Loss of boost mid-acceleration
09-18-2016, 05:08 PM
#1
Rennlist Member
Thread Starter
Loss of boost mid-acceleration
First time posting on here in a long time, so hi! to those of you I remember. 
My 951 is having an issue where when I'm running full boost it'll lose boost, and I'm not sure what's causing it. Wondering if you guys have suggestions for what to check.
It happens somewhat consistently when I'm flooring it from a stop, but generally it doesn't happen till third gear. I'll go through first and second fine, get up to maybe 4k rpm in third and then all of the sudden it will lose all boost - it feels like I suddenly let off the gas. It will rebuild boost relatively quickly but then after a second or two at full boost it will again release the pressure. This first started occuring when I was running 19psi but I've dropped down to 13psi/stock chips and it's still happening.
I've been having issues with high idles and rough running due to the ICV sticking open (assuming it's because of the wiring, see pic) so I was thinking this could be the cause but it holds and then drops boost so suddenly it's almost like something is triggering the bypass valve. I used to have all the vacuum routing on this thing memorized but that was the better part of a decade ago so I'm drawing a blank as to where a vacuum leak would have to occur to cause the bov to trigger randomly. Or is a failing/shorting ICV something that could open fast enough to cause an instant release of boost pressure like I've described? All the vacuum lines were replaced with silicon ones back in '07 and look to be in good condition, billet bypass valve, original ICV, Lindsey adjustable boost ****.
Thanks for any help!
I pulled the intake to fix the ICV and it has of course turned into a much larger project due to finding some leaking coolant lines, age-cracking hoses, and fraying wires, so I won't have the car running again for a week or so to test any fixes.
My 951 is having an issue where when I'm running full boost it'll lose boost, and I'm not sure what's causing it. Wondering if you guys have suggestions for what to check.
It happens somewhat consistently when I'm flooring it from a stop, but generally it doesn't happen till third gear. I'll go through first and second fine, get up to maybe 4k rpm in third and then all of the sudden it will lose all boost - it feels like I suddenly let off the gas. It will rebuild boost relatively quickly but then after a second or two at full boost it will again release the pressure. This first started occuring when I was running 19psi but I've dropped down to 13psi/stock chips and it's still happening.
I've been having issues with high idles and rough running due to the ICV sticking open (assuming it's because of the wiring, see pic) so I was thinking this could be the cause but it holds and then drops boost so suddenly it's almost like something is triggering the bypass valve. I used to have all the vacuum routing on this thing memorized but that was the better part of a decade ago so I'm drawing a blank as to where a vacuum leak would have to occur to cause the bov to trigger randomly. Or is a failing/shorting ICV something that could open fast enough to cause an instant release of boost pressure like I've described? All the vacuum lines were replaced with silicon ones back in '07 and look to be in good condition, billet bypass valve, original ICV, Lindsey adjustable boost ****.
Thanks for any help!
I pulled the intake to fix the ICV and it has of course turned into a much larger project due to finding some leaking coolant lines, age-cracking hoses, and fraying wires, so I won't have the car running again for a week or so to test any fixes.
09-20-2016, 09:18 PM
#2
Instructor
My car experienced something like this before and it was a simple issue of plugs being worn/over gapped causing weak ignition higher in the rev range under load. My car would start/drive around town nice and smooth. If I stood on it, 1st and 2nd would be fine, by the time I hit 12-15 psi in 3rd it would act like your car.
I'd look at plug wires for shorting or plugs out of spec first. No fix there, remove the TPS and test it through its sweep to be in spec.
I'd look at plug wires for shorting or plugs out of spec first. No fix there, remove the TPS and test it through its sweep to be in spec.