David Green

Can anyone help ?

I have a 1986 951, it has high miles and I don't think it has been modified although I can't be sure.

Basically under hard / full acceleration in 3rd / 4th / 5th the stock boost guage is reading beyond 2 bar, the car really puuls and then cuts out, it then will continue again on smaller throttle settings. I have fitted an aftermarket boost guage which I confess I am not sure I have fitted correctly as on normal idle it shows -0.4 bar rather than zero. Under the conditions described the boost will register about +1 which I reckon is giving a net +1.6 bar.

I have gone over the various hoses I can get to and they all seem to be OK.

Any ideas ?

Mike1982

At idle you should be under a vacuum. What does the gauge say when you don't have the car on? It should say 0. That -.4 just means there is no boost, at 0 that means you are at no vacuum and no boost and anything above 0 means you have some boost level. My car at idle has about 18-20 vacuum just idling when warm, only 12 when cold.

CPTdooberhead

It sounds like you may have aftermarket chips in it- also, the car is supposed to read in the negative at idle, that is under vacuum conditions. On the stock boost gauge 1BAR reads as atmospheric pressure (0PSI).

Mike1982

It also sounds like you are hitting the over boost problem that can occur when the car sees it is getting to much boost, the cutting out.

David Green

With the engine switched off it read 0. When at idle it read -0.4, the highest I have seen it go to is +1.3 - So it looks like I am getting nearly two bar boost if In understand how the guage works.

Jake951

If you are seeing +1.3 peak boost, then that is 1.3 bar, which is 1.3x14.5=18.85 psi boost. Boost is measured with respect to 0. If your car has stock chips, then you are definitely hitting the overboost cutout protection, which cuts out above 12 psi or so. Peak stock boost is about 11 psi.

You really need to know what kind of modifications, if any, have been done to know whether 1.3 bar is too much for your setup.

David Green

As far as I know it is standard. Under the bonnet looks standard. How else will I tell if it is not ?

Jake951

You'll have to open up the DME and KLR boxes and look at the DME and KLR chips themselves to see if they are aftermarket or stock chips. Some aftermarket chips are clearly identified with the vendor name (Autothority, Guru, etc.), but I'm not sure all of them are. The stock chips have a factory code number on them, although I don't know what it would be for your car.

Sam Lin

One likely possibility if your car actually hasn't been modified is your Cycling Valve fittings snapped off, so it can't reduce your boost any. I've seen that on most of the cars I've looked at.

Sam

Kurt

I had a similiar issue (and I'm addressing it after the holiday).

Here are some more suggestions:
https://rennlist.com/forums/showthre...hreadid=134375

David Green

My workshop manual doesn't show where or what the cycling valve is - Where on earth do I find it and indeed test it ?

86-951

The cycling valve is under the intake manifold. When I had this problem, there was a hole in the hose between the cycling valve and the wastegate. Find this hose and it will lead you back to the cycling valve.

David Green

Found the Cycling Valve and it was broken - only one connection was made. I've replaced it and now have a different car - much smoother but pretty tame compared to what I had before. I'll have to upgrade the wastegate and chips - Seems like you rate either Vitesse or Guru. Thanks to those who responded and helped me out.