1999 Reads 0% throttle at idle [Archive] (2024)

jacknsb

12th July 2019, 13:14

Hey all,

Been struggling with some issues with my 99'. The car surges/jerks/bogs at low RPMs and is especially noticeable from a stop and in first and second gear.

It's throwing at P0140 (bank 1 sensor 2), P0171 (too lean), and a P0301 (cyl.1 misfire).

My old TPS was only reading between 0-7% when I revved the car while it wasn't moving, so i replaced the TPS. Bad news is, the new TPS did the same thing. However, when I drove around and data logged the new TPS it accurately read everything from 0-100%.

The issue is... the TPS is supposed to read 10% at idle on Miatas, not 0%.

Would appreciate some more guidance.

Car runs great with TPS unplugged.

(Other parts replaced in the last 15k miles: NGK wires (3 times), 3 sets of spark plugs, fuel filter, battery, CAS and CPK, fuel injectors, air filter, cleaned MAF, cleaned throttle body, front and rear O2 sensors (TWICE), deleted Catalytic converter, fuel pump, timing belt, coil pack (oem))

Edit: Image from data logs. Was in first and second gear trying to keep the throttle as steady as possible... went WOT in third gear. https://ibb.co/sHxJZpR

MMoondog

12th July 2019, 15:19

Step 1. Where did you get the information the TPS should read 10% at idle. Mine reads 0 and always has. I do know that a range from 0 to about just under 20% is considered normal on SOME other cars. Mrs. MMoondog's Camry sits at about 17% at idle and is considered fine.

But I think the Miata is supposed to be zero. So check your throttle cables and make sure they are returning the butterfly plate in the Throttle Body to completely closed and have the proper tension on the cable. Mine goes way up over 70 percent approaching WOT. Check your cables again.

I am not sure how a P0140 or P0301 would be related to the TPS.

Are you saying that if you drive the car with the TPS unplugged and it is running great that those two codes don't come back, but as soon as you plug the TPS back in, they do?

On a 99, you can swap wires and see if the P0301 follows to a different cylinder.

You deleted the catalytic converter? So how did you accommodate the TWO oxygen sensors? Maybe the 99 doesn't have the same stock O2 sensor arrangement. What would I know?

jacknsb

12th July 2019, 15:30

I've been reading online that a Miata should read 10% throttle at idle.

As far as de-catting goes... only the front O2 sensor controls the fuel trims while the rear O2 is simply for emissions. This car has had this issue long before it wasn't stock.

96Formula6spd

12th July 2019, 18:37

MMoondog

12th July 2019, 19:02

On my torque app mine reads 0% when the throttle blade is closed

Mine too. Always has and I have been looking at that specification through Torque Pro and other software and scanners too. Its zero at idle.

I go with the practical, since I tested the TPS according to the manual on mellens.net/Mazda and it is working perfectly.

The TPS specification at idle or otherwise is just a relationship between the position of the butterfly plate full closed to whatever you open it to while you rev the engine.

At 10% it SUGGESTS that your butterfly valve is not closing all the way. Maybe it is suggesting something else.

I did some searches just to see what you might have read. Lot of information out there and some of it is definitely off base. We on this thread who mentioned zero at idle could be wrong too.

Well, good luck with the issue.

Logical diagnosis is best, but as you mentioned, the car is now far from stock, so I will just sign off here.

I do hope you find all the solutions though.

sholzy

12th July 2019, 20:48

sholzy

13th July 2019, 14:31

MMoondog

13th July 2019, 20:52

The butterfly appeared to be fully closed. Just to make sure, I backed out the throttle stop screw 1/8 turn at a time until the screw was no longer touching (3/8 turn total). The number dropped to 9.8%. My guess, without taking off the TP sensor to measure resistance, is the internal resistance is slightly higher on this particular sensor compared to a TP sensor that reads 0% throttle.

It happens. Sometimes these sensors are strange. Just for laughs though, did you check the tension on the throttle cable?

In the end though, I have concluded that there can be a small range at the bottom of reported position. If the idle is fine and you can get to WOT, why worry?

sholzy

13th July 2019, 22:32

It happens. Sometimes these sensors are strange. Just for laughs though, did you check the tension on the throttle cable?

In the end though, I have concluded that there can be a small range at the bottom of reported position. If the idle is fine and you can get to WOT, why worry?

Yep, a small amount before testing, and plenty of slack after backing off the screw. The engine idles is fine. So, yep, not worrying.

Thinking about getting the cheap, $29 TP sensor O'reilly's has just to see if there is any difference in measurements (ohms and %).