Mi16 Fuel Pump Relay Operation

[James504 12]

Hey, just a quick question about the Fuel pump relay (actually in a 405 Mi16 not a 205.) Vehicle has symptoms of hard to start when warm (when cold seems to start ok, but not perfect), takes approx 5-6 seconds of cranking when is sputters into life then settles into a good idle. When ign is switched on Pin 2 of the fuel pump relay is not earthed by the Ecu. Only after cranking again approx 5-6 seconds does it start getting earthed and intermittently at that, not a solid earth as shown on a powerprobe. Should this relay be activated when the ign is switched on before cranking ie- prime the fuel pump? Continuity on the wire between pin 2 of fuel pump relay and pin3 of ecu is ok. Added external earths to ecu earth wires, still same problem. Cleaned harness earths, changed cas, changed relays and holders etc etc, tried searching forum but nothing that I saw was similar to this problem. Cheers.

[jackherer 543]

If you can get access to an oscilloscope check the signal from the crank sensor at the ECU plug during those 5-6 seconds when it is cranking and not earthing the relay.

[Ryan 99]

The ECU can't turn on the fuel pump unless there's a signal from the crank sensor. So the relay coil is only earthed when the engine is cranking or running.

[James504 12]

Thank you for the replies. I had failed to understand the ecu operation fully. I was expecting the pump to be primed before cranking, but now I can see with this ecu that's impossible. I have now resolved the problem, I ran a new feed from the cas straight into the ecu. This fixed the relay earth(earthing as soon as it started cranking), however the car still exhibited the same hard to start symptoms. At this point I connected a fuel pressure gauge (something I should have done first) and found that even when pump was pre primed to 3bar it still took approx 5-6 seconds to start. I left the car sit for 20mins and watched the fuel pressure drop to 0. At this point I figured injectors of fpr, but given the smoke emitted when it finally did start I put my money on injector(s). They were replaced and the car now starts perfectly.

Edited September 24, 2014 by James504

[petert 549]

That's correct, there's no prime function when you first turn the key. Check that +12V is getting to ECU pin 37 via the injector relay M783. If the ECU doesn't see +12V it won't start.

[petert 549]

Another problem can be the fuel pump non-return valve. If a replacement pump is installed without the valve it exhibits similar symptons.

[James504 12]

Cheap china pumps not coming with them? I had seen those $39 070's on ebay. For now its case closed thankfully, all 12v were present to inj/ign and pump relays, just no earth to the pump relay. I found the motor harness wiring extremely hard and brittle ( I had to repair a few connectors- coil, amp, cts, injectors), I had not seen one this bad. In the future I dare say it will need to be replaced.