Alex Sol

After four or five sessions of testing various components we found that when we unplug the sensor next to the idle air control valve which controls the idle/wide upon throttle all the symptoms seem to disappear.

Is this an ecu issue or sensor issue or maybe ground. Is there any spec for multi meter to check resistance or operating volts amps etc

This has been a very tough diagnostic

We thought it was

Fuel injection - Got signal

Ignition - all new wires plugs rotors checked belt to secondary and did vent kit conversion

Spark - everything checks out and in proper order

Coil - New msd 8202

Injectors - flowed and tested

Heads - vacuum checked for leaks intake and exhaust

Air flow meter - rebuilt

Iacv - New

Crank position sensor. - New including oVal plug update

Fuel - fresh 91 octane

Dme - checked for cold solders and suspect it is not communicating properly with wot idle sensor

Any thing else needs checking. ??

Alex Sol

what does the idle / wot switch do?

from driving without the sensor plugged in, the engine does not seem to chop throttle / fuel and has little or no engine braking once throttle is completely released...

an odd thing that happened last week when driving in the morning and idle jumped to 3,000 rpms while driving and wouldn't respond to blipping the throttle. i shut of the engine, restarted and it was back to 'normal' which is abnormal misfire an wouldn't hold idle on decel then stall.... cars next to me thought i wanted to race bc i had to keep blipping the throttle which would not hold any steady rpms but would respond to me blipping and blipping and blipping... quite embarrasing...

ECU issue or sensor issue?

before removing this idle micro switch sensor, there was serious afm slapping from vacuum but not flames. i have experience one incident of blowing the intaake plenum apart and that was due to faulty wires and old caps / rotors

Vandit has suggested i check the fuel pressure regulator and it seems to be fine. what was the test? that there was no fuel coming out the vacuum hose.
should i swap out?

injector swap: prior to pulling the plug off the sensor, engine would misfire and idle with lots of hunting and around 300-1000 rpms.. and with the engine idling, we would pull out connector to injectors one at time.

cylinders no. 1 and 4 seems to make not difference plugged in or unpluggled.

cylinders 2,3,5,6 would stall.

after pulling the plug to idle sensor..idle would settle in nicely at a 800-900 rpm range and pulling injectors off all six cylinders would cause engine to stumble or stall when pulled out one at a time.

should i be servicing some of the vacuum parts / sensors?

replacing some of these same?

robt964

The idle & WOT switches are just simple micro switches.
The WOT switch will play no part in the idle control.
Now you say that when disconnecting the connector to the idle switch, things start to behave. That proves that the ECU does 'see' the switch but there is still something wrong. Switch contacts can degrade over time.
First off, when the the throttle shuts, is the switch firmly depressed?
If the switch contacts are making a poor electrical contact the signal can 'buzz' which I imagine would create trouble for the ECU.
Disconnect the connector to the switches and with a meter measure the resistance accross the pins for the idle switch. (You may have to work out which is which or alternatively I can check mine tonight and tell you.) With the throttle closed the reading should be zero. Throttle opened, it should be infinity.

Alex Sol

rob

the throttle does make full contact with the idle switch and i'll back it off a little more just to be sure... i'll make the adjustment from the tunnel...

perhaps i'll look into getting a new idle micro switch. the WOT is fine and the engine seems very happy at 2,000 rpms and above now that the idle switch is unplugged. prior to this i could not hold the throttle at any position ie: 2k, 3k, 4k.... being a new engine, i'm trying not to go into the 5k range too much...

so measuring resistance across pins means? set multi meter to ohms and test the two pins that stick out when connector is disconnected?

We made sure there was contact on the idle micro switch and that the micro switch would 'click' when the throttle is pressed...

if you have some numbers to compare, that'd be helpful!

robt964

>prior to this i could not hold the throttle at any position ie: 2k, 3k, 4k

Hmmm so its not just an idle thing. Could be that the switch is still conducting even when the throttle is opened.

I'll test tonight and tell you which pins to check.

My engine is sat on a pallet in the garage so everything is easy to get to

HiWind

Hey Alex - glad to hear you caught the red herring and are busy scaling it

https://rennlist.com/forums/964-foru...on-sensor.html

had similar problems ie throttle Pos switch and idle etc.. so did some reading on it
also check the 964 thread at Weiners Rennsport systems

Alex Sol

sooooo close...

but still have: suspension, hvac, stereo, exhaust, etc etc to do....

i'm confused from your link, do you have a tiptronic with 3.8L conversion?
because it seems the manual trans 964s do not have tps - throttle position sensor (not to be confused with crank position sensor)

JasonAndreas

Before you buy/replace any other parts, you can test everything in the car with a multimeter BUT do you yourself a HUGE favor and either borrow a bosch hammer or pickup an OBD1 interface (like Tore's) for use with Doug Boyce's free Windows software. Life is so much easier when you can look at the Actual Values menu on your laptop with the engine running.

HiWind

i'm pretty sure its the TPS - its on the opposite side to the throttle cable on the TB and its pic'd in my post.
I did ask that question but the thread went on a new tangent so I never found out 100%! :blush:
That thread also mentioned test pins etc... My car is a manual 1990 w 3.8 993RS internals.

Alex Sol

jason, thanks for the tip. i have a line on a theo jennisken scan tool that i will be borrowing to do some more diagnosis....

hi wind, if you read Loren's comments, you'll find that he said the Tips have TPS and suspect the manual trans do not. i was scratching my heading and wondering why/if you upgraded to 3.8 993 RS and were running Tiptronic... not that there's anything wrong with that... haha.. Jeremy Clarkson would have lots to say about that but me.... i believe you can do whatever you want to your own car.

Alex Sol

so i dropped into the shop (i had to move out of my garage.... 1. wife says so 2. it can get to minus 10 deg C here 3. it's nice to have a hoist....)

and thought to try to 'short out' the idle / wide open throttle switch. - it has three prongs

started it up without the idle /wot switch plugged in. engine roared to life and settled in around 3,000 rpms, then dropped to 1,500 rpm

i plugged a wire into the ground (guessed it was the middle) and tested wot side first and engine roared.. i plugged into the other side and the idle dropped to about normal 800-1000 rpm and the same old issue of afm slap, misfire, rich condition came right back.

so i think this rules out that the idle microswitch is faulty.

but then, what can it be?

i will have the diagnostic tool to test and will have a look at the Tore's (never heard of this before) and like the idea of 'free software'....

robt964

Hi,

These are the two pins you want to test.



Set the meter to test 'continuity' or resistance (ohms), It doesn't mater which way round you have your probes.

With the throttle shut you should have full continuity ie zero resistance. With the throttle open you should have no continuity ie infinite resistance.

Whilst there you probably also want to check the WOT switch. Instead place one probe on the centre pin, and the other on the RHS pin. The readings should be the reverse of those for the idle switch.

Hope this helps.

Alex Sol

rob, i'm pretty sure it is not the microswitches because when i dropped by the shop yesterday and tested with engine running and with a small wire, i tested the wot and idle contacts. and with this 'jumper' the engine had the exact same characteristics as when plugged in...

ie: with idle/wot 3 prong plug out, the motor idles at 3,000 rpms then eventually drops... but runs smoothly with no afm slapping, and all cylinders firing (test by ear and when driving, feels like it has full power and then some)

with idle/wot 3 prong plug back in, the rev immediately lower to about 1,000 rpms and it misfires and i hear/see the afm slapping and tends to stall on deceleration and cannot keep a steady rpm...

cannot hold steady at 2k 3k, 4k rpms etc... instead i hold the throttle steady and the motor immediately jumps to 4,000 rpms then down to about 2,000 rpm then wants to stall if i don't keep blipping. holding throttle steady will cause a stall... crazy eh??

Alex Sol

driving is the same: with the idle/wot plug out, i can hold the rpms steady and the car drives smoothly with just a wee bit of hesitation at around 1,500rpms... only problem is when i chop the throttle, i do not get any engine braking just slows down from the friction between road and tires and wind...
pretty scary... like riding a two stroke bike on a racetrack... no compression on decel

with the idle/wot back in: stalls, not smoooth, struggles to get up to 2,700 rpms then runs okay over the 3,000 rpms mark but does not feel like i am running full power

i hope to get the diagnostic tool (theo or hammer) within the week

Indycam

I'm thinking you have a leak in the intake .
Engine braking comes from the throttle butterfly being closed . If you have no throttle butterfly you have no engine braking , as in a diesel .

Do you have a vacuum gauge you can see / read while driving ?

http://en.wikipedia.org/wiki/Engine_braking