Why can't I idle at stoich?
 

I have some ideas, but I'm hoping for more input in case I've missed anything.

1997 NA 1.8L w/ MS1 in parallel (hi-res 11d)
DW 350cc injectors, DW200 fuel pump, Rotrex supercharger

What's happening:
Sometimes the car will idle just fine around 14.7 AFR, but often it will oscillate the RPM, MAP, and AFR--especially if I turn on the fan (blower motor). It actually sounds/behaves a bit as if it could be missing on a cylinder. If I adjust idle VEs so that it idles rich, the idle smooths out significantly and seems very solid, barely affected at all by turning the fan (blower motor) on.

Possible culprits:
- Terrible dead-time on injectors -- With a very slow latency (1.48ms), I'm wondering if these injectors just don't handle the shorter PWs necessary for idling at stoich very well?
- O2 sensor in need of calibration/replacement -- I've been letting the car idle pretty rich for the past month while sorting out other aspects of the tune. It's possible that I've screwed up the calibration or even ruined the sensor and am no longer getting an accurate reading at idle? (I don't think that this is the case, but it may be possible?)
- More serious engine issues? -- I'm sure that letting it idle stupid rich for a month doesn't do great things for the engine. Is there any sort of mechanical engine issue that could cause or contribute to this sort of behavior?

I've attached a short datalog that shows it idling relatively stable around 12.5 with blower motor off and on, and then idling wildly around 14.7 with blower motor off and then even worse with blower motor on.

Anyone with more experience care to weigh in?

shitty injectors with slow dead times coupled with too much fuel pressure ran by an MSI in batch mode installed in parallel.

also just a bad tune will do that. you might THINK you're at 14.7 but in reality, you have two adjacent cells that are just coocoo caca way off from where they're supposed to be even though the space between them is fine.

stable GVE and Timing Advance = puzzle piece.

Do you know what your fuel pressure is?
If it varies at idle from one condition to the next, that'll be a problem.

And, high fuel pressure causes injector dead-time to increase, making the dead-time in the tune wrong, making idle difficult. Double whammy.

I know.. I know. :(

I will take another look at my VE and Spark tables, and look for anything that seems out-of-place with them.

I'm wondering if maybe it's just missing on one cylinder (possibly a fouled plug?) which is causing a misfire or just poor burn, resulting in the oscillation. But you'd think that would only get worse with more fuel? It could be that one injector is a bit crappier than the others at short opening times than the others too, which seems to make more sense to me.

I think I'm going to have to install a gauge and eliminate this unknown variable. In theory it should be at or very close to stock when at vacuum/idle, but I suppose that the DW200 pump could be bumping the pressure up a bit or even causing a surging effect, right?


Thanks for the feedback so far guys! This gives me some stuff to look into, and perhaps eliminate if possible.

We would suggest taking a look at the new battery offset data on these injectors. See link under the "Battery Offset" tab in "Injector Data".

1990-05 : 350cc Fuel Injectors | DeatschWerks

Also, make sure that your fuel pressure is staying consistent and check any available load tables since it seems you're having a problem under a higher electrical load.

Is this just for the 350? Or have other sizes (IE: 1000cc) had new offsets?

Most of us idle a bit rich of stoich -- basically we set the VE to give the best idle quality, not to target an AFR. What's so important about 14.7 in the idle region?

mpgs!

You get zero mpg at idle no matter what your afr. :giggle::crx:

unless you're going down a hill and don't have fuel overrun cut on, then it's good for mpgs? otherwise good for average mpgs

yes, but not all zero mpgs are equals.

Some zero mpg are more equal than others.

that's what i was going for.


2.3ms at say 900RPM at idle is signficantly more fuel than 1.8ms at 750RPM.

You guys must live out East. There's no idling here in W. TX. :skid:

I'm curious what happened to your yellow injectors? no more 600/800 cc injectors?

Thanks for the feedback. I will compare the new offset data to my current value (MS1 only offers a static value rather than a dynamic table). I too think that I may need to install a fuel pressure gauge for diagnostic purposes, if for no other reason that to eliminate unknown variables.

Emissions testing! :cool: :vash: (And less importantly, I do actually spend a lot of time idling because the combination of traffic control devices and oblivious drivers in this area results in frequent stoplight intermissions on my daily commutes.)

I just had a "what if I'm just a moron?" moment. Can someone verify if I've understood or misunderstood how the values are expected in settings for MS1?

In the "Standard Constants Page 1" section, is the "injector opening time" expecting to see the latency value for the injectors at the average running voltage or at a specific constant (though not explicitly mentioned) voltage?

Similarly, is the "battery voltage correction" looking for the value from a specific constant or from the average operating range?

I don't think these will necessarily solve my problem, but it would be good to eliminate any errors that may be arising from incorrect settings.

injector opening time is the deadtime in ms at 14.0v

battery voltage correction is the ms per volt the deadtimes change, typically you'd use .20 ms/v

out of air: So if your injectors see 13.5v when running, and are rated at 1.2ms at 14v and 1.5ms at 13.5v a value to use would be 1.5 ms/v.

PWM current limit should be at 100% and PWM time at 25.5

OK, good.

OK, so injectors are rated at 1.4ms at 14v and 1.5ms at 13v, but commonly see voltage varying from 14 to 13 volts.

Wait, I thought it expected the value at 14.0v?

I'm a little bit confused about this one. Based on the published latency values, I'd expect this to be 0.1ms from 14-13 volts at 43 psi (see above). But it doesn't quite match the newly updated battery offset chart posted by DeatschWerks. If I'm reading it correctly--which I may not be--the offset from ... No, I have no idea how that table is meant to be interpreted. Why are there negative values?

I read that in the manual. So thankfully I at least had/have that right!


BTW, thank you for holding my hand a bit on this. Sometimes I think I've got it about figured out and then I get new data that makes me second-guess everything all over again.

OK, so to clarify on the value entered into MS:

The expected value is at 14.0v but if your injectors generally see 13.5v, it's OK to use the 13.5v value... as long as the voltage offset value is correct. Right?

Can anyone tell me how to decipher the "battery offset" chart provided by DeatschWerks? [LINK] Why are there negative numbers on the table?

EDIT: I think I figured it out. I think that it's using 12.6v as the baseline for offset and either adding or subtracting depending on the fuel pressure and voltage. Does that sound correct? Can anyone verify this deduction for me?

EDIT2: Ok, the math seems to work out based on the above interpretation, so my next question is:

Based on the provided specs (assuming 43psi fuel pressure), at 14.0v the latency is 1.42ms and at 13.0v the latency is 1.47.
The difference across that one-volt-range is 0.05ms.
So I would enter 1.42 as the injector opening time and 0.05 as the voltage offset?

Does this methodology sound about right? I will need to verify my fuel pressure still and then adjust my numbers if it's above 43psi, but does my method for determining the values sound right?

sounds right.

Awesome, thanks for the confirmation!

OK, so I finally got around to testing the fuel pressure. It matches spec for the '97 at idle (~36), as expected. About 41psi with the vacuum line removed, somewhere beyond 60psi under a few lbs of boost.

Do I use the value at idle or the value with the vacuum line from the FPR removed for my MS latency values?