8 replies to this topic

[gerglmuff2]

symptoms: car starts and idles 100% fine, when you open up the throttle it revs for a second and then dies back down, and then revs for a second, and dives back down. its pretty rhythmic. when driving, this manifests as bucking and barely accelerating.

stock engine, with headers, test pipe, stock ECU. (not a spec miata)

no codes on the ECU

things i've changed from a known good running car (i have several miatas): plugs, wires, coilpack, AFM, fuel pump, fuel filter, fuel rail (injectors and regulator), and the ECU. car has no cat, so it can't be a plugged cat. i also ohmed out the TPS, it appears fine.

the car was supercharged at one point and still has a timing delay box installed, but it was set to zeros. and when it was unplugged once, the car threw codes and would not run. i have changed nothing on the box.

i have no idea what to do next.

 

[gerglmuff2]

oh i also checked and cleaned the main ground. 

[Steve Scheifler]

Still sounds like the TPS or the AFM or the ECU. Swapping them doesn’t mean it isn’t the connector or harness to them. Unfortunately that can be a huge pain. Did you get it this way or did it start suddenly out of nowhere?

[gerglmuff2]

was running an ice autocross, and it started under heavy braking. or rather, when getting on the power after the heavy braking. 

TPS ohms out properly. im thinking my next move is replace the intake manifold and throttle body with the one off my running car. maybe something inside the throttle body? 

idk, im at a total loss. i literally have no ideas left. 

[Steve Scheifler]

Ah ha!!! Let it idle and pump the brake twice. Doing that a lot will always make the engine stumble but one or two shouldn’t. How were the brakes? That booster and lines can be a huge vacuum leak. There’s also an in-line check valve going to it, pull the hose and suck/blow to see if it works. Also look at the rubber plug covering the large unused vacuum nipple on the back of the intake manifold.

[gerglmuff2]

i mean thats how it started, now it does it all the time. i dont think there is a vacuum leak there and the brakes feel fine. 

im suspecting the TPS and or the TPS wiring. lots of threads on miata.net point to problems with the car "redlining" at 2000-3000 rpm like mine is, and it being either a bad TPS, or bad TPS wiring. 

[Bench Racer]

A couple years ago I had a similar issue. Did the TPS ohms check and it checked ok. Went to the Mazda Shop Manual and did the setup using feeler gauges and the engine ran fine. Seems as though I also checked the butterfly at closed.

[gerglmuff2]

solved it!

connection to the TPS wiring/plug was bad. you can find the pin-out diagrams for the ECU, and i found one of the leads from the TPS wasnt actually connected. must have failed on the ice. 

[Ron Alan]

Late seeing this but just something of note for future people reading. My NA1.8 car had very similar symptoms...Start and idle fine(even rev) but then stumble the minute you tried to drive and put load on the system. Turns out my 8 year old after market Fuel Pressure regulator had failed...specifically the rubber diaphragm inside needed to be replaced! Turns out I only had 10-15lbs of fuel pressure.