Hunting a misfire - Injector impedance question

[Dan88t]

Hey, so I've been chasing a misfire, occurs at idle, not on every rev, randomly. Started a few months ago and has gotten worse over time to the point where now I can't ignore it anymore.
Spark/coil is good.
Compression is good (170-180psi in every cylinder).
A/F (at idle) stays around 14.7-15 even when misfiring.
I was going to swap injectors around because I thought the misfire was in cylinder 1 but it's hard to tell because it's intermittent, pulling any plug makes it run worse.

When I took out the injectors I tested their impedance, (they're RX-7 high impedance 550's) and 5 of them are 12.8-13 Ohms and the #2 injector is 24 Ohms.
The high resistance of that injector is causing the misfire, right?? I really hope so because I have no other ideas

Thanks!

 

[Nick M]

The wrong injectors (which is what you have) will not fire as fast. Your symptom, and pulling one plug at a time, just proves it is several small things that are getting worse.

 

[Dan88t]

No, that's not it. What do you even mean by "wrong injectors" ? Is anything but the stock 440's is wrong? The injectors have worked for years, not to mention they're the same 550's that many many 7M owners have upgraded to...

Anyway, I borrowed a 660cc injector from my buddy who has a mkiv, figured that even tho it's 110cc's more than the other 5 in there at least the impedance matched so I could test it. Ran great, no misfire at all, so if anyone stumbles on this thread in the future - if one injector's resistance is out of whack with the others then that's your problem...

Now I just have to decide what to order - probably upgrade all 6 since the 550's are pretty much maxed out with my GT35 @ 16psi

 

[Nick M]

No, that's not it. What do you even mean by "wrong injectors" ? Is anything but the stock 440's is wrong? The injectors have worked for years, not to mention they're the same 550's that many many 7M owners have upgraded to...

Just what I told you. The high impedance (resistance) injectors do not fire as fast, this is why Toyota put the others in the high performance engine that needs a lot of fuel.

Now, what would that have to do with what you said was a gradual change, and therefore not related to an injector not firing as fast, as soon as you put it in.

Started a few months ago and has gotten worse over time to the point where now I can't ignore it anymore.

Or did it not get worse, and that is a placebo symptom? And for the record, being maxed out on your injector and the GT35R won't change a miss at idle.

 

[Dan88t]

The gradual change was most definitely due to the injector getting worse as time went by, I'm guessing if I keep ignoring it then the resistance will climb until it's completely dead.

When you install the high impedance injectors on the 7M you remove the resistor pack. Yes, older high-impedance injectors are slower than lows but that's not the issue here or you'd hear from the thousands of owners who've upgraded with high-impedance injectors

Who said anything about maxed out injectors having anything to do with my misfire at idle? I said @ 16psi... I mentioned it as an aside at the end of my post.

 

[Nick M]

Hey, so I've been chasing a misfire, occurs at idle,

Thanks!

Who said anything about maxed out injectors having anything to do with my misfire at idle? I said @ 16psi...

I did. Being maxed out will not factor into your idle mixture. Since you are clearly lost with such a simple problem like a lean idle miss. I am letting you know that has nothing to do with it. But as of now, you are on your own.

Let us know when you have it figured out. Which from your posting will clearly be a while.

 

[Supra mn]

http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?Section=EM&P=4

I have the rx-7 550cc injectors as well. You can't just unplug the resistor pack, you have to solder the 4 wires on the harness side that plug into the resistor pack. Or do what I did and find a plug from the junkyard to solder and plug it into the wiring harness.

Maybe you've already done this, in which case I'd re-read what Nick said, he knows his shit.
If you're going to upgrade your injectors anyways, do what he suggests and get low impedance.

 

[Dan88t]

Wow, and here's why I just lurk here and don't post.

Of course I know how to install the injectors, they've been installed for years and working fine, with the proper wires tied together.

Nick is more interested in arguing something completely different than providing anything useful at this point. There are some knowledgeable users here who I've gotten great help from before - this is not one of them, just somebody who had a wrong answer to start and is trying to keep from looking stupid now.

I already told you that it's figured out - I guess my original post should have said "I had a annoying misfire, I think I found the problem" then my next post would have been "Yep, tested with a borrowed injector and it solved the problem, time to order a replacement" I guess I was just feeling like starting a conversation on a quiet Saturday morning when I located the cause of my misfire more than asking for help... didn't expect the condescending and misinformed replies

Nick, I'll say it again, I never said being maxed out had anything to do with my misfire - not one time, not once, never ever ever did I say that my maxed out injector had anything to do with a misfire. Never. You made that up as a way to try to seem like you aren't totally clueless... I'll say it one more time and maybe you'll get it - I never once said that the misfire had anything to do with my injectors being maxed out...

 

[hvyman]

Well you should have pointed out the fact that you have no resistor pack considering the stock injectors are low impedence. The only info nick has to go off of is the things you posted which bypassing the stock resistor pack was not on there.

 

[Dan88t]

That's true, and that's my fault for not listing all of the mods I have. I was just thinking that it would be obvious that I bypassed the resistor pack but I should have mentioned it. Really I was just bored and wanted to talk about how I found an injector with more than double its specified resistance, I probably should've gone about it differently or just watched tv or something.

I've had this car since '97 and been messing with it ever since, sometimes I forget to mention the basics when talking about modifications with others - nowadays it's mostly just a summertime trackday/autocross/cruise-night car so in general, when I'm talking to other guys about it, I'll mention stuff like injector upgrades without talking about the resistor bypass or aftermarket ECU or any of the other ancillary mods...

 

[SideWinderGX]

There's no need to remove the resistor pack when installing high impedance injectors, the injectors will run just fine with it still installed. They are literally plug and play. I ran mine for a few years this way and so have a few other people.

High impedance injectors take a slightly longer time to open and close (about half a millisecond), which is negligible in all scenarios that would come from the OP. Just because Toyota used low impedance injectors does not mean that must be the only injector type you use...in fact many of the cars produced today (including high performance cars) use high impedance injectors. They are not 'wrong' :biglaugh:

Dan88t: the injector with 24 ohms of resistance is bad, that definitely needs to be replaced.

 

[hvyman]

Yes but the ones that use high imp injectors do not use resistor packs. And the resistor pack esentially makes the injectors high imp.

 

[miekedmr]

communication problems... he never claimed he was going to fix the idle miss by upgrading, just that he needed 1 new injector and was thinking of taking the opportunity to upgrade all 6.

Dan88t, depending on how much you love/hate your car, you may be interested in http://ifuckindrift.com/ they have track events over in Owego where you can do laps around a track. It's mostly about drifting but sometimes people do time trial kind of runs.

I live in MA but my car is parked in conklin. I work on it on weekends here and there. Hoping to make it out to the oct 12th(ish) event they have on that site, if only just to show off the car.

 

[miekedmr]

There's no need to remove the resistor pack when installing high impedance injectors, the injectors will run just fine with it still installed.

They may still work but they are getting less current than they are designed for in that case...
The resistor pack is meant for lowering the current through low impedance injectors which would otherwise draw too much from straight 12v. High impedance setups should have the resistor pack removed.

As for what the real world side effects are, of running high impedence with the resistors... I can only theorize that with the slow opening Nick mentioned, the fuel metering would be off. The car might be more susceptible to misfires or pinging, or bad emissions results.

 

[3p141592654]

Yeah, everyone is using high impedance these days, they are cheap, and can be corrected in the ECU for the low duration effects. The Bosch EV14s are the poster child. $20 each at the dealer for the ones used in the Ford GT350, ~ 550cc/min.

Truth is, the low impedance injectors were never properly utilized here because the drivers in the ECU are not peak/hold, but constant current. The only benefit was slightly lower inductance and higher Q which helps a little on dead time, but not a dramatic amount.

 

[3p141592654]

There's no need to remove the resistor pack when installing high impedance injectors, the injectors will run just fine with it still installed. They are literally plug and play. I ran mine for a few years this way and so have a few other people.

That is unfortunately complete BS. If you leave the resistor in it is essentially the same as running the injectors at 8-10V instead of 14. That results in a lot bigger hit than 500ms, which, by the way is less than the dead time of the stock low impedance injectors at 14V.

To give you some data, these are the dead times of the stock 440s at a few voltages (taken right from the ECU lookup table).

8V 2.16ms
12V 1.136 ms
14V 0.624ms

The reason your hack with high impedance injectors and resistor in place works, is because the ECU corrects as best it can, and you didn't need to pass a comprehensive drivability and emissions qualification, which would have shown how off you really were running.

 

[SideWinderGX]

That is unfortunately complete BS. If you leave the resistor in it is essentially the same as running the injectors at 8-10V instead of 14. That results in a lot bigger hit than 500ms, which, by the way is less than the dead time of the stock low impedance injectors at 14V.

To give you some data, these are the dead times of the stock 440s at a few voltages (taken right from the ECU lookup table).

8V 2.16ms
12V 1.136 ms
14V 0.624ms

The reason your hack with high impedance injectors and resistor in place works, is because the ECU corrects as best it can, and you didn't need to pass a comprehensive drivability and emissions qualification, which would have shown how off you really were running.

What you're saying is you don't like what I've suggested, but it will work because the ECU corrects itself. Thanks.

Also, the speed at which the pintle lifts is related to voltage, not current. You know this. The negative aspects you're trying to refer to aren't as bad as you're making it out to be, which is why the ECU is able to correct itself. Voltage stays the same, less current, everything still works.

 

[3p141592654]

Where to begin.... the ECU cannot correct the out of spec dead time, as that is hard coded. You might ask yourself, why is there a dead time correction map, if the ecu will correct for it anyway. The answer is, it cannot correct for it, but it can use the one lever it has which is the short term fuel trim to try to patch the mess on the fly.

The problem, however, cannot be properly solved with fuel trim alone. The error caused by the low voltage is a strong function of the requested injector duration. At idle, where requested durations are typically 1.5ms, the ECU will need to double that to compensate for the actual 2ms dead time vs the expected 0.6ms. Using the built in fuel model alone, it would get no fuel injected at all at idle. That means we need a +100% correction at idle. But now you crack the throttle and requested duration increases to 4ms, and now your new +100% correction gives you 8ms of duration, which is too much fuel and you bog and run rich. After a few Ox sensor cycles (~3 s later) you might get the correction back to where it needs to be but now the throttle probably has moved again and the cycle repeats. The ecu is always 2-3 seconds behind on its correction, which is no big deal if you use your car as a stationary electrical generator, or a cement pump, but pretty shitty for a car on the road.

Bottom line is the ECU is unable to keep the fuel where it needs to be, and your running erratically rich and lean as you drive along. If that is acceptable to you then great.

I don't have a clue what your other comment about pintle speed vs voltage is about. Since the resistance is fixed, current and voltage are linearly related (ohms law) so use which ever one you like, it makes no difference. The current and the voltage at the injector will be less, ohms law you know.