Miata 1.6T throttle

[BlueStreak]

I recently purchased a 91 miata 1.6 with a home brewed turbo utilizing the g4+ ECU, the throttle seems very binary in its operation, there is no smooth rolling on or off, it feels like all or nothing and makes the car very jerky especially at lower speeds. The ECU shows the fault 11 for the MAP sensor, but looking around I am unable to find such senor anywhere, it looks like the stock MAF connector is capped/stowed away, and there appears to be an IAT probe just prior to the throttle body. 

Here are some other notes:

-When cold started, the car drives normally and feels good for about 2-3 minutes it begins to behave as described.

-There is an open crack on the exhaust manifold just before the turbo that leaks, it will be fixed soon.

-The attached log is from my drive to work this morning, a few minutes into it there are some small oscillations in engine speed which was where I was rolling on and off the throttle.

This is new to me so thanks for bearing with me. 

Log 2023-02-17 8;56;41 am.llg

[Adamw]

The problem appears to be your TPS sensor is either failing or there is a bad connection to it.  The MAP sensor is working ok at least in that log. 

This flakey TPS causes several quite different behaviours at different times so I can see you would have some very poor drivability and probably confusing symptoms:

• In some areas the TPS cuts in and out between 0 & 100% (example in blue rectangle) so you get no accel enrichment at all, this will give large flat spots when trying to accelerate, in other areas it starts to work but is very noisy, this makes the accel enrichment go wild (example in red rectangle - you can see accel fuel is dumping ~120% extra fuel in here), this will make it excessively rich which gives very poor throttle response also.   
• In some other areas where the TPS has dropped out and is sitting flat on zero, this causes the overrun fuel cut to activate where it shouldnt be, this causes two further effects, 1) a large oscillation due to the ORFC bouncing between on and off (example in green rectangle) and 2) in some places I can see you are trying to accelerate but since the fuel cut is active you are going nowhere (example in yellow rectangle). 

 

 

 

 

 

[castillaricardo]

Does the car have the stock TPS? Is it manual? 90-93 manual NA6 Miata’s have an on/off sensor instead of variable 0-5v sensor. In this case you’ll need to install a VTPS, but if it already has an aftermarket sensor then it’s not working right as Adamw said

[BlueStreak]

Thank you guys. This is very helpful! It’s a manual transmission. It is not a stock TPS, the p/n is REPM314208, I’ve found a replacement that will be in later today. I noticed when removing the current TPS that’s been sort of jerry rigged into the throttle body, there’s a little bit of movement on the throttle before it moves the sensor itself. 

[castillaricardo]

That looks like the TPS off an NB Miata definitely not working right, so that’s your issue.

[BlueStreak]

I’ve replaced the TPS with a new one and did the TPS setup to configure it. I can see the AN 1 - TP (main) V fluctuating between ~1.5 - 4.95 volts smoothly. 

When I start the car now, it initially idles ~2000rpm for a few seconds then kicks up to 3000 and starts to drop every 2 seconds and kicks back up to 3000rpm. The AN V1 at GND fault still exists as well. 

Here's a log I took just now

StartIdleRev.llg

[Vaughan]

You're bouncing off the overrun fuel cut, either you have too much base idle position or you have an air leak post throttle body

Re AN V1 at GND fault still exists as well would need to see your tune, most likely something to do with your An Volt 1 error low voltage or the fault not having been cleared

[Adamw]

Was just typing the same as Vaughan.  Too much air from somewhere.  It could also possibly be the throttle stop/blade adjustment as well as those that Vaughan mentions.

As for the fault code, did you clear it after changing the sensor?   

[BlueStreak]

Out of curiousity I threw the old sensor back on and it idles just fine now, didn’t see any signs of leaks post TB. The voltages with this sensor don’t look as smooth, they drop to zero when I run through 2000rpm and the car bucks. The wiring looks good to the sensor as well. 
 

Thank you for the reminder to clear the code! 

[Adamw]

The only way the engine can reach 3000RPM+ with the throttle closed is if the required air is coming from somewhere else.  The only extra air the ecu has control of is the idle valve.  If you are sure there are no leaks and the new TP sensor isn't preventing the throttle blade from fully closing then the idle valve would be the next place to look - reduce the base position or temporarily turn the valve off to see if the idle will come down below target.

[BlueStreak]

I went and checked everything over again this morning, no leaks and the mechanical adjustments seem good, the throttle plate rests fully closed. I placed the new TPS back on, did the setup and cleared the codes, everything looked fine, voltages were smooth between ~ .2-4.5. 

when I started the car, the fault came up and I noticed the voltage was at 10, if I tapped the pedal it went down to 0, and the readings were all over the place. Perhaps this new sensor is the problem?

Tried it again, with the engine off and running the tps setup everything looks fine, once the car is started the voltages get out of hand. 
 

The TPS grounds to the TB, maybe there’s some interference there? 

[BlueStreak]

I put a new exact replacement TPS on and everything works just fine now. I really appreciate the help guys!