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If you are replacing the AFM wiper area, the cars from the same era have much much cheaper AFMs
Thanks tdiquattro. I bought a second-hand AFM (the previous owner was fitting throttle bodies). I opened up the new one and tested it and the output looks nice and clean throughout the range. The resistor track (see below) looks to be in much better condition than my original one (see earlier post).
Originally Posted by ras62
You need to get rid of the code 24 alarm.
Thanks ras62, I will investigate this further. I haven’t run the engine for more than a few seconds (scared of another over-rich backfire), so I can’t tell what the O2 sensor is doing when hot at the moment. I'll collect some data from it as soon as I can. I wonder if the backfire damaged the sensor.
I’ve turned my attention to fuel pressure. I hooked up a gauge to the fuel rail and made a jumper switch which bridges pin 30 and 87b of the DME relay.
As soon as I power the fuel pump the pressure zips up to 5.5bar (80psi) and stays there whilst the fuel pump is running. When I stop the pump the pressure drop straight down to 3.6bar (52psi) this pressure remains for the recommended twenty minutes. As you can see from the gauges below all of the pressures are significantly higher than the handbook ("Technical Data Without Guesswork™", Bentley Publishers) recommends.
From this, can I deduce that the fuel is not leaking past the injectors and that the fuel pressure regulator is not responding correctly to inlet vacuum? or the vacuum pipe is blocked?
I tested my fuel pressure as well. Mine was 3.5 bar running and 3.5 bar at 10, 20 and 30 minutes post shut down.
I then found out that my after market ECU has a prime pump delay of 3 seconds on start up before it will allow the engine to run. I've yet to test it, but that would appear to be a cause of my hot start issue. I'm adjusting it back to 1 second and see what happens.
Based on the pressure levels and the possible hot start fix I have not changed out my FPR. My attention is now on measuring my AFR with a wide band sensor and then looking at my map.
I could be wrong with this assumption but I'm looking at the injector pulse width, as with correct pressure, an over rich mixture must be the injector pulse width being too wide, thereby allowing too much fuel in the cylinder. Obviously there is no adjustment of this with the standard ECU, so your problem could be something giving the ECU incorrect information.
I tested my fuel pressure as well. Mine was 3.5 bar running and 3.5 bar at 10, 20 and 30 minutes post shut down.
Cool, good to have a comparison. Hope you sort your hot start problem.
I did another test this evening – It makes no difference to the fuel pressure if vacuum is applied to the FPR or not... so I'm pretty certain the FPR is jammed in the closed position (increasing the fuel pressure).
I've ordered a new fuel pressure regulator, fuel filter and O2 sensor. I will post news of my imminent victory soon... possibly.
If it's not the fpr it could be the fuel injectors. i had my originals serviced and suspected they were not done properly then bought a set from www.fiveomotorsports.com and they worked the same. turned out to be my dizzy wrongly indexed and 964 ran fine after.
i've had hesitation and stalling from my 964 and very unnerving to drive. usually happens when coming to a stop and stalls as if i had a lightweight flywheel where the revs drop off and stalls - but i don't have said lwf...
i would heel toe and blip the throttle when coming to stop... annoying and soccer mom in minivan thinks you want to race her... and... maybe you do!
interested to know if it's the fpr...
the test for this fpr (fuel pressure reg) is relatively simple with basic fuel pressure gauge. if it runs at 50psi and goes to 55 psi after stopping and holds at 48 psi after 20mins, then it usually means it's okay... some kind of test like that. going from memory only.
I read somewhere that fuel is burned on the overrun/closed throttle to keep heat in the exchangers? (for winter) could this be misbehaving in some way?
Ah yes, I see what you mean. I'd never heard of that, but then I just stumbled across this thread. Which mentions:
K-13 on the CCU controls something labelled "DME Coasting Shutoff"
I wonder if that's why so many people suffer with the same hesitation problem after coasting downhill.
In combination with my high fuel pressure issue, that would make sense.
Originally Posted by Alex Sol
i have spare afm but you are a little far...
Thanks Alex. A kind thought, but I think the second hand AFM I've just got should be fine.
Just to update this thread. I finally solved the problem.
Having diagnosed the overly high pressure in the fuel system I replaced the FPR with a brand new unit, renewed the fuel filter and retested the pressure. Nothing had changed it was still too high.
After tracing even inch of the return pipe I finally found a kink in the rubber pipe leading from the fuel pump. I replace this section of pipe and this solved the high pressure problem and the hesitation symptom. With the engine running smooth I also replaced the oxygen sensor, in case it had been damaged by the backfire. I also disconnected the CCU wire K-13 ("DME Coasting Shutoff") simply because I didn't like the idea of the engine running rich just because the cabin heater was turned on.
I suppose there's still no easy way to replace the AFM on an L-Jet car with a MAF?
Seems as if all the projects to try and create a kit for this (for 928s) eventually petered out without results.
I know there are some kits out there for BMWs of the era to convert their similar Bosch AFMs to a MAF - and I think some people in the 944 community have leveraged these for their cars. It's my understanding that the output voltages from the AFM for those particular LJet systems is different than on a 928 - and I also have no idea if these kits work all that well on an E30, E28 or 944 once installed.
Do we know of anyone who has successfully ditched their AFM on a 928 (again, without resorting to a mega squirt or haltech system)?
04-20-2016, 04:26 PM
