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4.3l 15000 Cranks But Won't Start

Atheist

By Atheist
in Ask A GM Technician

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Atheist Enthusiast

Atheist

Posted
Posted

I know that it is a bit long but it reads easy and I am very stumped. I did include cliffs at the bottom. Please help!!

 

I purchased a single owner GMC Sierra with the 4.3L v6 about a month ago from a guy who had let it sit for 6 months prior to purchase. The truck was nearly spotless and ran great for the first two weeks which included a 700 mile road trip.

 

One day a few weeks after owning it I was driving and the truck just lost all power and slowly idled down to a shut off on me with only 380 miles on the tank. I have a long bed so I thought i had a 34 gallon tank and since I had measured my fuel economy on the first two tanks to be 18mpg I was very surprised at this. In any case, it would crank but not start so a buddy came down with some gas and it started right up afterwards. I assumed that was the issue and filled it up but to my amazement it took only 24 gallons total to get the truck back to full. I assumed I must somehow have the smaller tank and that the manual was wrong.

 

The next day I was driving down the road and it hesitated a bit then shut off in the same way a bit later. I had some tools with me so I pulled a plug to check for spark and sure enough I had a nice strong spark on a clean plug but the car would only crank and not start. I could hear the pump priming and fuel entering the rail. While I sat around thinking it over I tried cranking it again (10mins after it shut down originally) and it fired right up.

 

I noticed after that that the car had been hesitating and running poorly occasionally but running great at other times. However, one time I started my truck up, drove about a minute to jump someone who looked stuck, and the truck would not start again when I went to fire it up before connecting the jumper cables. It just cranked and cranked however I could always hear the pump priming each time I reset the key. I called a buddy to come and get me and while I waited I just cranked it for a while. After a while it wanted to start but was running around 300rpm and obviously very rough. My buddy came and got me and I went to the store and got some HEET. I added two bottles, shook the tank, and immediately went to start the truck. Fired up and ran great. However, I do not think it was the heat as it started first crank essentially and only a minute after I had added it.

 

I drove it another 600 miles in one trip and it ran great, no problems. Then one night i parked it and when I went out the next morning it wouldn't start. Odly enough, it had only been 12 hours where before I have let it sit in the cold UP for 3 days. I pulled it into the garage to tinker with it and threw a heater under it. I started by pulling the intake and spraying carb cleaner into the throttle body and it didnt even want to start. HOWEVER, I checked the same plug again and it was still sparking fine. All of the wires were plugged securely into the cap, too.

 

I pulled the fuel lines over the engine and primed the pump and a bunch of gas flowed however when I reconnected it, it still wouldnt start. I let it sit for the night (not a heated garage and no heat on it) and came back the next day and once again aftwer a bit of cranking it was running rough around 300rpm.

 

An hour later I went back out and cranked it a bit and it fired right up.

 

Cliff notes:

-4.3L V6 which sat for 6 months with a full tank

-Runs great sometimes, even up to 600+ miles continuously

-Will occasionally hesitate/lack power

-Occasionally shuts off while running or will fail to start after being shut off

-Typically, after a while of cranking it will run at 300rpm. Then after a while later of sitting it will start right up and run fine

-Fuel pump PRIMES every time, fuel does spray out of the rail. Pressure unknown

-Confirmed spark at the plug

-DID in fact start on start fluid.

 

UPDATE ONE:

Checked the fuel pressure - came back at 60psi during priming. Pulled the cap -- it and the rotor looked fine. Going to go have the coil tested at `THE ZONE`.

 

UPDATE TWO:

We sprayed starter fluid instead of carb cleaner into the TB and it started to try to run so it is in fact working to that extent. New coil did not change anything. Fuel pressure is a constant ~61psi during priming and cranking. It does spark.

 

I do not know if the injectors are firing. If they are, I think it is probably bad gas sediment even though I have put several tanks in. Or some issue with the TBI that I do not know about. Who knows about TBI???

 

 

I'm stumped! Here are my thoughts on parts it may be:

 

Fuel pump:

Fuel pump checks out at 62psi.

 

Something else:

??? I have a lot of experience with cars but I am new to the GM/Truck world. What else could it be???

 

THANK YOU!

Zembonez Senior Enthusiast

Zembonez

Posted
Posted

Pull the distributor cap and look at it. I bet it has white chalky corrosion in it. My 01 4.3 did a very similar thing. Turned out that it was water dripping off of an ac line that runs across the top of the distributor. Over time the moisture corrodes the inside of the cap and the truck will run like crap. stall, or simply not start. You'd still get spark at a plug... but it is enough to make it have problems. Mine was towed twice and the dealer added insulation and covered the AC line the second time and it never did it again in 60K miles. (there was a service bulletin out on it)

 

Replace the distributor cap and see if that fixes it. If so, insulate the AC line and redirect the condensation to drip somewhere away from the distributor.

pennzoil29 Senior Enthusiast

pennzoil29

Posted
Posted
Pull the distributor cap and look at it. I bet it has white chalky corrosion in it. My 01 4.3 did a very similar thing. Turned out that it was water dripping off of an ac line that runs across the top of the distributor. Over time the moisture corrodes the inside of the cap and the truck will run like crap. stall, or simply not start. You'd still get spark at a plug... but it is enough to make it have problems. Mine was towed twice and the dealer added insulation and covered the AC line the second time and it never did it again in 60K miles. (there was a service bulletin out on it)

 

Replace the distributor cap and see if that fixes it. If so, insulate the AC line and redirect the condensation to drip somewhere away from the distributor.

 

:lol: It could possibly be a crank/camshaft sensor. jmo

Zembonez Senior Enthusiast

Zembonez

Posted
Posted
Can someone help me find a reason that the car wouldnt start even with carb cleaner???

If the cap is all corroded, it may fire but at the wrong times. Have you pulled the cap?

Atheist Enthusiast

Atheist

Posted
  • Author
Posted
Can someone help me find a reason that the car wouldnt start even with carb cleaner???

If the cap is all corroded, it may fire but at the wrong times. Have you pulled the cap?

 

 

Checked the fuel pressure - came back at 60psi during priming. Pulled the cap -- it and the rotor looked fine. Going to go have the coil tested at `THE ZONE`.

 

Any other ideas???

zaac Enthusiast

zaac

Posted
Posted

When you're cranking it over and it won't start have you noticed any of the gauges jumping around? This indicates a PCM problem.

Atheist Enthusiast

Atheist

Posted
  • Author
Posted

 

 

 

When you're cranking it over and it won't start have you noticed any of the gauges jumping around? This indicates a PCM problem.

Negative, they are all good.

zaac Enthusiast

zaac

Posted
Posted

You could have someone try to start it while your listening to the injectors with a stethoscope. You should hear them clicking. Someone mentioned crank sensor. I don't know if a failed sensor would still give a spark at all or spark at the wrong time. These newer vehicles can be a real pita. I'm having starting trouble also but only when it's real cold. Mechanic said it was the fuel pump so he changed it and $850 later it still won't start. You're better of changing parts yourself rather than paying someone else to do the same thing. Sure, once in a while you'll change the wrong thing, like a coil, but in the long run I think it saves money and you learn more to boot.

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