[Blue_Bora_TDI]

Hi all,
I recently solved some issues around rev hang and stuff. Fixed it but now I've got a new issue:

When you let the car sit overnight, it always takes a good 4 to 5 seconds to start. I have to physically hold the key on the start position and wait for the engine to jerk alive.
As soon as that initial start is done though, it starts super quick.
Also when slightly hot or with just rest heat on the car(letting it sit for a couple hours) makes no difference and the car starts up super quick.

Only when sitting for 10+ hours does it occour.
Ive had a look at some of the sensors, the MAP sensor deviates slightly between 930 and 938mbar constantly when the car is off. Doesnt seem like an issue to me.

Ive replaced the injector seals and cleaned them ultrasonically.
The car has a cold air intake with dedicated shield box.
No moisture comes even close to the intake.
It's a 2003 1.6 16v Golf 4 highline, engine code BCB. Marelli 4MV ecu.

 

[ttg4l]

MAP sensor reading is fine - it's meant to be atmospheric pressure with the engine off, so around 1 bar is spot on.

Has the issue only started after cleaning the injectors? If so, I'd suspect it's slowly losing built up fuel pressure overnight - maybe one of the injectors just needs a good push back into the rail, or the FPR is leaking.

 

[Blue_Bora_TDI]

Have no fuel leaks. Forgot to add but it was already present prior to doing the injectors.
Even when letting the fuel pump prime the cold start is just as bad.
And yes, map reading with engine off is atmospheric. But its constantly jumping around a few mbar.

 

[ttg4l]

Does your coolant temp read correctly when viewing live data? If the ECU is getting a signal that the coolant is warmer than it actually is, it'll put too little fuel in and you'll get poor starting.

 

[Blue_Bora_TDI]

I'll have to check tomorrow.

I haven't seen any weird temp readings on the cluster, i remember I looked at the temp when the engine had been idling for about 5 minutes and saw 50c. (Tad too high for just 5 min of idle on an ice cold engine, it was -4c outside).
I'll have a look at the readings tomorrow morning before i head into work. That way it has sat for a while and it should be ice cold(it's -6c forecast tomorrow 🥶)

 

[cedar]

I bought a 1.6 16V to use while I took my old Mk1 off the road to do some work on it, that was six years ago, the Mk1 is finished and I've still got the Mk4, I have spent longer working on the Mk4 than I did on the old Mk1 and it always seems to be a struggle.
I had the same trouble as you with the cold starting, a new set of Bosch coils improved it vastly.

 

[Blue_Bora_TDI]

Update: started her this morning and fired up straight away with no issues. Upon unlocking the car I heard the fuel pump whizz away which I normally don't hear.
It started with no trouble.
I'll try tomorrow morning to see if it'll start bad once again.

I could get a set of 2nd hand coils, maybe some red top coils if they fit. I know a common fail point on the BCB's are coils.. maybe it'll fix it with new coils and some new plugs,
Any other ideas??

 

[cedar]

I did read somewhere that the red top coils are no different to the standard ones.
I bought four for just under £80 from 'Carpartsinmotion' on ebay, that was with the 15% discount offer.

 

[adam-]

You should always hear the fuel pump on door open/ignition.

 

[Blue_Bora_TDI]

I found the reason for the bad cold start...

The alternator feed wire had burned and melted the fusebox ontop of the battery.
I have replaced both the alternator feed wire and fusebox. After a day, the bad cold starting has disappeared.
My guess on the fault: bad connection on the fusebox side.