[T3] 69 FI trouble shooting

[Jim Adney]

On 19 Jul 2012 at 7:47, R. David wrote:

> [[[Was there a gauge on the fuel pressure while the engine was dying? If so,
> was the pressure holding at 29 right up to the end?]]]
> 
> Steve said the gauge was on it when it died the previous night, but not
> installed last night. We didn't want to install the gauge on a hot engine
> last night, but we will now as this check is worth repeating.

I only asked because dropping fuel pressure is exactly what this 
sounds like, and an inadequate fuel supply will also not respond 
properly to increased load.

> I like the suggestion to re-check the valves, as these could have a reaction
> to a warming engine. That's the thing here; it takes this motor getting to
> running temp. before it starts it's death spiral. 

If the valve adjustment had only been done before the engine was 
first started, it could have been swayed by rust or crud that built 
up since the engine was last run. This stuff would have worn away 
quickly when the engine ran, resulting in improper valve clearances.

> I thought about the trigger points, but I couldn't see how they would
> work when cold but not work when hot.......

If the trigger points are maginal, they can work at one temp and skip 
as they warm up. This is a long shot because that generally results 
in a complete miss or the engine dying, not slowly weakening.

> and then the motor would fire with spraying a flammable into the IAD,
> so I kinda ruled out the trigger points to myself....but I could be
> wrong. 

That tells you that the ignition system is working but the FI is not.

> I forgot to say we did check voltage at the battery, and we could get it to
> over 14V with about a mid-range rpm setting. We will check AT the generator
> though, thank you. We also swapped out the coil with a known good one and
> that didn't matter. His Bosch points and condenser are new (although that
> doesn't guarantee anything).

If you checked voltage at the battery, that's enough. I just 
suggested checking at the generator because that's an easy place to 
make a quick test. The battery test is the real test.

If the dwell and timing are okay then the points are good. If the 
condensor or coil were going bad that would show up as misses, not 
weakening.

> When I saw it was a heat induced death, we immediately checked the
> resistance of both temp. sensors, but they were in spec.....so I thought
> maybe the brain reacted wrongly (is that a word) to the lowered ohm
> readings???

The temp sensors are a good suspect. If this '69 still has it's head 
sensor below the #4 exhaust valve that can cause a problem as the 
wire gets bent sharply and tends to short to ground. That, however, 
also stops the FI completely and instantly, not slowly weakening.

I don't expect the brain to react oddly to slowly changing 
resistance. These brains have been REALLY reliable, but I have good 
tested B brains here if you run out of options.

One more thing to check might be that the distributor is really 
pushed ALL the way down and fully engaged in the drive gear. That can 
stick and slip as the case warms up.  

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