AAN buggy engine overboosting/backfiring

[Stefen]

Yes, it was Paul. I guess the only way to do it correctly is on a rolling road as it is only a single seater.

The MAP sensor is stock and may be damaged by the high boost, but the ECU has not raised a fault code to confirm this. I know you have some ones that allow higher boost. Maybe it would be best to solder one of those in.

[prj]

Yes, it was Paul. I guess the only way to do it correctly is on a rolling road as it is only a single seater.

Well, the boost can be brought under control by repeatedly doing 3rd gear pulls with a datalogger. Or at least observing the gauge visually.
Although with a such light vehicle I imagine third gear will just spin the wheels...

The MAP sensor is stock and may be damaged by the high boost, but the ECU has not raised a fault code to confirm this. I know you have some ones that allow higher boost. Maybe it would be best to solder one of those in.

The stock MAP sensor's maximum rating is 600kpa, this is from the Bosch specification. It will not be damaged by 2 bar pressure.

Either way - you need to either go and do a remap on a rolling road - or to get the boost under control you need to get a chip programmer. Boost control is the one thing that can be gotten to a good level remotely - I or Paul can help with that if need be.

[Stefen]

Tonight we disconnected the N75 and found that the wastegate was opening at 0.4 Bar. We also tightened the spring to bring the boost to 1.1 Bar approx. Both times the engine did not backfire.
The fuel pressue stays at 6 Bar at idle. When at full revs at 0.5 Bar boost it dropped to 5 Bar. When at 1.1 Bar boost at full revs it dropped to 4 Bar.

[prj]

Your fuel pressure is a bit low. The FPR is set at 4 bar, if pressure falls to it or below, it won't have enough fuel.
You measured fuel pressure before the FPR as I understand.

Also, if you remove the N75, your turbo will not spool well and if there is knock, the ECU can not reduce boost to save the engine. You should be winding down the spring to 0.35-0.4 bar not removing the N75.

The last time someone told me what you are telling me now, he had to rebuild the engine.
Just saying.

[s2_bo]

Tonight we disconnected the N75 and found that the wastegate was opening at 0.4 Bar. We also tightened the spring to bring the boost to 1.1 Bar approx.

That doesn't sound save at all!!
Your buggy on the other hand looks mental! Cant wait to see a pic of it running

[Stefen]

We disconnected the N75 to establish what the wastegate wanted to do on its own. To see if it was working at the correct pressure. You think that slightly less is better? Maybe a weaker spring is better?

Certianly to have the N75 in place is what we want, it is necessary to get the best out of the homefries.

We also tried to get above 1.0 Bar on 4 cylinders, but she wouldn't do it. So the only thing now is to replace all the coils or swap in a new coil/POS.

[Acidburn]

We also tried to get above 1.0 Bar on 4 cylinders, but she wouldn't do it. So the only thing now is to replace all the coils or swap in a new coil/POS.

4 cylinders

[Stefen]

4 cylinders

Yes, cut the fuel and if it stops backfiring then you have found the bad coil.

[s2_bo]

We disconnected the N75 to establish what the wastegate wanted to do on its own. To see if it was working at the correct pressure. You think that slightly less is better? Maybe a weaker spring is better?

Certianly to have the N75 in place is what we want, it is necessary to get the best out of the homefries.

We also tried to get above 1.0 Bar on 4 cylinders, but she wouldn't do it. So the only thing now is to replace all the coils or swap in a new coil/POS.

When you tried to get above 1bar on 4 cylinders did you also unplug the injector of the cylinder you where leaving out?

[Stefen]

No, I only pulled the plug on the injector. Should the coil be disconnected too?

[Acidburn]

Yes, cut the fuel and if it stops backfiring then you have found the bad coil.

ok missed that you was trying to find a bad coil, i'd just noticed a couple of times you'd mentioned 4 cylinders.

[prj]

We disconnected the N75 to establish what the wastegate wanted to do on its own. To see if it was working at the correct pressure. You think that slightly less is better? Maybe a weaker spring is better?

As I previously said - you should fit the stock spring, so that wastegate pressure is between 0.35 and 0.4 bar, not more!
If that doesn't solve your issues, then your boost chip will need adjusting - I could send you something to try and adjust it based on your feedback - that means you'd need to have a chip programmer, an eraser and be comfortable about pulling the boost chip 3-4 times from the ECU (usually takes a couple tries to get right).

What fuel pump are you using? You need to solve the fuel pressure issue. Lots of air and little fuel means a melted engine real fast, but I guess you know that.

[Stefen]

The fuel pressure gauge is in the regulator.

Okay, I'll have a closer look at the wastegate, to see if it can be brought down a little with a different spring.

I have a background in Computer Science (which may explain the inexperience with engines!) so chip removal/re-programming should be okay.

The fuel pump is 'GS342 HRC 255 LPH'. See eBay item 250751306311

[prj]

The fuel pressure gauge is in the regulator.

Your fuel pressure should be set at 4 bar.
At idle, that would be 4 bar minus vacuum manifold pressure.
When on boost or out of vacuum your fuel pressure should be 4 bar plus the boost amount.

I have a background in Computer Science (which may explain the inexperience with engines!) so chip removal/re-programming should be okay.

Alright, but let's get the other things worked out first.

The fuel pump is 'GS342 HRC 255 LPH'. See eBay item 250751306311

I am sure that pump is okay, I am running one myself with more boost than you, and I have no issues. The voltage is solid at the pump right?

So back to the FPR - if it's an adjustable FPR - please ditch it and get the stock AAN FPR if you still have the fuel rail...

Otherwise at least try to set it so it is about 3.5bar at idle. If it is showing 6 bar at idle then you are flooding the engine, and the banging is because the mixture is so rich that it can't ignite!
In fact, your numbers on the FPR don't make any sense to me at all.

Once we get this FPR stuff sorted, we can talk about fixing boost.
Fixing the fuelling is the first step.

[Alastair]

I agree, with regards to frp. Instead of fuel pressure increasing as the boost rises it actually decreases. That must be leaning out the Fuel Injectors at full boost.
The vacuum/Pressure side of the Adj Fuel Reg mustn't be doing the job properly or the Fuel Pump can't withstand the high boost and flow.
Can confirm that the plenum pressure is connected to the adjustable fuel Reg that regulates the fuel pressure as boost increases.