Low voltage limit for fuel injector operation 87s4
 

Trying to figure out a gremlin that has me stranded..AGAIN.

Car has left me stranded more than enough times with this similar problem.

When starting, the fuel injectors appear not to fire. I know this as i have a fuel pressure gauge and when im cranking and turn my fuel pump off with my wired in switch, the pressure does not drop. It always drops when i kill the pump as i do this a lot when the car has a tendancy to "flood" in the summer.

Is there a low limit where the injectors wont pulse yet the car will still crank?



Theres a nice 928 parked at fresh n easy if anyone wants it...and can start it!

Tony not exactly sure what you mean by this "Is there a low limit where the injectors wont pulse yet the car will still crank?".

The Injectors (opening and closing) Pulse has nothing to do w/ the ability to "crank" turnover the engine. As you know 12v's is applied to the injectors via the harness & relay, its the ground that is "switched" on and off via the LH.
Have you tried an O Scope or a Noid light on the Injector wire(s) ?

Dave

A weak battery can cause this. The starter will use what power the battery has and there is not enough for engine management.

This is true even in a running car you'll have issues, I was hoping Tony was using a charged up battery.

The opening time of the injectors is automatically lengthened by the LH ECU as the battery volts drop. So if you can crank at a reasonable speed, then the injectors will be fired.

This is to compensate for the extra time it takes for the injector to open with low volts. However, if you have set the predicted opening time of the injectors too short when Sharktuning, and then compensated with longer Tinj when tuning, you may get a situation at low volts when the injectors may not open sufficiently long ?

The only possible issue I see is that the injectors get their supply differently than the starter & CE panel. The direct from the battery feeders go to the EZK and LH (and on to the injectors). If those connections are not good they may be getting less voltage than you expect and during cranking you will have the lowest voltage of all, may be enough the rest of the time...

Clean and tighten the additional connections to the battery +ve post, & maybe put in fresh DI/LH & MFI/EZK relays.

If it cranks really slowly - voltage may be too low anyway - can you monitor voltage at the jump post with a DVM when cranking (tough to do). Is it better with the battery charged overnight?

Alan

Thanks guys...i will look into that stuff. My gut says its a volatge issue of some sort.

My dad went back to the parking lot later in the day..5 hrs or so and it started right away.

I dont care if there are issues...not knowing the cause is what bugs me.

Tony are you saying this is a no start when hot/ warm issue, and it always starts when cold?

Dave

Sounds like a iffy ground. Years ago my 86.5 would not start until it cooled off. We traced the issue to the connector in the spare tire well. The wires running to the connector were badly frayed.

Tony, what injectors do you have?

What John said above is quite likely. The injectors take time to open, so if you want 1.0 ms worth of fuel (for example) and the injector opening-time is 0.9 ms, then you need to pulse the injectors for 1.9 ms to get the right amount of fuel. The LH adds the 0.9ms when it calculates the pulse-width from the maps, but the cranking pulse-width includes the opening time.

The opening time gets longer, with low voltage. And the LH compensates for this by making the pulse longer, based on what ST's opening-time setting is.

So at low voltage i.e. when cranking, if the opening-time setting in ST is shorter than the actual injector time, then the injectors will open more slowly than the LH calculates, and-- if the voltage is low enough-- may not actually open at all. This is especially true with large injectors because you don't need more fuel to start the car, only when running on boost, and the cranking pulse-width gets pretty short-- which magnifies any opening-time errors.

Please post (or PM me) with the injector part#, there are specs for some and not others. Also try a longer cranking pulse-width, and then reduce the higher-temperature values in the LH cranking map, to avoid hot-restart flooding. This stuff is all inter-woven, and it helps a lot to start with the correct opening-time number.

Tony,

I'm new to this forum and a new 87 S4 owner and about two months ago I learned a bit about injectors and the ECU's. After a lengthy thread that I started concerning issues with my car not starting even with a new and fresh battery I decided to send my ECU unit to have it tested and sure enough. It needed a new chip, came back, installed it and cranked it over a few times and wah-lah!!!! Had my car running again.
Maybe, just maybe your chip might be going out to the point where you may e won't b able to start it at all. I don't know how much patience you have for waiting to send it in and have it tested but it might be the issue.
If you're interested on who I sent it to let me know.
Also, did the whole volt meter and noid light deal and I had some voltage to injectors and light turned on but just lightly which was giving me hope it wasn't the ECU. But that was the case for me.
Ricardo Vega II
928 S4 Black