I had a fuel pump installed last year when my wife had it die on the highway. unfortunately it had to be towed to the nearest "gas station". the guy swore it was a Delphi as I requested but he left the empty Japanese box in the back seat.


2 weeks later had fuel pressure issues. after starting it would idle and jump around at 50 PSI. when driving I had bucking and stalling problems. I own 2 other blazers (yes I do!!!) so I hooked up the FP gauge at the Schrader valve and both trucks started and idled at 60 PSI and when accelerating they would go up past 60. both never dropped past 60.


I believe the poppets open at 40-45 PSI? once it's running it should never drop below 60. hope this helps. everybody agree?


and by the way i'm going to court with the mechanic as I type this.

 

You are testing regulated fuel pressure and that sounds ok. But definitive test for fuel pump is dead ended fuel pressure and dead ended leakdown. Just follow instructions in the sticky. Alternatively you could punt because of time you are noticing to build up pressure seems excessive, but just so you know that you have not truly proven anything yet and it's both expensive and a pain to install. Best wishes however you decide to proceed.

 

OK. Rather than speculate, I just looked at a General Motors reference document. It's General Motors Reference G083, "Subject: Fuel pressure testing and fuel injection operating checks for all FI engines," of table G083-2 "PFI Fuel Pressure Specifications.," Part 3 of 4 on page G176. I'm not sure if this is published by GM, or if it's training material based on other GM publications. For the 4.3L V6, VIN "W" version from 1996 to 2001, it says "Key on Engine Off" fuel pressure must be 60-66psi, and fuel pressure at idle must be 55-64PSI. It lists the same pressures for VIN "X" for those years. I didn't know they made the VIN "X" engine in those years, but that's what you have when you upgrade the injector assembly to the later version. Mine might be a bit low, or my memory might be faulty. I know it the pressure drops when vacuum is higher (no load, higher RPMs).

 

Not sure if you are responding to my post or not. I am simply saying that the definitive test for fuel pump is max dead ended output pressure and leakdown, NOT a regulated fuel pressure test as has been done. This will be a pressure far above 66 psi.

 

Les, my comment was more for the guy above you who expected 60psi + always. Apparently he owns 3 Blazers, and apparently all are 2002 models. That's the only year that specifies 60-66 PSI fuel pressure at idle. And I didn't want him to take me to court over my previous post.

On these vehicles (both the Blazer/Jimmy SUV's and the S-10/S15 pickups), I've tested fuel pressure at the filter inlet pipe (pump->gauge) for maximum fuel pump pressure as a diagnostic test. It's usually over 80PSI, and although I can't find it in GM's documents right now, but I'm pretty sure the specification is at least 72PSI (80PSI - 10%), and maybe a bit higher. Still, I'm actually really skeptical of this as a diagnostic test.

I did a little (lazy) research on fuel pumps for these trucks, trying to find exact specifications for output pressure and volume. Only one fuel pump manufacturer that RockAuto.com sells gives specifications anywhere. I told you it was lazy research, although I did also google each manufacturer name and part number with the keywords "specifications pressure psi flow gph". The one that publishes specs online was "Spectra premium" and they specify minimum output pressure of 79.75PSI, and minimum "free flow rate" of 48.6gph. Which brings me to my next point.

For every other vehicle I've done fuel system diagnostics on, there was a flow rate specification and test. A vehicle that had fuel system issues (misfires, other performance issues) and had "good" fuel pressure at the test port will usually fail the volume test if the fuel pump is the actual problem. "Free flow" tests can be run by hooking a hose to the "bleed" port on the test gauge, putting that hose in a 1 quart jar, installing a jumper at the fuel pump relay to run the fuel pump, then press the bleed valve and time how long it takes to fill the 1 quart jar. 45gph will take 20 seconds to fill a 1 quart jar.

On other vehicles, a "loaded" flow rate at the system pressure is given for testing with the system at operating pressure. That is tested by measuring the volume of fuel in a given time in the return line. To test those, you disconnect the return line somewhere, and put the end in your 1 quart jar, again time how long it takes to get a quart out, or sometimes the test is how much flows out in a fixed period of time. I usually do the math, convert the test volume/time into "seconds per quart" and use a quart mason jar.

30 gallons per hour (1 quart in 30 seconds) seems to be the nearly universal number for the return line tests. That spec is given for a wide variety of vehicles I've worked on in the past, with system pressures anywhere from 10-12 PSI up to 85PSI system pressures. That number, 30gph into the return line with pump running and engine off, is so universal that I'd consider it a "best practices" test and be very suspect of any fuel pump/system that fails that test, even if the manufacturer didn't specify that exact test.

 

Personally I only have specific Blazer experience with mine, a 2001 that had the AC Delco MPFI upgrade kit on it when I bought the vehicle - and I was trying to chase a phantom P0304 that really didn't seem to be connected to a real misfire.


When I tested fuel pressure key on/engine off it was 58 psi with the pump running. So I switched fuel pressure gauge source and again got 58psi. Per the sticky at the top of this forum that was too low. After testing the dead ended fuel pressure (I can't remember exactly what it was, but it was a lot), I replaced the spider assembly with another brand new AC Delco unit. It also tested 58psi, and ultimately did nothing to change anything with the vehicle. A lot of expense for nothing. I recently put my original back in to eliminate a bad solenoid as a possibility during a P0200 malfunction (it was a melted wire). So both of these spiders work just fine in my vehicle, even at a lower pressure.


But anyway I originally concluded that the actual pressure for a MPFI unit was much less dependent on fuel pressure to deliver fuel (without the poppet valves of the original design) and that the AC Delco replacement MPFI fuel pressure regulator was actually set to 58psi with no vacuum applied.


So I really don't think a specific regulated fuel pressure is critical, and I suspect you may find different pressure regulator settings from brand to brand of upgrade spider. I think if you have the upgrade, anything in the 58-66 psi range with key on/engine off is OK. I know this flies in the face of the sticky at the top of this forum, and I'm not used to contradicting Captain Hook so I have left this issue alone for a couple of years now. Still not sure if I have enough Blazer cred here to pull it off and have anyone believe me, but that's the way I see it.


Now the volume delivered past the fuel pressure reg into the return line is a valid test. I think the 1 qt in 30 seconds is probably a very valid spec.

 

Thanks for all the info Guys! I'm not sure which way to go now, I do know that I'm tired of throwing cash at this vehicle. It's strange though, A time of two when testing at the schrader valve, the key-on did nothing. After about 5 or 6 on/off for ten seconds or so, the pressure would max out at 60 psi and then drop to 55, then leak down to 51/52 psi.
I tried to do the dead end test just after the filter and couldn't scrape together enough stuff to do anything other than an "inline" test so I didn't bother.
I did replace a rough looking o-ring at the filter though.
Sometimes it runs great, other times it stumbles during acceleration and when cruising at highway speeds you will feel a little miss or something occasionally.
Later

 

Either check the return line volume for 30 sec with the pump running, and/or do the dead end pressure test. I think either test is going to lead you to the fuel pump if the pressure isn't building immediately with key on (as you have described). Its the only way you are going to know for sure. Let us know. Best wishes.

 

Here's a thought. The fuel pressure gauges I have used have a bleed line and an adjustable valve attached. Why not just hook up the gauge, jumper the fuel pump to run, and open the valve on the gauge just far enough to lower the pressure to 50psi and start timing. That way none of the fuel should be bypassing the regulator and you can catch all the delivered volume at 50 psi (below fuel reg open pressure). That would be pretty easy way to accomplish a fuel pump volume test (at 50psi instead of max pressure, but it really shouldn't matter much). If it won't deliver the quart in 30 seconds, fuel pump or fuel filter is bad.

 

The same GM document I got the "official" pressure readings from also said this: "When cycling the ignition switch to pressurize the system for testing, leave the ignition off for 15 seconds before turning it on again; or the ECM does not reenergize the pump relay." I'm not sure how quickly you were cycling the key, but if you had it off for less than 15 seconds, it could be "operating as designed."

Your filter has threaded fittings? The kit I rented/borrowed at O'Reilly had a fitting with the female threads that matched the filter inlet, so I hooked the gauge up that way.

For the flow tests, I'd either disconnect the return line at the back of the intake and put a hose barb in there, or I'd just use the gauge and do a "free flow" test with the bleed port on the gauge (look for 1 quart in 20 seconds, or 1 pint in 10 seconds). I wouldn't bother trying to "regulate" with the bleed valve.

Is your hesitation/misfire worse when the engine is hot?

Coils, iggy modules (next to the coil) and crank position sensors can fail in a "flaky when hot" manner. If you're not sure which, a canned "keyboard duster" can be used to cool parts when the problem is present. Turn it upside down, and squirt the cold liquid at the part you're "testing". If the problem goes away when you cool something, but comes back again when that part gets hot, that's the suspect part. Safety warning: the liquid in those "duster" cans is cold enough to cause frostbite. Be careful, don't get the liquid on your skin.

Also check the condition of the wires to the crank sensor, and the condition of the front cover where the sensor is mounted. If the plastic of the cover is distorted or warped, it can cause the sensor to be mispositioned and act flaky.