tapping the fuel pressure sensor output

Tonyakavw · 12-02-2002, 02:09 PM

Has anyone ever tapped the output of the fuel pressure sensor? In my quest to figure out why my car dies, I'm thinking of tapping the output of the fuel pressure sensor and using a cheap digital panel meter inside so that I can continually monitor pressure and see what happens when the car dies (or just before hopefully). Any idea what the output looks like? Is it a voltage that is proportional to pressure (linearly maybe?)

-Tony Long
73 914 AKA VW

philinjax · 12-02-2002, 02:12 PM

There is no fuel pressure sender in the 914 FI systems.
Phil

mike mueller · 12-02-2002, 02:23 PM

You'll have to install a fuel pressure gauge...mine is after the fuel pressure regulator and before the drivers side injectors.

I had mine mounted in the engine compartment.

Mike

Tonyakavw · 12-02-2002, 02:32 PM

I'd like to have a guage inside the car, as well as in the engine bay, but having one inside the car means running fuel into the cabin (which I think is a horrible idea) or use an electronic sensor. Any leads on sources of electronic fuel pressure senders?

I see that Pelican sells a "pressure sensor" for the 914-4, but its about $600...

-Tony Long
73 914 AKA VW

mike mueller · 12-02-2002, 02:59 PM

First thing you need to get is a manual for your car and read up on the Fuel Injection........see this page for all the info that you can absorb:

http://members.rennlist.com/pbanders/

then you'll know what "pressure sensor" Pelican is selling

http://www.egauges.com/eg_typeI.asp?Type=Fuel_Pressure
I have not seen a digital unit to mount inside, but I'd think that someone has to make one.

Tonyakavw · 12-02-2002, 03:18 PM

(slaps self on head)

okay... I have read through Brad's pages a bit before, but since I have an L-jet 1975 1.8 engine, I'm not sure what all is applicable. I do see now that the pressure sensor is not at all related to fuel pressure.

That places sells "isolated" pressure sensors, which effectively do what I want, but at a high cost, especially for something I'm using to troubleshoot.

-Tony

mike mueller · 12-02-2002, 03:39 PM

What are your syptoms? I worked on a 1.8 over the weekend that would die or stumble at about 3k rpms...

the last thing I tested (yes, I tested the fuel pressure with a $40 test kit bought from Kragens, hint, hint) was the CHT sensor, I was not getting the correct Ohm reading.

it ended up needing the CHT sensor tightened correctly since it would not conduct to ground all of the time....once tightened up in the head, the car runs great.

Repost you problems and we'll see if we can help you, I've had a few 1.8's as well as a euro spec 2.0 running the L-Jet

Mike

Tonyakavw · 12-02-2002, 03:53 PM

okay, the problem is highly intermittent, which makes it hard to diagnose. Basically after a bit of driving (maybe 15 minutes, maybe 25 minutes) the engine will just quit. This has happened on the freeway, and it sometimes can be restarted by popping the clutch, but not always. Sometimes it just won't start for 10 to 30 seconds even cranking with the starter. There may be some correlation with throttle, but I haven't had enough instances to know for sure. It seems though, that it occurs after acceleration and then letting it go to idle or low throttle, such as cruising on the freeway. Its almost as though after letting off the gas, the RPMs continue to fall, all the way to zero. Though other times it will just die spontaneously while idling or at low throttle.
Spark seems to be good, but that is not easy to observe when driving... The cap, rotor, and coil are new (maybe a year old) and last I checked, the fuel pump is at least functional. Though I ought to verify that with a pressure guage as you hint towards...

It would not surprise me that the CHT sensor may be part of the problem. I had the wire discconnect itself once which of course prevented the car from starting, but fixed that. Maybe its possible that at certain temperatures the sensor has some problem. I guess I should first replace that since its cheap and an easy thing to check off the list of things to eliminate.

mike mueller · 12-02-2002, 04:08 PM

I'd venture to say it's something electrical...with motor running, carefully play with the wiring...I had the relay which controls the fuel pump and injectors go bad on me once, it had an internal short and only certain bumps and dips would it cut out on me

Tonyakavw · 12-02-2002, 04:13 PM

Thanks for the advice. I'll jiggle things around a bit and see what happens. Another symptom is that it will die over and over again. For instance when I start it back up, it will have a hard time keeping itself going. Then after a few of these cyecles it will be fine. Not sure whether that makes it sound more or less like an electrical problem ..

mike95125 · 12-02-2002, 04:24 PM

Definetly sounds electrical to me. If your fuel is intermittent it would still be electriclly rooted. Mike has good advise. Check your ignition switch as well (just from experience) it can cause wacky problems with bad contacts and such.
Mike

dave blackburn · 12-02-2002, 04:37 PM

Mike.. Thats what my relay looks like, where did you get that nice new one?

jdogg · 12-02-2002, 05:14 PM

Mike-
Do you have any photos, or can you describe your fuel pressure guage install.
Also, in addition to knowing nothing about how to install a fuel pressure guage, I know very little about checking electrical systems. How did you check the CHT sensor. I have a multimeter, but haven't a clue about what I am looking for or how to get an ohm reading. Isn't the ohm reading temperature sensitive as well. I have seen on Brad's D-jet site an ohm value for a specific temperaure, but how is that corrected for actual temperature?

Tonyakavw · 12-02-2002, 05:45 PM

Most automotive temperature sensors are thermistors which are resistors whose value changes with temperature. Unfortunately the change is far from linear and may be negatively or positively related to temperature (NTC or PTC) so that as temp goes up, resistance goes down, or as temp goes up resistance goes up... Each manufacturer has a different way of doing it, and you'd have to actually test the thing over a range of temperatures and make a plot of the resistance versus temperature. This is why most temp senders and guages come as a matched pair, and also why they sometimes just have a red zone instead of actual numbers. Digital temperature guages generally have a lookup table to determine the temperature based on resistance.

pbanders · pbanders's Garage

Quote:

Originally posted by Tonyakavw
Most automotive temperature sensors are thermistors which are resistors whose value changes with temperature. Unfortunately the change is far from linear and may be negatively or positively related to temperature (NTC or PTC) so that as temp goes up, resistance goes down, or as temp goes up resistance goes up... Each manufacturer has a different way of doing it, and you'd have to actually test the thing over a range of temperatures and make a plot of the resistance versus temperature.

I did this plot for the 0 280 130 012 sensor as part of the analysis of the cylinder temperature compensation circuit of the D-Jet ECU:

http://members.rennlist.com/pbanders/ecu.htm

The sensor is an NTC thermistor, characteristics were exponential, as expected. The CTC circuit provides a cut-off effect when the resistance drops below about 300 ohms. Below that value, there is no additional effect on mixture as the engine is fully warmed-up.

Dave at Pelican Parts · Dave at Pelican Parts's Garage

CHT and air temp sensors on both the D-jet and L-jet systems are all NTC thermistors. As they get warmer, their resistance decreases. The L-jet sensors should be "somewhat close" to the D-jet ones that Brad has tested. Haynes also has suggested values for them.

Actually, not that much of Brad's work (so far) applies to L-jet systems; he has been delving into the secrets of D-jet. It is related to L-jet, but a lot of the parts are different.

I agree that the problem described is electrical in nature. I had a bad wire from the battery to the relay board which caused very similar symptoms, even down to "popping the clutch fixes it". Took a while to hunt that down.

If you want an electric fuel pressure gauge, I believe that a standard oil pressure gauge setup will work fine. Put a barbed fitting on the sender, run a hose to either a spare port on the fuel rail or to a "T" that you install somewhere between the pump and the fuel pressure regulator. Mount the sender to something that is grounded, run the wires, hook up the gauge.

I don't think I'd leave it there in the long term, but it should be fine in the short term.

--DD