[T3] Diagnosis help for a badly running 1971 VW 411

Jim Adney jadney at vwtype3.org
Sat Aug 31 14:37:50 PDT 2019

On 30 Aug 2019 at 12:57, Jens Vagelpohl wrote:

> I donTMt think itTMs a fuel delivery issue. That doesnTMt jive with the
> initial improvements after switching the MPS. I was thinking more along
> the lines of oewhat is it that you move/wiggle/influence when you switch
> out the MPS, like maybe a cabling or plug issue.

He should check to make sure that all 3 MPSs still hold vacuum reasonably 
well. It's possible, although unlikely, that the replacements picked this 
moment to fail.

A fuel pump relay that's going resistive can also cause these symptoms. I 
had one that would work just fine for miles, then start to fade out. That slowly 
reduced the fuel pressure until the car would barely drive. My accident I 
discovered that if I switched the ignition OFF and back ON, it would run 
better for awhile, but it took me a LONG time to track down the cause as 
being in the relay.

Relay problems tend to balloon quickly. Once you get a bit of resistance in 
there, then there's power going into heat in the contacts. Heat, makes things 
worse, so the problem gets worse, creating more heat... etc, etc.

They can be fine for a long time, then "suddenly" they go bad very quickly. 
And they can do this over and over again.

I don't want to be too emphatic on this. I've only had one relay go bad on me 
in more than 50 years, so it's a small odds thing.

It's often useful to put a fuel pressure gauge with a long hose, so you can 
watch the fuel pressure WHILE the problem is happening. The only real way 
to be sure that it's not a fuel pressure problem is to see the full 30 psi while 
the problem is happening.

If you see the fuel pressure drop while the problem is happening, then you 
know that you have a problem with the pump, wiring, relay, or filter.

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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