Starting voltage - how should it behave?

[Ian C]

Okay I finally did some more towards working out what goes on with the car when it starts, to try and diagnose this warm start issue that plagues a few people including myself.

 

Using an oscilloscope, I recorded injector #6 on starting. Now, annoyingly, I couldn't reproduce the hot start problem (might not have quite got it hot enough) but I did notice something that seemed odd to me:

 

These graphs show the injector behaviour when I start the car. The software is a bit pants, so let me fill in a few details. Each major X tick mark is two seconds. Each Y major tick mark is 5v. Injectors sit at full voltage and then drop to almost nothing when the ECU switches them a ground to fire them.

 

 

The odd thing I noticed is that when the car start, the voltage going to the injectors is about 10v. It takes five seconds, which is a very long time when it comes to EFI systems, to ramp the voltage up to where it should be at 14.3v. The engine note changes noticeably each time it steps up the voltage.

 

I have no idea if this is normal behaviour or not, but I repeated it about 6 times and it does the same thing every time. I'm wondering if the hot start issue is just because there isn't enough voltage oomph going to the injectors to fire them properly at that time?

 

-Ian

[mellonman]

glad to see your busy checking this issue and getting to the bottom of it,

 

this may have something to do with the fuel pump ecu too

[Chris Wilson]

Can't tell anything from that, the sampling rate is all wrong?

 

Look here:

 

http://www.tomco-inc.com/Tech_Tips/ttt39a.pdf

[Ian C]

Chris, yep, it says nothing about duty cycle etc, the objective wasn't to get the injector duty cycle or patterns at this stage so I didn't try and sort that out. Really I was just trying to learn the software, which is the usual disaster with these sorts of things, when I found this odd behaviour, so here it is In my world of problem solving, you hit the most obvious and suspicious thing you see first, so that's what I'm doing.

 

Digsy checked with People Who Know at his workplace and this isn't normal behaviour, so something is up. I need to check other system voltages on startup to see if they are all affected the same way (alternator?) or if it's just the injectors (resistor pack maybe). Recording it when hot and not properly starting would be good too I'm hoping if I can find whatever is causing this slow ramp-up and fix it, the hot start issue would go away as well. It feels plausible that it could be linked at the moment.