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Injector question?

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I am trying to learn tuning my car myself. I know it is difficult to do and I appreciate any advice that I can be given. The car is a 1g dsm with a 6 bolt 4g63 engine. I'm using the Vr4 G4+ plug and play ecu. One of my biggest problems has been the somewhat unknown injectors. The main problem I'm having right now is either an injector setting or something else. What is the reason that as soon as I let of the gas my injectors go full lean? It makes for very difficult driving at very light throttle situations. I would assume they should slowly go lean. I can try to upload log if needed. Thanks again for any assistance.

It isn't a fuel shut-off operating, is it? These normally operate on a closed throttle, and if mis-adjusted the ECU may be getting triggered while on light throttle?

I'd start with checking the TPS (Throttle Position Sensor) is correctly calibrated, then check the mapping on light throttle settings isn't causing the problem.

Another common problem is if there is an air leak - under load there will be close to, or above, atmospheric pressure in the manifold which will usually be mapped to compensate and you may not be aware of it. But under the high vacuum on light throttle, or overrun (coasting), the amount of air being sucked in can be significant compared to the amount being measured at the MAF.

With it idling, try spraying some engine start, or brake fluid, around the intake manifold and hoses - if something is letting air be drawn in, you may notice the rpm increasing if the spray gets drawn into the engine


Did you investigate the overrun fuel cut settings and other items Gord mentioned? Please let us know if you need further assistance.

I have had very little time with it but I did adjust the overrun fuel cut settings. One drive since and seemed much better. Now just need some new injectors. Having trouble with the unknown ones I have.

Sounds good! Yup, working with injectors that don't have known good data really throws off your process and keeps you from ending up with an ideal result. What injectors did you decide to switch to?

Haven't bought them yet. Looking at some FIC 1000cc injectors. This is for a 1992 Eagle Talon. Early 4g63 engine. Do you have a better suggestion for brand?

I'd use Injector Dynamics, so ID1050xds if something around that flow potential meets your needs. Their data is more complete and more accurate which will make your life easier. They also go so far as providing Link ECU specific data for those injectors.

You can copy paste the short pulse adder data right into Link software.

Then I create my own differential fuel pressure monitor using a math channel and use that and battery voltage as the axes for the injector dead time table, and then transfer one row at a time from the ID spreadsheet to Link software.

You can use the spreadsheet to tell you flow rate at your intended pressure as well, and then if you have a fuel pressure sensor fitted, hopefully you do, in fuel setup you can set fuel system time to FP Sensor, which uses live fuel pressure continuously to compensate fueling.

Interesting. You kinda lose me with the last part. Remember I'm just learning. Thanks for the advice on injector dynamics.

I have a fuel pressure sensor and log it also. Will have to refer back to this when I get new injectors.

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Generally it takes more time for the injector to open against higher pressure, or while battery voltage is lower, so the dead time/latency values change not only with battery voltage, but also differential fuel pressure as you'll see in the injector dynamics data.

Link lets you enter dead time as a 3d table rather than a 2d table where you can only give it dead times for a single base fuel pressure. In the real world fuel pressure doesn't maintain a perfect 1:1 relationship with intake manifold pressure at all times when you have a typical 1:1 fuel pressure regulator like you probably have, so accounting for variation in fuel pressure helps the ECU work with the injectors more accurately.

If you aren't yet familiar with the concept of differential fuel pressure, it's mentioned in HPA materials, but here's an article on it specifically in case this helps clear things up more: https://cobbtuning.atlassian.net/wiki/spaces/PRS/pages/638943275/Differential+Fuel+Pressure+Measurement+-+And+Why+You+Need+It

If you have further questions please let me know.

Thanks. I'm sure I will be back with questions.

Sounds good!

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