FW: Lack of Boost, '88 5ktq

["Southerlin, Russell S" <russell.s.southerlin@lmco.com>]

> Scott,
> I have been wanting to complement you on an awsome web page. I think I
> have read it from start to finish more than once!!!
> 
> ----------
> From: 	Scott Mo.[SMTP:scottmo@teleport.com]
> Reply To: 	scottmo@teleport.com
> Sent: 	Thursday, February 26, 1998 1:16 PM
> To: 	Russ Southerlin
> Cc: 	quattro@coimbra.ans.net
> Subject: 	Re: Lack of Boost, '88 5ktq
> 
> >Russ Southerlin wrote:
> >Sound like you simply have a tired wastegate spring if all other
> engine >functions are O.K.
> > I had a same thing when I first got my car a year ago.It turned out
> to
> > be nothing more than a tired (stock) wastegate spring.  I went the
> the
> > local Ace Hardware and found a heavy duty spring that fit inside the
> > diameter of the stock spring and then cut it to length by trial and
> > error was able to get consistently 1.4 bar of boost.  
> 
> If the ECU boost control system and components are working correctly,
> the ECU will add pressure on top of the Waste Gate (WG) diaphram using
> the WG solenoid and will overcome a weak WG spring. Normally if the
> boost is only controlled by the stock WG spring with the upper WG hose
> removed, you will get 1.2-1.3 bar. With the ECU assisting, the boost
> should be ~1.4 bar as is set by the boost map in the ECU EPROM. The
> boost map values change slightly at different engine RPM values.
> 
> When I was having these problems with my car I actually put a pressure
> gage on the WG F/V output to the wastegate to actually see how much
> pressure "assits" the ECU (WG F/V) was putting on top of the WG.  Too
> my surprise there was hardly any assist at any engine load condition.
> The pressure would just fluctuate about 1 or 2 psi when the full
> throttle switch was energized.  Now this 2 psi is equal to 0.136 bar
> which may be enough with a good spring to bring the total boost up to
> 1.4 bar if the "unassisted" boost pressure is 1.3 bar.
> 
> I guess the question is how much "assist" can the ECU provide through
> the WG F/V.  I think it is limited to the max duty cycle that the ECU
> can run the WG F/V at and I don't think it will operate it at anywhere
> near 100%.   Do you know what this value is?  I suspect  it is in the
> 1-2psi range. Therefore if your spring is tired and your ECU can only
> give 1-2psi of assist then you won't reach 1.4 bar.
> 
> I guess one test would be to run the car with the WG F/V hose
> disconnected and see if you get the 1.2-1.3 bar as you suggest.  This
> would give an indication whether or not the spring itself was tired,
> although you could not rule out some other cause (diaphram, WG etc.).
>  
> Putting in a stiffer WG spring is only masking a problem or fault in
> the
> boost control system. A defective throttle switch is a likely culprit
> but, for details on things to check out which can cause low boost,
> check
> out the ABC's of Running High Boost" a
> 
> Yes I carefully checked all my boost control systems.  I know the
> throttle switch was good as I also monitored the O2 sensor and could
> see enrichment when the full throttle switch was energized.
> 
> 
>  I suspect that there are many 1986-90 5000/200TQ's driving around
> with
> low boost due to faulty components in the ECU system.
> 
> I agree - you have to make sure all other systems are O.K.
> 
> Russ
> 
> 
>