Hey guys I been tracing this issue for a month but can't get anywhere. My 99 sierra 4.3 started running rough above idle so I did a tune up ( ...

Hey guys I been tracing this issue for a month but can't get anywhere. My 99 sierra 4.3 started running rough above idle so I did a tune up ( plugs wires cap and rotor) that dist help. It got worse and worse so far I have changed the tps, IAC, distributor, fuel pump, had the cats removed. I turn the key and it cranks forever before it finally starts. Once its running it runs perfectly at idle. Once u hit the gas and get it above idle it start missing and jump in and cutting out. Now here is the odd thing. If u unplug the tps it will start as soon as u hit the key and it runs smooth all the way through the rev range . does anybody have a clue what is going on here?

fuel filter, when have you changed it? and theres a plug on the back of the intake, go to oriellys or auto zone they have a fuel pressure gauge you can use to see if you have a leak from a line or injector or something else, i pulled the fuse for the ignition so the truck wouldnt turn on while doing this test, needs to be over 50psi, then if that doesnt lead to a problem, start replacing maf,airtemp senors, have you thought about getting it tuned to not have the cats yet?

no i havent thought about getting it tuned but thats not a bad idea. i just dont know where to even begin with that. i just replaced the fuel filter not three months ago. i replaced the tps again sunday night and it made no difference. today i used a multimeter to test the sensor and the wiring and it is all working as it should. i also noticed that when the truck is in nutral or park and u rev it it will start misfirin and cuttin out and buckin and goin nuts, but if u put it in drive it dont do it. hell i drove it a few miles to my cousins and it did good till i let off the gas to coast u could feel the motor miss and it even backfired through the exhaust a few times. also when i got to my cousins and got out of the truck it smelled extremely rich to the point that it made my eyes water and i was standing at the drivers door. if u stay on the gas hard it runs good or when u let it idle it does good. when i changed the distributor it had a new camshaft sensor in it already. all i did was put it in. could that be causeing all this being that i did not get a diagnostic computer to reset the camshaft sensor? any ideas and suggestions help thanks

I'd say timing issue except that you said when you unplugged the throttle sensor it ran good. Quit throwing parts at it and we can figure it out.

Check the fuel pressure, 'course running rich kinda tells me that'll be okay. I'd also scan it for current/pending codes, there should be something in there if it's running that bad.

I'd also check the timing even after what I've already said. That only costs you time. And while you're at it check for an intake leak.

Well Saturday I tried starting it and I ended up having to prime the fuel system back up and got it to finally start. I held it at 1000 rpm and got a ses light. Got code p0300. Cleared the code and held it at 1000 rpm again and the code came back. I let it sit for a few hours and my uncle came over and asked if it still had any fuel pressure. So I went to the Schrader valve and pressed the little button on and got nothing. That being said and everything I've read on here that points to fpr leaking so I pulled the plenum, but I don't see anything leaking. Could it still be the fpr or something else in there. What's my next move other than just replacing the fpr.

You can check the fpr by pulling the hose off and if it drips fuel it is bad. I bet your pump is bad. Need to check fuel pressure if the fpr does not leak a drop or two when you pull the hose off.

acfd813 wrote:Hey guys I been tracing this issue for a month but can't get anywhere. My 99 sierra 4.3 started running rough above idle so I did a tune up ( plugs wires cap and rotor) that dist help. It got worse and worse so far I have changed the tps, IAC, distributor, fuel pump, had the cats removed. I turn the key and it cranks forever before it finally starts. Once its running it runs perfectly at idle. Once u hit the gas and get it above idle it start missing and jump in and cutting out. Now here is the odd thing. If u unplug the tps it will start as soon as u hit the key and it runs smooth all the way through the rev range . does anybody have a clue what is going on here?

Hello Im new to the group and i have 2001 1500 4.3 that is doing the exact same thing, did anyone figure out what was causing this? thanks

Has anyone figured this out yet?

Mine started last week.

Random all cylinder misfire from 2% to 40% throttle. Misfire quits when I unplug the tps.

Same as the OP, you need to check your fuel pressure.

Walkers wrote:Same as the OP, you need to check your fuel pressure.

That's a very cryptic "i don't know".


55psi at all times.