Greetings all, yet another crank no start situation being posted here. I'm following GeorgeLG's article on the subject [https://blazerforum.com/forum/2nd-ge...ticle-103750/].

My wife was driving through a stoplight in town, hit the accelerator to speed up, and got no acceleration for at least 3 seconds. She was able to pull it into a parking lot it, and I got it towed back to my apartment complex. Only check engine code is O2 Bank 1 Sensor 1. Thus far battery and cranking speed seem normal, but no ignition. Fuel port on the rail in the engine compartment is low, ~45 psi, and goes down to about 35psi after ten minutes. I setup the max fuel pressure test adapter per Captain Hook's article [https://blazerforum.com/forum/2nd-ge...akdown-88305/], and pressure just after the filter is also ~45 psi, but holds steady there.

I have another Blazer available, which has a new ( less than 2 years old) Delphi pump. I'm planning on doing the same test on that Blazer just to make sure it's getting appropriate max pressure, then swap the pumps. Can I just swap the tanks without removing the pumps? Is there anything else I should check before doing the swap? The cranking has been strong, but the fact that the pressure isn't leaking at the pump gives me pause; could somewhat low battery voltage affect the pressure that the pump puts out? If I do swap the pump, does the drop in pressure at the test port in the engine compartment also indicate that there's still a leak further up the line?

 

The 45 psi result at the fuel filter says you have a fuel pump problem regardless of what happened at the fuel rail so solve that problem first. Sure, low battery voltage can affect the fuel pump but I dont know how low you have to go to get only 45 psi or if that would even be the result. You should always make sure your battery is charged before doing any of these or related tests.

Now the thing is that it could be the fuel pump but it can also be the power side or ground side wiring to the pump so once you get the tank down you should test the wiring before condemning the pump.

If the tanks are identical I see no reason why you could not swap tanks.

George

 

Thanks George, I appreciate the advice. How do I check the wiring at the pump, do I check the voltage across the harness on the vehicle side once I have the tank down? Are there other obvious checks I should do once the tank is down? Do I need to bring the tank down first to check the wiring, or is the wiring harness accessible with the tank in place?

The update thus far: since it is likely that there was/is an issue both in the tank and further up the line, I decided it was more than I would be able to do in the weekend, especially given that I'm not allowed to do any vehicle maintenance in the parking lot of my apartment complex. I had it towed to a shop in town (one of the few that is open on weekends), and after a few days they decided they couldn't diagnose it or do the work, but they had a mechanic from another shop come in and do a diagnosis, and his opinion was that it was the fuel injectors. However, they weren't able to tell me what testing he did, and he didn't mention any issues with the fuel pump, so the impression I got is that his diagnosis is just going off of the low pressure at the rail and the fact that the injectors are a known issue with this model year. Not having confidence in that diagnosis, a friend of mine and I pulled it over to another friends driveway, where I'm going to do the work, starting with the fuel tank. I'll most likely be getting starting this weekend.

 

Anytime a load (fuel pump in this case) is not acting right the load itself is suspect but so is the wiring and the resultant net voltage to that load, while under full stress. As vehicles get older the probability of wiring issues starts to go up from corrosion, repairs, critters, impact damage, etc. I don't know how accessible the harness is for testing while the tank is still in place, idid mine so long ago I dont remember.

The basic idea is to make sure that the net voltage at the connectors (as close as you can get to the fuel pump) is proper while the circuit is under a load. You measure from the positive wire to the negative wire right at the pump connector. This will catch any unacceptable voltage drop caused by either the positive circuit or the ground circuit. If the pump is sufficiently hosed it can itself cause a large voltage drop. Because of this, if you get an unacceptable answer then the next thing is to unplug the connector and substitute a load from say an incandescent test lamp and measure the net voltage again at key on. If you get a good result then its the pump. If you get a bad result then we need more testing to find the wiring fault.

What year is this truck?

George

 

Thanks George; it's a 2002 RWD Blazer. What should the net voltage be? So to test it properly, I'll need to scrape off some of the wire casing on both the positive and negative wires leading to the pump, as close to the pump as I can get, turn the key to energize the pump, and then take the reading? If the result is low, then unplug the pump and wire up an incandescent bulb; if the net voltage is low after tuning the key to energize the circuit is normal than it's an issue with the pump, if it's not, then it's elsewhere in the wiring?

 

The best way to measure is to back probe each wire at the back of the connector with t-pins or back probe leads while everything is still connected. If that's not possible then use a piercing meter probe into the wires and last choice is to remove some wire insulation but if you do that, seal the exposed areas back up with liquid electrical tape. The minimum voltage while the fuel pump id priming is 11.0v if the battery is fully charged at 12.6v.



If you get at least 11v then your done, if not, disconnect the plug and substitute another load like an incandescent test light. Ideally you want to load circuit with at least 1 amp (12 watt bulb) buy not more than 5 amps.


George

 

Thanks George, the diagram is helpful. I put in a charged battery (but haven't yet retested the pump pressure) and got the rear end jacked up and ready. Based on where the harness is, I don't think I'll be able to test with the tank in place, but I'm hoping I can just lower it a bit and get enough room to get at it. The harness looks like it makes a pretty tight seal, so I don't know that I can get T pins in while making a good connection on the harness, so I'll see if I can find some thin back probes for testing. I'll be doing some more work this weekend and let you know how things come out.

 

Those are generic wire positions btw, not intended to show your wire positions, just the concept of back probing


George

 

Greetings George,
Thanks, for the visual
The update: I was working on it about two weeks ago, I can access the harness, but I wouldn't call it accessible. I went last night to see if I could do the backtesting, I bent a pin to see if I could get it in place and reattach the harness, but the harness won't connect all the way; so I think I won't be able to backtest with it in place. Since the net voltage in operation should be 12v, if I drop the tank and wire it directly to a full charged battery, will the pump register the correct pressure (assuming that I create an adapter to capture the flow straight from the pump) the same as when it's being primped? I don't think it will be possible to backtest in place, and perhaps not even with it dropped since there's not much slack in the harness. Also, when I went this last time, the battery was totally flat, not even enough charge to unlock the rear hatch. I've got it on a 3amp Walmart trickle charger, I'm thinking my next step will be to drop the tank, and either get enough slack in the harness to do back testing or setup a pressure test with it wired up to the 12v directly. Any thoughts?
Cheers,
-MinorStraw


My attempt at a backtesting pin.

The pump harness, with tank in place. The rear differential and axel is all up in my armpit.
.

 

The likely hood is that this is the pump itself and most people get away with just doing the pump replacement. That said, there have been ground problems and there have been power wiring problems. Since this job is such a pain, especially in our driveways on our backs I like to see the wiring get confirmed so we are not back here talking about it again and possibly dropping the tank a second time. 15 gallons of gas weighs 90 pounds.

Given the crappy wiring access with the tank in place I think that you have a couple of options after you drop the tank:

1. Yes you can drop the tank and "jump" the existing pump and catch the fuel. You would have to use the same gauge wires and have good connections.
2. You could also insert a load into the harness via back probe on the pump motor power and ground wires and then measure the voltage, this is done routinely by good mechanics any time a load is misbehaving to make sure its not the wiring, like say an O2 sensor heater circuit. A favorite hack is to use an incandescent test light because it draws some current and will stress the circuit to some extent. I don't know the amperage draw on that exact fuel pump but it should be in the neighborhood of 5+/- amps when the pump is running, a few amps for sure. The amperage draw of an incandescent test lamp depends on the bulb but mine is 0.5 amps. Not a perfect test but will catch a bad wiring problem.
3. Insert a heavier load like a 50 or 100w cement or wire wound resistor and measure the voltage. Any value between 5-8 ohms will give you a 1.5-2.4a draw which is a more robust test
4. Just replace the pump with a Delphi or AC Delco brand and assume that there is no wiring problem. At least inspect and if necessary, clean and tighten the frame ground while your there.

Its up to you how far you want to go but I would at least do the $10 incandescent test light thing with a 0.5A bulb.

George