re. Did I find the weakest link?

[Ben Swann]

Cody,

These things seem to go whether or not you have increase the boost,  and I can't really see why it would fail due to overboost, as when closed, their is a metal valve.  There is also a rubber seal, so maybe that is blowing out/causing it to stick open.  I suppose high pressure may decrease the MTBF on such devices

I was running near 2.5 Bar on mine, and did not experience any problem.  To me the biggest cause of failure in these cars is accidents - which is what caused the untimely death of my '87 5ktqa.   The engine survived and will be transplanted into another.

Do you have a bypass valve?

Ben

[I've been waiting on my 5ktq to show me the weak points on these cars for a while now, and basically I have been just running too much boost. Until this Saturday I had been on the road for a year between 2 5ktq's (each for roughly 6 months continuously) running 2.5bar with not even the slightest sign of anything even dreaming about breaking,except about 2 months ago I had an ISV get stuck wide open which I though was just a plain failure by age. Saturday night though after a long 4th and 5th gear boost run I went to downshift and exit the highway and noticed my engine was idling at 2300rpm. After checking all of the vacuum lines and finding them to be perfect (was fairly confused at this point, all hoses ok, throttle fully closed) I remembered that last time my ISV went bad it did about the same thing. After squeezing one of the rubber connections with the engine running I found that the ISV was indeed the issue. For the ride home I plugged up the valve with a piece of a shop rag and turned up the idle with the TB screw. I'm going to go ahead and remove the valve for good now and plug the holes on the manifold.

Anybody else heard of ISVs failing because of being overpressurised? Maybe I'll see what I can do about repairing it and find out exactly what went bad inside.

-Cody Forbes
http://www.500tq.com
'86 5ktq
'87 5ktq EFI/K27 and now manual idle adjustment
'88 80 4cyl
'88 90 4cyl]