As part of my ongoing Delorean instrumentation/automation project, I decided to install a full-time fuel pressure monitoring system. Many common starting and performance problems of our cars are most easily diagnosed by measuring fuel pressure, yet this is not a procedure that very many owners are equipped to perform, and even fewer routinely carry the necessary equipment with them on the road. With a permanently installed fuel pressure monitor it would be easy, for example, to diagnose a hot start symptom as an accumulator/check valve problem, since the failure of the fuel system to hold rest pressure would be obvious.
Fuel pressure sending units are readily available from performance car instrumentation suppliers. I bought mine from Dakota Digital. I bought a 5 bar (75 PSI) sender (Dakota Digital part number SEN-10-2). The price was $23.37. This is actually a VDO #360-003 unit, which reads 10 ohms at 0 bar and 180 ohms at 5 bar. A wider selection of sending units is available from Continental Imports, but Dakota's prices are excellent.
The hard part of this project was figuring out a clean and safe way to tap into the fuel system at an appropriate place. We are talking about pressurized gasoline here, so I didn't even think about hacking t-connectors into an existing hose with pipe clamps. The VDO sender (and most others as well) has an 1/8" NPT threaded pressure port. The Delorean fuel system uses variously sized metric banjo fittings. How to get from one to the other?
The key to this is the use of a double banjo bolt. This is a variant of the standard hollow banjo bolt with two sets of ports so that two banjo fittings can be stacked, one on top of the other. You can see one of these coming out of the bottom of the stock fuel distributor.
There are two different pressures in the system that would be useful to monitor: primary pressure and control pressure. I chose the latter for this project, since problems in the primary pressure would be reflected indirectly in the control pressure, whereas the converse is not necessarily true. Control pressure is measured at the hose that goes from the top center fitting on the fuel distributor to the Control Pressure Regulator. The banjo fitting on the distributor side of this hose is a 8 mm single banjo bolt, meaning we need to find an 8 mm double banjo bolt to replace it with. This turns out not to be a common part. After much searching, I finally found a company called MotorVac which makes a family of wierd and wonderful machines to clean out the insides of fuel systems (improperly-stored-Delorean-resurrectors, take note). Their accessories department has all kinds of special fittings for use with their product. They had the bolt in question (item #060-2710) for $3.64. (As an aside, they also seem to be the cheapest place in the world to buy crush washers for the banjo fittings. For example, 8 mm washers are $.16/each. This project requires 3 of these. Crush washers re not reusable--new ones must be used each time--another reason why a permanent pressure guage is a good idea).
Now we need to somehow get from an 8 mm banjo fitting to the 1/8" NPT port on the pressure sensor. No problem: the friendly folks at Baker Precision Bearings were happy to make me up a custom length braided-stainless-steel-covered fuel rated cable with an 8 mm banjo fitting on one end and -3 BRA fitting on the other. My 24" hose cost $20.00. They also provided a -3 BRA to 1/8" NPT adapter for $3.00, so I was all set.
So, the whole kit costs around $50 and looks like this:
Installation was pretty trivial. I removed the existing single banjo bolt from the center port on the fuel distributor (WARNING: there may well be pressurized fuel in the system. If you try this, be sure to follow all the safety procedures outlined in the shop manual when performing this step). I replaced it with the double banjo bolt, adding the new hose as shown below, sealed with three crush washers. I ran the stainless steel hose along the firewall to the area below and to the left of the ignition resistors, where I mounted the sending unit. I had an assistant crank the engine briefly to purge the air from the line before tightening it down onto the sensor--catching the fuel in a container. Finally, I tightened the hose/adapter/sensor assembly down. One should not use teflon tape here, since it could disrupt the necessary ground connection from the sensor through the hose to the engine. The NPT fitting has a tapered thread that seals via deformation of the threads. It will seal just fine, although it does take a lot of torque to get everything together.
Here's what the finished product looks like:
(Click to enlarge)
So, what can you do with such a setup? There are several choices. The easiest thing is just to connect up an ohmmeter between the terminal on the sensor and a good ground. A resistance reading and a little math will tell you the pressure at any time. My intention is to continuously monitor the pressure using a PIC microprocessor with an A-to-D converter. If you want a more turnkey solution, you could buy a complete in-car fuel monitoring setup from Dakota Digital.
It would, of course, also be possible to monitor the primary fuel pressure in the same way. For this application, though, it would probably be best to chose a different sender with a higher pressure range, since normal primary pressure is pretty close to 5 bar and the sensor I used would not give much head room. I may eventually decide to use dual sensors to monitor both. Another possibility would be to use a similar setup to monitor the automatic transmission fluid pressure.
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