Surging and erratic boost

Phil and Judy Rose pjrose at
Sun Dec 31 13:16:57 EST 2006

At 11:56 AM -0500 12/31/06, rguzz at wrote:
>186k miles 200tq TAP chipset (yeah I know) there is a worsening 
>surge intermittently beginning at around 2.5-3k rpm. The external 
>boost gauge as I run the car a bit hard will hover at 16 psi, 
>sometimes very steady other times wildly

>I'd like to clean, check function of all sensors (hall, etc) but I 
>sort of think these function on an all or nothing basis.
>Could this be as simple as a boost leak? I don't hear one and power 
>seems good. Should I replace the WGFV on a trial and error basis? 
>Where else should I look?
>Thanks for your input.

When I hear "surge" complaints, I tend to suspect the oxygen sensor. 
Does the surge seem most pronounced while at steady throttle? How old 
is that sensor? Likely to be bad if more than 4 or 5 yrs old.

I think that achieving 16 psi on an external gauge indicates that the 
WGFV is unlikely to have failed completely (absolute WGFV "failure" 
results in a limit of about 0.4 bar boost, i.e., about 6 psi) 
although I suppose the valve could simply be erratic or sluggish.

As to your boost gauge behavior: how much boost deviation (+/-) do 
you see when your gauge is "wildly" varying? My own (Lehmann-chipped) 
200 has always had a fluttering boost gauge needle when it reaches 
the boost limit of about 18 psi. At that instant the needle flutters 
rapidly at about +/- 1 psi (seen with a new wgfv), but I have no 
leaks, no surges, and good fuel mileage, so I've decided that this 
behavior is "normal", at least for my "el cheapo" gauge. But in 
contrast to what you describe, at the point when true boost limit is 
reached in my car, the gauge needle always flutters and is _never_ 
steady. I imagine that if my car was failing to achieve true maximum 
boost from time-to-time, then the gauge might behave like yours does.

I recommend you check more carefully for a pressure leak; set up a 
careful pressure test of your intake manifold and you may be 
surprised at what you find.

*  Phil & Judy Rose           Rochester, NY  *      
*        mailto:pjrose at       *

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