2002 Chevy Blazer fuel pump stopping

Tiny
TWBBER
  • MEMBER
  • 2002 CHEVROLET BLAZER
  • 6 CYL
  • 2WD
  • MANUAL
  • 170,000 MILES
IIve repaced 3 fuel pumps. The last time it went out, I jumped the pump motor to the car battery. After it was uninstalled. It worked fine. Bought a replacement motor, insatlled it, and the blazer idled rough, and stalled, and wouldnt start again. A weeek later, my other blazer fuel pump went out. I installed the new one in it that wouldnt work in my blazer, and it works fine. Ivr replaced fuel pump relay, wiring harness for fuel pump, fuel filter, inspected wiring for damage, wiggled wires when it wil start to see if it stalls ect. Nothing happens. Ive sprayed starting fluid into it, and it has spark. Its like the fuel pump is not getting power, or something is shutting it off. Sometimes it will start, and most of the time it wont. What do I do next?
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Monday, April 26th, 2010 AT 3:56 PM

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Tiny
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  • EXPERT
Hi twbber. Welcome to the forum. Did you check the pump's ground wire? Do you have a plastic or steel fuel tank? Did you measure for battery voltage at the pump? It may only be there for a few seconds after turning the ignition switch to "run", then again during cranking. If the voltage is missing, be sure you're grounding your voltmeter or test light to the body, not the tank. Trucks that had metal fuel tanks often lost their fuel pump ground due to rust on the mounting straps. It was common practice to drill a hole in the flange outside the welded seam, and attach a separate ground wire with a self-tapping screw.

Caradiodoc
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Monday, April 26th, 2010 AT 4:08 PM
Tiny
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It has a metal tank. Ive cleaned grounds, even added one into the flange of the gas tank. With key turned forward, I couldnt find a hot wire while checking the wires connected to the harness(was unplugged) using test light. I plugged everything back in, and the blazer started again. Went out last night. Wouldnt start. I dont have a voltage meter. What problem do you figure is causing this problem. Thx a million
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Monday, April 26th, 2010 AT 4:26 PM
Tiny
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Since you have a test light, let's start at the fuel pump relay. That will split it into four circuits. Attach the clip lead of the test light to the battery POSITIVE terminal, remove the fuel pump relay, and probe the four terminals in the socket. Don't push the probe in too far because it's easy to spread the terminals causing them to make intermittent contact. The test light should light up on two of those terminals. Sorry, my book doesn't show the pins or list the pin numbers.

Assuming you find good grounds on those two pins, move the test light's clip lead to the battery's negative post, then check for voltage at the two remaining terminals. One should have full battery voltage all the time. The other one gets battery voltage from the Engine Computer to turn on the relay. Voltage should appear there at some point; for sure during engine cranking, and possibly whenever the ignition switch is on.

If one of those powers or grounds is missing, we have to figure out which circuit it is and troubleshoot it. If you're missing one of the ground circuits, you can idnetify which one it is by observing if the test light reacts the same when the fuel pump is disconnected.

Caradiodoc
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Monday, April 26th, 2010 AT 5:41 PM
Tiny
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Ok. Just checked fuelk pump relay post. Have 2 lit up while connected to positive. Only one lit up while connected to negative side of battery. Car would start. Took relay out. Tapped it. Put it in another way. Not sure if this matters. Car started! LOST
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Monday, April 26th, 2010 AT 6:22 PM
Tiny
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I meant car wouldnt start after I checked terminals
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Monday, April 26th, 2010 AT 6:23 PM
Tiny
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If you have the gray square relays with four terminals, it can be plugged in either way and work just fine.

It sounds like the two ground circuts are ok. One is the ground for the relay coil itself. The other one is reading to ground through the fuel pump motor.

That next test where you found battery voltage, if it is there all the time including when the ignition switch is off, that is the voltage supply for the fuel pump motor. The conclusion is the high-current circuit including the motor is ok. What appears to be missing is the 12 volt turn-on voltage for the relay. Since this is intermittent, a good suspect is a stretched terminal in the relay socket. Pushing the relay sideways while a helper cranks the engine might get it started, but it should also cause the pump (and engine) to quit while it's running.

The next thing is if the Engine Computer is intermittently not supplying the turn-on voltage. The question then becomes is the problem in the computer or is a signal to the computer missing. GM used a number of different circuits over the years. ALL Chrysler vehicles use one circuit that is very similar to one used by GM, and that involves the Engine Computer. Signals must be received from the crankshaft position sensor and camshaft position sensor. That tells the computer the engine is rotating and the fuel pump needs to be turned on. Other GM systems run the pump from a terminal on the oil pressure sending unit. (Low oil level can cause a stalling problem before the warning light turns on on those systems). With the Chrysler-type system, an intermittent sensor is a common cause of a no-start, but it is WAY more common for the problem to show up after the sensors get hot from normal engine operation.

What you must do next is test for voltage on that fourth terminal. It should appear for a second or two right after turning on the ignition switch, then it might go away until the engine is cranked. The first few seconds is to be sure fuel pressure is up and ready for starting. The voltage must be there again during cranking. Here's the key. If the voltage appears for the first two seconds, the computer is working. If it doesn't appear again during cranking, suspect the cam or crank position sensors.

Caradiodoc
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Monday, April 26th, 2010 AT 10:37 PM
Tiny
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I ran the check on the fuel pump relay circuit. All ok. Took out the crank sensor and attempted to clean it with cleaner. Put it back in. Is it possible to "clean" it? Replaced cam sensor a cpl of months ago.
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Wednesday, April 28th, 2010 AT 2:00 PM
Tiny
CARADIODOC
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Those sensors are magnetic. Dirt and oil won't affect them, but metal filings will.

I was thinking about why it doesn't stall after it's started and running, then it occurred to me that I completely forgot about the Passlock sensor. It's GM's version of anti-theft and causes a lot of problems with no-starts. I never experimented with them or learned how to test them, but I recall hearing a lot of stories about broken wires in the steering column due to flexing when tilting the wheel. The system disables the fuel injectors though, not the pump. Also, the red "Security" light on the dash will be on if the system is preventing starting. Be sure to check for that light.

Getting back to the crank sensor, before replacing it, check the voltages on its three wires. The light green wire should have 12 volts and the purple ground wire will have around 0.2 volts. The yellow wire is the signal wire and will have a pulsing voltage but it can be hard to test without an oscilloscope. You can try a cheap digital voltmeter but the reading will bounce around if the signal is being produced during cranking. If the voltage stays near 0 or 12 volts durig cranking, suspect it is defective.

Caradiodoc
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Thursday, April 29th, 2010 AT 1:56 AM
Tiny
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THx for all your help. I trouble shot those wires. Appear to be fine. So what Ive decided to do, since its an older blazer. Spliced wire into power wire(gray) to the igition fuse. Started right up. I guess Ill leave it like that. Not having to worry about all the sensors ect. Thx again forall youre help.
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Thursday, April 29th, 2010 AT 9:39 AM
Tiny
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You're welcome. Sorry we couldn't find the original problem, but sometimes we would rather be driving than standing there thinking about it.

Caradiodoc
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Thursday, April 29th, 2010 AT 2:12 PM

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