Car pops under vacumm conditions


New Member
May 25, 2001
I have had this problem since last year. With the car in neuteral, reving the motor, as soon as I let off the gas there are a few small pops through the exhaust. The wideband shows +-1 percent correction, air fuel around 13.5/14.0. Last year, I had a few instances were I would down shift (my wastegate is vented to atmosphere) and a big flame would shoot out the waste gate, followed by a big bang:eek: As for the down shift goes, there is just about a back fire everytime:confused: The motor I have in now is brand new, rings, valvetrain(swapped to solid roller), etc. This has only been a problem since the speed pro swap, any conclusions to this problem

Anything you can think, shoot my way
Let me also add, while maintaining a particualr RPM, say 2500 there is a consistant sequence of poping through the exhaust.:mad:
If you are spitting flames out the pipe, you are either running way too rich in that area or you are dropping a cylinder, loading it up, and firing it FULL of fuel. Short of some horrid mechanical problem in the valvetrain, I would say that removing a significant amount of fuel in the "no load" areas of your fuel tables will significantly improve this problem for you.
Just a couple of thoughts.

13.5 seems rich to idle- why do you have to have it so rich?

Are your plugs firing properly, or are you running plugs that might be too cold?

Also might want to check your timing- if the timing got goofy, you could be firing late (when the exhaust valve is opening). Maybe advance it a bit?
First thanks for the posts.
I have tried bringing fuel out of the VE table and the poping seems to get worse:confused: Right now my crank referance angle is set at 6 degrees, LT1 application. Besides the coil and the ignition modual everything else is new. I run autolite 104 plugs, never had a problem, also tried NGK's, same result.
Anymore thoughts


A few more pieces of the puzzle, when I start the car cold, there is no poping. Also when I was running out of gas and the fuel psi started to drop, the car was poping worse. Not sure if this helps or just makes one scratch their head harder.
My experiance with the optispark is, they either work or dont. You will also experiance extream roddeo action when problems exist. I replaced it last year, has about 5K on it.
seems like a lean condition if its popping through the blow off....
you say its not popping when cold only when warm? makes me think your cold enrichment fuel is keeping you straight until it warms up and then it goes lean after its taken out. just an idea...
my bad.....gotta pay more attention to what I'm reading. being a belt driven guy I don't get the luxury of havin a wastegate. have you tried leaning out the high vacuum numbers in the ve table?
I had to re-seal the intake because of an oil leak. So have not been working on it lately. I might have to re-post if no body reads this. When I rev the motor my air fuel dives into the 12.0's because of the AE VS TPS rate of change table. Im not sure how low I should bring the table. My AE vs map is set at 0. How do most of you guys set your AE table VS tps rate of change? Also what would you say is a good air fuel for the 2500rpm and under, I was thinking maybe 15.0's
The best way IMO to get the AE fuel tables right is simply by feel. I wouldn't worry about the a/f ratio when you whack the throttle. Just give the motor what it wants. The a/f ratio will recover. As far as numbers in the AE fuel tables, because they are an additive pulsewidth, it depends on how much injector you have. I usually set the tables so that at the highest rate of change, you are approximately doubling the fuel applied at an idle.

You probably should have something in the AE vs. MAP rate of change table in a speed/density setup. Just make sure to set the AE MAP limit to around 110-120 so that AE fuel doesn't get applied as a result of increasing boost. That can really throw things off.


For an N/A application, where do you generally set the AE MAP limit (with a 1-BAR MAP sensor, obviously). And how do you like to slope the AE vs. TPS and MAP tables if the max is 2x the idle fuel. I'm having a stumbling problem when accelerating from a rolling start that I assume is due to AE as well. That's next on my 'tuning list'. :)