Fuel guage issue

[Lightning]

Recently my fuel guage stopped working (needle pointing at 12 o'clock) so I replaced the unit in the tank which fixed it. I noticed last week an oddity though and am wondering what would cause it. First of all, I'm in Florida so most of the roads are very flat. If I am at a stand still at a light then the guage appears to read properly. Once I start driving the guage will start going down, the longer I drive before a stop the further down it drops. I've seen it low enough that it sits below the E yet the indicator of being low on gas did not light up and it was indicating half a tank when I was stopped. There is a local bridge and going up or down the bridge does not affect this at all. Once I come to a stop again then I'll read half a tank of fuel.

Ideas? The guage for electrical power shows 13V when running and I'm not sure of a good way to test the issue while driving.

Sam

[PJ Grady Inc.]

This has always been a problem with aftermarket sending units as adapted to a Delorean fuel tank. There is no baffling in the tank so most senders will read lower when driving as the fuel sloshes to the rear of the tank. You did not mention whose sender you are using but the problem is common to most (if not all) of them. Dampening the electrical input sufficiently to compensate for this usually creates other issues and slows the gauge reading considerably.
The "local bridge" readings cannot be explained as easily....for that you'll need to consult a palmreader;)
Rob


Recently my fuel guage stopped working (needle pointing at 12 o'clock) so I replaced the unit in the tank which fixed it. I noticed last week an oddity though and am wondering what would cause it. First of all, I'm in Florida so most of the roads are very flat. If I am at a stand still at a light then the guage appears to read properly. Once I start driving the guage will start going down, the longer I drive before a stop the further down it drops. I've seen it low enough that it sits below the E yet the indicator of being low on gas did not light up and it was indicating half a tank when I was stopped. There is a local bridge and going up or down the bridge does not affect this at all. Once I come to a stop again then I'll read half a tank of fuel.

Ideas? The guage for electrical power shows 13V when running and I'm not sure of a good way to test the issue while driving.

Sam

[Lightning]

I have Houston's unit. The only reason I mentioned the local bridge is that I expected that if it was the fuel sloshing around that it would change as i went up or down one of the sides. I wouldn't expect 30mph on a road to shift enough fuel to cause it to read below empty though and return to half a tank without triggering the low fuel light hence wondering if something else may be the culprit.

[dustybarn]

I have Houston's unit. The only reason I mentioned the local bridge is that I expected that if it was the fuel sloshing around that it would change as i went up or down one of the sides. I wouldn't expect 30mph on a road to shift enough fuel to cause it to read below empty though and return to half a tank without triggering the low fuel light hence wondering if something else may be the culprit.

I have Houston's unit too, and I noticed the same thing. However, the low fuel light would sometimes trigger when the fuel sloshed on acceleration.

Here's what I did:

On the plastic bottom of the sending unit are two holes that allow fuel in and out (on mine, one of these was already blocked with a screw). I tapped the other hole, degreased it, then totally blocked it with grey Hysol epoxy (the threads provide mechanical support for the epoxy, and make it less likely to fall out should the bond fail).

After the epoxy hardened, I carefully drilled a tiny hole through the epoxy with a drill bit in a handheld pin vise. IIRC the hole was something like 1/16" or so. Although the fuel can still flow in and out, it takes much longer for that to happen, and the sender doesn't respond to fuel sloshing as much as it did.

[Jonathan]

I have Houston's unit. The only reason I mentioned the local bridge is that I expected that if it was the fuel sloshing around that it would change as i went up or down one of the sides. I wouldn't expect 30mph on a road to shift enough fuel to cause it to read below empty though and return to half a tank without triggering the low fuel light hence wondering if something else may be the culprit.

Do you have the dedicated fuel sender as a replacement or the pump and sender combo?

Don't the warning light and gauge run off the same physical level float going up and down in the housing in the original design, whereas the pump/sender combo has a separate sensor for the gauge and another one for the warning light?

Your problem sounds more than just the typical poor response to fuel sloshing around because you aren't seeing the warning light coming on. On the other hand, do you know for sure the warning light circuit and bulb are ok? Maybe something got coincidentally bumped loose and now the warning light is trying to come on and you just don't know it?

[mluder]

Slight sideways tangent - don't mean to hijack the thread...
My warning light doesn't seem to work. I replaced my binnacle a few months ago and verified all the lamps were working at the time so I know the bulb is fine.
Is there somewhere down the line (close to the tank without going inside) that I can force a trigger before diving into the tank?

Cheers
Steve

[Lightning]

I do not have the combo unit. Last time I know my tank was low the warning light came on, it has just seemed odd as I didn't recall the previous unit acting this way. I like the idea of pulling it and adjusting it for amount of flow.

[Chris 16409]

When I got my DMCH fuel sender, I only had one screw installed as well. What I did was simply install a new screw in the second hole, but not tighten it completely. I left some of the threads exposed so fuel could leak past them, which still allowed fuel into the sender tube. I no longer get the needle movement when accelerating and breaking.

[Timebender]

I recently installed a new sender, one of the nice metal ones from DMCH, as my original plastic sender wasn't working, and when I took it out, pretty much deteriorated.
After I put in the new sender, it took a while for the gauge to register when I filled the tank - like maybe a half hour after driving, though it never registered a full tank, or close to it (more like 75%), but then I don't know if I'm getting a full tank when I fill it up as fuel nozzles around here tend to shut off mid-fill, or every couple gallons (which is what's supposed to happen at at full tank).
I recently pulled the sender out slightly to check if the rubber seal was not seated right, as I'd smell fuel on what I'd consider a full tank - turns out that was actually a loose fuel pump boot which I fixed for now. But the sender now only shows a half tank, or less, after a fill-up - and sometimes it shows a quarter tank after a fill. This morning it started showing 3/4 of a tank, and as I drove it gradually went down - but still showed a half tank after driving about 30 miles. However at my place of arrival, it suddenly went to above empty, and when I came back later, the fuel light was on.
So I went to get gas but it only let me put in 7 gallons.. which I assumed would be full- as last time I wasn't sure it was full due to the shutting off fuel nozzle at the pump, until the gas started to come out the back of the car from the exposed fuel return line... (which I think should be plugged?).

Any ideas - bad gauge maybe??

[Bitsyncmaster]

Our fuel gauge circuit uses quite a bit of current. So any bad connections are going to upset accurate readings. The fuel gauge uses a double coil circuit so bad connections with the cluster will also upset accuracy.

I would suggest you try the change I made removing the fuel pump current from that shared ground with the gauge cluster. I did that to make the volt gauge read accurately but never thought about it improving the fuel gauge.

[Timebender]

That's an interesting idea. I'm guessing I'll have to get inside the binnacle to create a different ground for the cluster?

[Bitsyncmaster]

That's an interesting idea. I'm guessing I'll have to get inside the binnacle to create a different ground for the cluster?

I keep the binnacle ground stock and just moved the fuel pump ground from the inertia switch to frame ground. I connected to the frame at one of the shifter mounting bolts. To do this you cut the ground wire from the inertia switch and splice in another wire (from that cut wire going to the inertia switch) to connect to the frame. I had posted about that change on DMCtalk but it must have been the old .com forum or somewhere that I can't find it anymore.

[Timebender]

So did you cut the ground from the fuel pump to the inertia switch, splice in a longer wire to ground to the frame, then run a wire from that frame ground back to the inertia switch so it would still work?

edit: Found what you're getting at on the archives here: http://dmctalk.org/archive/index.php/t-4243.html'

So cut the ground wire to the inertia switch and then make it longer to get to the frame attached to one of the shifter bolts under the middle console. Are there any places closer that provide a good ground?

[Bitsyncmaster]

So cut the ground wire to the inertia switch and then make it longer to get to the frame attached to one of the shifter bolts under the middle console. Are there any places closer that provide a good ground?

You got it. Cut the existing ground wire going to the inertia switch. Splice onto that cut wire still attached to the inertia switch enough wire (use 12 AWG or larger) to attach to a frame ground. I did not find a closer frame ground but maybe there is one.

What that does is remove the fuel pump ground current from the instrument grounds. I did that to get the volt gauge more accurate but it probably also helps all the instruments. My result was volt gauge read about 0.3 volts higher.

[Henrik]

Slight sideways tangent - don't mean to hijack the thread...
My warning light doesn't seem to work. I replaced my binnacle a few months ago and verified all the lamps were working at the time so I know the bulb is fine.
Is there somewhere down the line (close to the tank without going inside) that I can force a trigger before diving into the tank?

Cheers
Steve

Here is a How-To that might be helpful:
http://dmctalk.org/showthread.php?2291-How-to-Inaccurate-fuel-gauge-readings-How-to-rule-out-the-actual-fuel-gauge&p=30914&viewfull=1#post30914

[Timebender]

Awesomeness. Thanks guys!

[Timebender]

Btw I have a new fuel sender that I installed, and actually the other day it did give me the low fuel light after a drive on the freeway, some hills, stop and go traffic, and one all out WOT for fun....

[Timebender]

You got it. Cut the existing ground wire going to the inertia switch. Splice onto that cut wire still attached to the inertia switch enough wire (use 12 AWG or larger) to attach to a frame ground. I did not find a closer frame ground but maybe there is one.

What that does is remove the fuel pump ground current from the instrument grounds. I did that to get the volt gauge more accurate but it probably also helps all the instruments. My result was volt gauge read about 0.3 volts higher.

So I'm guessing the big black wire is the ground - and will this area with the bolt by the brake pedal be good enough for frame ground, or do I need to run the wire under the center console to the stick shift and there's a bolt there somewhere? I'm looking for something easy to get to vs taking off the center console (okay,
Yes I know I can just take off the screws, shift knob and boot to take that off).

39177 39178

[Bitsyncmaster]

So I'm guessing the big black wire is the ground - and will this area with the bolt by the brake pedal be good enough for frame ground, or do I need to run the wire under the center console to the stick shift and there's a bolt there somewhere? I'm looking for something easy to get to vs taking off the center console (okay,
Yes I know I can just take off the screws, shift knob and boot to take that off).

39177 39178

Yes the big black wire is the ground wire to cut. Not sure about how good a ground that bolt by the brake pedal would work. Even if you found it was grounded via checking with an ohm meter the ground just may be through brake lines. Your really need a bolt that bolts to the frame to be sure. It's pretty easy to feed the new ground wire to the shifter without taking the console out.

[Timebender]

Thanks Dave. :)

[Timebender]

Reviving this thread. Turns out the only fuel pump was the inline one, and that's grounded to the frame in the front of the car. The pump that should be in the tank isn't- whoever did the conversion put in a big metal tube that acts as a siphon. So my best guess is the inertia switch doesn't work, because if I removed the fuel pump fuse, the inline one would keep running. And there are no wires even near where the in tank pump goes. If that's the case, then if I just cut the wire to the inertia switch, will the gauge work better? Here a pic of my tank where the siphon is.39926

[Bitsyncmaster]

I don't think the fuel gauge is as sensitive to the ground offset as the voltage gauge. So even if your fuel pump was wired to the inertia switch, removing that current probably won't change the fuel gauge operation.

You test if the inertia switch is working by pulling the "reset" knob on the switch. That should shut off the pump and unlock the doors.

[Timebender]

Good to know - I'll have t test it at least for the doors, as the inline fuel pump was failing (and there is no in tank pump for now). I'd like to maybe put the in tank pump back in, but have no idea what they did with the original wires that should be there. The power to the inline pump was hooked up to accessory power - actually to the RPM relay and was getting a signal to turn on when the key was on from the accessory relay 2's white ignition wire.

That ended up bypassing the fuel pump relay and I would guess the inertia switch. When you mentioned that moving ground from the inertia switch to the frame, I thought that was in order to help the fuel gauge read more accurately.

Being there's no fuel pump grounded to that, I wonder what else could be causing the gauge not to properly work - it's fairly new, but when filling up the tank, I show only a 1/4 tank - then about 30 miles into driving, it eventually shows 3/4 tank, and it holds for quite a while - it won't budge until I turn off the car, and then back on and it might show half a tank or less depending on how much I drove. After that it will show a 1/4 tank - and one time out of the blue (from a half tank and 30 miles in), it suddenly dropped to empty with the fuel light on.

I'll probably take it out, see if the float is moving inside, spray the connectors, put in back in and see what happens. If no luck it might just be faulty. I'll also test to see if there's 12 volts at the connector as well.

[Timebender]

Checked it today. I get 12 volts at the plug and the float inside sounds like it goes from end to end pretty easily.

[Bitsyncmaster]

That ended up bypassing the fuel pump relay and I would guess the inertia switch. When you mentioned that moving ground from the inertia switch to the frame, I thought that was in order to help the fuel gauge read more accurately.

The RPM relay is the fuel pump relay in stock wiring. There is no fuel pump relay. Now the RPM relay also drives the lambda relay which is also called the main fuel relay on the schematic. So I'm guessing your fuel pump was wired to the lambda relay but even that would only run with the engine running with stock wiring.

[Timebender]

My bad - I meant the fuel pump fuse. And I would guess to get the pump to run with the rpm relay and lambda relay, that's why they ran a second wire from the ignition terminal on the accessory relay 2 to pin 87b on the rpm relay.

Speaking of which, if the accessory relay is going bad, would that cause the engine to suddenly die?

[Bitsyncmaster]

My bad - I meant the fuel pump fuse. And I would guess to get the pump to run with the rpm relay and lambda relay, that's why they ran a second wire from the ignition terminal on the accessory relay 2 to pin 87b on the rpm relay.

Speaking of which, if the accessory relay is going bad, would that cause the engine to suddenly die?

The two auxiliary relays drive quite a few circuits. One defiantly would stop the engine if it shut off since it powers the ignition.

I wired my battery cutoff relay into that one AUX relay so it shuts off the engine before shutting off the battery.

[Timebender]

That's an interesting hack. How hard is it to replace those relays? Are they screwed in or?

[Bitsyncmaster]

That's an interesting hack. How hard is it to replace those relays? Are they screwed in or?

The two AUX relays are held under the metal plate with one mounting screw each.

[Timebender]

The two AUX relays are held under the metal plate with one mounting screw each.

Easy enough - what part am I looking for on Delorean's site? I see lots of different relays but not one that looks like that.

[Timebender]

Easy enough - what part am I looking for on Delorean's site? I see lots of different relays but not one that looks like that.

Found and ordered. Fingers crossed that this is the culprit to my engine just suddenly shutting off. I replaced all the other ignition components so here's hoping (and lots of praying).

[Bitsyncmaster]

Found and ordered. Fingers crossed that this is the culprit to my engine just suddenly shutting off. I replaced all the other ignition components so here's hoping (and lots of praying).

I've only heard of one AUX relay ever failing. They are 70 amp rated and only run about 20 to 25 amps in our cars.

[PJ Grady Inc.]

Found and ordered. Fingers crossed that this is the culprit to my engine just suddenly shutting off. I replaced all the other ignition components so here's hoping (and lots of praying).
In 35 years I have not seen one failure of either of those relays. You'd be the first!
Additionally I've seen poor quality new RPM relays with malfunctioning timer circuits that run continually with the key on. That is likely one of your issues.
Rob

[David T]

In 35 years I have not seen one failure of either of those relays. You'd be the first!
Additionally I've seen poor quality new RPM relays with malfunctioning timer circuits that run continually with the key on. That is likely one of your issues.
Rob

Because this car has a conversion motor (not a PRV) any failures can't be considered in relation to the general fleet. We have no idea how things were rejiggered to do the conversion, the relay could be overloaded. As for the fuel pump, again, we have no idea how they powered it. Speaking about the fuel pump circuit, we do see some of the most creative work-arounds there because many just don't understand how it is supposed to work so they jury-rig the pump to run as best as they figure it should. Timebender's car is a "one of" so do not draw any inferences from his problems to "stock" Deloreans.

[PJ Grady Inc.]

My bad - I meant the fuel pump fuse. And I would guess to get the pump to run with the rpm relay and lambda relay, that's why they ran a second wire from the ignition terminal on the accessory relay 2 to pin 87b on the rpm relay.

Speaking of which, if the accessory relay is going bad, would that cause the engine to suddenly die?

No it wouldn't but if the main relay failed the engine would die. However they are rated to hold about three times the load they carry so they don't go bad. What is it you're trying to fix here...an inoperative engine or a faulty fuel gauge reading as I've lost track?
Rob

[PJ Grady Inc.]

Because this car has a conversion motor (not a PRV) any failures can't be considered in relation to the general fleet. We have no idea how things were rejiggered to do the conversion, the relay could be overloaded. As for the fuel pump, again, we have no idea how they powered it. Speaking about the fuel pump circuit, we do see some of the most creative work-arounds there because many just don't understand how it is supposed to work so they jury-rig the pump to run as best as they figure it should. Timebender's car is a "one of" so do not draw any inferences from his problems to "stock" Deloreans.

Point taken but it still is not the main/aux relay's whatever the symptoms are IMHO.
Rob

[papanoel]

I have a houston unit as well that stays at halfway point. sometimes 3/4 but never lower than just below half.

They told me they would check it and fix it. I just havne't had the time or the place to take it out of the car and send it.

[Timebender]

No it wouldn't but if the main relay failed the engine would die. However they are rated to hold about three times the load they carry so they don't go bad. What is it you're trying to fix here...an inoperative engine or a faulty fuel gauge reading as I've lost track?
Rob

Well....

It was both - I actually had another thread about the whole RPM relay/fuel pump issue and they kind of got cross-pollinated.

That's due to the thought that the gauge wasn't reading correctly because the pump (in-tank) is grounded along with the gauges. Being there's no in-tank pump in my setup -which is odd as I thought there was, since DMC CA tried to install a new all in one a while back, but couldn't get it to work since they discovered there's an INLINE pump, they went and put it back the way it was. My assumption was they cleaned up the original pump and put it back in. I didn't even realize there is no in tank pump, but a siphon instead the fits in the boot. And there aren't any wires at all within view that would have gone to the original pump.
The inline pump had power coming to it, from the ignition, and was grounded at the frame. All this is under the big access panel for the clutch and brake cylinders. The pump, from what I can deduce, was getting powered by the RPM relay - which in turn the RPM relay was getting a signal to be on from a wire that was attached to the ignition post on the accessory relay #2, next to the white wire. They had that going to pin 87b on the relay in order to close it (like the tach signal would normally do I'm assuming). If I took the RPM relay off and jumped 30 to 87, with all power off, no key in, the pump would run.

And that brings us up to the fuel gauge is not misbehaving due to a shared ground with the fuel pump, as there is no in tank pump to cause the issue.

So now we're back on track pretty much.

It would be nice to put in a new pump in-tank — I have a new one, but there's no way I can see how to wire it up. So I'm going to put the mechanical pump back on the engine, and pretty much call that a day.

As to why the engine would suddenly lose power - I'm going to chalk that up to the RPM relay not driving the pump - however I could turn off the ignition and then back on and get engine power. Add that even after all the changes and essentially replacing everything in the distributor (except the pick up coil) and not getting spark sometimes - I want to point to something else.

When it runs, it runs well and it's a blast to drive.

It would be nice to know who did the conversion way back when somewhere around the Chicago area. I know who the dealership was (thanks to my sales certificate from the DeLorean Museum) and the original owner, but the rest is a mystery.

[Bitsyncmaster]

Just an FYI since your installing a mechanical pump.

Both the OEM RPM relay and my solid state RPM relays need to see a changing ignition signal to keep the pump running. The OEM unit is AC coupled on that signal and mine is DC coupled but has to see the signal changing in software to keep the pump running.

[Timebender]

The nice thing is by putting the mechanical pump on the side of the engine (Chevy 350), it's all analog.. as long as the engine moves, the pump pumps, and then no need for the RPM relay anymore.

[Timebender]

The OEM unit is AC coupled on that signal and mine is DC coupled but has to see the signal changing in software to keep the pump running.

And that explains why the one I got from you never seemed to work the way it was designed, as it was getting voltage from the ignition wire to close the circuit vs a signal from the system.