[T3] Temp sensor and MPS problems on new motor

[Jim Adney]

Jessica,

Okay, now I get that you're getting bucking and bad running at high 
A/F, but this doesn't coincide well with the poor gas mileage, which 
could also go along with low A/F. So what's really going on here?  

Some thoughts:

Could unburned gas be causing your A/F sensor to indicate high when 
it's really low? If so, this could mean that your problem is in the 
ignition system somewhere. Double check your SP wires, connectors, 
cap, rotor, etc. Look for arc tracks; swap cap & rotor, I know you've 
already tried some of these. Just a thought. 

If you've got a poor connection to one or more injectors, that can 
make them inject less or even not at all. This will be intermittent 
and temp dependent. For a '69 there are 2 things to check:  

1) The grounds for the injectors went to screws into the heads in 
'68-9. Those are heat-cycle failure prone unless you have Belleville 
washers under the screw heads. I can supply those washers. I believe 
you have one of Joe's repop harnesses, so he may have wired this to 
put those grounds at the center of the case, where they do much 
better. If so those will be fine. If not, you need the Bellevilles.

2) At some point in the Bosch injector production they changed the 
thickness of the male connector prongs in the injectors. They made 
them thinner. Replacement injectors will OFTEN make poor connections 
with original connectors. If you have poor connections at any of 
those connections, they will haunt you until you pull out the female 
pins and squeeze them very slightly down. After squeezing, check each 
pin for suitable fit ON its injector. Once again, if your replacement 
harness was made by Joe, he probably fixed this, but he might have 
missed one.

Finally, I believe you've checked the voltage available coming out of 
your FI main power relay, but check also the 2 wires in the engine 
compartment that bring that power to the FI harness. I've seen one of 
those wires with badly frayed strands, and that can starve one part 
of the FI system. (The 2 wires deliver power to 2 different parts of 
the brain: One serves the low current control section. The other 
supplies the much higher current injectors.)

Finally, I'd like to discourage you from misadjusting good components 
in an attempt to "fix" a problem that none of us understands yet. 
Something in your system is intermittent, and that's probably not in 
the pressure sensor. Temp sensors have been known to become 
intermittent as they temp cycle, but that's rare. Frankly, I suspect 
the brain. Since you're using one that has one KNOWN fault, one might 
wonder what caused that fault and what else might have happened at 
the same time.

I look forward to looking at the brains you're sending me, but 
ideally I'd like you to try a completely different one that I can 
send back to you.

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