hot potato - 2016 5.7 hemi rough idle stalls

Hello wrenches

I received a truck to go over a few months ago. as of now I am waiting to hear back from Star Tech assistance who is waiting to hear from their platform advocate. I am a tech in a rural FCA (stellantis) dealer and this truck has been to two other paces before it came to me.

I can dig up some more info to add to this thread late but here's the basics. Customer started the truck one day to find it had a rough idle. The took it to an indy shop who put in lifters and a cam only to have the original problem unchanged. It then went to another fca dealer who sat on it and put in a pcm and found no solution.



It seems to have a junkyard intake but I could not find any sign of an intake leak and we did put intake gaskets in it.
No sign of any contamination on the OCV screens.
I have confirmed that the cam and phaser are aligned properly.

I found it had map sensor wiring was messed with but checked out ok. The biggest problem I could find is the MAP sensor reading was up at about 50 in. mg. but on my manual guage it reads about 20 in. mg.

Other than that I haven't found anything wrong. I just checked the pcv hose seems to have excessive vacuum and when I block it off it stalls. Is this normal? we are going to check another truck to see if does the same thing.

any ideas would be appreciated.

thanks

What are your fuel trims at idle no load?

The biggest problem I could find is the MAP sensor reading was up at about 50 in. mg. but on my manual guage it reads about 20 in. mg.

20 in-hg is a good number for the manual gauge. But 50 in-gh is way to high at idle for the PCM.

In Global OBD what is your BARO PID with Key-on?

BARO PID minus the MAP PID will give you the intake vacuum as you guage reading would be.

So if you BARO was 29.2 in-hg and the engine MAP pid at idle was 9 in-hg. 29.2 - 9 = 20.2 in-gh is the vacuum in the intake and this should match your manual guage.

So if your MAP is 50, your engine thinks it's under extremely heavy load and if the Oxygen Feedback system is working correctly it will be subtracting fuel on your fuel trims. So your rough idle could because of this situation. I've never seen a MAP pid that high lol.

As far as the Vacuum on your PCV hose it should be minimal. There shouldn't be any vacuum in the crankcase, very little.

Slowly block off that pcv hose and look at the scan tool to see if it reacts the fuel trims.

I had one that had a broken fan blade made it feel rough throwing no codes.

I found it had map sensor wiring was messed with but checked out ok. The biggest problem I could find is the MAP sensor reading was up at about 50 in. mg. but on my manual guage it reads about 20 in. mg.

Do you mean 50 in. mg., or 50 inHg? Mostly wanted to clarify. While the MAP is reading 50-whatever, what is the MAP voltage indicated on the scan tool?

I agree with Paul that if the MAP reading was truly that skewed, there'd be a significant fuel trim problem. You didn't mention it, but are there any fault codes? Has this hot potato been tuned (that you can tell)?

I haven't checked that easy enough. I'll check thanks.

Hi. Yes that was a typo it is in. Hg.

What I was told by the tech assist gut. I have a "star case" open and he says the scan tool is not displaying correct and just go with my manual Guage. The voltages match the reading on the map. If I let it run it will set rich codes.

The barometric pressure reads 100 continuously which I thought was funny but I checked a good truck and it did the same.

Fuel trim is crazy. It jumps from -21 to 157.

I can add a screen shot of my flight recordings to show map and o2 data later.

The other thing is that blocking the pcv hose to the intake causes the it to stall. While blocking the pcv hose to a known good truck does not cause it to stall. We're going to try a good throttle body...

Also I don't beleive there was a tune. We reset the adaptive anyway.

I'll see what the fuel trims do with the pcv block off but I checked another same truck and it doesn't stall when I block it off. I was suspecting possibly bad on sensors but it ran the same with them unplugged. I'll add some screen shots of o2 and map data below. We're going to try a known good throttle body I think. Will update thanks

Have a look at the map value(s) in generic OBD2.