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Discussion Starter #1
Hello! I went to move my 96 yesterday, and when it started, the rpm kept rising to around 3k and stayed there. I can't drive it now, because I don't want to neutral drop it at 3 grand.
I swapped out the iac valve with two other known good ones, which didn't help. It also now won't start without throttle now. Any ideas?
 

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Check the throttle return spring. I worked on an LT1 recently where that spring had somehow come off.

Otherwise you have a hellacious vaccum leak. Like the brake booster hose cracked or something.
 

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Discussion Starter #3
The throttle touches the adjustment screw when closed, and I couldn't find any vacuum leaks. I think its the connector/ wiring, but I dunno.
 

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It's not going to idle that high without air. Getting it somewhere. Is the IAC being commanded to be that open?
 

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Discussion Starter #5
I don't think so, but if I cover the IAC port while it's running, the idle drops.
 

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The PCM does not get position information back from the IAC. The PCM just counts up/down and hopes that the IAC is moving in the direction it is ordered to move.
Did you try a IAC reset with any of the other IACs?

Reset the IAC valve pintle position.
Depress the accelerator pedal slightly.
Start the engine.
Release the accelerator pedal.
Run the engine for 5 seconds.
Turn the ignition OFF for 10 seconds.
Restart the engine.
Check for proper idle operation
 

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Discussion Starter #7
I tried to reset it via the procedure above, with no change.
Is there a way to check the wiring?
I've had a few issues with connectors and wiring since I've had the car.
 

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How are you covering the idle port while it's running? Are you running with the MAF disconnected and no intake on the throttle body? Exactly which hole are you covering?
 

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Discussion Starter #9
There is no difference whether the maf is plugged in or not. So while it was running with the intake off, I covered the iac port between the butterflies with my finger. The rpm drops to the point of almost dying.
 

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Discussion Starter #10
Figured it out, it was the dang TPS. Having to give it throttle to start was the giveaway.
 

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What exactly about the "dang TPS" was causing it?
 

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Discussion Starter #12
Not entirely sure, I just replaced it with a good one and it started immediately and idled perfect.
Is there a way to bench test the tps?
 

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I don't have my manuals here, but it will be a voltage or resistance value change when actuating. Someone check the FSM. There will be closed to open sweep value.
 

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I don't have my manuals here, but it will be a voltage or resistance value change when actuating. Someone check the FSM. There will be closed to open sweep value.
The TPS voltage when idling should be somewhere between .3 and .9 volts according to the FSM. Lower than .23 volts and you will have a MIL. Over 4.8v you will have a MIL. I could not find a chart in the FSM that detailed the resistance of the sensor. A scanner would be able to show the TPS voltage.
 

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Discussion Starter #15
The tps fell apart while I was handling it, I guess that confirms it!

I happen to know that it was a cheap replacement anyway.
 

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The TPS voltage when idling should be somewhere between .3 and .9 volts according to the FSM.
Although this is technically correct, to be more precise, 0.5V - 0.7V is preferable.
 

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Although this is technically correct, to be more precise, 0.5V - 0.7V is preferable.
Personally mine is at 0.67V at idle and 4.48-4.5V WOT. I slotted my TPS sensor to adjust the idle voltage.
 

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Discussion Starter #18
How is voltage being measured?
 

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How is voltage being measured?
With the OBD1 you can use any one of the free scanners. Freescan or $EEHack

Get a cable that you can plug into the OBD port. I see you have an OBDI swap, so make sure you get a cable that plugs into the OBDII port that is pinned for OBDI.
Check out the cables from OBD Diagnostics. They have the 16 pin cable to use with the OBDI PCM.

Later,
Michael
 

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Discussion Starter #20
EEhack shows TPS info in a percentage, 0 to 100.
 
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