Start and tach drops below 500

[Izzmo]

So, I have been having this weird problem lately. No matter if it is a cold or warm start (does not do it everytime, but just about everytime) the car will start and then immediatly on the way back down it will drop below 500 rpms and come back up to normal parked rpms. It does not ever die...

It's weird because sometimes you can feel how slow the rpm's are cause it waits forever till that next spark to get back in action.

This only happens at startup, after that it runs like a pro.

But anyways, I have cleaned the MAF and IAC sensors.

I have searched countlessly through the forum and haven't had much luck.

Could this be a crank sensor? Or maybe the TPS?

Thanks for the help.

 

[SHOZ123]

Get a new IAC. Or check out the TPS voltage.

 

[Izzmo]

How would you check the voltage on the TPS? Take the sensor out, start the car and view the voltage across the pins?

** Edit

Just did a check on RockAuto, guess Motorcraft still makes the IAC's. Cost me about $50-70. Not too bad I guess..

 

[SHOZ123]

To check the TPS you can stick some small wire in the back of the connector and measure the voltage. It needs to be on the car.

One wire is the 5V in , another is ground and the other is the signal return which is what you want to measure. It should be ~.90v to .98v with the key on and throttle shut. ~4.85v at WOT. As you slowly open the throttle all the way the voltage should rise steadily with no sticking points or sudden changes.

 

[Izzmo]

Thanks for the tip, I will check this out.

 

[Izzmo]

Alright, so how the **** are you supposed to stick a wire in there? The plug is pretty sealed and the rubber goes into the plug to where I cannot strip it back... or am I just ********?

 

[SHOZ123]

I use 20 ga solid wire like found on fluorescent light ballast. You just poke it in the back......

Like this

 

[Izzmo]

lol alright, let me try that.

 

[Izzmo]

Well that, my friend, is a big PITA. So, I just dc'd the plug, and routed wires from the sensor to the plug and bam! Problem solved.

Seems not to be my sensor though.. so I'm guessing it's the IAC? Is there any way to test that with voltages?

 

[SHOZ123]

You can data log the IAC Duty Cycle and Idle Interrogator. But that does not mean it is what it is doing.

 

[Mr Anonymous]

You mention you're running a tune, any chance you're running with wired-open IMRC's?

 

[Izzmo]

They are not wired open, I gutted them all together. Are you saying this would affect it?

 

[SHOZ123]

I could add some rpm at startup for a few seconds with an update.

 

[Izzmo]

I'm going to go check my voltage really quick again on my TPS and let you know. If it still comes back fine, I think we should go ahead and do that.

 

[NebraskaSHO]

I have .16 V either closed or WOT with the key on. Will check again tomorrow.

 

[Izzmo]

I don't understand how you guys check that so easily.. it's a PITA for me to check that

 

[SHOZ123]

Stick a piece of 20ga solid wire in the back of the connector.

 

[Izzmo]

I tried that.. I tried 22ga as well.. the little bastard wouldn't get in there!

 

[SHOZ123]

 

[NebraskaSHO]

Ok you stick it in the top wire plug correct? I got .21 closed and .22 open. Used an aluminum twistee tie stripped of the plastic insulation. Maybe that skewed my readings?

Paul, PM your email so I can send you a short video I took if you want it.