13 replies to this topic

[ZeroX12]

Hello,

 

I bought a 1990 Spec Miata in March. I hope to do my first race Memorial weekend.

 

When I bought the car it idled around 1800-2000 rpms. The seller said it always did that. Now I am new to Miatas, but not these engines. I have done Rally Cross with 1.8L Escort GTs for years and years. I also have tuned fuel injected cars myself so I'm very familiar with ISC strategies. However this one has me stumped. After a lot of work, I now have it idling at 1500 rpms...

 

After much testing, there is only one way I have found to get the ISC to go into closed loop, and that is to start the car from completely cold and don't touch anything. It will fast idle as is normal on start up, then 15 seconds later come down to ~900rpm. As soon as you touch the throttle the RPMs will climb to around 1500 and never go down again.

 

What I checked:

Searched for vacuum leaks, blocked hoses, etc. Did everything except remove the manifold. No changes except when i block the ISC tube.

Checked air bypass screw (was turned all the way in), this doesn't make a difference because car never goes into closed loop

Coolant temp sensor - matches voltage and ohms range as car warms up
Neutral/clutch switch - measured from the ECU connector and is correct (clutch switch bypassed, but ECU still get 12v when in gear, 0v neutral)

Throttle position sensor - throttle blade closed  the TPS is at exactly 5v, and the IDL is at 0v, changes to 12 with slightly open thottle

Removed and cleaned the ISC, valve is clean and moves with key on

Removed and checked the thermostatic air valve, also blocked it off completely with no impact

Removed and cleaned throttle body, even reset all the stop screws so the throttle blade comes back to perfectly closed

Set Airflow Meter back to stock (I think, hard to tell), appears previous owner adjusted the airflow meter quite a lot. This did help a bit, but only because the AFM is restricting air flow

Base ignition timing is ~14 to ~15

 

Basically checked everything here https://www.miata.net/garage/isc.html and here https://www.miata.net/garage/ECU%20Pin-Out%2090-93%20MT.pdf .

 

Sorry for the long post, but this is driving me crazy. At this point I wish I could just hook a tuner up to it so I could know what the ISC duty cycle is at. Is it worth it to just replace the ISC? They are so expensive.

[Bench Racer]

 

After a lot of work, I now have it idling at 1500 rpms...

 

After much testing, there is only one way I have found to get the ISC to go into closed loop, and that is to start the car from completely cold and don't touch anything. It will fast idle as is normal on start up, then 15 seconds later come down to ~900rpm. As soon as you touch the throttle the RPMs will climb to around 1500 and never go down again.

 

What I checked:

 

Throttle position sensor - throttle blade closed  the TPS is at exactly 5v, and the IDL is at 0v, changes to 12 with slightly open throttle

 

Removed and cleaned the ISC, valve is clean and moves with key on

 

Set Airflow Meter back to stock (I think, hard to tell), appears previous owner adjusted the airflow meter quite a lot. This did help a bit, but only because the AFM is restricting air flow

 

 

Sorry for the long post, but this is driving me crazy. At this point I wish I could just hook a tuner up to it so I could know what the ISC duty cycle is at. Is it worth it to just replace the ISC? They are so expensive.

 

Going through your check list ass backwards:

 

If you had an non screwed with OEM AFM that might be a positive.

 

With the engine warm and ignition key in the run position, connect and disconnect the ISC wire connector and listen for solenoid to function upon disconnect and connect.

 

The info I'll include comes from a couple years ago when my 1.6 would idle normal cold. As I remember, after warm up and I'd step on the accelerator pedal to approx. 2200 rpm and hold the pedal steady, the engine would oscillate back and forth between 2200 and 1500 rpm. Release accelerator pedal and would idle normal. I checked a bunch of stuff to the factory shop manual including the throttle position sensor and everything checked ok.

 

Read some info someplace and I the went to the diagnostic box and shorted TEN and GND together the engine is now in open loop and the ECU ignores inputs from the ignition rpm, temperature and throttle position sensors. This open loop is the same as when engine is cold. Warmed the engine and the oscillating issue disappeared. Because I had previously installed  a new thermosensor at the rear of the head I felt that was not part of the issue. I installed a new throttle position sensor. Oscillating eliminated.

[OrangeCrush86]

Tried a new TPS. No change. Just ordered an unmolested AFM so we will see what that does. Last resort is a new ISC...

[Steve Scheifler]

I am curious to know what you conclude.  Many of us have had similar issues and resolved them only by replacing the ISC.  I've tried cleaning, testing, tinkering first but quickly took the word of others before me that it was a waste of time.  So far I have been surprisingly fortunate with two good used units, but these things are getting so old that seems less and less likely today. 

[davew]

You have 3 idle speed adjustments on the car.

 

1-base minimum throttle stop. This is the little set screw with a 5mm jam nut. It is hard to get at without holding the throttle wide open. So do it with engine off. This should be in contact with the throttle linkage at closed throttle. I set the idle speed at about 800 rpm with the bleed screw turned all the way in (closed)

 

2- bleed screw adjustment, should be 900 rpm hot

 

3- dash pot. The little vacuum pot that slows the throttle closing. Make sure this is not hanging up and keeping the throttle open. These get bent often when the motor mount breaks on a side impact. Look for marks on the underside of the hood as evidence of a broken mount in the cars past.

 

Check ignition timing. Set to 14* for diagnostic work. Then adjust later on a dyno. Should also check to verify TDC is accurate.

Get an unmolested AFM. People who don't know how to properly adjust them, can get some wacky results.

Check throttle cable freeplay. There should be a small amount of slop in the cable at the throttle body.

 

Make sure all the basics are working properly before you dive too deep into electrons.

 

Dave

[OrangeCrush86]

I disassembled the TB and set the throttle stop. It stops the plate exactly at closed (there is a little bit of wear here, but not enough to cause the high idle).

 

I know it isn't the bleed screw. Car idles at ~2000 with the screw turned in all the way.

 

I checked the dashpot when I had the TB off. It works like I would expect for an old car. Anyways I can turn the throttle plate to 100% close with my hand and it's still at ~2000.

 

I installed a new sealed AFM from a `90 1.6L and the idle is still the same.

 

Last option is to try a ISC. The only question is should I risk spending $400+ dollars on a new/reman one or test my luck on an ebay unit.

[OrangeCrush86]

I bought an almost new ISC from ebay. Now the car idles pretty consistently at ~1500. So that seems to have reduced my idle by 500, but there is still no way I can get to 900.

 

Very irritating. I don't know if there is power steering switch on my 90, but I guess I didn't check that one. I have heard it can have a big affect on idle.

[OrangeCrush86]

I found the PS wire and grounded it. That increases idle so that isn't it. When the car really warms up it will will drop idle to 1100. I'll throw a $20 temp switch at it and call it good enough for me.

 

BTW, I found out that some ISC on eBay have the coolant nipples reversed and you need longer hoses to install it in a Miata. If you buy a used one from online make sure the straight coolant nipple is next to the electrical connector.

[TylerQuance]

Have you plugged the ISC hoses completely so it can't let any air into your manifold? If you verified the 3 things Dave listed and you say the throttle blade is shut completely, it has to be getting air from somewhere to be idling at 2k. If you plug the ISC hoses and it dies or idles down, you're on your way... if not, find where else your engine could be getting 2k RPM worth of air.

 

Edit: I just saw you got it to idle down to 1100. I'd be happy with that too... actually would prefer over 900 to keep oil pressure up.

 

tq

[OrangeCrush86]

Have you plugged the ISC hoses completely so it can't let any air into your manifold? If you verified the 3 things Dave listed and you say the throttle blade is shut completely, it has to be getting air from somewhere to be idling at 2k. If you plug the ISC hoses and it dies or idles down, you're on your way... if not, find where else your engine could be getting 2k RPM worth of air.

 

Edit: I just saw you got it to idle down to 1100. I'd be happy with that too... actually would prefer over 900 to keep oil pressure up.

 

tq

 

I have plugged it before. If I take a vice grip and block the ISC air hose the car nearly dies (400 rpm). So I know the ISC is commanding the fast idle.

[gerglmuff2]

I have plugged it before. If I take a vice grip and block the ISC air hose the car nearly dies (400 rpm). So I know the ISC is commanding the fast idle.

 

its either commanding it, or its leaking. 

[luvin_the_rings]

Is your ECU correct?  We had an automatic ECU and harness in the car and it would always idol high.  I'm guessing it was because it was expecting the load of a torque converter when it wasnt there? (we have a 5 spd in there).  

 

the ECU should say B61P on it. 

[OrangeCrush86]

Is your ECU correct?  We had an automatic ECU and harness in the car and it would always idol high.  I'm guessing it was because it was expecting the load of a torque converter when it wasnt there? (we have a 5 spd in there).  

 

the ECU should say B61P on it. 

 

That is a really interesting idea I didn't think of. I'll check that.

[Ron Alan]

93 has a different ECU for the 5 speed...at least in CA