Beetle Convertible

[Marcas]

The length of time my fuel injected (and fairly standard) convertible turns over before starting has been increasing over the past year. If I cut the engine immediately and restart it, it starts immediately end will also do so if left for an hour or two. However, if left for a day the problem re-emerges. I have owned the car for 25 years and it is running as well as ever, except for a rather variable idle speed (I have searched without success for an air leak). Any suggestions would be greatly appreciated.

[Marc]

Still running the original fuel pump? How long since you've changed the filter up front? It sounds like it may be taking longer than normal to build sufficient pressure to keep the engine running.

It's hard to hear the pump running over the noise of the starter but if you crawl under there and grasp it while an assistant cranks the engine you can feel if it's coming on smartly or if it's laboring to get up to speed. Unfortunately for you, it appears that this'll only be meaningful a few times a day since the car has to sit idle for hours for the symptom to recur.

The pump receives its power from terminal 88d of the double relay (behind the rear seat). The relay closes while power's applied to the starter solenoid or when the flap in the airflow meter moves open slightly once the engine starts pulling air. There's a blocking diode inside the relay to prevent the signal from the AFM feeding back to the solenoid. IF that diode's forward resistance goes high, it could inhibit the relay from closing right away when you start cranking...but power to the pump goes via the same contacts & wiring in either state, so that's about the only "relay" problem I can imagine being responsible. I would clean & tighten the grounds at the relay mounting point though, it's conceivable that they could have high resistance that's "cured" when the vibrations of the cranking/running engine pass into the body.

You can check for the presence of voltage to the pump from the relay using a test lamp at its plug. If it's intermittent when cranking, get a new relay....but from this distance it seems more likely to a problem at the pump itself (or we're wandering around in the wrong forest and fuel pressure's not the issue - but that's my gut feeling from here).

[Marcas]

Marc, thanks for taking the time to give me such a detailed response. I can't honestly remember when I changed the filter but it could have been over a year ago, so it is probably due a change. I replaced the pump as a precautionary measure years ago when it was still straightforward to get a new one from my local Bosch supplier (I live in Belfast, Ireland). I still carry the old one under the back seat so could swap them over. A few years ago I was driven mad by an intermittent starting issue which you correctly identified as a fault in the double relay so that is a fairly new part, but worth checking the grounds as you suggest.
I may not get to look at it in any detail until the weekend so will post again if I find anything.

[Piledriver]

It's EFI, but the easiest things to overlook are the mechanical bits.
We look for an electrical fix, but that's not the actual issue more often than not.

It could also be a little rust or wear in the pressure regulator seat, allowing it to bleed down completely...
Or a drippy injector or three for that matter, same pressure loss effect, plus minor flooding causing hard restart after awhile.

You can put a different model pump in it and it will run fine, but the pressure regulator is a key part of the tuning.

Oil smell like gas by any chance?

After ~40 years of use those injectors and regulator have to be on their last legs.
They do wear, like everything.

I know direct replacements are available, and I'm pretty sure far more modern injectors that would drop in exist.

Also the CHT sensors (largely controls cold start/warm up) are notorious on those, swap that, or at least check that before replacing anything else. The internal ground contact is just in spring contact, not soldered or welded (ohmic contact) and they corrode.
That can cause all manner of weird, just like flaky grounds.

[Marcas]

I really appreciate those additional suggestions. I did a very major engine rebuild about 13 years ago. At that time or slightly later I replaced the injectors with new Bosch ones and the pressure regulator. I also replaced the screw in CHT sensor about three years ago when I was trying to locate the hard starting when hot problem which turned out to be a faulty relay. That, of course, doesn't mean that any of these components don't need replaced again. Will do some checks.

[Marcas]

Checked the fuel pump today after checking the valves: 38psi with the vacuum connection to the fuel pressure regulator disconnected, 30psi with it connected. It seemed to get to this pressure very quickly. I tried changing the filter but it seemed quite clean and it made no difference to the pressure readings.

[Piledriver]

Don't forget to leave the gauge connected to see what happens to residual fuel pressure.
Theres a spec for how fast it can drop and how far.

[Marcas]

I'm affraid I didn't think to do that

[Marc]

Another thing that's quick to rule out is poor contact between the CHT sensor & the head (that could "cure" itself when the head warms up).

[Marcas]

I checked the ease of starting on Sunday and Monday morning and it was fine both times. It looks like it was the fuel filter after all as Marc suggested. Once again thanks to Marc and Piledriver for your willingness to help.