What in the PCM determines where to idle?

[94-6spd]

Car starts and idles perfectly but any rpm change such as a blip of the throttle or slight rev will cause it to bypass the desired idle rpm. It'll just keep dropping rpm down to zero. We've thrown almost 30 idle revisions at it and nothing changes except spending more money on sensors to try. I've tried IAC, TPS, and map sensors. We changed the TB and even the injectors. Although it shows no codes I'm concerned the cam or crank sensor could be causing an issue.

My question is what sensor makes the pcm know where to stop dropping rpm as it's coming back down to idle?

[5FDP]

Post tune and logs.

[blindsquirrel]

https://forum.hptuners.com/showthrea...g-back-to-idle

[94-6spd]

https://forum.hptuners.com/showthrea...g-back-to-idle

We've done all of that. Nothing helps, I'm trying to think out of the box here and figure out where else to look. At this point there's no way I think it's in the tune. That's why I'm looking at other things like other sensors or pcm.

[94-6spd]

Post tune and logs.

No need to at this point. We've tried unconventional things to try to correct this issue so looking at the latest logs would possibly cause confusion. I'm concerned with sensors or corrupted files being the cause.

[blindsquirrel]

The only sensor that tells the PCM the engine speed at idle/return to idle would also cause it to not run at all if it were causing an issue. Cam position sensor is ONLY used during cranking, to determine #1 compression from #1 exhaust, since those two positions are identical as read by the crank position sensor; and for misfire detection after it's running (same deal, it needs to know which half of the crank's 720 degrees it's in before it can know which cylinder is firing). There is no 'idle sensor' you can parts-cannon at it and fix your problem.

That was your thread asking the same questions I linked to.

[Matt Vardaman]

Post tune and logs.

If you would do this we could help.

[5FDP]

Put a stock throttle body back on it. See what happens.

[94-6spd]

Put a stock throttle body back on it. See what happens.

It has a stock TB on it now but doing same thing.

[blindsquirrel]

"Doing same thing" means nothing if no one else can see what you see. It's along the same lines as "it runs bad how can I fix it" or "my engine light is on what part do I need to replace". This thread makes my head hurt.

[94-6spd]

"Doing same thing" means nothing if no one else can see what you see. It's along the same lines as "it runs bad how can I fix it" or "my engine light is on what part do I need to replace". This thread makes my head hurt.

I'm not sure you understand my question. I may have worded it wrong but I'm not a computer guy, in fact more of a mechanical guy and wanting to diagnose this issue. I'm 100% percent sure it's not a tune related issue at this point. Here's what happened last night after smoke testing the intake, installing new cam and crank sensors. There's a 3/8th vacuum hose ran from the passenger side valve cover to the port behind the TB on the LS6 intake. I was suspicious of this hose because I had pulled the oil filler cap off and it was sucking in a ton of air. After plugging both ports the problem disappeared. The engine will rev and come right back down to idle. What doesn't make sense is searching online I've found that most have this hose going to the TB. Why would this matter where it gets the vacuum from? I understand that the tb port is in front of the tb blade but there's still a large opening above the blade pulling the same vacuum.

[blindsquirrel]

Manifold vacuum should go to a PCV valve in one valve cover. The PCV meters the amount of air that can be drawn out of the crankcase. Then a fresh air hose from the other valve cover to a point between the TB and air filter. Blocking both hoses seals up the crankcase and that's Bad News.

[Carlfromkerikeri]

The right hand rocker cover line should be a ?fresh air? side of the ventilation system where it allows a controlled amount of fresh air into the engine, and the left hand rear rocker cover has a line that goes to the intake manifold (center left side) as the ?dirty air? side of the system to draw any crankcase oil vapour etc into combustion. Check that all lines are where they should be then post your tune and log for people to review and help.