Knocking, stuck fuel gauge '97

[NoFrills]

Have a 1997 4.3L Blazer, 6 months ago all the gauges went to the stops and the truck started sputtering and knocking and had no power, rough idle, stalled. The ses light came on. limped it to the dealer right then, showed the mechanic, when they hooked it up the computer they said they couldn't find anything wrong and it wasn't doing it anymore, but thought it was the K&N filter charger I had, the mechanic said he had previous problems with these on the same engine. I also have a custom single pipe and muffler exhaust. Since the engine was actually getting worse mileage, I removed the K&N (wasn't just my driving habits for the mileage either). No problems after that. 3 days ago I was on the highway doing 55, the gas gauge (Not the rest of the gauges) went to the stops on the full side (Beyond a full tank) and when I stepped on the gas it knocked and had little power, but i could cruise at 55 no problem. Got off the highway, drove the rest of the way home on city streets, no problem unless I tried to stomp on it, normal acceleration from stop lights it was fine. Let the truck sit off 10 minutes in the garage, fired up no problem and wasn't doing it anymore. Yesterday, the gauge goes all the way to the right again, no knocking this time, no loss of power, but the truck idles a little funny. Now the gauge can't seem to make up its mind. Its sits on the stop then seems to work, then goes full, etc. The truck has 112,000, the pump and sending unit were replaced with 60,000 due to a bad sending unit.. Same thing again? ignition switch? stumped by the inconsistent engine trouble. Thanks for any help or advice in advance.

NoFrills

[ricksza]

First, the fuel level gage signal comes from the VCM to the cluster.

If there is high resistance in the fuel level sensor circuit, it will go to the extreme high. That could mean a break in the wiring anywhere from the VCM back to the sensor ground, or even a bad sensor itself. There are a lot of problems with the sensors going bad because of a reaction with the additives in the newer fuels. Now GM sells the sensors by themselves to install on the tank unit.

About the engine trouble, the ses light doesn't come on for no reason. Were there any codes at all? The ignition switch could cause it though. You might need to scan all the computers (VCM, SIR, ABS) for a clue.

[NoFrills]

OK, well, the ses light came on now, and it ran fine over to a friend's house. Went I went to leave it stalled and putzed and sputtered and won't run longer than 30 seconds. So, its dead in the water for the moment. I will be getting codes hopefully tomorrow, wether it runs or I need to tow it. Everything else is normal on the truck. Any ideas?

No Frills

[ricksza]

If it starts, runs for a few seconds and then shuts down, I would say that it sounds fuel related possibly filter or pump. But I'll feel better after checking the fuel pressure and codes.

[Fireplug]

This sounds a lot like a bad ing switch

[BlazerLT]

1.) Bad Ignition Switch
2.) Clean your MAF sensor. There are two wires on it that take the air intake reading and could have been gunked up with filter oil from the K&N. Spray TB cleaner onto the two wires and dissolve the crap off of them so your MAF can take proper readings.

Also, go get the codes read.

[NoFrills]

Well, it wasn't the ignition switch, I changed that tonite and cleaned off the MAF sensor, it actually runs now normal now though, but the ses light is on and the gauge is still stuck past full. I don't trust it enough to drive to the garage so it gets towed tomorrow, then I'll know codes. At least I swapped out a potential future problem. Thanks for the help so far and the great directions on how to change the switch. I will keep you updated.

No Frills

[BlazerLT]

So it drives normal?

You should take it to autozone and get the codes checked.

[NoFrills]

Well, tried to take it to Auto Zone, and it died a block from the house. Had it towed to my mechanic. I'm not sure the code number (I wrote it down and now I can't find it) but they said the MAP sensor was bad. It's sitting in their lot for now (been up two nights plowing, can't deal with the truck right now)

No Frills

[ricksza]

The MAP (Manifold Air Pressure) sensor could be bad or just caused by running bad. It's sensing low vacuum when it expects high vacuum.

[BlazerLT]

There would be a check engine light if that problem was present.

[NoFrills]

Well, the garage finally got it to act up. The MAP sensor was completely gunked up, so it was replaced. Now they found that when the truck acts up, the computer is only pushing out less than 5 volts to 3 of the sensors and that this is why the truck runs poorly, though they still aren't sure about the fuel gauge. And, yes, forgot to mention, the check engine light was on before. So, sounds like a new computer for me, any other ideas?

No Frills

[ricksza]

The computer uses a 5 volt reference so depending on if the truck is running or not and what other accessories are on at that time, it may be normal. But again a bad sensor on that 5 volt reference circuit could bring the voltage down also.

[BlazerLT]

Very true.

How much below 5 volts would be the question of the day.