Speed Sensitive Idle?

[shift1313]

Okay guys this is a weird one. Ever since I put my car back together ive had a weird problem. My normal idle is around 750rpms, nothing wrong there. If im moving, even 1mph the idle is around 1200rpm. At first I just thought it took a bit to idle down but i did a little test at a stop light the other day. Sitting still in N, foot nowhere near the gas pedal it idles smooth at 750. Let off the brake and start to roll, it jumps up to 1200. As soon as i stop it goes back down to 750. Same rpm every time. And it doesnt bounce around. It goes up and down to the same spot every time. Its not just the tach, the exhaust note goes up as well.

 

Has anyone had this happen before?

 

 

All the standard questions.

 

Yes i reset the ecu, tps and isc.

Yes the fuel filters are all clean.

The injectors are clean and they have new orings.

I plan on rebuilding the TB as soon as i can, just havent found the time yet.

All vac lines are fine(only 3) and all open ports are capped off.

No other weird problems have come up.

[jwrape]

I know on some cars, don't know about the Quest, the ECU will elevate the idle while in motion to keep the rpms up for accessories like the brake booster and lights. I have noticed that on all my Stick shifts. I don't know if the older cars had the same thing but my 99 Mustang, my 91 Mustang both did it. The idle was high while coasting and would return to norm when stopped.

[shift1313]

Interesting. Ive never had another car do it, and i dont remember this car doing it a few years back.

[Indiana]

the throttle position at cold idle the tps will be above .5, when hot it will drop to below .5 then the ECU takes over and move the idle motor

 

the adjustment on the back of the throttlebody, adjust that one again

 

you set up the throttlebody with the butterfly 100% closed and how can the motor idle? well it must depend 100% on what is being sucked in through the PCV valve, that blowby is unmetered air and its dirty, left over unburnt gas, oil, the MAS and the ECU still use the idle map and put in the same amount of fuel but its effected in the AFR since so much blowby is being burned not fresh filtered air

 

change the adjustment for that other screw, its not the fixed one I don't know what they call it but you can't have the throttlebody setup to idle the motor from the pcv valve and when you change the vacuum a little as you press the brakes the intake gets a surge, the motor gets a surge, its depending too much on that pcv valve then it sucks too much into the booster and you get that surge then the idle motor tries to take over then it goes back to rest and goes back to sucking on the pcv valve

 

try this, pull your pcv valve hose off at idle and see what happens

 

there is another voltage range for the idle motor position nobody checks, its what is changed when you adjust that other screw, its in the ECI checker instructions of your service manual and it will get the throttleplate opened more and when you're done the car shouldn't die with the pcv valve hose off -or the oil cap but I know most don't remove the oil cap when the car is running cause oil blows out everywhere

 

you don't hear this very often do you? all you hear about it tps voltage

 

no consider check valves instead of pcv valves and cars that decel on the dyno and what crap gets sucked in from the motor and who detonates on decel on the road or blows smoke on decel... its all relative and that back adjustment screw it usually ignored and almost always is messed up

[shift1313]

thanks! thats very informative. Ill be sure to check over that stuff this weekend. I understand what your saying about the pcv and a surge etc but wouldnt that make the idle raise when im on the brake and not off the brake?

[SpEeD147]

I had pretty much the exact same problem on my 84 technica after i rebuilt the throttle body a while ago.

 

I have the 84 service manual, and i found that there is a calibration sequence involving primarily the isc that you have to go through in order to sync everything back up. i'm not sure if its the same sequence for the 88-89, but you could try looking it up in the 88 manual at http://www.starquestgarage.com

[shift1313]

thanks speed, i have the 84 factory manual and i have reset the tps/isc a few times with no change. The really strange part is if im stopped on a down hill foot only on the brake the idle is fine, as soon as i start rolling, no throttle, no clutch nothing just rolling is when the idle jumps.

 

I messed with it for a little bit with no luck and its not that big of a problem for me to spend time tracking it down. Soon im going to rebuild the TB and see if that makes any difference(it needs to be done anyway).

 

thanks for the reply

[mikec]

One test you can try:

Locate the ISC motor connector. It'll be one of the connectors on the rear of the throttle body. On 83-86 cars, it'll be the only connector on the back; on 87-later cars (those that have the "bump" on the flat metal bracket at the rear of the t-body; this bump covers the Motor Position Sensor) there are two connectors... find the one with 2 wires going to the black cylinder at the rear of the t-body and a 3rd wire going "somewhere into the bracket." Unplug the connector... but add your own jumper to re-connect that 3rd wire. That 3rd wire is the idle/nose switch input to the ECU.

 

Now go for a drive. With 2 of the 3 wires disconnected this way, the ISC motor is disabled - the ECU can't change the idle RPMs at all. If your idle RPMs stay constant when stopped versus rolling then you know your bug is something commanded by the ECU. If it still varies, it's NOT the EFI system.

 

Are you sure it's not the brake system screwing with engine vacuum? The power brakes use engine vacuum of course, as does the ABS claptrap. Leaks in those systems affect the idle... and brake booster diaphragms often fail by getting minor cracks in them... when you apply the brakes the pedal movement moves the diaphram, exposing (or sometimes hiding) leaks as the folds in the diaphram move. Also, a totally freaked-out ABS might be pumping away as you're "just rolling" enough for the speed sensor to say "we're moving" to the ABS. When the ABS pumps a lot, it uses a lot of engine vacuum. For testing, it's safe to unplug the fat vac hose going towards the ABS and plug both ends. If that cures the idle RPM variations you've got ABS issues. Do NOT disconnect the fat vac hose going to the brake booster for a test drive though - the pedal effort to stop will be WAAAYYY higher than you're used to... dangerously high. Instead, unplug the fat vac hose at the booster and instead connect a vacuum pump & guage to the booster (engine off). Apply vacuum to the booster and see if it holds (it should). Now have a helper push the brake pedal a little bit... re-test the vacuum, making sure it doesn't leak down again. Apply more pedal, make sure there is still NO leakage. If the booster ever leaks, it needs to be rebuilt/replaced.

 

mike c.

[shift1313]

thanks for the reply mike. ill try to drive it with on isc this weekend. As for the ABS, i have no abs in the system anymore. Im running the vac line straight from the manifold to the booster with that little filter unit inline. The abs module is completely removed but now that you mention all that of course the abs light is on. Maybe that in itself is telling the ecu to bump the idle to change engine vac?!?!?

 

if you look in this pic you can see the red line goes into that filter unit(is it a check valve?) and straight to the manifold.

 

http://i268.photobucket.com/albums/jj39/shift1313/Starion/EngineBayNov2.jpg

[FlattopMike]

One test you can try:

Locate the ISC motor connector. It'll be one of the connectors on the rear of the throttle body. On 83-86 cars, it'll be the only connector on the back; on 87-later cars (those that have the "bump" on the flat metal bracket at the rear of the t-body; this bump covers the Motor Position Sensor) there are two connectors... find the one with 2 wires going to the black cylinder at the rear of the t-body and a 3rd wire going "somewhere into the bracket." Unplug the connector... but add your own jumper to re-connect that 3rd wire. That 3rd wire is the idle/nose switch input to the ECU.

mike c.

 

Just an FYI: The 83 doesn't have an ICS. Or, at least my 83 doesn't. I only have a single wire going to the idle nose switch.

[Richie_Rich]

I can't offer any new suggestions, but when I had my '86, the first problem I had was similar to this.

It turned out to be the TPS/ISC sync.

 

During my intial tests, I would stop on flat road idling smooth in neutral.

I'd open my door and move the car with my foot. (This would take the vaccum draw from the brake booster out of the equation.)

It would start to do the up and down cycling after few feet or so.

Somehow, the ECU know when the car moves.

I'm guessing it the wire coming off where the speedo cable connects to the trans.

 

There is a speed senor type device in the truck in front of the spare tire, but I thoguth was an inertai sensor of some sorts.

[shift1313]

the sensor in the trunk is the accelerometer for the rear abs:)

 

im going to try something similar by using the ebrake and take the vac out of the equation:)

[mikec]

Hmmm... now that FlattopMike mentions it, I do remember reading the 83s used a different mechanism to control idle RPMs. Instead of an ISC motor they have something like an idle air valve. That's basically an ECU controlled solenoid to feed air into the intake manifold (i.e. past the throttle plates). The ECU opens/closes this thing rapidly, adjusting the ON vs. OFF time to control idle RPMs. If that's the case, these things sometimes get gunked up and need to be removed/cleaned.

 

And unless the 83s have another unqiue feature... there is no ECU feedback for ABS vacuum. The ABS claptrap is totally separate/independent. If there is no engine vacuum for some reason, the ABS will be useless.

 

There are multiple speed sensors on StarQuests. One is a magnetic switch on the back of the speedometer; there is a round ring attached to the spinning part of the speedometer that has 4 magnets. This pulse generator switch feeds the cruise control and might also go to ECUs. There is a true pulse generator on the output end of the tranny too.

 

mike c.

[FlattopMike]

Hmmm... now that FlattopMike mentions it, I do remember reading the 83s used a different mechanism to control idle RPMs. Instead of an ISC motor they have something like an idle air valve. That's basically an ECU controlled solenoid to feed air into the intake manifold (i.e. past the throttle plates). The ECU opens/closes this thing rapidly, adjusting the ON vs. OFF time to control idle RPMs. If that's the case, these things sometimes get gunked up and need to be removed/cleaned.

mike c.

 

 

Wow, if there is such a thing, I bet I tossed it out with all the polution control crap. The only idle control I know of is the fast idle that is activated by the coolant.

[shift1313]

just as an update over the weekend when i was trying a few things trying to get my IC setup working right i was still having this high idle issue. It had nothing to do with the brake pedal/booster setup. i tried it using the parking brake and it did the same thing. Sitting still the idle was great, let off the ebrake and starter rolling, feet nowhere near any pedals and not in gear and the idle jumps up. pull the ebrake to stop and it drops back down as soon as i stop.

 

crazy!

[jd_stazza_brendan]

If its like the 82-82 setup on Australian Starions the Dashpot (think thats what tis called) gets stuck and can cause idle issues. Have a loook on Austarion.

[Komeuppance]

My car idles up on decel in neutral... 1100-1200rpm or so when I'm moving and just about stopped.

 

As soon as I stop completely, the idle goes back to where I set it.

 

I'm pretty sure this is normal, and to keep the engine from dying on sudden throttle let off's, and accessories goin.

 

My car is an 86 BTW.

 

-Robert

[Szester]

I have owned a audi 1.8T, mustang 3.8l, and my 87 conquest. ALL of them do this, dont worry about it.

[Indiana]

disconnect the speedo cable under the car at the tranny and see what happens

 

loosen up the throttle cable some more

[Tim_C.]

Also, if it has a higher flowing air filter, just a little movement can increase air flow enough to bring it up like that.

[Technology]

I was reading through this really old thread, and saw that Richie Rich had posted, so I checked the date, and sure enough it's old as dirt.

 

Some old members who don't post anymore give it away. Tim, you have to be careful to only bump super old threads that have only members who still post. Just a tip.

[Tim_C.]

Yeah, thanks. It was already on the top, so I didn't think to check the dates. I wonder how it got up there?

[BlowinUp54B's]

It was already up, whoa.. I read it before Tim posted in it.

[shift1313]

what do you mean super old member:)