Just looked back and saw I was responding to some posts while you were posting another one. So just to insure I wasn't being confusing, the vacuum gauge for the back pressure test is different than a MityVac. The gauge for back pressure is only a single hose with a gauge at the end. You remove an upstream O2 sensor (before the cat) and hook up the gauge. Then run your engine as described. On this test your looking at pressure. If pressure builds (over about 1-2 psi at idle) that indicates your exhaust is clogged to some level. As mentioned previously, I say use a vacuum gauge because most you get at an auto parts store show single psi accurately also. Basically the needle goes both ways (left & right) Whereas a compression gauge isn't good at showing such a low psi. Your parts store may have a exhaust pressure gauge for this purpose. You just want to look at the face of the gauge to see if it has single digits with hash marks for even lower readings in between each single digit for psi. HTH.

 

I'l give the back pressure a test, but I don't see how that could be the problem. The truck runs worse with a full tank of gas, and better when it's almost empty. Seems like with a clogged exhaust, that wouldn't make a difference.
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Yes, I agree, but I think there's an additional problem. The fact it stalls out like it does "or falls on its face" and the rich trims from yesterday's just makes me think it's not breathing right. So if there is a combo of that and high vapor from fuller tank, could be a combination. Its another diagnostic to rule out what it isn't to see what it is.

 

So I went through the rest of the procedure, and everything checked out fine. I never got the purge solenoid to open, even with the engine up to temp. But it wasn't open when it wasn't supposed to be. And at 2500 RPM, I had about 20" of vacuum at the throttle body port

 

OK, I'll draft up the steps to test the solenoid and the circuit from the VCM. Do you have a DMM?

EDIT: Can't do it till later though...getting ready to replace house water heater.

 

Yeah, I've got a multi meter. Appreciate you going through the effort for me.

Have fun with your water heater. Those suck to replace. Had to enlarge the opening at my sister in laws house. Apparently a new 40 gal is way bigger than her ancient 40 gal.

Edit. Hope no more tests require a mighty vac. Went to close the truck door, and forgot I had it hanging there, so I could see it from inside the truck. Crunched it, now it won't pump up past about 15" and the needle drops all the way down, instead of stopping at zero

 

Oh man, that sucks. Sorry to hear it. Try taking it back and see if they'll replace it for you. My local chain store is real good about that. Also, does your engine have a MAF sensor or just a MAP sensor?

 

So, I got to thinking. This problem started after I changed the pump and injector. So I popped the plenum off. Looks like there's been some washing around each injector port, and under the fuel lines. But with the pump energized, there's no visible leaks at all.

With the pump running, fuel pressure is only like 52 psi, which I remember it being higher after I changed the injector. Also, it falls off as soon as the pump shuts off.

It still confuses me, though. Low on gas, it runs better than when it's full. If it's not evap related, why would fuel level make a difference?


Here's what washing I'm talking about. Seems like something is leaking, but I can't see where

 

I only have a map sensor, no maf

 

Well fuel pressure is low, some reason had in my head you had changed that. I saw something when I was researching about a fuel wash and smell. But I can't remember if it was your engine or the other one. I'd have to try and find it. At the time I think I thought it was the other engine set up. Try Google, you'll probably find what I did.