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I’ve had a warm starting issue with my car for several years and also with a couple of different engine setups. I have recently switched to flex fuel which has definitely not made the situation any better. I now run a small lithium battery but have had the same issue with using a pc680.
Car will start fine when cold and hasn’t been run for months but if I drive it somewhere and then try to restart it will crank for ages and not even try to start then kill the battery. I can’t data log it while cranking as the ecu power is interrupted and won’t store anything. Now running a 5-0 ignite cam sensor but had the same issue with the Nissan 360deg sensor.
My tuner is great and can provide all the Motorsport and tuning solutions I need but has limited time and knowledge with the link software as he is mostly a motec tuner. I have attached my tune if someone could have a quick look at the basics and see if anything stands out it that could affect this problem would be greatly appreciated.
Car is a s13 Silvia running a sr20 stroked to 2.3l with a gtx3076gen2 making 550hp on 23psi.
Perhaps due to vapor lock. Does the fuel pressure remain when engine stopped?
It has a single intank walbro 535 with no check valve and -8an lines so doesn’t hold fuel pressure but this is new since the e85 tune. This problem is still the same when it had a walbro 460 and oem fuel lines.
In order to prevent vapor lock, residual remaining fuel pressure when the engine is not running, is the basic requirement.
It may be "vapour block", which is when the fuel boils in the fuel line leaving only vapour. You could fit a spring loaded (so it isn't accidentally left on) toggle or push-button switch that powers the fuel pump independent of the ECU's power.
By doing this you should be able to flush out the vapour and replace it with fresh fuel - you will probably hear a different note from the pump when the lines are filled with fuel.
Another possible cause is if you are using cranking enrichment for cold starting without any temperature compensation. This could mean the engine is getting a very rich mixture, akin to flooding, and it can't ignite it. You could try reducing the enrichment and/or checking if there is a temperature vs cranking enrichment option.
A third is the ECU may not be getting the voltage it requires ACROSS IT, to operate correctly - that means not just the power side but the ground/return side needs to minimise voltage drops. This may seem strange, but because of heatsoak, cranking speed, and other issues it is possible for the starter to draw more current than when cold, and this increases the voltage drop at the battery with the follow-on issues with voltages in the rest of the wiring.
A fourth someone else will have to think of...
I don't have much experience with flex fuel and Link for the cranking table, but here's a couple things I noticed.
First of all, the cranking tables are located in the "Fuel" and "Cold Start" menu. In your tune, the Pre-Crank prime is set to inject fuel when you hit the start button. The values in this table seems really high compared to what I usually sees. You are opening the injectors between 100ms and 200ms depending of ethanol %, it looks like a lot.
Second, in the Cranking Enrichment table, the numbers are the same between 0 and 80% and then change at 100%. I'm guessing you are only running E85 and not really using the flex fuel capability. If so, you can change the numbers in the 80 and 100% column at high temp to see if it improves.
Lastly, you have a really clean engine bay! Nice work!
Frank has made some great suggestions, and I agree that cranking fuel really should vary with ethanol content level.
Also, over supplying fuel can cause a no start condition just as under supplying fuel can.
However, you mentioned your ECU is losing power during cranking. That's a big issue which can certainly prevent startup and should be resolved before going down other avenues of resolving this.
Thanks guys. I will touch base with my tuner and see if we can try a few of the suggestions. I’m not sure why the ecu Power drops out well cranking. It is a link plugin ECU in wiring specialties harness. Also strange how it starts fine when it hasn’t been run for a couple of months, but then drive to a petrol station and try restart and I get along cranking periods and rough start up.
If its an OEM ecu header give that a jiggle and make sure its fully seated, even if its bolted tight it may not be making great contact. Its an issue I can only think to blame on reproduction headers tolerances being slightly different.
I have fixed a very similar issue on a W/S harness, they put a capacitor in the middle of their harness between ground and power and it failed and was causing a 2 volt drop at the ecu, even with the engine running. I wont start ranting but if you're about to tear into a W/S harness there's a few other things I'd fix while you're there...
That's all potentially for the voltage drop issue, those PC680's are a bit of a comprise. I've done quite a few SR20 S chassis with front mount intercoolers and that battery is one of the few that fits and allows piping, but you really only get 2 or 3 good cranks before its falling off.
I'd also agree with Frank and Mike's comments
Good stuff Tyson. I've also had nothing but issues with those batteries, but one aspect of it is easily checked. See if the terminals themselves are getting loose.