Intermittent Fuel Pump Fault - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Alan2b · 10-03-2018, 12:29 AM

New member on here.
I have a 2002 Boxster 3.2s which cuts out intermittently with no fault codes or check engine light.
Initialy won't re-start and just turns over but after 5~10mins will re-start and run fine for days before the next episode of cutting out.
Recently seems to be that if I don't wait long enough, it will fire up and run for a few seconds before dying (but waiting longer it will eventually start and run fine).
I had thought I had pinned down to the fuel pump relay having a break in the windings (fine when cold then break opening and breaking circuit when hot) which would explain the time lag before the engine will fire up again. However so far on long term (24hrs) bench test the relay has been fine and never dropped out - I have a replacement on order tho to completely rule out.
I'm still thinking along the lines of a faulty relay, but think the fuel pump could also be the culprit. Are there any other relays involved with the circuit that supplies the fuel pump?
Any thoughts or ideas would be very much appreciated.
Alan

Oggie · 10-03-2018, 03:45 AM

I had almost exactly the same symptoms with my 996. It turned out in my case (after replacing the fuel pump without any improvement) to be the crank position sensor.

The car would run fine for a little while (around 15 minutes) and it then would simply cut and roll to a stop. It would then not start until it cooled a little and then it would go again for a couple of minutes until it did it again. There were no codes recorded in the OBD, which is why we first replaced the fuel pump. It has since been running fine after the sensor was replaced about a month ago. I'd bet that this is actually your problem.

Lapister · 10-03-2018, 08:36 AM

There is a bung on the fuel rail where you can check the fuel pressure to eliminate the FP.

Alan2b · 10-03-2018, 08:47 AM

Hi
That sounds hopeful. Do you know if the rev counter still displays engine revs when the crank position sensor goes faulty? Thinking rev counter is fed from crank sensor - just that in my car revs still appear to display correctly.

911monty · 10-03-2018, 09:53 AM

Fuel pump relay is likely to solve the problem. Bench test may not simulate the load. The fuel pump has minimum load when idling and goes to max load at full throttle. This is where most relays heat up and drop out. I may be opinionated due to old 911 days but a fuel pump relay was always carried as a spare. Good Luck

Alan2b · 10-04-2018, 12:23 AM

Thanks
Relay is cheap & replacement arrived yesterday. Will fit and see how it goes. Fingers crossed.
Intermittent faults are such a pain - each time it has stopped has been different!
3hrs into long motorway drive
2mins after starting back from a shopping trip
30mins into A road driving
Seconds after starting back after a visit to friends.
After coming back to life it can be anywhere between 50 and 500miles before it fails again.

Oggie · 10-04-2018, 12:50 AM

Quote:

Originally Posted by Alan2b

Hi
That sounds hopeful. Do you know if the rev counter still displays engine revs when the crank position sensor goes faulty? Thinking rev counter is fed from crank sensor - just that in my car revs still appear to display correctly.

Rev counter displayed fine right up until it cuts out. I don't recall the behaviour as it shut down, sorry.

BYprodriver · 10-04-2018, 10:34 AM

Failing fuel pumps run hotter, often the performance is directly related to fuel level in the tank since the higher the fuel level, the more cooling liquid around the pump. The catch 22 is the higher the fuel level when the pump fails the more difficult it is to replace the pump &/or sending unit.

Alan2b · 10-05-2018, 04:16 AM

Any way of checking? Was thinking of breaking out the wiring from the pump relay connection and keeping an eye on the current being drawn. (and hopefully see whats going on at the point the engine cuts out. - Thinking this might be easier than trying not monitor fuel pressure).

Alan2b · 02-19-2019, 09:01 AM

Hi

Just an up-date - I rigged a pressure gauge to the fuel rail and ran this back to the dashboard so that I could monitor it while driving.
At last it failed while I could see what was going on.
Normal pressure (around 3 bar) suddenly just dropped away (as if pump off) but definetely still power going to it.
Trying to re-start, pressure would build to about 1 bar but wasn't sustained and engine spluttered to a stop. After about 15mins delay / re-trying, pressure back up to 3 bar and engine running fine - but time to change the pump methinks!!