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Anyone good with trucks?
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Anyone good with trucks?Posted:

Took
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I have a 99 5.3 silverado and it has a random misfire in cylinders 5&7.
I have changed fuel injectors, coils,plugs and plug wires.
Air filter, fuel filter and put a new crank sensor in it...
I dont know what it could be, also put a new head gasket set on it
Any help would be appreciated
#2. Posted:
002
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What makes you say you have a miss? Is it running horribly / getting bad gas mileage?

To me the three things that are going to cause a miss are the fuel system, ignition system, and the engine in general.

For the fuel system, it sounds like you have injectors / filter covered, now look at the fuel pump. The reason I asked the question above is because if it's running horribly it's probably a fuel issue in a clogged injector, pump, fuel pressure regulator, or filter. Since you have the injectors and filter covered, now look towards testing / replacing the fuel pump and fuel pressure regulator.

It sounds like you also have the spark plugs in order, but make sure the gap is right. Also did you notice if the old ones where wet? If they where you'd have coolant or oil getting in there and that's your issue, but with a new head gasket that shouldn't be a problem. Basically when looking at the spark plugs you're looking for cracks, oil / coolant, carbon tracks on the insulator, and the center electrode to see if it's clogged. On the COP (the wires for the spark plugs) you're looking for cracks or burns, and carbon tracks where it goes on the plugs.

Now as far as the engine its self, the miss you're experiencing could be low compression in those cylinders which can be caused by a bad piston, or the walls are corroded. If you did the head gaskets you should've been able to check this. Going and getting a compression test will tell you if your engine is the issue (pistons and piston walls). Also look for vacuum leaks. anywhere you know air goes, spray some water on it to see if air is coming out. This includes the manifolds, intake, etc. A common problem for big engines is the manifold studs will break and make a vacuum leak. While you're in the engine, take off your air intake and clean the throttle body, if they get dirt they'll cause issues. I don't think it'd give you a cylinder specific trouble code, but weirder things have happened lol.

My priorities would be as follows:
1. See if a manifold stud is busted
2. Inspect the old spark plugs / wires for what I listed above
3. Clean the throttle body
4. Check for vacuum leaks
5. Test the fuel pump (and check the fuse)
6. Test the fuel pressure regulator
7. Get a compression test

EDIT:
If all that checks out, look at lifters (you should hear a ticking if the lifters are bad), then look at valve seats, guides, springs, etc. Then look to re-learn the CKP (you'll need a scan tool, a mechanic should have no problem doing it for you).
#3. Posted:
Took
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Cam and lifters seem good, im going to get a compression check soon and go from there

I already cleaned the throttle body and the two injectors on the cylinders that are misfiring are new now

Im thinking its something to do with the fuel
Getting too much or too little.
Thanks +rep
#4. Posted:
002
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If it's a fuel issue you still have the pump and pressure regulator to check.

Quick question about fuel, did you go to a different gas station on your last fill up / was a truck dumping fuel into the gas stations tank? Some gas stations (Arco / AM/PM) are known for bad gas. When any gas station gets more gas, it makes all the crap at the bottom stir up and you get dirt and debris, etc.
#5. Posted:
Took
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Would bad gas cause a random misfire in two cylinders?

I bought this truck and it wa setting for two years i need to get it on half a tank and put some treatment in it.
#6. Posted:
002
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Bad gas will wreak havoc in a vehicle. What makes you say there is a miss? If there's a check engine light and that's what the light says, then it could be bad fuel, but to be honest that's a hope and a prayer lol.
#7. Posted:
Took
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The gas that was in it was in it for 2 year, i put 20 in it but never put any kind of treatment in it, now its back on the e with a miss

And its got a dead miss
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I'd drain the gas tank ASAP. That old of gas is bad, it may have screwed up your engine because I can almost guarantee you there is water in it. As the gas sits in your tank, expanding and contracting with the weather, it will evaporate, but the tank is closed so a lot of condensation (water) goes to the top and drips down.
#9. Posted:
Took
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Drain it and anything else i should do?
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After you drain it, fill it up and hope that was your issue. With a dead cylinder I doubt it was but there is no doubt in my mind that bad fuel will cause issues.
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