[T3] loss of power & sputtering … need WOTG

[Jim Adney]

On 28 Jul 2012 at 15:34, Vern Creamer wrote:

> Not too long after I returned from the Invasion, my 72 Fastback MT FI
> started stuttering, stumbling, missing once the engine got warm.  Here
> are things I've tried; looking for suggestions as to what I have
> missed.  I am not an accomplished mechanic by any stretch; no
> suggestion is too elementary.  The car runs OK when cold but has less
> power than normal; when it gets warmed up, it's a real challenge to
> drive at all ... barely made it home when testing. 

> 3.  replaced spark plugs gapped at .028" (2 of the old ones were at .040"
> upon removal; all 4 were the right light tan/gray color; #4 gap full of
> dry, gritty material) ... ran better after this, until it gets hot.

Somethings wrong with #4. I suspect this is the root of your problem. 
It could be that there's a broken wire in the injector connector, or 
it's not getting spark. Check its SP connector to make sure it's got 
about 1000 Ohms resistance; it may be open and not letting spark 
thru. Use the inductive timing light here. (see below)

If that cylinder isn't working, it's not warming up, and the FI temp 
sensor is on that cylinder, so it will be running the other cylinders 
richer, because it thinks the engine is still cold. This alone will 
make the engine run poorly.

> 4.  readjusted the Throttle Position Switch

This is pretty much a set and forget item. Way too many people waste 
major time messing with this and often just mess it up. On a '72, you 
need an Ohmmeter or the Bosch tester to do it right. I have a writeup 
on correctly setting the TVS if you're interested.

> 6.  retimed the engine following the Bentley instructions for the 72
> including idle adjustments with the vac lines plugged and then after
> the timing was set and the lines replaced. 

Did that go okay? The '72 instructions are difficult and often things 
go wrong that keep you from doing them correctly.

> 7.  tested the battery voltage at mid-range RPMs ... approximately
> 13.8V 

That seems low. Where did you measure it? It should rise to above 14 
V at the battery, but you have to give it time to charge the battery 
first, so it's best to do this after a good warmup drive. That may be 
difficult until you get your main problem fixed. 

Since you have a new VR, I don't expect this to be part of your 
actual problem.

> 9.  tested the coil output by holding the wire from the coil near the
> engine block; spark is orange 

I know all the books say to look at the color, but it's really 
irrelevant. The color depends on what elements are in the arc. It's 
really a question of whether it's there or not. I like to use a 
timing light with an inductive pickup to check each SP wire for spark 
with the engine running. It's easy to move the pickup from one wire 
to the next to verify that spark current is actually getting thru the 
wire.

> I am due for a fuel filter replacement, which I'll do this afternoon.

If you've been driving with a broken overflow hose, this could be 
part of your problem. Otherwise, I never replace the filters.

> Some questions:
> 
> 1.  What other kind of tests can I do to the coil?

Spark problems are generally binary, either 0 or 1. If things go well 
for awhile and then the engine develops a miss or stops completely, 
that could be a spark issue. Poor running, all the time is usually 
something else.

> 2.  Fuel pump issue?

Always a possibility, especially if you've been driving with a 
cracked overflow hose. To test this, connect a pressure gauge with a 
long hose so you can watch the pressure while you drive. The pressure 
should be constant at ~30 psi. If it droops, suspect the filter or 
the screen, then the pump.

> 3.  Voltage Regulator replaced 13,000 miles ago, last May, with a
> solid-state unit 

Not a lot of experience with these, but I suspect they'll be 
excellent.