Hi Bob,
The teflon tape is definitely contributing to the problem since it insulates the sender, which needs to be grounded to function properly. You are likely getting erratic readings since teflon kind of falls apart when it gets threaded and you likely have somewhat of a ground that may vary due to temperature and any
oil that seeps onto the threads. I assume that you are using the Classic Instruments gauges and senders that came with the Lister like I am, so I pulled their installation instructions off of the web to show you that they specifically recommend not to use teflon tape. I followed this advice and have no problems.
GM Installations - OHV Engines
The correct location for the pressure sensor on most GM-V8 engines is under the distributor housing at the rear of the block!
Use the Brass Bushing Kit from Classic to allow the Sender to be mounted at a 45-degree angle pointing towards the driver's knees.
This allows Sender to clear the back of the intake manifold, the underside of the distributor housing and firewall.
* CAUTION *
DO NOT use Teflon sealing tape or Gasket Cement on the threads of your Pressure Sender or the Brass Bushing Kit! The threads are tapered and self-sealing and should be tightened just enough to prevent an
oil leak!
Do NOT overtighten!
The url for this and more on guages is:
http://www.egauges.com/Cla_Inst.asp?...resssender.htm
Alan