Bad fuel pressure reading

[chanik]

I've had on ongoing issue with spiking fuel pressure on my Efis, annoying since it sets off the alarm. It started after my annual with Roger and continued sporadically after he replaced the sender. I had attributed it to heat since it tended to happen when the OAT was high or the engine sat around hot. But after my latest checkout with Jeremy, it was more constant and uncorrelated to temperature. It couldn't be the engine as the swings are too extreme and too rapid. So, time to debug: The VDO 360-043 sender is a 10-180ohm sensor (180 is 30psi) so pressure spikes could be erratic connections as this is low impedance. So I got a 51ohm resistor, removed the sender faston spade lugs and spliced it in. The sender measures a constant 8.5ohms with engine off even when I hammer on it etc but I found it was hard to make a good connection with the DMM clips. The EFIS also measured a solid 6.5PSI with this resistor plugged into the faston lugs even as I twisted the leads around so that connection also seems solid. It turns out the problem was that the brass lugs on the VDO were heavily oxidized and the faston lugs had loose tension. After crunching the lugs together with pliers for tighter grip, sanding a fresh contact on the lugs and applying pro-gold, the fuel pressure is now behaving. I was surpised at how bad the contact resistance had become on the sender, but there are some cautions about these for other VDO senders from Rotax like this one where they recommend saftey wiring these:

http://thermostasis.com/includes/templates/thermostasis/downloads/troubleshooting-rotax.pdf

What a pain to find, but at least now I know to look for this. BTW, the pro-gold really should be required treatment also for the grounding bolts FD uses (terrible design practice) where they ground two dozen connections to a couple of little bolts on the firewall behind the right side EFIS. Roger has found this to be a source of problems so often that it is virtually his mechanic's refrain: "Check two grounding bolts and call me in the morning"

http://store.caig.com/s.nl/it.A/id.2019/.f?sc=2&category=82

This stuff has a well-deserved cult following in the electronics industry.

[Roger Lee]

Hi Kurt,

 

Your right that many times poor connections especially grounds are a problem. If your gauges swing high, bounce with wide swings or peg out on the high side then it is time to look at connections.

 

Any of the senders or probes with the spade type connector (i.e. CHT, fuel pressure, oil temp) should be on the first to check items if you start to have a bad reading.