So my Polo's got this problem, i'll try and explain it but i warn you this might be a long one...

The basic problem is that the fuel pump does not always prime when the ignition is turned on, it is usually fine when dead cold, but after that first try it can take probably ten on's and offs of the key to get the pump to prime. Obviously without the pump priming the engine won't start.

I took a guess at this being a relay, what persuaded me is that sometimes i can hear one or both (not too sure) of the relays mounted by the ecu buzzing quite loudly but not clicking in, if you tapp them when they're buzzing they click in and the fuel pump primes. I have spares of the four pin fuel pump relay, i tried these which made no difference, the second relay (five pin ecu relay) was harder to get hold of but today i received one that some members on here kindly pointed out had the same circuit just different terminal positions.

I made up some temp loom to try this relay and it made no difference, in fact this one wouldn't click in at all. I quadruple checked the wiring so i knew that was correct. I then grabbed the 12v tester (light in a screwdriver with an earth cable type) and started seeing where there was power etc, everything seemed correct. When i touched the black/white cable on the five pin relay (terminal 86A) the relays clicked in and the fuel pump primed. Not due to a loose connection but due to the tester earthing that terminal i guess? The connection was definitely not loose.

Looking at the wiring diagram in the Haynes it appears that cable goes straight back into the ecu. The ecu itself was okayed by an injection system specialist almost a year ago who plugged it into their test computer etc. Its running a standard limiter performance chip.

From previous fun I'm aware that its most likely not just the fuel pump system clicking in but its just the one you can hear. Everything runs through the ecu therefore with no power to the ecu...

So basically does anyone have any ideas of a good place to start? Having written this and looking at the wiring diagram i'm going to check the engine earths tomorrow, anyone know where the ecu itself earths too? The battery terminals were cleaned a few days ago, the only other small earths i can think of are the ones on the inlet manifold (obviously i'll check the main engine earths too).

But i was also wondering whether this could be a dodgy ignition switch? According to the diagram a black wire goes from the ignition switch to the coil and two relays. I'm imaging thats the main switched live? In which case i guess if i test the black wire at the coil end i'll get my answer.

Yeah so if anyone has any ideas they're much appreciated! This has been a bit a of a brain storming session so i will report back with results from tomorrows tests...

Cheers!

The cable going back into the ECU tells the pump to run when it detects the engine has started. Turn the key, the pump primes, then the ECU turns it off and awaits the RPM signal. Found this out when fitting the bike carbs and retaining the original wiring.

the trigger wire for the relays is a thin (0.5) black wire split between the ECU and Fuel Relay, GT has a 5pin fuel relay too.

ECU earth is on the passenger wing I believe, I'm just going to check the wiring now.

Cheers Cymro, that does make sense (about the rpm signal to the black/white wire going to ecu). The pump didn't run continuously though when i touched this, it just primed.

I will do some testing of the black wire then today if thats the main switched live.

I can't say i recall any earths on the passenger side inner wing though, only loom earths i can think of are the ones going straight to the battery and the two bolted to the inlet manifold.

Another question, does anyone know where the 'lambda probe heater' is? Number 27 on the Haynes diagram, this is shown to be in the fuel pump circuit. I've had this engine out and everything apart and i can't recall it. This is shown as a seperate component on the diagram to the Lambda probe.

Its internal to the probe.

There is a earth on the engine hoist point that I think comes from the main loom. I know that when its loose it causes engine issues.

Cheers for that Cymro, much appreciated!

I don't think GT's have the earth on the engine hoist point though? I think its just spi's that have that, the earths on my GT go to the inlet manifold.

It is fixed now though! Honestly i don't think anyone's been turned on by a fuel pump priming before It starts when you want it too!!

Basically it did turn out to be the relay I cleaned all engine earths, battery connections, all the way back to removing the passenger rear light and cleaning the earth that's behind that (guessing that's where the fuel pump earths?). The relays seemed to work fine when i had the test screwdriver attached, however if i wriggled the five pin relay the pump would prime....

Tried the new relay with the correct wiring, still wouldn't work, then i switched over the two low tension switching wires figuring i had nothing to loose and it works! (pins 86 and 85 that is) works perfectly now every turn of the key.

For anyone that finds this thread whilst searching, the relay that is suitable for replacing the Polo GT and G40 five pin relay (discontinued in 2010) has the part number: 165 906 381

I bought mine off ebay, listed as the ecu relay for:

VW GOLF MK3 GTI 2.0L8V & 2.0L16V

(ENGINE CODES 2E, ADY, AGG & ABF)

FROM 1991 TO 1998

This does however require the terminals being removed and changed to different places in the mounting block, one needs a small spade terminal fitting too. It also needs sealing with some silicone or something.