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

Jim Adney jadney at vwtype3.org
Sun Aug 11 20:46:27 PDT 2019


Since it's a VW, and a close relative to a Type 3, I think this is an 
appropriate place to ask this question. I'm not nearly as familiar with the year 
changes in Type 4s as in Type 3s, but the FI is similar enough to make 
troubleshooting straightforward.

Check that the pressure sensor holds vacuum and that it's hose is 
connected on both ends.

Check the system voltage at medium rpm. If it's below 13.5 V, replace the 
voltage regulator. This is an alternator system, but it has a separate VR, and 
those VRs also wear out.

Check the mechanical advance and make sure it's free.

Did the fuel filter get changed at the same time as the pump? If not, it may 
be clogged (or the filter screen could be clogged) making the fuel pressure 
drop under demand. (Check by hooking up a pressure gauge you can watch 
while trying to drive.)

Where is it? I can be bribed into a house call, especially if it's in a direction 
that my wife finds interesting. Would be expensive, but not as expensive as 
what you just spent on air fare.

Ask the own to save the old parts. They're worth keepingif they are the 


On 11 Aug 2019 at 12:24, Jens Vagelpohl wrote:

> Hi all, hi Jim,
> Not directly Type 3 but closely related in this case.
> I´m currently looking at a very low mileage all original 1971 VW 411. Original engine, original fuel injection. The car itself is in beautiful shape otherwise and I took at 2.5 hour flight yesterday with money in hand, my plan was to complete the purchase and drive back to Fort Worth where I currently live. Unfortunately, the selle had not fully explained how badly it runs right now and I had no choice but to leave and fly back. The seller wants to do more diagnostic work, he has been working off some Bosch guides and the Harold Glenn guide to test individual components, but thereTMs obviously something he hasnTMt hit yet.
> I am hoping for some input about likely causes and troubleshooting strategies for the following:
> - starts up pretty willingly, but idle too low and stumbling
> - constant backfiring into the intake when I push down on the accellerator
> - no power when driving, I think I got it up to 50
> - backfiring doesn´t change much as the engine gets warmer
> - keeping the accellerator in one position, inclucing all the way down, causes more backfiring than actual speed increase
> - the only way to get accelleration for split seconds is to "pump" the acellerator, by backing up and pushing down carefully, at that moment the engine runs well and accellerates for a sub-second period, and then returns to the no-power situation.
> These issues apparently developed over the last year or so, the owner told me when he had the car first it would cruise at 75 on the freeway without issue. He has already replaced the fuel pump and verified with a pressure gauge that there is no fuel pressure issue. He also replaced the injectors and all fuel lines. On the ignition side he did the points, plugs, plug wires and coil. Oil is changed and valves are adjusted.
> Any help is highly appreciated. If the car wasn´t located just over 4 hours from Jim I would already try to bribe him into going there to look at it ;-)
> jens
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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