ChevyNo1

Went to the track last nite with my good 'ol buddy Ron Larson ( aka Kimchee & Rice for those that know him! ). Put my ET Streets on, added 4.5 gallons of 100 unleaded to 2.5 gallons of 91, giving me an approximate 97 octane mix. Started the nite running 6.5psi and doing some tuning. Got the car to 11.96@116.7. Turned the **** a bit more, and the boost went crazy! Set two high input MAF codes because the boost was spiking to like 12psi and I could feel the car stumbling, then I got out of it. I reset the codes, but decided to be done with racing just in case I had blown off a hose on the actuator. It didn't make sense that the boost went that high with the slight adjustment I made. Anyway, I didn't reset the computer. We're driving home and I'm testing, the car works fine up to 6psi, but as soon as it hits 7, the car is stumbling again!! The only explanation I can come up with is the PCM frieked out on the run, and now at 7psi it is doing something, like pulling a lot of timing or something like that. With 97 octane I shouldn't be having this problem.

Ideas anyone??? <img src="images/icons/confused.gif" border="0"> <img src="images/icons/confused.gif" border="0">

- Dug

Blackbird

check your plugs is an easy thing to do.

I had a Thunderbird Supercoupe that started doing that. Anytime it got into boost it would stumble and buck like a motha. Replaced the plugs and wires and whammo bammo, good times.

Might not be it, but it's an easy check for a loose plug wire or bad plug.

ChevyNo1

Yes, I planned on that too . . . it's just weird that it's at 7psi that it has a problem, even 6.5psi it seems fine . . .

- Dug

Phate

iTrader: (2)

Well, you might have toasted the plugs a bit on the 12psi run and now you are blowing out the spark over 6psi

Scott

y2khawk

iTrader: (5)

Dug

What fuel pump do you have? Can't remember the injectors either.

Spiking sounds like a wastegate issue. What's your method for boost control? VBC, manual controller, etc.

by the sound of it, it's a fuel delivery issue.

ChevyNo1

Yea, I thought about if I toasted the plugs, but would they still work, even at 5psi? Cause they seem like they are . . .

Harlan, I've got the pump that comes with the Stage II kit, which I believe is a Walbro. I still have stock injectors, but I have the additional two that I use for the extra fuel. I'm using a Greddy Profec-B. I think it just got really sensitive in the range I was in, and I turned it too far. Before the boost spiked, I was running 7psi fine . . .

- Dug

Phate

iTrader: (2)

I have the Profec-B too and I was really suprised by how much the boost goes up when you use the dial on the right...I've seen 10psi or so with just a little movement, but mine wasn't a spike.

Dug,

Where did you mount the solenoid for the Profec-B? I had mine mounted near the ***-pipe, but I moved it to the firewall behind the intake to shorten the lines...it was a pretty tight fit though.

Scott

ChevyNo1

Well, I don't really think mine was a spike when I think about it . . . it was just allowing too much boost. I don't use the 'high' setting **** at all. Even when turned down all the way, it's too high, I only use the low. I have the sharpness **** all the way clockwise, that seems to get rid of any spike problems I have. My solenoid is close to the front of the car, on the driver's side . . .

- Dug

Cool28

iTrader: (2)

I would say maybe you melted a plug alittle. And at boost it blows out the one spark plug that messed up?

ChevyNo1

Could be . . . but I'm still thinking the computer has something to do with it. This has happened once before ( boost too high ), and the car ran like a dog until I did a PCM reset, and voila, working better again . . .

- Dug

y2khawk

iTrader: (5)

With the high boost i'll bet all the injectors went static and your pump started to lose pressure. 12 to 13 psi is about where the 340 starts to fall off on flow, especially w/ the base at near 60 psi.

Pretty often setting a high MAF code will force the PCM to run speed density only. Freaks the PCM out, and it tries to run off a sensor it thinks is still working. A little doggy on tip-in, but....

The MAP sensor is only 1 BAR, as soon as you boost and you're in default speed density, you don't read any of the added airflow. BAD BAD BAD <img src="gr_eek2.gif" border="0">

I concur, check the plugs. a lean spike is murder on plugs among other things.

ChevyNo1

Thanks, Harlan, I will check everything out. At 6psi it still runs very strong with no apparent problems, just at 7psi it starts stumbling.

- Dug

Cool28

iTrader: (2)

I dont know i pluged in my maf the wrong way one time and still ran fine in boost or not. This was 200+ miles till i figure it out lol. It was alittle laggy but it ran good even in boost. DO you still have the supplied FPR. I didnt like mine so i got a new one. It helped on fuel. But i dont know what i'm pushing on FP now though(no FP gauge). I think its around 75psi though <img src="gr_eek2.gif" border="0"> .

ChevyNo1

Well, I haven't pulled the plugs yet ( yah, I know <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" /> ), but I did the PCM reset this morning ( still same gas in it ), and on the way to work, I slowly turned it up to 8psi, no problems, no stuttering. I can only assume it was the PCM then . . .

- Dug

Dave

think about it: if the plug worked at intermediate boost then after exposure to high boost misfired, it's likely that the plug has been eroded so that the SAME voltage as was developed earlier can't sustain an arc over the greater distance (caused by overboost) under the conditions of normal boost (where a higher voltage gradient is required).
Remove and examine plugs. Not enough to just replace them; was it a lean condition that eroded them? Ifso, just changing the plugs without upgrading the fuel supply will put you right back where you were