Hello All,

I am new to the forum so bear with me - I need some technical assistance. I should start by saying this involves a Jeep, but I figured it would likely reach the most people here and the engine is the part that I need help with, which began life as a 5.2l magnum v8. The engine is now a bored and stroked 390ci / 6.4l v8 with edelbrock heads, big gulp throttle body, MSD ignition, comp roller rockers, etc. etc. running through a 46RE transmission built by Martin Saine, and tuned by Ryan from Flyin Ryan Performance (originally tuned by hemifever but I was much happier with Ryan's work).

With that out of the way, here are some symptoms of the problems which have plagued this engine since it's conception in my garage 4 years ago:

1) While cruising down the road (say 40mph), if I shift the transmission in neutral with foot off gas, it does not return to idle but instead remains around maybe 2000-2200 rpm. In line with this, it tends to pull itself down the road slightly even when I'm not pushing the gas - not much but it is noticeable.
2) Accelerating from a stop the engine has a hesitation, sometimes more, sometimes less, but it has always been there. At most this is only half a second or so but it makes the Jeep feel "less than punchy," let's just say.
3) Even with repeated tuning and fuel trim adjustment at low rpms, the AFR likes to go lean at idle occasionally. This one is hard to describe because it's been largely overcome via tuning, but the times it happens would be when you come to a stop if you let it idle for a bit - it will slowly go into the red (eventually comes back after maybe 10 seconds). I can usually tap the pedal a couple times to try and train it (in my mind) and it will eventually come to find it's happy place around 14.7 but this is still a weird symptom.

So... I don't know about #3, but #1 and #2 seem to be dead giveaways for a vacuum leak (in my mind). As such, I have done a ~5psi homeade cigar smoker test into intake manifold and exhaust and only managed to find one insignificantly cracked hose. I've sprayed a can of starter fluid around every seam I could think of and never got any change in idle speed. So here I am, does anyone have any ideas? I want to finally get to the bottom of these gremlins! Is there something else that could be causing these symptoms or perhaps I should take it to a shop with a real smoker and get some professional opinions? Any help is appreciated. And yes, every sensor has been replaced with genuine dealer parts since the engine build.

 

What intake manifold you have on it? Still using the kegger? Is the plenum gasket blown??

What cam do you have in there? What does manifold vacuum look like at idle? Compression ratio?

 

I have a 390 tuned by Ryan also. Regarding #2 and #3, have you checked fuel pressure? I wonder if the fuel pump is degrading. I have a Bluetooth ODB2 device coupled with the android Torque app that allows me to read vacuum real time as well as other engine parameters read by the engine computer. I have seen similar issues that I've narrowed down to throttle body adjustments and aftermarket sensors. I'm running Mopar sensor now with no issues

Ken

 

I have an airgap intake manifold with a Hughes grind cam 218/228 duration about .550 lift both valves. Compression ratio should be theoretically 10.1:1, doing a compression test only yielded about 165psi from my memory though, I was told it isn't necessarily a direct relationship due to different overlap with aftermarket cam. I have never tested the manifold pressure because I wasn't sure what to do with that information - I know it's less than stock but because of different setup I want sure what I'd compare it to.

 

And I replaced the fuel pump for good measure shortly after all of this started and checked pressure at idle, that is exactly where it should be.

 

I do have a snap on scanner that I use to set the fuel sync. It can give me readouts from all the sensors from the OBD2... would any of that information be helpful in troubleshooting? Thanks ahead of time for the help everyone.

 

Did you adjust the throttle body idle settings? Per Ryan, the IAC valve response time is slower than needed with the added air/fuel strokers require. Here's the procedure and a better explanation copied from one of his posts.

Ryan Hogan
"Okay, here's the deal.
This goes for JTEC controlled vehicle, does not matter what engine it is.
When you add a aftermarket camshaft, and/or cylinder head(s) and/or a different throttle body, your airflow requirements at idle/off idle, change.
Additionally, the IAC needs to be open less, else you get throttle "hang" on deceleration.
Both of these can or will be affected by air fuel ratio as well.

This is the case for all TBs, F&B TBs specifically often need to be opened up (even though it says in the instructions specifically not to) using the idle set screw to add this extra airflow required.
This is because the IAC cannot move fast enough/doesn't have the resolution to compensate for this.
Usually, a 1/2 turn is a really good starting point, but should be adjusted specific to the vehicle.

The full procedure (if you don't have a scantool) to set the blades is this:
Start vehicle.
Let it come to full operating temp.
Shut vehicle off.
Disconnect IAC at pigtail.
IAC should be closed when key is off/engine is off. If not, replace with MOPAR IAC.
Start vehicle again.
Adjust idle set screw until idle rpm is ~200 rpm above commanded (650 commanded in most cases)
Shut vehicle off. Reconnect IAC connector.
Done."

 

Seems to me that process would have you idling at 850 with a fully warmed engine...... as the IAC can't reduce airflow beyond being completely closed.......

 

Logic would dictate that, but it doesn't. Not sure why though. Idles just fine at 600-650

 

I would dearly love to understand how that works........ Wonder if the PCM doesn't actually close the IAC completely when you shut the engine off....... or something similar.