Solved: stalling when braking uphill

[Forrest]

1994 Wentworth Auto Estate, B200FT. The above problem had been gradually getting worse for years. I first experienced it at least six years ago. It also featured in my thread from last winter when I fixed a whole host of other issues with the car but this fault stubbornly remained.

https://www.volvoforums.org.uk/showt...=313229&page=3

My concerted attempt to resolve it over the past week has now succeeded!

Although braking hard uphill could pretty much be relied upon to induce a stall, I think the key symptom was unreliable idle when coming off the throttle at speeds between 30-40mph. Sometimes idle would drop to the 1,000 - 1,500 rpm range as expected. At other times it would drop below 500 rpm and either stall or recover belatedly to something under 1,000 rpm.

At the weekend I cleaned the throttle body. It needed doing (picture 1), but didn’t solve the problem.

A bit more internet research on this and other forums yielded suggestions to test the throttle position sensor - which I had checked was clicking as the throttle closed while I had the TB off the car. Also an implausible suggestion to “adjust base idle” which I felt was best ignored.

Then I found this thread, and bingo!

http://www.turbobricks.org/forums/sh...d.php?t=260444

My new compressor bypass valve diaphragm arrived today and was fitted in about 20 minutes this evening. About the easiest job going. Best to cover the holes in the structural beam immediately below to avoid it swallowing any dropped bolts. Undo the three small bolts and remove the CBV assembly. Clean up the area with some carb cleaner and reassemble with the new diaphragm. I couldn’t find torque specs for the bolts so went for a conservative 6 ft lbs. I suspect it’s a good idea to tighten them evenly to ensure the cover sits perfectly flat.

There was indeed a small hole in the old one (picture 2). Problem now resolved.

I think the science behind it is that without the CBV opening when the throttle closes the mixture goes lean due to excess pressure in the inlet. However, I’m happy to stand corrected. The important thing is that it’s solved a very irritating long-standing problem.

[TonyS9]

Ah yes, this is also described as stalling after acceleration (which includes going uphill). Basically the boost pressure isn't being releived for idle, the MAF doesn't read any air flow or there is too much and no extra fuel is injected. The engine can die from a weak mixture. If you don't close the throttle for long enough to lets the pressure disipate it will be ok.

The dirty throttle body normally only causes problems on startup, after that the ECU remembers the compensated idle valve position.

I had the same turbo problem in a family members car after a turbo overhaul. The (cheap) overhauler damaged the diaphragm and didn't replace it. Took about 2 weeks to figure it out, noone online could understand what was going on, but now I have a much better understanding of why the dump valve is needed as a result.