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Re: [T3] erratic fuel delivery


On 9 Oct 2005 at 23:20, shalom@ucla.edu wrote:

> I have a 70 squareback (fuel injection) and I can't figure out why the car will
> run for an hour and then in the middle of driving it shuts down.  Then I can't
> even start it for a half hour.  It turns over but won't continue running, like
> it's not getting enough fuel.

Don't assume that it's a FI problem. You should verify that you still have 
correctly timed spark when it still won't run.

When it quits, does it cut off sharply, or does it lose power slowly over a few 
seconds? If slowly, does it just get weaker and weaker, or does it start to 
missfire more and more?

> Could it be the pressure sensor? I checked the resistance across both coils of
> the sensor and they are both correct at 90 ohms and 350 ohms.  But, I read that
> sometimes the rod between the coils will get stuck and will give this erratic
> behavior.

It is not likely the PS. They are quite reliable. For your '70, however, you 
should put a clean hose on it and suck on yours. Make sure it holds vacuum. The 
70-1 PSs sometimes get cracked diaphrams which keep them from working right.

How is your gas mileage? How do the spark plugs look?

> What about the fuel pump?  Can the pump work sometimes and not work at other
> random times like the problem I'm having?
> 
> I have yet to check if I have 28psi in the fuel system.  Maybe the regulater is
> not working.

It is probably not the pump, and it is almost certainly not the regulator. The 
brain is probably also fine.

The first thing I would do, would be to connect a voltmeter to the output of 
the fuel pump relay. Those sometimes develop poor contact resistance and then 
they heat up and get worse. If you see the voltage delivered to the pump 
dropping lower and lower just before the failure, then you've found the problem 
and you should replace that relay.

If the overflow hose next to your gas tank filler pipe is broken, you could 
have water in the gas from driving on wet roads. This will clog up the filter 
and cause the fuel pressure to drop. Dust and dirt in the gas will cause the 
same problems.

Watching the fuel pressure while driving would spot either of the last 2 
problems. Watching the pump supply voltage would tell you which problem it 
actually was.

Whatever you do, don't just start replacing suspected parts. Almost everything 
here can be tested, and shotgun parts replacement is the most wasteful, time 
consuming, and expensive way to troubleshoot.

-- 
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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