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84 928 fuel line shuts off intermittently

First time to the posting forums but I have an issue that has eluded me:

Basically while the car starts and runs, once in awhile the fuel pump seems to be told to stop. The place I take it believes that is a electro/mechanical issue.

We have replaced relays 15/16 XV/XVI gone through the connections adding dielectric grease and flipping the fuses over. But still even while driving the fuel will shut off. It has only happened 4 or 5 times over the last 2 years but I would like to get this gremlin out of the system.

Otherwise the car is in great shape.

Any ideas?

I have done work since but too long to list, I am hoping that this is something that someone has seen before.
Old 10-16-2018, 09:49 AM
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You should post your location...someone may be near you who knows the P928!

Brilliant - A lot of issues can be electro/mech. It's a car, and there is only electro or mechanical involved plus the fuel. Cleaning electric connections, due to car's age, is usually helpful and theraputic

The big control panel with relays and fuses have a lot of contacts on the backside; one recommendation is to pull it and clean them. Also, clean the system grounds. Additionally, clean the contacts at the pump(s) and the grounds in the back. You could focus on the circut's involved; but, it's best to do all.

Then focus on the EFI system...
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Old 10-16-2018, 10:01 AM
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added location, Boston Mass area

OK, I haven't done the backside yet. will look into when I get a chance. As far as EFI, I had an issue last Nov. with the hot wire ( some critter left his mark), so it has been replaced as has the temp sender, found a broken/chipped tooth in the twin distributor, replaced all parts. Tested relays 16, 17 (power supply to LH) and triggered LH and EZH. All seem to be OK, while testing. But doesn't necessarily rule them out. As far as the replaced parts, car responded with acceleration throughout each gear. Also replaced the alternator 2 years ago as it would no longer charge.

The car has a 90's aftermarket alpine alarm/radio/ phone wired in and we tested its draw which was minimum, so it was ruled out.
Old 10-16-2018, 10:56 AM
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does the car stall and refuse to start again when this happens? - how long does the condition last? - seconds, minutes, hours? - intermittent problems like these are hard to pinpoint if they don't last long enough to get a tester on it - i had an old pontiac for the kids to drive that refused to start on the road sometimes - they'd call and we would drag it back to the house - then it would start - took months to figure it out - keep an electrical tester in the car and when it happens see if you can find where the power to the pump stops - try at the pump first then the fuse and relay area - if there is always power at pump it might be a pump or fuel line issue - good luck
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Old 10-16-2018, 10:58 AM
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So this last time was the longest and had to be towed from downtown. So hours at least, with a new battery which did crank every time I tried. Once all I did was roll the car and it started. I agree the intermittent makes it the problem. I did try and disconnect the battery for a few minutes the last time and hoped the system would reset, but it still just cranked. And this is the reason for the post I am looking for some new ideas before I send it back in to the shop so at least I can feel there is a shot at not being a black hole. The pump is something I have been thinking as well as a fuel filter issue. I would hope I could recreate the problem but have yet to get there. I am not a pro around these systems. I try what I know which is mostly 60's type cars (or limited electronics), which leaves me with a lot to learn on this car.
Old 10-16-2018, 11:21 AM
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You could pull the pump and connect some juice and see if it stays on pumping. Have a pan/jar with fuel to cycle through the pump while you are testing. I have a CIS so can't help much on the EFI side of things.

I have heard of the Alarm systems mucking things up with starting/running???
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Old 10-16-2018, 12:14 PM
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Tach Signal -

Hey Euro - Intermittent problems are always hard to chase down...Have you determined if there is a specific length of driving time involved when the fuel pump quits ?

Does your Tachometer appear to be reading consistantly ? It's input signal comes from the Ignition Control Unit and is AlSO sent to the Fuel Pump Relay XVII on pin 31b. The electronics inside this relay interprets the incoming Tach Signal as = The Motor Is Running so "Keep The Fuel Pump Relay Energized". If this Tach signal is lost (Poor Connection) or the Fuel Pump Relay electronics are intermittently failing - the Fuel Pump will shut down.

Good Luck and Keep Us Posted - Michael
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Old 10-16-2018, 02:29 PM
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i re-read your original post - do you hear the pump running while car is running and then hear it stop when problem occurs or did repair place say the pump is stopping? - not that they are mistaken, it's just that while we are all giving you ideas for why the pump is stopping it could be other issues like the injectors or plugs are not firing - just wondering
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Old 10-16-2018, 06:52 PM
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Guys, Thanks for the posts! Relay XVII was tested when the car started rich - "safe mode" when the hot hire had failed to trigger EZH pin 25 which passed, as well as pin 15,30,7, and 19. But that had to do with a separate failure, hot wire and temp sensor replacement. ( I was hoping they were the same problem, but no such luck). There has been no specific length of time 1 was at the shop after the alternator was replaced and the dimmer unit was bypassed, the car was idling, 2. was driving the car at slow speed around 15 mph road. 3 was on rt1a while driving for 25mins at around 40/50mph. and the 4th time was driving down town out of gear heading to a rotary to stop. While driving the way I noticed is the tach drops to a flat read (consistent to engine rpm I assume). However after trying to start after the event, there is no tach movement.

On the pump you can not hear the pump and it is stopped. But then it starts again at some point.
Old 10-17-2018, 09:57 AM
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Make a switched jumper wire with male spade connectors on both ends.

During your next episode of non-running: Remove the fuel pump relay and install the switched jumper wire. One spade in terminal 30 and the other in terminal 87.(look at the diagram on the side of the relay for location. Basically terminal 87 is the slot that perpendicular to all the others and 30 is the slot directly opposite that perpendicular one)

Turn the jumper switch on. The fuel pump should be pumping fuel anytime that switch is on regardless of if the engine is running, so be sure to turn it off when not directly testing.

Try to start the car.

I believe there are really 2 main possibilities for why the car is not running.

First possibility: Bad fuel pump relay. I personally have had a relay that was susceptible to heat soaking. The relay would get hot and fail. My roadside solution to this was to prop open the wooden door so that the relay would stay somewhat cooler and I could get home. The switched fuel pump jumper will remove that possibility from the equation. If the car never fails with the switched jumper, replace the relay and never look back.

Second possibility: Bad CPS/bad CPS connection. I think this is the more likely culprit. I have never experienced it, but I have heard of a heat sensitive CPS that would fail once warm. The only real true test for this involves an oscilloscope and someone to interpret the wave form. The easy test is to replace the CPS sensor and see if that fixes the problem.

Good Luck!
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Old 10-17-2018, 10:22 AM
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Sorry, clarification relay XVII triggered pins 25, 15,30,7 and 19. 31b was not tested. wrote that poorly.
Old 10-17-2018, 10:38 AM
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What is the part number on the fuel pump relay you are using?

is it 928.615.113.01? Or is it a standard 53 relay? A 53 relay only has 4 terminals.

My Euros use a 53 relay for the fuel pump.
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Old 10-17-2018, 11:50 AM
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the fuel pump relay has 4 terminals and has a part number of 171 937 503, with a 24 on it, so by assumption it is a standard 24. I only have 1 53 at the top left, many relays are 24.


The car only goes out on sunny days in New England,
Old 10-17-2018, 01:14 PM
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My understanding is that the "24" relays are VW parts and although they have they same circuitry as the "53" relay, they are rated for lower amps. Do Not take that as Gospel!Strictly hearsay.

That said, the 24 relay should work fine, unless your fuel pump is drawing more amps than normal, heating up the relay..........etc. etc.

But I would still remove that particular relay from the equation as a possible problem by using the switched jumper while testing.
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Old 10-17-2018, 10:00 PM
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thanks, makes sense!
Old 10-18-2018, 07:54 AM
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Any update?
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Old 10-18-2018, 09:50 PM
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Quote:
Originally Posted by stepson View Post
Second possibility: Bad CPS/bad CPS connection. I think this is the more likely culprit. I have never experienced it, but I have heard of a heat sensitive CPS that would fail once warm. The only real true test for this involves an oscilloscope and someone to interpret the wave form. The easy test is to replace the CPS sensor and see if that fixes the problem.

Good Luck!
This. I had similar failure mode on my 84 Euro. Replacing the CPS cured the issue.
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Old 10-26-2018, 06:31 AM
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Do you know for sure it's the fuel pump and not a loss of ignition? I ask because I've been told a million times "my fuel pump is dead" only to find out the fuel's fine and there's no spark.

If when it dies the power drops hard like the key had been turned off it's likely ignition, a loss of fuel pressure should give a noticeable lowering of power and not a hard stop of power.

If it has an aftermarket alarm system, even one that's been 'by-passed', that's something to suspect.
Old 10-30-2018, 09:28 PM
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If the CPS loses the crank rotation signal (either by bad CPS or bad CPS connector), the ignition (EZF computer) will shut down spark, which in turn shuts down fuel (LH computer). The LH will not energize the fuel pump if it doesn't get a "engine turning" signal from the EZF via the CPS. So, CPS failure results in no spark AND no fuel.
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Old 11-01-2018, 11:30 AM
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A new CPS seems to have fixed a similar problem on my car. It seemed like an strange failure mode to me, but I guess a tiny break can form in the coil and then it opens the circuit randomly when it gets vibrated just right, or expands due to heat.
Old 11-06-2018, 06:05 PM
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