Thom

Hi all,

After starting the engine from warm it runs lean with an AFR of ~18 for about 10 seconds until the mixture gradually gets richer and stabilises at a normal 14.7.
The engine accordingly stumbles during this lean mixture phase but when the AFR is nearing 14.7 again it idles fine again.

The engine starts every time and this problem only occurs only when the engine is warm. When starting from cold it idles fine right away with the correct AFR.

I have had this issue only ever since I replaced the Walbro GSL392 fuel pump with a Bosch 044, with no other modifications or any adjustment to the mapping. The 044 is fitted with the usual check valve.

Any thoughts on what could cause my set up not to be happy with the Bosch fuel pump, all other things being equal?

gruhsy

Bench test the pump.

fast951

Have you checked the fuel pressure when it runs lean?
Are you running the stock wiring to the pump?

Thom

Thanks for the replies. One may wonder sometimes if that once great board has gone dead.

The wiring is stock. I read in the past about the stock wiring being undersized for the 044 pump but about half a dozen friends run the 044 pump on their 3L 951 with the stock wiring and without any fueling issues whatsoever. I have not checked the fuel pressure but I would guess it is fine since I have set the fuel pump to prime the circuit for 3s when switching the ignition on.

fast951

As a first step, I would check the fuel pressure when it runs lean.

If the pressure is low, I would test the pump by wiring it directly to the battery..
If that's not it, you could have a bad pump (or check valve) .

Years ago counterfeit 044 pumps were on the market, did you verify you have a legit Bosch 044?

I have not seen a 951 experiencing vapor lock, but it's possible... Can you insulate the fuel lines?

gruhsy

Funny you mention the counterfeit. I was just thinking about posting that as I remember reading that on old posts here and some other sites about fake pumps. Turbos are another bad one for fakes from what I have heard.

gruhsy

I have seen pumps...not fuel pumps..... that start very slowly and take a few seconds to a minute to get up to proper operating speeds....usually faulty electrical internals or bad bearings.

V2Rocket

try a jumper on the DME relay socket for a few seconds to run the fuel pump with key off.
then start the car, does it idle properly or still lean?
that'll tell you if you have a "time delay" to getting proper pressure.

Chris White

I doubt its the fuel pump - the quantity of fuel for idle is extremely low. In addition the fuel pump should come on before you crank so it has item to build up a little fuel pressure. If it was the pump having difficulty building pressure this would be the same or worse during cold start.
BTW - how do you know the AFR for the first 10 seconds? wideband sensors take that long to come up to temp and they typically report full lean until they are functional.

divil

Probably not important for this thread, but I'm curious - are you sure about that? I was under the impression that the fuel pump only runs while the engine is turning.

Chris White

you might be right, I have been working on too many stand alone systems to remember what the stock system does! But I believe you might be right, it requires a crank signal to energize the fuel pump relay via the DME.

V2Rocket

stock 944 wiring setup, fuel pump only turns on while cranking and not before.

can get an Ftech9 DME relay which will prime the pump at key on though. or of course, different ECU.

Thom

The pump is a genuine unit from Bosch.
Whether the engine is cold or warm, fuel pressure remains unchanged at 3.0 bar at idle.
Injector duty cycle and every data observable in the ECU are fine. I have an IAT fuel trim table that removes fuel once IAT gets past an unreal value, but IAT never gets there when the engine runs, even just after starting it when warm which is only when the IAT sensor may get heat-soaked.
Since the problem appeared only after replacing the fuel pump I checked the fuel valve yesterday which seemed to work fine as confirmed by the fuel pressure not going down when shutting off the engine and starting it again. As for vapour lock I am not sure how only replacing the pump could suddenly cause it.
Short of trying another check valve or perhaps checking voltage at the pump I am not sure where else I could look at, and why would voltage in the electric supply to the pump fall when the engine is warm anyway? Again it worked fine with the old pump and it primes the circuit with the same vigour whether the engine is cold or warm.

Humboldtgrin

Have you checked your soft fuel lines and hard lines to the fuel rail, softly lines do fail internally and with more flow may show their weakness? Also the fuel filter. I replaced all the lines when I upgraded. Even the hard line as I found both the supply and return lines were smashed just behind the passenger wheel well. Also you may want to use thicker wires for your 044 pump seeing how the original wires are getting old. Maybe even use a relay setup for that pump.

Tom M'Guinn

When wires heat up, like the windings in the pump, their resistance increases, and when the resistance increases it will need more electrical power to pump as hard as a cool pump -- which you may or may not have available. How is your charging system, battery voltage at idle, and wiring to the pump? My 044 pump doesn't have any problems, with 3L, big injectors, 3bar, etc. I'd probably run temp wires (jumper cables?) to the pump to see if that helps, assuming your charging/battery are up to snuff. If that doesn't work, taking everything you said as true, you may just have a bad pump. That said, more often than not on these kinds of things, you may discover that something you checked a thousand times isn't actually right and/or that a squirrel is living in your FPR or something...