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Saab Mad
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For a while now my T16 has had a problem with high idling when warm and also pig rich mixtures. The rich mixtures made the car very foul tempered when cold, and a drunkard when warm :lol:

The high idle was less of a problem, but as I knew that something was wrong I really wanted to fix it.

When I turned up to Kodak, I wasn't expecting to do any work on the car but when I drove home later that day, both problems had been fixed. The reason for the rich running was straightforward, but the cause is still a mystery. I think I have unravelled the high idle cause, and as it's not something I've seen before I wanted to post it here.

None of the work I did on the car was anything new, or high-brow. Hopefully it might be of use to those with a similar problem. In particular, I'm hoping the photos will show some folk where stuff is in the engine bay.

Oh also I'm bored, and haven't used the digital camera for a while :lol:

Despite not intending to do any work on the car, I was encouraged to tinker about with the engine to solve the high-idle problem. I say high, it was about a steady 1200RPM on a warm engine.

I could only think of seven ways that the idle could be elevated: rich running; vacuum leak; idle-adjustment screw; dashpot wound in too far; throttle-stop screw; faulty auxiliary air valve or misadjusted throttle-cable. All of these I had adjusted before but with no improvement.

This is the throttle-body on a T16:


The idle-adjustment screw (centre of the picture) opens or closes an air bypass channel in the throttle body. Alex wondered if the channel had become blocked with crud, so I shot some carb cleaner into the channel.


To my amazement some of the carb cleaner exited the from solid metal on the other side of the throttle body.

It turns out that I'd been ham-fisted when previously trying to lower idle using the adjustment screw. I'd managed to puncture the throttle body using the end of the screw. In the photo below, you can just see cracked metal where the screw's pushed through:


This was potentially a vacuum leak. I pushed back the metal which seemed to real the hole. I'll do a proper and permanent repair soon using JB Weld. Be careful when winding in that adjustment screw!

Alex asked if perhaps maladjustment of the throttle-position switch was preventing the throttle-butterfly from fully closing. As I'd never adjusted the TPS, I didn't think this to be the cause of the high idle but given a lack of other options, I gave it a go.

The throttle-position switch (centre of picture):




The switch is adjusted by undoing two small bolts (7mm IIRC) allowing it to tilt and rise/lower. You can see each bolt in these pictures, they're either wide of the TPS:




Even though the switch was correctly switching the ECU between different engine maps, when unbolted it became clear that the switch was holding the throttle open very slightly. This allowed metered air into the engine, raising the idle speed.

The TPS informs the fuel injection ECU on the throttle's position. There are three positions: closed (idle), partially open (cruise) and wide-open.

Closed throttle causes the ECU to use a special idle map. Cruise means that fuelling's measured according to air metered by the AMM. WOT IIRC engages an RPM-only map. It is essential that the TPS is correctly adjusted so that the ECU can alter fuelling for different engine conditions.

The Bosch LH2.2 TPS is a switch, unlike later LH2.4 and Lucas cars that use a potentiometer. Because it's a switch, you can hear a quiet click as the throttle's opened to shift from idle to cruise and cruise to WOT.

A correctly adjusted TPS will switch from idle to cruise as soon as the throttle starts to open. Adjust the TPS so that when you rotate the throttle spindle, the switch clicks open immediately. Make sure it clicks again when the throttle's shut and that it doesn't hold open the throttle plate.

Adjusting the TPS is a PITA. Just when you have the thing in the correct position and tighten down the bolts, everything shifts slightly. Because only slight movement should be required to open the switch, tightening the bolts is just enough to cause maladjustment.

Because the throttle plate was now allowed to shut fully, I found that the throttle cable was too tight and that now was holding open the throttle. This of course then required further adjustment to the TPS.

I advise you to fully slacken off the throttle cable tension thing before you adjust the TPS:




The throttle cable should be adjusted so that it's only slack enough to allow the throttle plate to close.

Next I adjusted the throttle-stop screw:


Wind the screw down until it just touches the throttle stop and then wind it back 3/4 turn.

Since the dashpot is required mainly for emissions, I wound that fully back:


It's worth pointing out that I did almost everything in the wrong order, and consequently had to keep re-adjusting the TPS. To make the job easier, wind the throttle stop screw fully up; fully slacken the throttle cable; unwind the dashpot. Then adjust the TPS; the throttle stop; cable tension; dashpot.

I had suspected the auxiliary air valve, but this appears to be functioning correctly. I've cleaned it through with carb cleaner and WD40 on several occasions.


Onto the rich running. Two of the top suspects for rich running are: faulty NTC sensor and air mass meter.

We started with the NTC sensor, by reading its resistance with the car at normal operating temperature. Although initial readings indicated open-circuit (faulty sensor) later readings showed correct operation.

The NTC sensor is mounted in the centre of the inlet manifold, in between runners for cylinders 2 and 3. The top looks a bit like a fuel injector, but the electrical connector is blue:




One of the diagnostic tricks suggested by Alex was to short-circuit the NTC sensor with the engine warm. If the sensor's faulty, the fuel injection ECU will default to a cold-start enrichment mode. This means the car runs pig rich all the time. Since the sensor's resistance is inversely proportional to engine temperature, short-circuiting it means the ECU sees a hot engine and will not engage cold-start enrichment: mixtures should return to normal.

Make sure the ignition's switched off before short-circuiting any engine sensors.

Using John's Gunson meter, we found that C02 levels at idle were 8.7% :eek: when they should have been less than 3%. This reading stayed steady whether the NTC sensor was connected or shorted, so the conclusion was that the sensor was functioning correctly.

We then turned attention to the air mass meter. Using a pin-out diagram in the Bentley, Jon and I were able to check the meter's resistance. The reading was way above the correct which would give rich running, and explain the 8.7% C02!

Bentley specifies resistance of 380 ohms for the AMM, but this is for catalysed cars. My car does not have a catalyst so we tried setting the meter to 280 ohms.

The AMM is adjusted using a trim pot on the side of the meter. Jon commented that the most common failure mode of the meters is breakage of the trim pot. This would mean the meter does not respond to adjustment, and gets binned. However, the fault can be easily corrected using an inexpensive part, so don't be too quick to bin that AMM!

Jon also suggested blasting the meter's filament and thermocouple (heat sensor) with carb cleaner before refitting it to the car. Whilst the AMM's filament gets cleaned whenever the engine's shut off, the thermocouple does not. Muck stuck to the thermocouple will affect accuracy of the meter.

A prolonged and carefully aimed blast of carb cleaner soon removed all the dirt from the AMM's components.

We refitted the AMM and checked C02 levels. They were 1.5%, and we adjusted this up to 2.0%. Job done.

I might replace the NTC sensor and auxiliary air valve if they're inexpensive. After all, they've been working hard for 17 years!

Thanks to everyone who lent a hand at Kodak.

Sorry for the ramble. I warned you at the start it wouldn't be anything new ;) Please post corrections as necessary.

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the dashpot is a little spring and rubber valve that prevents the throttle from snaping closed too fast. when you let off the gas it lets it close 99%, and then about 3 seconds later (if it's working right) it closes 100%. primary reason for it as far as I recall is to prevent stalling, with the AC running.
 

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Wowee Matthew, you and the gang are SHARP! :D

As far as I know the dashpot is exactly what DeLorean said, I don't think that it's AC dependant though - it keeps the engine from choking or stalling if you step on the gas and then take your foot off all of a sudden.

Good photography, btw :D

aleks!
 

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well I can recall when I had my dashpot out all the way (someone told me it was only for emissions) when ever I ran the AC, if I would let the gas off 100% and it was at low RPM's the engine would just shut off. When the AC was not on, the engine RPM's would just fall WAY low (like 400) and then bump back up. adjusted the dashpot to spec, and the problems were gone. yeah, I would say preventing stalling is indeed it's primary function.
 

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A real tour-de-force, Matthew--well done :)

Just a few additions/corrections:

The TPS is the same for LH 2.2 and 2.4 systems (it's the 2.4.2 that gets the potentiometer); it's supposed to complete a ground at idle and WOT positions, and do nothing in between.

I also recently cured a high idle (in our 1990 non-turbo automatic) by removing the throttle body, and adjusting the TPS on the bench. This way, I actually had total access to those diabolically placed 7 mm bolts, and I could feel the switch click as the throttle was gently returned to idle. Worked like magic!
 

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Wow Matthew - that's a novel i need to go through myself sometime in the near future :)

ProfZ and Matthew - is there any way to get to the TPS bolts without removing the throttle body? HOW?? what sort of tools can one use? i need to adjust it myself, but i couldn't do it because i didn't know how to loosen up those bolts. Thanks
 

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what is that aux air valve by the way... it isn't AIC valve, right? i dont remember seeing it in my hood! hm.
 

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There is a big difference between the aux. air valve and the AIC valve.

The aux. valve is a passive device: It's on a timer so it progressively closes after starting the engine from cold. Once the engine is fully warmed up, it does nothing more to the idle.

The AIC is an active device: It's controlled by the ECU, and constantly adjusts idle speed to compensate for engine load changes at all engine temperatures.

The AIC setup also gives lower emissions at idle (if it's working properly), so that's why some markets got this instead of the simpler and cheaper aux. air valve.
 

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Today I went out to my SPG and tried to finally beat this TPS and AIC thing to death, profoundly inspired by Matthew's article here. :)

I took off the AIC valve, and it looked fairly clean. But I blasted it with throttle/intake cleaner and then with WD-40 to see if that would help my cold-starting instability. I'll report on that later.

I tried to get to my TPS, but I failed to find the right tools, so I decided to try to take the throttle body off to help access those TPS bolts. I took off the hoses, the dashpot, the microswitch underneath, and undid 2 of the 3 nuts that hold the throttle body in place, but I couldn't undo the 3rd one. It's frozen shut. I even sheared a little of its hexagonal pattern trying to undo it and decided to have a mechanic help me get it off next time.

Then I was struck by a great idea and was able to rig up a tool that was made of a socket bit inserted into a wrench-coupling bit inserted into a wrench socket, and that was good enough to undo my TPS bolts.

The good news is - my TPS is working! It indeed needed to be adjusted. I was successful in adjusting it so I could hear a click of the switch and the circuit across lead 1 and 2 was continuous and the throttle valve was not disturbed from its resting position at "closed". I put the bolts back on, put the microswitch arm back on, wound the dashpot back on. Checked the continuity on the switch again - TESTS GOOD! For both closed and open throttle. Started the car, and it was a perfect 850 or so. The idle was good, and the throttle response was instant. The RPMs came down gently to a good idle speed.

Thanks Matthew for all the useful tips!! They all helped me a lot while I was doing this.
 

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well, since there are two wires attached to the microswitch (by that i mean a small clicker under the main throttle valve pulley, which sits on its own metal arm attached to the side of the throttle body with two Torx screws) and it only clicks when the valve returns to its closed position, i would think that it could be used to control the AIC valve in some way - possibly opens it for idle mode or something... i think the microswitch is providing some kind of a short to ground or something. this is just my guessing, but i wouldn't be surprised if this is correct. i do wonder, however, how it supplements the main throttle switch (TPS)... Matthew said above that when the TPS is in "closed" position, it activates the idle map... but then, my car was idling just fine even when my TPS wasn't adjusted correctly and was giving the wrong continuity readings when the throttle valve was closed. so the microswitch could be a redundant feature.
 

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hmm, interesting guesses. anyone know for sure? this IS running into the ECU (I checked a wire harness I have in the basement)

interesting...
 

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then it must short some terminals in a circuit that regulates opening or closing of the AIC valve, i would think.
 

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The microswitch is part of the TPS assembly; it clicks "on" at idle and also at WOT (wide-open throttle).

At WOT, the TPS will kill power to the air conditioning compressor clutch temporarily, to give more engine power for emergency passing, etc.

The AIC is controlled totally by the ECU--no switches involved. HOWEVER, the TPS must signal the ECU to go into idle mode in order for the AIC to do its job properly. Failure of the TPS microswitch to click "on" when the throttle is closed (often just due to maladjustment) is where many high-idle problems come from!
 
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