#5 misfire, Stalls at operating temp....LOST!

Ok so here's the situation......99 v6 4.3. Fuel Filter/pump replaced, new plugs, no new wires, cap or rotor yet. Truck stumbles, bucks, jumps like nothing you've seen when idling, pulled #5 plug wire no change, pulled number 3 wire no change. It's as if this thing is runing on 3-4 cylinders. Fuell pressure within specs. When driving and you give gas and it boggs down then will come alive somewhat in the higher RPM range, once it shifts and goes to lower RPM's it starts bogging, bucking again. Once it's at operating temp, the vehicle dies. It will turn over but won't start until it cools down which makes me think it's a sensor of some sort. I'm trying to not go throwing parts at it. After fuel pump was replaced vehicle ran fine for a few days. I want o find the problem beofre finishing hte tune up? Has anyone had any similar experiences?

hello, i'd check compression you dont mention miles on engine.i would also check for a possible intake leak causing trouble

Another Airtex fuel pump? :nono: :lol2:
What is the fuel pressure doing?
Fuel filter check?
Look at dist cap and plug wire.
Good hot blue spark to all cylinder?
Any check engine lights or codes?

143,000 fuel pressue 55-65 63 during start up. I thought about fuel pressure, but it doesn't make sense to me...the bucking and jumping fuel pressure related...check! But how would fuel pressure cause the engine to die at or near operating temp? Which makes me think it's a sensor of some sort that's supposed to do something at operating temp? Does that make sense? It then won't start until the temp. lowers? Like a safety mechanism as if the car thinks its overheated? Keep the ideas rolling! Not sure what type of fuel pump, my buddy had it installed at a local shop. I've heard something about a 3 in. tube from the pump to a sending unit? Still doesn't make sense that it would stop at operatin temp......:banghead:

143,000 fuel pressue 55-65 63 during start up. I thought about fuel pressure, but it doesn't make sense to me...the bucking and jumping fuel pressure related...check! But how would fuel pressure cause the engine to die at or near operating temp? Which makes me think it's a sensor of some sort that's supposed to do something at operating temp? Does that make sense? It then won't start until the temp. lowers? Like a safety mechanism as if the car thinks its overheated? Keep the ideas rolling! Not sure what type of fuel pump, my buddy had it installed at a local shop. I've heard something about a 3 in. tube from the pump to a sending unit? Still doesn't make sense that it would stop at operatin temp......:banghead:


Always check fuel pressure cold and hot and after a 30 minute drive.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold and fuel pressure testing guide line.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.