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Over boost

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I have a 2006 Lotus Exige with a TVS 900 SC and I have a ECU Black PNP. My old logs showed I was putting out about 10 PSI. I just add a return fuel line and regulator with a 1:1 reference. Before I was hitting my AFR's. Now It is very rich and my boost has gone up to 15 PSI, this is too much for my motor. I expected to have to adjust my VE tables but I don't know where my over boost is coming from. At 100% TPS 4500 RPM I am showing a a MAP of about 198 kPa or 13 psi with a Lambda of .7, .8 is what I want. at 7000 RPM I am showing a MAP of about 219 kPa or 16 psi with a Lambda of .6, .8 is what I want. At any cruse speed I am at Lambda 1

Before my log shower at 8500 RPM my MAP was 177 kPa or 10 PSI. What could be causing the extra boost? Could it be because I am excessive rich?

Here is what I have checked.

I put a hand pump on my MAP sensor and checked if it is reading correctly, it is. I checked at 10 psi and 15 psi.

All pipping and intercooler is clear all the way to the intake

Checked timing with a timing light

checked cam timing

checked the exhaust is clear. I do not have a cat

I took the boost reference line off and went back to my old tune and still high boost and rich Lambda.

I could use any help


Did you previously have a dead head (returnless) fuel system on it? It may be that the base pressure remained the same (injector differential pressure dropped) with the old fuel system, and the ECU compensated by increasing the PW of the injectors to maintain the target Lambda value. Now that you have changed the fuel system and have a regulator that is raising the base pressure to maintain the same differential pressure across the injectors under boost, the ECU needs to be retuned to remove the compensations to the fueling.

Another cause of the rich fueling could also be if the old system changed the operation of the fuel pump under boost. If the pump was ramped up when the engine went into positive manifold pressure to maintain the differential pressure across the injectors, it may be over powering the regulator so that it cannot flow enough.

With the boost levels, check that you have all of the control hoses correctly connected to the control solenoid and wastage. It may also be that the increased mass flow from the richer mixture is driving the turbine harder, leading to more boost. Is the Boost control setup as a closed or open loop operation?

thank you very much for your response.

I did have a deadhead system and now it is a return line with a 1:1 boost reference and I did add a fuel pressure sensor. I will take a look at my PW and see if it needs to be adjusted. The fuel regulator is an AEM and I have a mechael gage on the system also. I can see in my loge the pump is not overriding my regulator.

I do not have boost control on my supercharger. With the pulley I have it should not be able to boost more than 10psi.


I was presuming that this was a turbo charged car with the boost changing. A supercharger behaving that way is different. Is there a bleed valve on the manifold that is not operating now? When you plumbed in the FPR did you tap into the inlet manifold?

No the manifold does not have any bleed valve. The top tub on the left is where I taped in for MAP reference, fuel regulator, and the IAT.

Attached Files

I think Steven means a bypass, and it looks like the S/C has one from the spec's? If so, previously it may have been opening at 10psi, but now not opening until the higher value, if at all.

Fuelling won't make any difference, but things like the above, improved air path to the supercharger allowing more air mass to enter it, greater internal restriction such as the cam(s) not varying as much if they're variable, the throttle butterflies opening more allowing greater air mass (sometimes happens with cable stretch that's then adjusted out), etc. Basically the S/C is just a pump and "boost" is just a measure of the resistance to airflow, so think about ways the mass/volume going in may be higher and/or the restriction may have increased?

It's unlikely, because there are usually rather significant clues to it, but it could even be down to belt slippage before.

With my pulley 10psi should be the mechanical max it can produce. The intercooler did have a little oil in it so I am going to give it a good cleaning but it was not that much. I will give the bypass valve a good look but I thought it only opened at idle. My Vtech does seem to be working, I can feel it when it activates.

Thanks guys

If cam phasing is really off and combustion delayed you could significantly increase intake side pressure due to exhaust expansion on overlap but that is a big change.

Could you have contamination in the vacuum line?

Slides, you're correct about my cam. I went back and checked my log. My intake cam is staying at 637 deg. I may have a bad cam position sensor or a problem with my VVT.I hope to have the car back together today so I can trouble shoot and fix it.

Good stuff. Hopefully no damaged valves if it's still running. That would explain running rich.

I wanted to come back and post the fix, incase anyone els should have a similar problem.

By looking at the logs we found the ECU was not reading the cam position sensor. I replaced it with know effect. I checked all the wires on the block that housed the two wires for the cam sensor. They were all tight. The I did a continuity test and found one was not reading. When I mover the locking bar to my surprise the wire was not all the way down. It is next to two pins I had done some work on. I guess some how I dislodged that one. I put it back in place and returned the lock bar. I guess the moral of the story is move the locking bar, it is the only way you know for sure the wires are all the way down and in place.

I hope this helps someone els.

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