Here ya go

George

 

I have a result.

I removed the purge solenoid altogether and plugged the vacuum port. Trims level off. Bank 1 LT was between 0 and -1.56. Bank 2 was between -3.51 and -3.91. These values did not vary too much with throttle. I reinstalled the solenoid and fired it back up and trims started drifting way negative again. Removed solenoid and plugged vacuum port once again and trims stabilized. The solenoid was not plugged in during the solenoid removed tests. I'm not sure how the computer deals with this in regards to air/fuel mix. I'm trying to figure out how to interpret this data. If the purge solenoid was stuck open, the computer should have thrown a code during a drive cycle correct? If the gas tank is in a vacuum state and won't build pressure that would throw the code? Like when you leave the gas cap off?

Edit: And if the purge solenoid was stuck open, why would trims go way rich if vacuum in the manifold drops during throttle? It would be sucking in less fumes from the tank?

 

The purge valve (near the engine) is normally closed and is only opened on an occasional basis to draw the stored vapors from the charcoal cannister to the engine intake. The vent solenoid (by the gas tank) is normally open to vent the tank for filling and draw down and then closed on occasion to pressure or vacuum test the system for leaks. A stuck open purge valve should essentially be a source of unmetered are creating a lean condition requiring a rich command and positive fuel trims. Yes the engine would eventually set a code as a large leak the next time it vacuum/pressure tests the system.

Was the rest of the evap system hooked up or did you just reattach the isolated purge valve?

George

 

All I did was remove the two bolts holding the valve in, popped it out, popped the bolts back in to hold that side of the plenum down, then plugged the vacuum port where the valve went.

 

Im wondering if you have two problems then. A leaking purge valve that should be normally closed and its sucking gas vapors into the intake. Try leaving the purge valve on but disconnect the evap pipe on its way to the back of the truck and see if you go lean and command rich

George

 

I can't actually figure out how to disconnect the tube from the valve. Looks like some sort of c-clip? Know any tricks so it doesn't go springing off on me?

 

No I don't know what that clip looks like. Maybe someone else can chime in.

George

 

Back at it again with more graphs!

I was able to disconnect the line running into the Canister Purge Valve (Thing on intake plenum). I was not able to blow through the valve so it did not appear to be stuck open. I installed the valve back into the intake plenum with the line disconnected and the electrical connector disconnected as well. I started the car and it started trimming a bit rich. I let it warm up for about 10 minutes and bank 1 got down to around -2.34, while bank 2 got down to -4.69. Not entirely bad, it's an old car with 190k on the engine, I'm not expecting perfection. After 10 minutes, I reconnected the electrical connection to see how the purge valve would operate. It was opening and closing quite fast. I could feel vacuum at the port where the line connects. I'm not exactly sure if the rapid opening and closing is intended operation. (Edit: I don't believe it did this pulsing with the evap line connected. I don't remember hearing the constant clicking.) Shortly after, I pressed the evap line back onto the connection of the purge valve and trims immediately dropped HARD. I let it run like this for a few minutes so it would show on the graph. I then unplugged the purge valve electrical connector and trims started turning towards lean again.

I'm not sure if this indicates a bad purge valve... I'm not actually sure what this indicates. I shook the charcoal canister at the rear of the blazer and it didn't sound like it was full of gas or anything like that.


Graph showing when I reconnected the canister purge valve.

 

I have been going back through this thread and I am confused about a few things stated here.

First some terminology. There are two solenoids. One at the engine called the evap purge solenoid and one more at the rear of the vehicle called the evap vent solenoid. At rest with engine off the purge solenoid should be closed and the vent solenoid should be open. The vent solenoid is commanded full on or full off with a dc voltage and spends most of its time open, only being closed for a routine vacuum test of the system to look for leaks and set codes. The purge solenoid is controlled by a pwm signal, a square wave with variable duty cycle (on vs off time). The vent solenoid clicks on and off slowly. The purge solenoid may chatter or click faster.

Some of the fuel trim terms are getting confusing. Lets say commanded lean/negative fuel trims or commanded rich/positive fuel trims. If we say rich fuel trims I dont know if we commanded rich for a lean condition or commanded lean for a rich condition. Also, you must always report both ST and LT fuel trims together, mindful of sign for a given bank.

Again, the purge solenoid is open and closed with pwm control. That means a square wave of variable duty cycle is applied which also means that the valve may be perceived to chatter or open and close continuously. More on time, more flow, less on time, less flow. The purge valve is supposed to stay closed until the engine reaches an operating temperature of 113F. After that the purge valve is opened under certain conditions to pull the fuel vapors into the engine from the charcoal cannister but I do not know the frequency of this process and the shop manual does not discuss this. It is also opened and closed in coordination with the vent solenoid to check for leaks. I think that this is a once per drive cycle kind of thing. First a check for large leaks (cant pull a vacuum) and then a check for small leaks (cant hold a vacuum).

One test would be to see if the opening and closing of the purge solenoid starts only after a cold engine warms up to at least 115F.

As to the severe negative fuel trims when the purge valve is operating and all of the plumbing is hooked up: does this truck get filled at the gas station with a bunch of extra squeezes at the gas pump to fill the system all the way up to the top of the filler neck? this kills the charcoal cannister by flooding it with liquid gasoline instead of just scavenging vapors. That may be why the system goes rich and commands lean when the purge valve is operating. That or there may be a leak that allows gas vapors from the tank to be pulled in. Do you have any evap system codes?

In a post a while back you said that fuel trims were different between and uninstalled purge valve with no electrical connection and a sealed intake port and an installed purge valve with no electrical connection. That implies an open purge valve with no electric signal. In another post you said that the purge valve appeared closed with no electrical signal. Can you clarify this?


George

 

I have no evap system codes. The only codes before I started troubleshooting were Rich on both banks and random misfire. Longterm and short terms were both around -25 when it started dying on me while driving.

I don't do a lot of extra squeezes when filling up, although I filled up right before this happened and the pump did that thing where you have to hold it at an exact low flow rate or it will stop. I believe this indicates the vent solenoid might have been closed? I fill up with the truck off.

Unplugged and removed appear to result in the same trimming, between 0 and -5. When I first started poking at the purge solenoid, I might not have waited long enough for a difference to start showing in fuel trims when unplugging it. I've been very sick lately and just now getting better, head is a bit clearer now lol.