[T3] Bad day at the (FI) office

Daniel Nohejl d.nohejl at gmail.com
Tue Mar 29 04:15:34 PDT 2016


> the ECU really needs to see below 100 ohms (approx) to go full lean, (no warmup enrichment) otherwise the pressure  sensor wont track properly.
> below 100 and the ECU essentially sees it as a short.  Many different sensors were used but their cold resistance was the difference, so it changed "how much choke" your engine had at a given cold temp.

Well, on Sunday my sensor read 87 ohms without the resistor so then wouldn’t I need a resistor in order for the ECU to not read the sensor as a short? Or am I misunderstanding what the effect of the ECU reading the sensor as a short would be? In other words, it sounds bad to me but perhaps you’re saying that below 100 ohms just registers as warmed up and it doesn’t matter if it’s 99 ohms or 1 ohm.

> adding a shunt resistor mimic's this, but adding a series resistor gets you over 100 ohms and richens up the hot mixture, this can mask the real problem and is not a good way to adjust hot mixture as then the hot mixture can wander somewhat and you dont want that.  These sensors were just for gross cold warmup adjustments.. they are not that accurate or linear.

Ray Greenwood has been telling me something different for the last few months now….at least since November when I first put a resistor inline with the temp sensor. He says that the CHTS shouldn’t read below 70 ohms because then there’s no room for it to offer any additional enrichment on, say, a hot day in August at 70 mph on the Thruway. In other words, he suggests that the CHTS continues to influence the mixture well after warmup. I’m not really sure which is true: either it does or it doesn’t (or maybe it partly does or partly doesn’t). As mentioned, I can use a variety of low ohm resistors and see what effect that seems to have on the AFR at idle and while driving after warm up. 

Anyway, in general, things seem to be better. We went away over the weekend and on the way out of town, we leaned out the MPS a wee bit closer towards its original position. Our highway cruise AFR is kind of lean….anywhere from 14.5 to 16.5 but our MPG over the 260 miles was a modest 25.8. I imagine if we were too lean, our MPG would have been much higher. 

Toward the end of the trip, I started noticing that the idle AFR went back to rich even though we hadn’t touched it again and that the idle sounded too high. When we got home, we took out the dwell meter and timing light. First, the idle was at 1020 rpm so we brought it down to 850 but it was unstable and hunted slightly so we brought it up to around 900 which is where it seems happiest. We noted that the dwell had changed from 48* to 52* in 1600 miles. I’ve never had that happen before. Also, the timing was a few degrees advanced….closer to the second timing mark than the first. We tried to reset the points but there was such a big tit on one side that it was hard to get the feeler gauge through so we just put in new points and re-did the timing. Once we did that, the idle AFR went from 11.5 to 13. Seems like the advanced timing was messing up the MPS and the a/f mixture. 

I’ve yet to have a set of points go bad on me in only 1600 miles. Admittedly, I only have about 70,000 miles of VW driving under my belt but still….Part of me wonders if repeatedly having the key in the ignition in the on position while toggling the fuel pump prime switch hastened the death of these points. Maybe I can power the switch from an always hot fuse and see what happens?




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