has access to what the EGR sensor should be. You can then grab a DMM and test the voltage, remove the sensor, make sure that the voltage vaires as you move the end of the sensor in and out.

good luck, I hope it is some help.

Also on a side note, after 20 years, your numbers are pretty darn good!

 

The egr valve is activated by a ported vacuum signal (vacuum signal at throttle body, only present when throttle is open). manually opening the valve with the truck running at idle speed should produce a very rough idle, nearly stalling or stalling. If no rough idle occurs during this test, the passages in the egr system are clogged. Test for a vacuum signal at the egr valve by hooking up a vacuum guage to the vacuum signal line to the egr. No vacuum signal indicates a problem between the valve and the throttle body, possibly the thermo or ported vacuum switch. Temperature activated, closed when engine coolant is cold. Check when engine is fully warmed up. The timing should not have to be rearded from factory spec to pass the emissions test. Hope this helps you. Dan.

 

Peter94 - If I didn't mention it before, I did apply vacuum to the valve, and it does hold open. I also pulled the valve off, and checked from the opening in the manifold all the way to the opening in the intake..... nothing, everything is clear. You are correct, and there is an EVP sensor on the back of the valve. I thought it could be bad, but I swapped 2 others with the same result. I am starting to think I am not getting enough juice from the charging system/battery. It is only 2 yr old, but it does lose voltage over night. I just put the meter on it while running and I only get 13.6v instead of 14.x plus. Don't know if it is related, but I thought it might be of interest.

y2kfordparts - As I mentioned, the valve hold open just fine. I don't know how much vacuum it is getting while running since I don't have an inline vacuum gauge. I did notice the way the hoses run out of the manifold and into the valve is kind of odd. It loops around into this plastic chamber/PCY looking thing and then into the valve. I think I will need a drawing to accurately explain it.

 

Hey Y2kfordparts. I thought the EGR ran off of manifold vacuum? Or are you talking about the older style that did not have any actuators, and just had the valve hooked strait up before the TB/carb? Otherwise I think his is manifold vacuum and is controlled by the solenoid.

13.6V is just fine for your battery by the way.

Code 31 is caused by the sensor on the top of the EGR having low voltage. the next step we need to take is to figure out what the min and max reading for that sensor are supposed to be. Then hook up a DMM to it and see what the reading actually is. If it is a little bit below specification, you need to remove the sensor, and push the rod in with the DMM hooked up. If the voltage changes like it should, then you need to add some epoxy or a spacer to it to get it to the correct lenght.

If the output voltage of the sensor does not change when you move the rod in and out, then you most likely need that sensor. Otherwise it can probably just be corrected by adding epoxy to the end of the rod to make it longer, thus bringing the voltage up and back into range.

When you remove the sensor you will understand what i'm talking about. It really has nothing to do with the egr solenoid.