Well guys, the hits just keep on coming! I've been slowly working on a cold start problem lately, the blazer would always start up just sometimes it would take a long crank in the morning (getting worse as of lately). This morning when I started the blazer up I noticed it dipped 2 times right when it started, like it was close to dying. It hadn't done this ever before and my mind went straight to possibly a fuel delivery issue, however, it seemed to be running great after those 2 dips so I proceeded on my way to work. The blazer made it 3km from my house before I stopped at a red light and it died, I tried starting it again and it ran very poorly and then died when shifting into drive. I got it towed home then tried starting it again, and this time it was running flawlessly and the computer confirmed by telling me no misfires and the fuel trims all looked in check, sensors all looked good with it running in closed loop. After work I started it up again and took it for a drive to get it up to operating temp, all was great. I will test the fuel pressure when I get a chance and report back (I'm thinking tomorrow morning since that's when I've been having problems so far). I'm kind of suspecting the fuel pump since the rest of the fuel delivery system is new, I will need to run some tests and see if I can catch it acting up to confirm though. Les, George, and everyone else, I'm looking for your help to get this one sorted out and get my blazer back on the road quickly! Let me know what you guys are thinking.

 

I think your are on the right track with testing the fuel pump. Start by testing at the fitting next to the distributor. With KOEO, after the fuel pump pulse you should have around 58 psi. This should hold to somewhere around 5-15 psi of original for around 10 min.

 

Thanks Les! I'll test fuel pressure tomorrow morning before I leave for work and let you know what I find. The thing I'm having trouble understanding is how the fuel pump could go from running very poorly, to flawlessly, with doing no work or anything. That question goes for the whole fuel delivery system. I guess the fuel pump would be the most likely for this as I have heard of knocking on the gas tank to get a dead fuel pump to work and get you home etc... I'm also wondering if it's failing like this how would that show in the fuel pressure test? I wish I was more on top of it when it died in the middle of the road and listened to the fuel pump noise while it was acting up.

 

Those dang intermittent problems can be difficult..... sometimes have to catch it in the act with test equipment to know for sure. If it is putting out anything over 58 psi at the test port at the time, then the pressure regulator will regulate to 58 and you see nothing wrong.

You could stress the fuel system and make it do maximum output by getting the required fittings and testing at the fuel filter per the sticky at the top of the 2nd gen forum. It might be putting out just enough pressure to run good sometimes and then it falls on it's face. For example it you test maximum pressure output and it is only 65 psi, then the pump is definitely bad. These pumps can put out over 100 psi (sticky says spec is 73-108 psi). One dated error in the sticky is with the newer generation fuel injectors they do not require a specific pressure (ie 60 psi) to "pop off" and deliver as the injector itself is electronically opened. Also on the newer fuel injectors, fuel pressure is regulated to 58 psi with KOEO, and less after engine starts.

Or you could just take an educated guess and spend the time putting a new fuel pump on. Be careful not to break plastic hose fittings when dropping the tank. I have heard that AC Delco pumps are the only way to go. I have never put a pump in my Blazer.

 

Thanks Les. Got out there and did a leak-down test, results follow. 57 psi after cranking (close enough to 58 I will say that is the regulated pressure and 1 psi error on my gauge), the leak-down was amazing stayed within 5 psi for much longer than 10 min (about 20 min then I disconnected the guage). I'm now off to the parts store to set up my dead-end contraption, hopefully they have everything I need. Might be able to get it done today will have to see the time when I get it all set up. To what you said about taking an educational guess, it definitely sounds like a fuel pump issue hey? I have just been wrong so many times before and it feels so good when you finally see firsthand the problem and see exactly what's going on. It will sure be nice if the dead-end test gives me something to go off.

 

The holding pressure verifies the check valve in the fuel pump and the pressure regulator are just fine.

Like you say, let's see what the dead end test tells us.

 

So I called around and seems nobody has the 1/4" MFL to 3/8" hose barb adapter near me, I think I will have to order one online if I decide to pursue this way. Is there any other way I can test the fuel pump without the contraption?

 

Nope.... AFAIK.

 

Thanks Les, I will see what I can do about that adapter, maybe just get a non-flanged one from in town and see if I can make that work. I got back out and took it for a drive after I realized I wasn't putting the dead-end tester together today, and to my surprise, I noticed something interesting. So the fuel trims were pegged long term -12.5% on both banks at idle (different from the last time I checked and said everything was looking good!). Then I was driving it around and of course, the fuel trims were bouncing all around everywhere (not a big deal). The interesting part was when I got home and started playing with it some more. I was looking in park holding at about 1200 rpm and the fuel trims were looking decent, then I let off to idle and the short terms immediately went way negative (-30% on both banks! long+short) and I could feel the engine start running rough and hesitate a bit when I pressed the throttle. This is the weird thing, it held at -30% for a few good seconds (short+long), then went to -20% long term on both banks with the short term bouncing around. Then returned to -12.5% long-term after a minute, and I could not repeat this after every time. I got it to show me the -30% fuel trims twice and both times it was right after I was holding the rpms at 1200-1500rpm looking at the fuel trims. It's also weird because high negative fuel trims are the opposite of what you would expect to see from a weak fuel pump right? I would expect to see positive fuel trims and then going even more positive under load? The leak-down test confirmed I don't have any leaking injectors or any leaks under the plenum that would cause it to run rich, so I'm not too sure what to make of what I was seeing on the computer this evening.

 

I generally agree with your fuel trim assessment if it is detecting fuel/air mix correctly. Is it carboning up plugs from running rich?

On the other hand, if the PCM thought it was too rich and it really wasn't - then taking away fuel would make it run poorly. Still I wouldn't think it would stall.

Neither of these leads to fuel pump. But maybe you should go ahead and complete the test on it. Let's think on this a bit. Maybe look at temp and maf and map sensors, etc.
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