so s5 fc, cheapo fuel pump, custom large exhaust, new oil, newer plugs+wires.
sometimes wont rev, just bounces when push the gas.
only does it sometimes.
been driving for a week or so with no problems.
then did it again today, so had to walk home like 4 blocks or something.
idle isnt great but never stalls.

anyways, its really ticking me off that it only does this sometimes.
fixed the only vacuum leak i knew about, going to look for more on sunday.
also going to replace the fuel filter cause not sure if thats ever been done.

anything in particular i should check?

heres old post with vid.
https://www.rx7club.com/2nd-generation-specific-1986-1992-17/engine-fuel-problem-video-incl-946677/

 

check if the vacuum line to the fuel rail is hooked up or has cracks and ****. replace fuel filter too. its a fuel problem definitely. a vacuum leak wont let it idle that low in most cases.

im taking out my na fuel pump if ya need it. i can just give it to you if you wanna pick it up and this time it works lol

 

you sure about that?
haha, yeah you swapping to turbo again right?

you dont have an air box do you?
i need to go to springs here in a week or two to test drive a miata with my girl.
might stop by if you're available.

i'll do my fuel filter first and see if that helps the problem and then pick up the pump from you.
anything you need that i doubt i'll have?

 

does it only do it when the engine is warmed up? if so, get it to normal operating temp and when it does it again, unplug the coolant thermo sensor plug from the back of the water pump housing. if it fixes the hesitation and runnability, replace the sensor. i had the same problem.

 

^yea that too. i hate how s5's have to be soo finnicky about their electrical ****.

yea, another turbo swap on the black fc this time. i do have an airbox available but i dont have a afm for it.

 

thats fine i just need the box.
all my screw holes have busted.

 

is there a way to test the sensor?

 

Do you have a CEL on? Limp mode?

 

not really, unless you have the specs of it which are not in any repair manual. haynes doesnt even mention this sensor. i would suggest just getting a new one for $20 and see if it fixes it. if it doesnt, then you will have a brand new coolant thermo sensor you wont have to worry about.

 

if it were in limp mode it would happen all the time, not just sometimes.

 

im guessing a coolant thermoswitch is the same as a water thermoswitch and in the fsm there is a test you can do.
i might try it.
we'll see.

 

make sure you test the one on the back of the water pump.

my car would do EXACTLY what yours is doing: when it started throw a fit all i did was disconnect the sensor and then it ran perfect. this is how i knew it needed to be replaced. if it keeps doing it after you unplug it, then you know to move on to the next step.

 

ok thanks.
we'll see after this weekend.

 

There is a testing procedure in the FSM. When there is a problem with the Thermosensor the fault usually revolves around the wiring/connection rather than the sensor itself. There are two pins at the sensor and often times one of the pins will pull back within the plug and thus makes poor contact. You can test the sensor reading at the ECU and compare to spec.

 

Not entirely true. I used to own a 91 n/a. It would go in/out of limp mode from time to time. Ran codes, OMP was pooped. Replaced it and never had limp mode again.

 

no limp.
no codes.

 

i'll check connection when i remove for testing.

 

so...i dont think its the thermoswitch cause, i dont have one?
looked where it should be, found the connector for it but found nowhere for it to connect to.
so im guessing i dont have one so it must be fuel or i am supposed to have one but for some reason dont.
not even a hole for one.

 

The "Water Thermosensor" is connected to the green plug.

 

So it is this one? I have almost same idle problems as OP.

 

so tried disconnecting that with the problem happening, made no difference.
just going to assume its a fuel related issue and do the pump and filter anyways.

but what you're saying is that the fsm is wrong?

also noticed this today(see attached)
could that maybe have something to do with it?
the white stuff doesnt seem to get worse but still odd.

 

There is a difference between the Thermosensor and Thermoswitch and the green plug is connected to the Water Thermosensor which runs to pin 2I on an S4, which is one of the two places to test the sensor. If the connection to the sensor was problematic then disconnecting it does nothing. I did not say your problem is a function of this particular sensor but there are basically only two ways to test its functionality.

 

You have coolant residue forming at a connection which is failing at one end of the BAC. Just redo it and clean the muck up.

 

so, where would i find the specs to check the thermosensor at the ecu as stated before?

 

In the Fuel and Emission Control section of the FSM is an example of where you can find the two methods of testing the sensor as one involves testing the voltage output of the sensor at the ECU(there will be a list of all pin positions of the ECU, Water Thermosensor position included with the voltage value relative to specific operating conditions), while the other method requires removing the sensor and placing it in water of various temperature levels and reading the ohm level.