Stumbles under load, need help please.

[crank182]

95 formula firebird, LT1 350 5.7, auto.

My car knocks under acceleration. Sounds like a metal banging noise. Revs fine over 6k in neutral and doesn't skip a beat. can't figure out the problem. Idle fine in park, Stumbles in drive around 400 rpm's, almost dies. Here's whats new:

MAF sensor
IAT sensor
Plugs w/ msd wires
Throttle position sensor
Idle air control valve,
all new vaccum lines
PCV valve
new thermostat stock temp
new air injection system under recall
new oxygen sensors
rebuild throttle body kit and cleaned

It ran fine until i changed the plugs and wires. I ended up cracking some vaccum lines so i replaced all of them and it ran like crap after that. I tripple checked the plugs and wires. It does the same with the old ones and i'm sure they are on right. It doesn't miss a beat with no load on the engine, no missfire, nothing. Please help this is getting expensive. All of the above was replaced trying to fix this problem.

Oh it also gets pretty hot when driving it on the highway when the knocking noise starts. It stays cool and runs fine in the city when i barely give it any gas. Thanks, chris.

[FormulaForce]

I have a similar problem that no one can resolve. I have the exact same 1995 formula car, except mine is a 6-speed. Without OBD-II, my mechanic can't find the problem because no codes are being generated. I've done as you have in replacing things like the spark plug wires and ignition module. No change. Mine stumbles as you accelerate, like the whole ignition system is cutting out for a half-second. It gets worse as the car warms up. I'm betting we have the same problem, whatever it is. I might try a website my dad knows about that has all the technical bulletins and recalls for each year car. If I find something, I'll let you know. In the meantime, maybe some of the other guys will help with info.

[crank182]

Well, i'm going to replace the knock sensor today. A couple people said to try that. Also since i'm at it i'll change both coolant temp sensors just for the hell of it. I know it has to be a sensor or something because just like yours as it warms up it gets worse and if i unplug the mass air flow sensor and throttle position sensor it runs perfect for some reason but then the check engine light is on and i have to get it smogged at a california test only center which it won't pass then. I think the computer is trying to overcorrect itself. I replaced both of those as well as all the sensors above. I took it to a pontiac dealership and they tested the computer and said it was fine so i'll keep trying to figure this out. Also i'm getting the cat and muffler replaced soon anyway.

Anyone else have this problem or something similar and how was it fixed?

[crank182]

I talked to a really good mechanic today. He said there is a couple reasons my car would do what it does.

One was the knock sensor as i already guessed. He said that it plays a part in advanceing timing. When it warms up it will add to the timing advance if it is malfunctioning and will keep getting worse as it tries to adjust as the engine heats up and will cause a piston slap like sound because of the early spark. Thats the short version.

The other was the fuel pressure regulator. If it is malfunctioning, when the engine is under load or quick acceleration it can cause the pressure to the injectors to drop to almost nothing until it catches up. That makes sense too and maybe it's a combination of the two. I'm also getting a new cat and exhaust put on after i replace the sensors.

Well i'll keep updating as i go. I should have the sensors by the 6th. If anyone has some input that might help i would appreciate it and i'll update if i get it going so maybe i can help someone else.