My idle has been crap for a few weeks now. Would hardly sit a 900 but bouce around from 1000 and 2000 rpm. Occasionaly the SES light would come on when the car was stopped. But I'd start driving and it would go out. Im assuming it didnt stay on long enough for it to be stored in the computer, as I have tried to pull the code to no avail. Well today I went out with the T/B cleaner and clean the T/B and the IAC. And when on a test drive, but the problem was not fixed. I was able to get the light on long enough to get the code -24 (VSS). First off.. What does Vehicle Speed have to do with idle? Secondly...My Haynes book says code 24 only applies to V8 motors, WTF?
Also, where is the VSS. I believe its on the instrument cluster behind the spedo. Is that right? Any help would be greatly appreciated.

 

What year is your car? mines an 89 and its on the trans. BTW..i completed a t5 swap and dont have a VSS right now and mine doesnt idle (when it did) very well either.

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1989 Firebird
2.8 v6, t5 swapped in

Ram Hi-po clutch, Cold Air, 1.6 rockers,Lakewood LCA's, Lakewood Lift bars, Wonder Bar, 3" y-pipe, Random Tech Cat, 3" I-pipe, Flowmasters, MSd 6a, MSD Coil (Fireball in future) Accel 8.8 wires, MSD Ignition Module, Auto Meter Gauges (Water, Clock (needed one) Fuel Pressure, and 5" monster tach) 4th Gen Seats
--For Sale--
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" I'd rather run last in a full out race, than to NOT run at all ".

 

well the VSS is a fundamental thing that is the final output of the comp. It tells the comp how fast you are moving. It also controls the speedo...Look at the speedo and if it does not register then it IS bad. The VSS itself is in the computer chip so...
My haynes is somewhere in my garage buried at themoment so I can't help there but, if the egines idling funny first check the timing...though it does not sound like thats your prob. It could be the IAC(idle air control valve) mine acted up for arounf 2 months then worked itself out after a long and hard highway drive...a 988 mile trip doing 80 plus... it went away then... but I did learn that the parts stores know absolutely nothing about these things... I tried to replace it and went through 4 of them and still did not get the right one. I got one from the dealer and the problem went away for 2 days then came back so it was not the IAC. And the part went back.


Ok a question...does the engine surge up and down or go there and stay?. If it surges then it may be the timing. If it stays then its the IAC.
I am off tomarrow and will call a friend up at the dealer and check on that code for ya...

 

It's an '86 MPFI 2.8 w/5-speed manual

 

Sometimes it will surge between 1000-1500 or so and sometimes it will stay put at 1500. When it surges, it is more likely to turn on the SES. Also my spedo is cable driven...

 

well mine is a 92 3.1 5 speed and I have don't have a cable. It sounds like its in the tranny but I don't know for sure. I would think its near the drive gear for the speedo. But as I said I will ask about that code tommarrow...ok so 86 MPFI 2.8 5 speed... code 24...

 

hey greame you try those shifts yet?.....

 

Huh? Im lost..What shifts?

 

The VSS is not in the computer chip. The computer chip uses the VSS signal, perhaps that's what was meant.

Luke, on your cable-driven speedo, you're right- the VSS is the yellow box on the back of the speedometer. Our speedometers can work when the VSS system is bad. Code 24 does apply to the V6 motors. As to what's involved with that code, I don't know off hand. If nobody comes up with anything, I'll post info on it tomorrow.

If you cleaned the IAC and the idle's still poor, maybe your TPS is out of adjustment. Tom K wrote up an article about adjusting the IAC and TPS on the tech board. You probably don't have to play with the IAC or minimum idle speed adjustment screw, just check out the TPS. You'll need a digital multimeter ( http://www.harborfreight.com has some cheap digital guys ), a Torx screwdriver, and some wire to make "jumpers" with. Don't follow the spec listed in the tech article, V6's have a slightly different spec. Our 2.8's use a spec of 0.55 volts +/- 0.05 volts.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
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the chirping 3rd...???? I noticed you were in the other thread...or is yours an auto?..

 

well I talked to the guy at the dealer and he said that the VSS should not make the engine idle funny but it is not uncommon for it to happen...So that was a big I dunno from him... He said those are alot of throw parts and see what happens.
I would guess that the computer could get confused and run eratically. But you would think that it would see the O2 readings, the MAP/MAF readings and see the car is not moving and not have a load input from the TPS and correct itself. But then again who knows GM did make these things...

 

Luke, here's the info! The VSS mainly deals with the TCC lockup (automatic transmission), so it shouldn't be affecting your idle. Code 24 sets if all of the following happen, for at least four consecutive seconds. These conditions normally occur (besides #1, of course!) during a "road load deceleration"...

1. CKT 437 (VSS line) is constant
2. Engine speed between 1400 & 3600 RPM
3. Less than 2% throttle opening
4. Low load condition (low air flow)
5. Not in park/neutral (auto trans only)

Diagnostic? First, verify the speedo works (you did), then raise the drive wheels off the ground & block the fronts. Cruise control should be off. Backprobe the ECM connector for VSS signal, CKT 437, with a voltmeter to ground. CKT 437 is pin A10, a brown wire, at the ECM. Start & idle the engine in drive. The voltage should be varying between 1 and 6 volts.

Less than 1 volt? Check CKT 437 for a short to ground. If okay, check the ECM for a bad connection. If okay, you've probably got a bad VSS.

8 to 12 volts, and steady? Check CKT 437 for an open. The ECM sends out a 12 volt signal on CKT 437, and monitors it. When the VSS rotates, it pulses a "ground" on CKT 437. So if there's a break in the brown wire, the ECM is only putting out 12 volts, and not getting the VSS signal.

1 to 6 volts, and varying? Well, this is the normal condition, so you've got an intermittant problem. Check the park/neutral switch out if you've got an automatic, and also check for a bad ECM connection.

If you've never checked your TPS, it could be reading too high (or too low) for an idle condition. Too bad you cleaned your IAC system, I would've said to do that.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
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Thanks, Tom, for all the info! Ill have to hunt down a digital volt-meter and get to work!

 

Sounds good! Definately let us all know what you find! If you're after cheap, check out http://www.harborfreight.com for multimeters... I got my cheap-o for $7. It does the job for now!


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
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