ATTENTION BOOSTED GUYS: Running rich underboost? Consider the throttle body.
 

First off I want to thanks Teamrx8 for the tip and my SARX brothers for the hand wrenching.

I had an issue where all of the sudden my car started going pig rich and my STFT and LTFT at idle were -20 +20 % (should be within 3% of 0 +/-). Right away I assume a boost leak and so I grab out trusty home made boost leak tester made up of a fog machine and go to work. Well after testing everything in sections, I found no boost leaks or vacuum leaks. So then I tested the Synapse DV thinking it may be leaking internally, and nope it was fine. So then I start thinking compression but zero oil in my catch can and the car starts perfectly and idles perfectly (despite wacky fuel trims) when fully hot. Fuel pressure was good, oil pressure was good, everything was good.

Anyway, I get to talking Teamrx8 and recalls a situation a guy had where the symptoms were similar and that he ended up replacing the throttle body and that fixed it. Well, I recently purchased a new throttle body from RP @ AFE just to replace mine that had 10+ years and 143,000 miles on it.

So I slap the old throttle body back on and go for a spin and everything is perfect. I have a few throttle bodies laying around, I could have saved myself a lot of hassle just swapping the throttle body in the first place, :lol:

Just thought I would share in case anyone else is chasing an issue like this. I had a issue similar to this that was tune related but since I was recently retuned and the car was running well, I knew this was not tune related since it happened so suddenly.

Oh and my fog machine/boost leak tester that works great.


https://cimg0.ibsrv.net/gimg/www.rx8...ad460eafd5.jpg

any chance you can make a write up or a list of parts you used for the fog tester...it would be a great tool to have..

(Should this be over in Major Horsepower? :) )

I'm curious as to theories on what was actually happening to produce that effect? The dealerships love to replace throttle bodies, but it has so rarely been actually involved that I'm curious about what was wrong when it really was involved.

I can take detailed pictures of it, it is really simple but I'm not sure on the thread sizes and such, we just went to home Depot and got the parts to work. There used to a be a website with a write up but it is now gone.

I almost put it there, wasn't sure. I have no clue, the new throttle body seems just fine and there really was nothing to indicate a throttle body problem. If team had not mentioned it I would have never considered it, especially since it is new.

Internly the electronics are an advanced TPS system. You can get to them by drilling out the rivets on the side cover plate and lifting them off. Other people have usd the RX8 TB on different vehicles, but swapped the electronics from their factory TB to make it compatible.

+1 .... doesn't make a lot of sense .

hOw about you explain it then . Smart arse

Of course it doesn't make sense. It was just another lucky fluke guess that flies in the face of everything you might think it would be, but wasn't.

Feel better now?

I guess the two ... most likely(?) sources of failure would be either the control motor or the control circuit board.

In both cases though, it's puzzling why there would be a rich condition. Any throttle control issue would inherently mean that as far as the airflow or MAF is concerned, the throttle position is commanded. Or... does the throttle position matter for the fueling map? Electronically that is. A factor in the calculation. I don't think I've ever seen or heard that it is, but I could see a connection if it is. Assuming that idea is just hairbrained, I don't see how the end result of the throttle position could possibly affect the AFRs on a tuned map.


So that means that the failure had to be something other than throttle position. The only other thing I could think of would be a vacuum leak, but you proved that out as not present.

Dunno. I'm certainly curious.

Translation

https://cimg2.ibsrv.net/gimg/www.rx8...898e7c9adb.jpg


actually it is deep enough ...


https://cimg4.ibsrv.net/gimg/www.rx8...6cf40cef2a.jpg

Well, that does answer the question. I guess it's a failsafe mode, loss of TP Sensor info = probably defaulting to enough fuel for WOT operation. The older Mazda ECUs do this (read up on the Mazdaspeed Miata factory ECU problems and the TPS disconnection patch), but I wasn't drawing the connection there.

Thanks.

That makes since. It probably has to do with the max calc load/bario/temp tables and them being set excessively high.

There are dual, but offset value, TPS signals that are supposed to help prevent problems or at least provide a backup if one fails, but something is happening that the system doesn't seem to recognize or avoid. It's not a common problem, but have seen it a handful of times. Never recognized it fully until the last year or two. Even this instance helps clarify it for future trouble shooting. Not just an FI issue either, happens with NA engines too.

Translation: saw it in the manual . Still no clue but at least I sound clever .

So thinking about it more, I would suspect that for an N/A car, the symptom of this failure would be running extremely rich in part throttle situations, with the difference between what you would expect and what you are getting becoming further apart with lower MAF values, and closer together with higher MAF values. Probably still rich at WOT, but probably not excessively rich. Of course, it's also possible that it is just a sensor signal trouble issue at max throttle plate deflection, the same kind of way that the HVAC temp control knob starts having issues at each extreme first, in which case the problem will only manifest there.

I'd be interested to know what the various pedal/throttle position sensor readings were for the failed one vs the new one.

For boosted guys running different maps and injectors, how far off you could be, could be rather wild I imagine.

I know that before I swapped to an aftermarket ECU in my MSM, unplugging the TPS sensor to smooth out the nasty fuel handling logic worked fantastically because fuel was always available, but made me run so rich in part throttle, idle, etc... that the car was literally reeking of fuel.

On a dyno, the WOT AFRs were the same with the TPS plugged in vs with it unplugged.

More like you don't understand the role of throttle position.

I knew enough to help solve the problem when the solution made no sense to you. Not sure why you have to be so bitter about it. :Peace:

I was only reacting to your put down of me. I wasn't the one claiming to know how it worked yet you chose to have a dig at me without coming up with a full explanation yourself .


After giving it some thought I suspect what is happening when the TPS is faulty is the same as when you unplug the maf (or have a faulty maf). The ECU uses the values on the 'max. calc. load' table to work out how much fuel to inject .

Come on guys.....let's try and keep the personal stuff out of it. You are both a wealth of knowledge....and can both be egomaniac. ....embrace it and move on 😊

Yeah, I agree with this. My curiosity has moved on to wondering if the failure mode is more than just the on/off of unplugging the MAF. Like does it only trip the failure mode at certain failure points of throttle plate position? and when it sees a valid position, it returns to normal?

I'm also curious if it is flipping to a failsafe mode, why it doesn't produce an ECU flag/code/whatever to actually tell someone. :)

Well yeah it does make sense. So who dug at who first; the chicken or the egg? :p:

To be fair, I admitted that it ws only through experience over time that I became aware of the situation myself. 10+ years of intimate RX8 experience and you start picking up on stuff not necessarily printed in a manual ...

I've stated more than once that reality always trumps theory ....



.

All I know is that the swapping the TB fixed it. The only other thing I did was replace the TB connector (well Hoss-05 depinned it and replaced it with a new one) but that was months ago and everything was working great until recently. The TB I bought from RP was new, but it could have been dropped or something, who knows.

The car is running great again except for the fact that it doesn't love this 100F weather for very long before I have to back it off a bit.

It needs some internal cooling effect ;)

OK, I read somewhere that the RX8 TB has a reverse to shut the throttle faster than if the spring shut it.....

I can't find out where the fuck I saw it at, but I wanted to check it.

Most of what I can find says most other TB even with TPS 1 & 2 use PWM +/- and do not invert the signal.
IE PIN + or PWM
IE PIN - ALWAYS

What I seem to remember is that the RX8 one throws a negative duty cycle to slam it closed faster.....

PIN 1 70% DC +
PIN 2 Ground

Throttle Pedal = 0

PIN 1 Ground
Pin 2 70% DC+ (until TPS reads closed)


I know I didn't make it up about finding it, but is it bullshit?
I also hear something about it being a stepper motor and 1 step is 1' of butterfly angle?

For the car, I don't care as much for troubleshooting, but I am trying to move one on a bench with a micro-controller for a boost project and precise boost amount. (instead of a blow off valve, it is a precision bypass valve with ultrafast responses not triggered by the presence of pressure)