Black Top low idle issue

[bigbacon]

Did compression test. Forgot to warm up engine first so I need to do it again but the semi-warm result was
1: 200
2: 200
3: 207
4: 200

Never sure how long to let it crank though but those numbers seem ok to me.

All plugs were out during the test and throttle was wide open

[Davegt27]

Nice read on this subject
Good help also

To explain a little of the why the engine idles at 500 rpm go into the history of FI, basically it’s called the basil idle speed
What it means is FI motors will not run if the throttle plates and no bypass air are getting around the throttle plates
So most FI cars crack the throttle plates ever so slightly to allow enough air for this basil RPM number

This also tells me you have a good motor with no leaks
Next the FI system has to modes cold start and regular running and often this is used to troubleshoot the FI system

In older or original type L-jet FI system for cold start they used and auxiliary air valve and thermo time switch
The auxiliary air valve was used to bypass air around the throttle plate to give the engine the proper idle

On later cars like the 20V the auxiliary air valve gets replaced be ISCV (I am guessing it means Idle speed control valve) this ISCV is controlled by the ECU

It is a stepper motor like what was used in old PC printers

You guys followed all the steps and eliminated almost everything but the ECU
Hate to say this but it’s probably the ECU (maybe bad ground)

But you guys are correct go for the easy fix first (that means don’t replace the batt for a blown fuse)
ECUs are pretty expensive so you would want to do all the checks that you did

Also get all the questionable items taken care of like the wiring and O2 sensor
Add a fuel pressure gauge if you can

Hats off to jondee86 for giving good advice


Davegt27

[bigbacon]

But does what I am seeing with the O2 sensor mean it is bad? that is one thing I am not understanding. I want to eliminate it as a possible problem but I'm not sure if my methodology of testing it was correct or not even though I followed the BGB but continued on with the troubleshooting steps even though it passed the initial test which according to the BGB means it is an ok sensor.

[jondee86]

If that is normal then the O2 sensor seems to be functioning properly.

Page FI-71 indicates that the VF to E1 voltage should be between 1.8 and 3.2 so
it looks like your O2 sensor is within range.

Did you try reversing the wires on the ISCV ?

Cheers... jondee86

[bigbacon]

i am 100% positive the ISCV functions fine. It does exaactly what is it suppose to do when I turn it on cold. I can hear it do exactly what it would and close up as it warms up.

[jondee86]

If that is the case, then the ISCV must be getting the wrong signal as
you originally theorized The fact that the low idle rises back to near
normal when you remove the clip (de-power the ISCV) bears this out.

This indicates an ECU related problem. Best bet would be to try another
ECU to see if that fixes the problem, and go from there.

Cheers... jondee86

[bigbacon]

If I could find another ECU to try I would go that route but I just can't justify right now 200 bucks for a used one that could have issues as well and/or it may not even be the problem.

[bigbacon]

Just to check. Fi71 is just when the power is on not with the car running which is why i am confusedbwith the o2 sensor test.

[jondee86]

As far as the Toyota test thing goes, I have to agree it is not exactly clear.

As I see it a 4-wire sensor will have..
1 x battery power supply (B) for heater
1 x heater monitoring/ground wire (HT) to ECU
1 x pulse generation (OX) no idea WTF this is for !!!
1 x O2 sensor output (VF) @ 2.5 +/-0.70 volts

Looks to me that your sensor output should be fluctuating, but I have already
outrun my understanding of O2 sensors and BT ECU's Apart from that it
appears that the sensor should be OK. The theory seems to be that the ECU
changes the mixture from rich to lean, and the O2 sensor reacts by telling the
ECU that the mixture is lean. The ECU then increases the fuel and the mixture
goes rich... and the O2 sensor tells the ECU the mixture is rich. The ECU then
cuts the fuel etc etc etc and so on

If the sensor is working, then maybe the ECU is not doing its part of the deal ??

Cheers... jondee86

[bigbacon]

I found a cheapish ECU to test with although it will probably take weeks to get here.

[bigbacon]

OK! thing I got somewhere today and...it has to be a funky wiring issue.

So I stated before that I kept seeing a tiny bit of smoke from somewhere every time I plugged the battery in but I could never pin point where it came from it always seemed like it came from the battery...well....it doesn't.

I went to test the STA voltage today. I setup my multimeter, turn it on, set it to DC voltage. I put the battery back in the car and hook it up and ...smoke....and this time it didn't stop. it didn't stop until I unplugged the battery.

Looks like it comes from under or at the distributor but I unplugged the distributor and there is still smoke which leads me to the starter.

Something is shorted and cooking down there so at this point I need to pull the harness out and see just what is going on.

[bigbacon]

Ok here is what I think was happening:
The starter ground melted through the sheath, which was crusty as hell anyway, and decided to also ground itself to the starter casing.
It was like having the multimeter hooked up to it all forced it to be a bigger issue. I am actually really unhappy with this whole part of the wiring harness with the way the other starter connector and the fan switch wiring come out together with this big fat wire. The temp switch wiring has to go back with the rest of it behind the starter and back out to the sensor.

http://mr-2.info/images/P1050858.JPG

[jondee86]

Thar fat wire to the starter is live at all times (when the battery is installed)
and it is not fused. So important to make sure that it is secure and can't rub
thru the insulation and short out. If it does it can easily start a fire, and if
it does not burn your car to the ground, it will kill your battery instead.

The STA signal sends 12V to the ECU during cranking. The ECU then adjusts
fuel, timing and the ISCV to help the engine start.

Cheers... jondee86

[bigbacon]

it was definitely secure but it was definitely shorting out against the starter itself. interesting it was never contentiously smoking UNTIL I actually attempted to test the STA voltage. I never even got to the cranking point just plugging the battery in this time was enough to get it going and keep going until I unplugged the battery.

[bigbacon]

No change with repairing this. still idles like **** when warmed up.

[bigbacon]

just to test. I tried unplugging the O2 sensor to see what would happen. Car sat for maybe 10 minutes. Turn it back on, 1100 idle minute later it drops to 900 and starts running rougher, another minute or so later back to 600 and really rough. It isn't even a slow progression down from those number it just drops, again, almost like a switch or command is sent to the ECU or whatever to say do "X" and it does it.

I think I'm going to try getting a new temp sensor and see. I mean the temperature is one of the things I would think would tell the car some vital information on what it needs to do to idle where it should.

also, what do you think of this. STA voltage with IGN ON (not cranking) is -5.5v

[jondee86]

The STA pin takes a signal from the starter relay supply i.e. it only sees
voltage when the relay is energised (during cranking). The expected voltage
at the STA pin during cranking is 6V or greater. If you have voltage on the
STA pin at any other time, you have a problem.

Since AFAIK the STA signal is only there to tell the ECU to get into startup
mode, By way of an experiment, you could disconnect the STA wire and see
if that makes any difference. It is possible that 5.5V is close enough to 6V
to keep the ECU in startup mode continuously. Not sure what effect that
would have, but it ain't gonna be good !!

You will also need to try and figure out why you have voltage on the STA
when the engine is not cranking. If it is coming up the STA wire, backtrack
and see where it comes from. If it is output by the ECU, check your ECU
grounds to make sure there is next to zero potential difference between
the ECU ground point and the battery negative terminal.

Cheers... jondee86

[bigbacon]

notice it was NEGATIVE 5.5 volts while IGN was ON not while cranking.

[jondee86]

Have you got an analog multimeter ? Because I don't think you get negative
voltage on a digital meter ? In any case we are really talking about the difference
in potential between two points. The convention is that ground should be at zero
potential, and the battery positive at 12 volts.

The difficulty arises when you get high resistance in ground connections, or 12V
backfeeding into a system that has multiple grounding points. This can generate
strange voltages as the current flows thru relay coils and resistors.

Cheers... jondee86

[bigbacon]

i actually a pretty nice digital meter

[jondee86]

i actually a pretty nice digital meter

Aha... just checked with mine and the negative voltage is only a polarity
thing. Depends on which way round you use the probes. It may have some
use when working with a system that has a floating ground, but can be
ignored for basic automotive testing.

And since this is a little bit interesting ... I have added an explanation
I found that helps explain the floating ground concept...

Voltage is just the difference between the electrical potential between two wires.
In most electrical systems, for efficiency's sake, one of those wires is actually
connected to the ground, or Earth. This allows the other wire to have the minimum
potential necessary to achieve the desired potential between the two. In terms of
automobiles, each component in your car requires 12 volts of electrical potential
to operate. If one wire is a true earth ground, you must supply 12 volts of potential
to the other.

However, if you supply 100 volts to the ground wire, and 112 volts to the other,
your electrical devices won't know the difference, and will operate happily on the
12-volt potential difference.

Don't know if that helps, but it will take your mind off food for a couple of minutes

Cheers... jondee86

[bigbacon]

ok I see. I swapped the them around but yea, tried it again and 5.5v between STA and E1 at the ECU with IGN set to ON and NOT cranking the engine. this sholdn't be right? I see no indication in the BGB about STA to E1 voltage other than when cranking.

Pulling the AM2 fuse cuts the voltage so what else is part of that ciruit?

[jondee86]

Digging into this I discover that the factory GTS ECU ST (start) wire works
differently from the AE111 BT ECU STA wire. The GTS ECU actually supplies
12V to close the COR during cranking until the AFM flap opens and energises
the second coil in the COR. The AE111 ECU does not supply 12V to the COR.
It only monitors for 12V on the STA pin during cranking.

Now 5.5V is the kind of voltage you would expect to find downstream of a
relay coil, so I wonder if there has been some shonky wiring done to get the
COR functioning ?? Anyways, it is probably easier to do a bit of experimentation
rather than try tracing all the wiring.

This what I would do... you can take it or leave it Track the wire that
connects to the STA pin and cut it before it reaches any splices to other wires.
Insert a bullet connector so you can hook it up again if this doesn't work. Run
another wire from the starter solenoid and plug it into the wire that connects
to the STA pin. Now you have a wire that puts 12V on the STA pin during
cranking, and only during cranking.

Leave the other half of the STA wire hanging for the moment, and try starting.
If it doesn't start it will be necessary to check to see if the COR is getting 12V
on one coil during cranking.

Cheers... jondee86

[bigbacon]

too much work for me....

I'll wait for my backup ECU, test with unplugging/changing various sensors and if nothing changes then I am pulling the harness and sending it away to get a new one made.

[bigbacon]

ok.. I need to apologize here...I was an idiot with the whole STA voltage.....it was really 5.4mV....not sure why I just overlooked the "m"

cranking voltage was like 9.7x so that is ok and it drops back down to 5.3mV

running the VVT unplugged changed nothing as well.

still the same thing cold idle 1500....warms up to 1000 starts to run a bit rough, turn off car and back on...runs really rough and idles 500-600

[bigbacon]

New ECU didn't change a thing.....as expected

I give up at this point.

UNless there is a shop somewhere near me I can tow it to who knows the engine and the car....I'm done messing with it. I may end up just dropping the BT out of the car, selling it to someone else and finding another full 16v.

[jondee86]

Yes... unfortunately there are 101 small things that can upset the
running of a 20V. The ECU is a bit more sophisticated than the 16V ECU,
and the engine itself is tuned a bit closer to the edge. Some things are
obvious and some faults can be a lot harder to uncover.

For example, one owner installed a one-way valve in the MAP sensor
tube, thinking that it was a pulsation damper. Since the two items look
much the same, it took a while to figure out why the engine did not run
so good. As you have tried a different ECU with the same result, it is
fair to say that there must be a faulty sensor or sensor input.

If you can find a technician who is familiar with Toyota 20V engines,
that is probably the best bet.

Cheers... jondee86

[bigbacon]

I keep asking around about placing to bring it but no one want to help....

they won't touch it even though they are places that KNOW the MR2 and KNOW the BT and have done BT swaps many times in the past.

Pretty much at this point either sell it, find another good 16v and put it in or crush the **** out of the car so that no one can enjoy it.....perfectly good rust free, original mk1 in amazing shape but i can't damn use the thing...

worth **** to sell it so kind of the only two options....I mean I an just angry as hell and I bet the dude that sold me the swap is laughing his ass off.

[jondee86]

Back in your first post you said that the engine idles fine with the ISCV unplugged.
Why don't you just use it like that, or is there some other problem ? Leastways if it
runs OK with the ISCV unplugged, you could sell it like that making sure to tell the
new owner it has a small problem that needs sorting

Cheers... jondee86

[bigbacon]

it won't pass an inspection and it bogs SO BADLY under 3k RPMS it is nearly impossible to get into first gear without bouncing the clutch around and slipping it until the RPMs get high enough.

it is like raising the RPMs artificially doesn't solve the problem.