For the past 2 years I have been trying to eliminate a low RPM heavy throttle stumble which seems to plague some LT1s. I just replaced the O2s (AC Delco) in a final attempt to get rid of it... no luck

I can't go to the strip like this... there's no point if I can't launch the damn car

I have replaced the Opti (tested bad), plugs (20K ago), wires (a few years back but tested good), coil (just on principle), carefully cleaned the MAF (was dirty), cleaned all grounds in the engine compartment, checked EGR (bad, blocked off), checked all vacuum lines (twice), injectors cleaned, fuel filter replaced, coolant temp sensor replaced (twice to make sure). There's no codes (I mean, there's no SES light).

The engine runs great otherwise, and there is no stumble when cold.

I have had it

Unless someone else has any suggestions on how to get rid of this stumble, I am done even thinking about ever taking this thing to the track again.

 

How about the fuel pump maybe your have some type of fuel starve at WOT and takes a little time to pick up?

 

Describe with exact details when and how the stumble occurs.

 

Check your wires on the passenger side by the manifolds. See if they are melted.

 

hook a fuel pressure gauge up and see where it goes

 

When you say cold, do you mean open loop or just when it's cold outside regardless of open/closed loop? You say low rpm HEAVY stumble. Can you clarify what low rpm is < 2000 rpm and what HEAVY stumble is, backfire and try to die, little hiccup like cam surge?

 

The engine warmed up, I'm stopped at idle (A4 trans). I then quickly open the throttle until it's wide open. The engine will fumble around, at approx 1500 RPM, for maybe 2 - 3 seconds, revving slightly slower and faster around 1500 RPM like it's stumbling and fumbling around. As the revs slowly rise (maybe another second or two) to around 2000 RPM, everything "clears up" and the engine revs perfectly & all is OK.

With the engine warmed up and the trans in neutral, if I quickly give it a heavy throttle, it will act basically the same, perhaps clearing up a second or so quicker.

With the engine COLD and the trans in neutral, if I quickly give it a heavy throttle, the engine is almost instantly responsive and without a pause, the revs rise very quickly.

 

What are your mods and who did your tune? For it to do that in neutral with no load on the engine is very strange to me.

 

Have you had a chance to put a scanner on it? Also, how/when did it start happening? Just out of the blue one day or has it progressively gotten worse

 

sounds to me like open loop it runs fine, closed loop you got a problem. Time for a tweak of the tune. Might just need to raise the idle a couple hundred rpm and maybe add a couple degrees timing at idle. I'd give that a try. A carbed engine that stumbles when the throttle is stabbed usually get fixed by bigger squirters because the instant WOT makes for no vacuum to pull fuel in, so mechanically the bigger squirter pumps fuel in until the vacuum can pull it. You may need to go into the WOT part and tweak that but I'd try the idle rpm and timing bump firts. Sometimes thats all it takes. Don't give up, it's a learning experience.

 

If his spark is grounding or arcing out somewhere or hes got a fuel filter thats as old as the car...sometimes its the simple stuff you dont even think about. Hang in there you'll figure it out, some good suggestions in here so far.

 

Thanks for all the quick replies.

My mods are very minimal, as in my original sig... roller rockers, CAI, cat back, that's about it. I have a stock tune, never been touched except to accommodate 3.42 gears and 160 degree thermostat, done via a Hypertech.

Never had a scanner on it. The problem slowly appeared and progressively gotten worse. Cleaning up the MAF, grounds and the new O2s have made it slightly better.

I agree, it acts EXACTLY like a carbed engine that needs more of an accelerator pump shot.

Fuel filter is recently changed.

It does seem to run fine open loop.

I'm not into this enough to do my own tuning/tweaking, so I'd need to bring it somewhere. I just would wonder why a virtually stock engine would need a special tune to run OK, particularly when it ran fine a few years back... ?

EDIT: One other problem, it is VERY difficult to start when the temps go below freezing. Have no idea if this is related.

 

Does it stumble any at low RPM driving? I would have the battery check, might replace it is old. I worked on a Ford Focus that would stumble like you say and sometimes at low rpm cruising. It turned out to be a bad battery and the engine wasn't getting enough voltage at low RPM, might also be why you are having problems starting the car when it is cold.

Taylor

 

Check your fuel pressure as someone stated above, your pump could be on the way out, also...is your cat plugged maybe?

 

I am thinking fuel delivery too.