High Idle-Throttle Sensor Question

[Highway Q45]

I've now got the plenum/injector work done, no vac leaks, and the idle is still high at both cold start and warmed up. I remember when I removed the throttle body for cleaning I loosened the throttle sensor switch but did not remove it. I'm not sure I installed it in exactly the same position. Do those slotted holes on the sensor switch effect the baseline idle by rotation or position on the TB? I didn't move the AAC idle adjustment or throttle cable adjustments. The symptoms are: 1. 1500-1750 rpm cold start-nuetral 2. 950-975 rpm warm-in gear 3. 1000-1150 rpm warm-nuetral

There are no other driveability/shifting problems other than the fast idle. Unfortunately my 91 FSM I bought from AZhitman won't arrive for a few days. My mini manual is missing so many pages it's near to worthless. Any ideas appreciated or recommended testing I should do? Steve

[AZhitman]

Aw, man! I coulda looked it up B4 I stuck it in the mail...

Sorry brotha.

[forecast]

There is an adjustment on the IAC valve to close it down some. Use a screwdrvier to do that until the idle is normalized.

[Highway Q45]

Forecast,Thanks for the info. Yeah the ACC idle screw is the typical idle adjustment. Before I adjust the idle at the AAC I just wanted to make sure of the throttle sensor question. I would think that with all things being left "as is" during the plenum work that the only variable is that throttle sensor which I moved. Could the cleaner plenum and, TB, and plenum effect idle? Does the ACC idle adjustment make up for the throttle sensor error or is there a sweet spot you set the throttle sensor at and then fine tune the idle? Sorry I wish I knew more about how they interact. I also wish I had a manual with all the pages. My wife doesn't even notice it but I can't leave it like that knowing there's a simple answer out there. All and all the Q now runs like a scalded dog, never would have thought a good intake/injector cleaning could make such a difference. Tommorrow I'll try rotating the sensor at idle. If nothing happens I'll try the AAC adjustment. Thanks Again

Steve

[Q45tech]

The problem is [if someone ever misadjusted the IAC bypass screw to raise the idle to compensate for a dirty throttle body].

The REAL idle speed is critical and the interior tach is not accurate enough to tell the difference between 650 and 675 rpm....or even 700 rpm.

The TPS must be adjusted hot so that the ecu sees 0.40-0.45 volts after the the engine has been at 176F or hotter......but not above 194.9F for at least 10 minutes [to make sure the hot wax fast idle cam is totally disconnected].

Then you set, reset, or leave alone the IAC bypass screw [usually 2.5-3.0 turns out after you have bottomed it............what you want is the IAC percentage [pulse duty cycle] to be no less than 10% and no more than 15% [idle in park, AC OFF]........then test to see that the IAC % goes up when in drive and up even more when AC is turned on [30-35%].

The ecu depends on having full and precise control of IAC, if IAC is not set right the ecu has to change the base timing [you don't want this to happen especially in summer as retarding ignition increases the heat load which may get you above 194.9F which decrease the advance more ........a cycle that just get worse as the coolant temp rises hotter and hotter the timing get lower and lower.............and eventually you can get in thermal overload and runaway.........where the cooling system can't control the temperature rise. [primarily due to rad not being 100% brand new].

All this from a mis adjusted TPS and improperly set IAC.

AS you all know in drive, AC on, stalled traffic eventually the system will overheat..........some cars take 5 minutes in 100F some 15 minutes, some 20 minutes of not moving.

Mu definition of overheating is for the coolant to get above 212F.

[Highway Q45]

Thanks so much Dennis. Very good write-up. Not to sound dense but is the TPS (throttle position sensor?) you refer to the same as the throttle sensor in the manual? If so, I can't find a page with the adustment instructions (EC-130 to 131 and EC-246). All it shows is how to test the TPS, not adjust it. I think I'm clear on the testing procedures, I just need to know how to adjust it. How and where is this adjustment made before adjusting the IAC. Is there a TPS position adjustment made at the TB mount with the two mounting screws or do the slotted holes do nothing? Sorry to hound you with all these questions but I know how important these small things are to a Q and just want to do it right. I wish I'd never loosened the TPS in the first place or at least marked it's position against the TB.Thanks again for all the help.

Steve

[Q45tech]

Go to EC-146/147 where it tells you how, but the resistance method is less accurate than reading the voltage the ecu actually sees via a Consult. The Consult has a special setting proceedure for TPS [throttle position sensor].

Think about this: the hot wax cam opens the throttle partially [minutely] when the coolant is cold....[and as the wax melts slowly the Throttle slowly closed down to normal closed position]...thus the ecu knows that the Throttle blade is not in its normal closed position by the TPS voltage.........instead of 0.44 you might read 0.6-0.7volts at cold start up.

The throttle is not 100% closed [at the closed position] as the tiny sloped groove at bottom and the edge shape of the blade lets a tiny amount of air slip under the throttle blade. This air plus the air that the IAC manual bypass lets slip around the IAC and the extra IAC pulse width combine to make the total air necessary for some rpm.The IAC manual air bypass and the TB should allow 600 rpm worth and a 10-12% IAC pulse should make up the other 75 rpm.Perfection and double checks are the name of the game with idle speed control.The 90-95 ecu is dumb as a rock and only knows what the sensors tell it. Whereas the 1996 ODB2 ecu has the smarts of an early home computer [say 1990 vintage] it can run some software code and guess better.

Getting everything perfect is also important as when you decelerate [lift foot off throttle] or as rpm increases the IAC duty cycle goes to 90% to supply enough air to run the engine at say 1300-1500 rpm so the engine does not die as the throttle is snapped shut............acts like the old fashioned dash pot on carb engines.

Hopefully you can see how easily dirt in IAC or TB can effect the smoothess of idle and deceleration.........the ecu knows what to do but it expects all system to work as new....be clean!

[Highway Q45]

Dennis thanks for taking the time here on Fathers Day. Your description of the TPS/IAC interaction is very detailed and helpful. Unfortunately my 91 mini-manual relates to the Crank Angle Sensor (CAS) at EC-146/147. I guess I'll wait until my big book arrives from AZ this week unless you know the mini manual page. I now can see how the smallest "out-of- spec" tweak can effect so many downstream control functions; that's why I was reluctant to just use the IAC idle adjustment to take up the idle slack.

Just out of curiosity does the rotational sensor placement on the TB matter or are the slotted holes just for having leeway in engaging a device on the throttle plate shaft?

Where in the hell did you learn all this stuff? Truly amazing! Kind of makes me wonder how many previous Q owners gave up with dealership service and dumped a perfectly good vehicle due to little nagging problems that never got fixed correctly. Working on the 95 Miata is like wrenching on an old Ford truck compared to the Q, cheating the timing only takes a paper clip and good timing light!

Thanks Again,Steve

[Q45tech]

The slot on the TPS is how you set the correct position.

The problem or the solution is that automotive technicans are not engineers or they wouldn't be working for $50k a year. Being an engineer by trade I try to determine what the designers had in mind.The Q is not anymore complicated than other performance V8/V12 of that era.......BMW/Jag/Ferrari/MB were all nightmares in 1990.

Comparing a domestic or import I4 or I6 - V6 to a performance V8 isn't fair.

All I know is that after 13 years and 250,600 miles the engine has never had much changed other than spark plugs, water pumps, hoses and guides..........sure lots of multiple cleanings and reseals.......the oil pan has never been dropped and all the sensors are what came with the car.....except the O2 sensors and oil pressure switches.

Hopefully the engine can last 15 years [Jan 2005] and 300,000 miles.

[Highway Q45]

Congrats on the 250K milestone, if Q's could talk they would all like to have you for an owner! I can see where an engineering background has paid off for you. They must cringe when they see me coming. Thanks for all the help on the TPS, now I know "what" to do and sure have a better understanding of "why" I'm doing it. It will be so nice to now have a FSM manual where 1/3 of the pages aren't loose at the bottom of a box.

Again, thanks for taking the time to help me out.Steve