MS2 when driving suddenly fuel delivery goes lean

[alice]

Having a problem with the MS2 unit where it can be doing fine but the car will occasionally (and with disturbing frequency) suddenly run lean and start popping. When this happens I've been able to shake the unit, flip it over, hold it at an angle, etc. and get the fuel kicking in again. Have been able to reproduce the issue several times when coasting down some steep hills in second gear, giving it no gas. All the sensors seem to be working, and continue outputting readouts to the computer when the problem occurs. I see air:fuel ratio suddenly kicking into the red zone very suddenly, car starts sputtering and losing power, then often a quick shake will bring it right back. Have checked, rechecked, and even redone all the wiring for the relay cable multiple times.

Any guesses as to where to start debugging? If it's a possible cold solder joint on the PCB, where might I look?

[devastator]

I've had problems with fuel delivery since the day I installed MS, and only recently, cured them. My suggestion is that you start from the basics. Install a fuel pressure gauge and watch it when the engine goes lean. It's possible that while you are beating up your MS that whatever is altering your fuel pressure is going away.
BTW, the cure on my fuel system was a larger fuel filter and never had anything to do with MS.
If you are still having problems, report back with a datalog so we can see what the injectors are doing.

[alice]

Thanks for the response, devastator. Spent more time on it and fixed a bunch of other things on the car that may have affected fuel delivery (and really needed fixing anyway) in the course of debugging. However the sudden dropout of fuel issue persisted. Then by chance I pushed down on R37 on the PCB while it was happening and fuel kicked back in immediately. The solder points look good but it just so happens that the resistor is missing the thermal tape that attaches it to the heatsink. Have no idea how this resistor is wired into what circuit... but to anyone who might be more knowledgeable, could this resistor overheating be causing this kind of problem to occur? Anyway it hasn't happened again since last night so I'm feeling pretty optimistic! Maybe now I can actually start tuning.

[devastator]

Then by chance I pushed down on R37 on the PCB while it was happening and fuel kicked back in immediately.

I hope that fixes things for you.

but to anyone who might be more knowledgeable, could this resistor overheating be causing this kind of problem to occur?

You might try posting this question on the "MS-II and V2.2/V3 Main Board Assembly, Testing, and Troubleshooting" forum. I'm sure someone there will know the answer.

[Matt Cramer]

It sounds as if the resistor may be partially burned out. If its resistance becomes too high it will cut off the injector ground.

[alice]

Hi Matt, it actually happened again and touching the resistor immediately fixed it. Also it possibly felt slightly warm to the touch. Since it happened twice, I am thinking that this must be the problem. From what you've said it sounds like this behaviour makes sense so thank you very much for the validation! Do you think that resistor might actually be burned out and need to be replaced? I've ordered some thermal tape but I'm wondering now if I need to replace the resistor.

[grippo]

If it feels warm, that is fine, but not very hot to the touch. What I would do is measure the resistance and if it is what it is supposed to be, then I would reflow the solder on both ends of the resistor. The fact that touching it makes it work would seem to imply there is a cold solder joint there. Of course it could also be a crack inside the resistor, so when you test it move the resistor back and forth with the meter on it. If the resistance goes high then it will have to be replaced.