Yes, VIN W is correct. I'll check those valuse you listed tomorrow.

I guess I don't really understand exactly how the evap works. I disconnected it from the intake, so even if it was bad, and allowing too much fuel/vapor into the engine, it shouldn't have affected the engine performance.

Am I missing something? Or is it plumbed into the intake elsewhere?

Edit: Not sure if i mentioned in the original post, but to clarify the symptoms: It starts and idles fine after getting gas. But opening the throttle more than a crack, and it falls on its face and sometimes dies. Oddly enough, if I manually shift it, and stretch the gears out (shift around 4k rpm) while having the same light throttle, seems to be the best way to accelerate when it's acting up.

 

My suspension is that after fill up fuel vapors are getting into the intake when they shouldn't and when you crack the throttle the mixture is too rich and is dumping it on it's face. As an example, a pretty well known issue with Hyundai Sante Fe's is when the purge valve is stuck open, immidiately after fill up the car won't stay running unless you left foot the brake and keep the rpm up to about 2k for the couple miles or so, then drives fine. Symptoms aren't exactly the same as yours and this isn't a Hyundai. But just as an example. Sadly, I'm not as familiar with your EVAP as I am for later models. I wish for your sake Captain Hook was still around, I think he had a '95 and could probably nail the problem in about two seconds. Maybe try doing some search criteria in the forum using posts from him and key word 1995 EVAP?

 

 

Hey Red, I was reading through your post and I noticed that you stated you taped off the hose going to your intake but didn’t state anything about sealing the nipple on the Purge Vent valve/solenoid the hose came off of? Again, I am not real familiar with a ’95 EVAP system but you may have created an air leak if the solenoid itself uses the engine vacuum to “push” the vapors into that hose. I don’t know whether the solenoid is solely a valve with the vacuum originating at the other end of the hose (under the throttle body I believe) or not. Also, how well did tape seal off that hose? Could you have had an air leak through the tape? When doing vacuum testing at hoses I use vacuum nipple plugs turned upside down and inserted into the hose. But I do believe I figured out something that was throwing me off concerning the tests on your system.

Later models have both a Vent Valve Solenoid (located near the fuel tank) and a Purge Valve Solenoid (located at the engine). The Vent Valve Solenoid is a “Normally Open” (N/O) solenoid that only is commanded closed by the VCM when the system is self-checking for leaks. I couldn’t nail down why I wasn't showing any tests for yours and then it dawned on me that possibly the ’95 doesn’t have an EVAP leak test, therefore it would have no need for a Vent Valve. The leak test may not have been added to Blazers until following years.

The tests I show for a ’95 are for the Purge Solenoid and a Vacuum Switch Test. But the Vacuum Switch Test is not a self-test. It’s a DC voltage test done with a DMM. Just to make sure it works. A basic description of how I think your EVAP system works:

There is a third fuel line that comes off your fuel pump and goes to the Vapor Canister. The Vapor Canister is located up in the rear fender by the filler neck in front of the rear bumper. It’s a rectangular plastic black box about the size of a thick Bible. There should be two fuel type lines going to the canister and probably a hose about 5/8” I.D. As mentioned, one of the fuel type lines comes from the tank. As fuel is added (or expands) in your tank, the vapors are displaced into the Vapor Canister. The canister has activated charcoal in it and it traps the vapors.

The second fuel type line runs up to your engine to the Purge Valve Solenoid. The ~5/8” hose on later models goes to the Vent Valve, but I don’t know where it goes on your truck. Can you trace that and see where the other end goes?

The purpose of that ~5/8” hose is to allow air into the canister to displace the vapors on their way to the engine. Picture sucking on a straw. If you put your finger over the end you would not be able to suck in any air (or vapors). So that hose allows air to go into the canister so the vapors can be sucked out and into the engine.

I think you are having one of three problems: 1) the Vapor Canister is bad; 2) the Purge Solenoid is not sealing…Note: the Purge Valve is a “Normally Closed” (N/C) solenoid and commanded open in percentages by the VCM. So when your truck is not running, it should be closed. As stated, I suspect either the Purge Valve is not closing all the way when your truck is off. OR another thought is it is opening too much when the VCM commands it to open. Since, the percentage it is to be opened is based off several factors (like engine temp for example), there could be another component that is sending the VCM bad intel (but I doubt this since you are not getting codes for other things); 3) the third possibility is that the EVAP line going from the Vapor Canister to the Purge Valve or from the Purge Valve to the intake is cracked or dry rotted.

 

Rock2p, I appreciate the effort you'e putting into this! We're out of town camping but I'll be back tomorrow, and will be able to provide you with more info then.

Here's where I'm at currently on the truck, though. Starting at the port on the underside of the throttle body, that hose goes to the two "modules" mounted on the intake plenum. The line that goes, I assume, to the rear of the vehicle is unhooked currently. Also, I have been leaving the gas cap off. The truck still runs like crap, but I have been able to keep more gas in the tank, and it is much closer to being driveable, lol.

What's weird, is I haven't gotten a single code for the gas cap being off. Even with driving 150+ miles to the state park we'e ccamping in.

 

Funny how a little bit more info adds. Your gas cap comment made me think maybe the '95 doesn't have a Fuel Tank Pressure Sensor. So I looked up a fuel pump on Rockauto and from what I saw of the image, I didn't see an FTP sensor. Then I fired up one of my tools and sure enough, there is no test for an FTP sensor. So that should answer why you're not getting a code for your EVAP system (small or large leak) because of the fuel cap.

But what I did notice is that there appeared to be four connections on your fuel pump. Three smaller ones and a large one that could be the size of that one hose I mentioned. Do you remember when you changed your pump if there were four lines as described going to it?

If so, this is a really different set up from later models and I'm wondering how the Vapor Canister is able to vent without a line being vented from atmosphere. I think when you want to work on it next though, we should try to map out your complete EVAP system so we can get some good direction.

One other question, based off your other comment...are you able to fill up your truck, or does the pump keep shutting off before it's full? If so, that is a classic bad Vapor Canister symptom.

 

The fuel pump/sending unit only has two fittings on it. Pressure, and return. However, IIRC, the tank itself has two vents on it. One at the front, and one towards the rear, closer to the pump. But I'm absolutely certain the sending unit only has two fittings

Also, I'm able to fill the truck up, no problem. I've been keeping it fairly low, since it seemed to help with how it ran. But before I made that connection, I was filling it all the way up.

 

Red, I put together an incomplete EVAP map/diagram for your Blazer as I understand it to this point (see pdf attachment). If you can answer the questions on the map for me I can do a better job that might not only help you but someone else in the future. Some comments:

-Your VENT VALVES are not to be confused with the VENT VALVE SOLENOIDS on later models. They work very differently. Yours just direct vapors into the VAPOR CANISTER during a refueling event or other fuel expansion event. We need to know where the second VENT VALVE is connected to. (Q1)

-On your VAPOR CANISTER there should be three connections, all labeled (stamped) on the canister itself as “PURGE” “TANK” & “AIR”. Where does the “AIR” hose terminate? Is it connected to anything on the other end? (Q2) I don’t think it is and is probably just secured up inside the body of your truck and open to atmosphere. The AIR hose is used to provide air to displace the vapors in the canister as the vapors are being purged (vacuumed) from the canister on their way to the engine.

-From what I have gleaned the VACUUM DIAGNOSTIC SWITCH is only used to inform the ECM that there is flow present. It is a N/C basic switch (0 volts drop) that I think is opened by vacuum from the PURGE SOLENOID. Once the ECM sees 12v due to switch being opened, it considers flow present. Otherwise you get the no flow code. The VACUUM DIAGNOSTIC SWITCH seems to be called a lot of different things, but I believe this nomenclature is the correct one. Also, on all but one image I saw of this switch, they all seemed to have three hose connections. Hose connections on left and right are obvious, but where does that third hose on the bottom of the switch connect to? Or, is your switch like the one I saw that only had the left and right connections and nothing on the bottom? Saw many images from different parts sellers and only one I saw had only two connections.

-We’ve been going down the trail of the vapors flooding the engine due to a faulty solenoid. But I found a thread on the forum that listed the same exact problem as yours (below). The guy said the VENT VALVE was the problem and he changed it and fixed the problem. If that’s the case, this means the opposite and of flooding since if the VENT VALVE is bad (clogged) that would mean no vapors are getting to the engine and it would actually be very lean, I would think. This makes me wonder if the ECM in a ’95 is registering a refueling event (based off the fuel level input) and is opening the PURGE SOLENOID more than regular percentage after refueling expecting a great deal of vapor and instead is getting nothing but AIR? Doesn’t sound right to me, but I guess it could be. Do you have a blue tooth dongle or any scanner that provides live data where you could watch the O2 sensors after a refueling event? Maybe having your wife or somebody drive? Just so we can see what they are showing for the first mile or so. Seeing the percent the PURGE SOLENOID is open after refueling would be fantastic also. Comparing before and after.

Anyway, take a look at the diagram and let me know if anything is not correct and please respond to the questions so I can make a better one. Also, while you’re taking a look at things, inspect all your lines for holes or cracks. See if the VAPOR CANISTER is cracked or broken also.
Once we get this all nailed down, we can start looking at some more specific tests (unless you find the problem beforehand of course).

EDIT: This is apparently for a '99, different type of Vent Valve so I don't think it applies to you: https://blazerforum.com/forum/2nd-ge...-problem-9834/

 

Dude, you rock! I'll get a couple of your questions answered tonight when I get home from dinner with my parents, the others I'll get to work figuring out after work this week.

I have an obd scanner that shows live data, I'll hook it up and check it out next time I fill up. Running lean actually kinda makes sense. I was thinking flooding also, but unhooking the evap from the manifold made no difference whatsoever. Running lean, and expecting vapors that aren't there might explain it.

 

Ok, Q1 and Q2 I'll investigate tomorrow after work.

As for the others:

Q3: That line does not exist on my truck. The "vacuum diagnostic sw" as you labeled it only has two lines on it.

Q4: That line connects to a port on the underside of the "throttle body." It is before the throttle blade, so it's not connected to full manifold vacuum.

Also, I appreciate the level of effort you're putting into this, man. That map you made is pretty awesome. Hopefully when we get this figured out, this thread can be of help to someone in the future.