Car jerks after boost?

[lxsv8]

View attachment 5747

View attachment 5748


Help please!
Question for those in the know.

I have a centrifugal supercharger fitted to my 1uz in a soarer. If the car is driven sedately I have no issues, however as soon as the foot is planted and I achieve boost the car will jerk and RPM's seem to hunt upon de-acceleration. If the foot is planted again there are no issues and the car is a real beast until I back it off. I must stress that there are no flat spots or jerks when accelerating. So far I have replaced the blower itself, AFM, ECU, TPS, ICV, plugs, leads, fuel pump, fuel filter, injectors, tried different air filters (pod type), cleaned throttle body, changed Bypass valves (even tried adjustable plumbed back BOV,s) reset ECU, tried a different configuration of pipework through the intercooler, changed intercoolers. When being dynoed it was unable to be tuned as the tuner said the air fuel ratio was exteremly lean when this jerking started and they could not tune it out through the piggyback ECU. (the whole car was bouncing around on the dyno). The result was still an impressive 311 rwhp. The guy who installed the blower on my car has exactly the same setup on his car and has never had an issue. (everything the same exept he has stock injectors and mine are 378cc although we believe this shouldn't be an issue. Any ideas anyone. It is driving me crazy and has been an expensive exercise so far.

 

[lxsv8]

UPDATE.

The car seems to be surging as soon as the car is backed off from most speeds now.(not just after boosting) Especially noticeable at cruising speeds of 110kmh (65mph) straight after the accelerator is released and the car slows down without braking. Could this be a vacuum problem? How could I test this? I have no issues at all with either starting or idling.

 

[Zuffen]

Given you're blowing through the AFM (MAF)and T/b do you think the surge is being caused by excessive manifold pressure.

Do you think a blow off valve could help in this situation?

 

[lxsv8]

Rod the AFM is before the blower.There is a plumbed back adjustable BOV that you can not see in the picture between the throttle body and the input to the blower (after the AFM) and another one (bypass valve) between the output of the blower and the input of the blower. (also after the AFM).

Brett

 

[stevechumo]

I think you haven't mentioned to test the fuel pressure regulator. And what fuel pump are you using? It seems like the fuel supply has a problem.

 

[rms]

Rod the AFM is before the blower.There is a plumbed back adjustable BOV that you can not see in the picture between the throttle body and the input to the blower (after the AFM) and another one (bypass valve) between the output of the blower and the input of the blower. (also after the AFM).

Brett

The Toyota ECU has fuel cut on decel. This is triggered by shut throttle, low flow through the AFM and above a minimum RPM/speed.
This surging can be felt in a std Soarer/LS400 going downhill with a very slight throttle opening at around 100kph.

The AFM is seeing compressability effects/reverse flow ? turning fuel cut on/off and giving incorrect A/F ratios.

 

[morris]

I would guess that the jerking is caused by the car leaning out and not having enough fuel to burn. Fix the lean condition and I bet the jerking goes away.

 

[lxsv8]

The fuel pump is a walbro 255l/hr and a sard rising rate regulator. -however these were put on after the problem and did not change a thing.

How do I get more fuel into the car when I decelerate?

 

[rms]

The fuel pump is a walbro 255l/hr and a sard rising rate regulator. -however these were put on after the problem and did not change a thing.

How do I get more fuel into the car when I decelerate?

The AFM is what controls fuel delivery. Stop the AFM seeing flow at the wrong time and you will fix the problem !!!

(1) Disable the BOV and the bypass valve.

What happens ??

(2) Vent BOV and bypass to atmosphere.

You should have a VERY rich mixture on shut throttle.

 

[stevechumo]

The fuel pump is a walbro 255l/hr and a sard rising rate regulator. -however these were put on after the problem and did not change a thing.

How do I get more fuel into the car when I decelerate?

Did you check the vacuum line to the fuel pressure regulator that it's hooked up correctly? You may go back & trace all the connections related to the fuel system.

 

[lxsv8]

RMS I have removed both BOV and bypass valves but same problem, however was not too keen to do this because of possible compressor surge so I have put them back in. I had not thought about venting to atmosphere as both these are fed back into the system after the AFM. That should not make a difference should it as the AFM will not be reading these?

Steve the problem existed before I went to a larger fuel pump and the rising rate regulator. Do you mean the factory regulator which is still in the system?

 

[stevechumo]

RMS I have removed both BOV and bypass valves but same problem, however was not too keen to do this because of possible compressor surge so I have put them back in. I had not thought about venting to atmosphere as both these are fed back into the system after the AFM. That should not make a difference should it as the AFM will not be reading these?

Steve the problem existed before I went to a larger fuel pump and the rising rate regulator. Do you mean the factory regulator which is still in the system?

Yes. It seems like the fuel is maxxed out (either for the pressure or the flow capacity) at boost and got stuck & bogged when you release the gas.

I don't know what you mean by the bypass valve, but I believe only the BOV is needed before the throttle. And the best way is to vent it to the atmosphere for simplicity. I do this all the time in the turbo systems very successful. I don't think it'd help much if you feed it back to the supercharger's compressor inlet. It's because the supercharger always needs to spin to create boost. What would you mean by the bypass valve? I think the bypass valve is refered to the one used with the root supercharger like the Eaton.

 

[rms]

I had not thought about venting to atmosphere as both these are fed back into the system after the AFM. That should not make a difference should it as the AFM will not be reading these?

When the throttle is shut from a boost condition the volume of compressed air in the intercooler and piping is released between the blower and the AFM. This will then flow to the lowest pressure side which is out to atmosphere through the AFM untill the pressures equalise. The AFM is fooled for a period of time.

If you vent to atmosphere, on shut throttle the blower still pumps the volume of air relative to engine rpm, not throttle position. The AFM sees this flow and thinks you still have open throttle.

The Toyota ECU triggers fuel cut on decel by throttle position, there is NO fuel injected on shut throttle above around 2200 rpm. Does the surging stop if you lightly touch the brakes ? This disables fuel cut and unlocks the torque converter.

Still think your problem is fuel cut/throttle position and AFM related.

 

[lxsv8]

If you vent to atmosphere, on shut throttle the blower still pumps the volume of air relative to engine rpm, not throttle position. The AFM sees this flow and thinks you still have open throttle.

The Toyota ECU triggers fuel cut on decel by throttle position, there is NO fuel injected on shut throttle above around 2200 rpm. Does the surging stop if you lightly touch the brakes ? This disables fuel cut and unlocks the torque converter.

.

This IS exactly what happens and yes if the brakes are lightly applied the surging stops immediately. So how can I fool the ecu (or piggyback) into disabling fuel cut?

Just rereading your reply. - So if I vent to atmosphere I should be okay and the surging will stop as the AFM will think open throttle and not enable fuel cut?

 

[lxsv8]

RMS - My local mechanic on the GC (and I believe you know him well) is fitting a high speed stally very soon (2800) Will this change the unlocking RPM of the torque convertor. I am assuming the 2200RPM you mentioned is the stock lockup.

 

[rms]

This IS exactly what happens and yes if the brakes are lightly applied the surging stops immediately. So how can I fool the ecu (or piggyback) into disabling fuel cut?

I think the only way of fooling the Toyota ECU is by rotating the TPS so the ECU thinks you have some throttle opening and will not initiate fuel cut.

The piggyback idle settings would need to be remaped to suit.

Good luck.

Erol

 

[morris]

Do you mean the factory regulator which is still in the system?

So you have the factory psi regulator still in the tank? Is it still regulating fuel pressure? If it is still part of the fuel system, how does your rising rate PSI regulator ever get a chance to allow PSI to rise above the setting of the stock regulator which is supplying it?

Not sure if I misinterpreted what you were saying in the previous post but if you still have the stock regulator in the system that is a problem as PSI will never go above the stock setting.

 

[rms]

Just rereading your reply. - So if I vent to atmosphere I should be okay and the surging will stop as the AFM will think open throttle and not enable fuel cut?

No. It is the throttle position and rpm that initiates fuel cut. Leave the BOV and bypass as they were. Try rotating the TPS SLIGHTLY.

I did not mean the torque converter stall speed but the lock up clutch mechanism. If you are changing the stall to 2800 rpm you may have to disable the lock up clutch.

Send me a PM and I will give you a contact phone no. Will be easier to explain.

 

[lxsv8]

So you have the factory psi regulator still in the tank? Is it still regulating fuel pressure? If it is still part of the fuel system, how does your rising rate PSI regulator ever get a chance to allow PSI to rise above the setting of the stock regulator which is supplying it?

Not sure if I misinterpreted what you were saying in the previous post but if you still have the stock regulator in the system that is a problem as PSI will never go above the stock setting.

The way I understand it (please correct me if I am wrong) The rising rate regulator is placed in the fuel return line in series with the stock regulator and used to increase the pressure of the fuel delivered. Factory is about 35psi and a RRFR enables higher pressures to be delivered at different levels of boost. A quote from the Lextreme product page "The regulator installs in the EFI return line after the stock regulator and overrides the stock regulator at full throttle or when under boost. "

 

[morris]

Are you positive there is a return line. I am pretty sure that the Lexus all have intank regulators and no return lines in stock form. At least the GS IS and LS do. I am pretty sure that the SC is the same way.