I have had my TB drilled out to 82mm. On top of that, I have added a different throttle elbow that should ensure better air flow. Everything works but for some reason, the engine stalls after a period of 'hard deceleration'. The idle speed seems a little low (around 500rpm). Some questions:

1. Is it likely that the TPS needs adjusting? If so, would for example a clockwise totation raise or lower the rpm at idle? In the new configuration it is really hard to get to so it is good to know in advance.

2. What does adjusting the TPS do? I thought the engine ECU governs the throttle valve position based on rpm (and not pedal or throtle valve position).

3. Any other things affect idle rpm?

 

I have been with the same problem last two years, with two different bored TBs. Very annoying. Some dangerous situations when happened on the highways. Back to orginal TB - no problmes anymore.

 

It has been posted here often in various threads that once really shouldn't mess with the air induction system because these cars are so tightly tuned from the factory that doing stuff like you did will make the car run no so good.

 

not the dude in this post went back to his orig TB and it was fine.

 

There was a recall on the early throttle bodies due to issues similar to yours.

See post #5:

https://www.jaguarforums.com/forum/x...t-s-b-s-27320/

Maybe your problems are related to that?

I also have a bored TB but have not experienced any issues. To note, years ago my dealer replaced my TB due to the recall even though mine was fine and I wasn't even aware of the recall.

 

Wow....should have known this a couple years ago. $$$$ paid for this case. TB changed maybe 10-15 times. The first bored TB was brand new bought from the UK. The second one was used from the USA. Both had same problems. The orginal TB works fine....it's just so, so small for the requirements One way is to use just electronics from stock TB and other parts from the bored one.

Willafizz: Your orginal TB was bored out and the problem started after that?

 

Hi and thanks for the feedback. My TB was exchanges about 8 yrs ago because of some incident that thad occurred in the US from what I understand. Anyway, it always worked fine and I don´t think this is the problem. What I see is simply that after a longish deceleration (braking seems to help), the revs dip really low and sometimes the engine dies. A simple restart and I´m back in business. I also note that the idle rpm is not much above 500 which appears to be a bit low.

I understand that adjusting the position of the TPS may correct the issue and set the idle rpm at a more reasonable level. Actually, I am not sure exactly why - I would have thought the ECU sets the butterfly valve to an angle that ensures the target rpm, regardless of the TPS.

 

Is your throttle bore clean? Could the butterfly be sticking occassionally?

This is a known issue in TBs with dirty bores..and dirty edges on the butterfly valve. Yours is likely fine since you've been paying attention to it, but worth asking.


.

 

If you monitor TPS value when the problem comes, you will see about 4% for idle. Normal is bit over 2%. AFR goes to rich side 12-13 from normal 14.7. Engine restart resolves the problem. Just big smoke comes from the pipes if you have needed to roll too long time to stop the car from highway speed and you have much unburned fuel during the restart.

 

We are not talking about stock TB problems. We talk about Maxbored TBs. This is not a dirty issue. My first bored TB was brand new. Absolutely clean. TPS value should be tested with oscilloscope from stock working TB and check if the bored one match or not. Also the TB can be recalibrated but it has a risk for bigger problems.

 

Thanks. Yes, the TB is real clean! I would never allow it to be otherwise, believe me!

Anyway, I have ordered an OBD-II reader. I will look at the key data when I get it. By the way, the software called something like Enginuity - is that appropriate for this sort of thing?

 

I have checked around a bit and am now quite convinced that the problem needs to be cured by adjusting the position of the TPS. In my setup, the TPS is pretty inaccessible which will make things pretty tedious. I am contemplating adding an adjustable load (resistor) to each of the two TPS outputs to get them to read a fraction less than they should. This should be the electrical equivalent of adjusting the sensor mechanically. If I remember correctly, there is a bit of space at the top end (WOT) and so the small adjustment should not cause issues here. This would only work if the sensor is passive and I have a sneaky feeling that the sensor will use some kind of buffer circuit. If that is the case, it can still work but it would require an active circuit (couple of op-amps in a simple configuration). If I decide to try this out and get it to work, i can post the circuit for others to use!

Before starting, anyone done something like this??