Odd rough idle vacuum leak. Help...

[ponkotsu]

Seems like this keeps getting worse. I've got a rough and hunting idle. I've checked everything:

Plugs
Coils
Spark
Vacuum leak
O2 Sensors
Cleaned throttle body
Cleaned intake. Upper and lower.
New OEM IAC
PCV system is functioning correctly

So today I connected to Scan XL. With this tool I was able to confirm that my O2 sensors are all working correctly.

But based on troubleshooting theory, it says I've got a pretty good vacuum leak. Long term fuel trim is 10 - 20%. When I raise RPM to 2k, the long term goes to zero and short term goes high. It's got pretty equal percentages on both banks. That's text book intake vacuum leak. So I smoke tested again. And again. I've put hose clamps on everything that even seemed like a leak.

Only thing left is EGR. It is leaking, but only very slightly. From videos I've seen on youtube, the small amount of leakage is pretty normal.

I did the garden hose test also. I was getting what I thought was a reaction at the rear of the left cylinder head, but it was hard to tell. Can't imagine anything leaking there other than the head gasket, and that would only cause a lean condition on one bank.

If anyone has any ideas of where a vacuum leak could hide let me know!

 

[Sixonemale]

It does sound like a vacuum hose leak. Though it does not sound like your problem, way back when I solved a rough idle by changing spark plug wires.

 

[ponkotsu]

One thing I thought about is that a smoke tester may not always find vacuum leaks because it applies positive pressure. Some leaks are one way. For example you might have a loosely fitting PCV hose that leaks under smoke (pressure), but actually seals adequately under vacuum because the hose is squeezed onto the fitting. Thinking it's time to get out the carb cleaner and go old school on this thing.

 

[ponkotsu]

This is a decent video:
https://www.youtube.com/watch?v=WikqVH_QrTA

Using old school starting fluid and watch the O2 sensor. This guy says that smoke machines are only good for finding the big leaks.

 

[tubeless]

I hope I am rong about yours, but it sounds exactly like the probem I was having with mine exactly.
Turned out to be the timing chains. I did all the trouble shooting that you have described.
04 4.0 v6.
I took it to a Ford mechanic and he figured out what it was. He is a Ford mechanic with his own shop.
I also did a lot of reading on here and had an idea of what it was.

 

[ponkotsu]

I hope I am rong about yours, but it sounds exactly like the probem I was having with mine exactly.
Turned out to be the timing chains. I did all the trouble shooting that you have described.
04 4.0 v6.
I took it to a Ford mechanic and he figured out what it was. He is a Ford mechanic with his own shop.
I also did a lot of reading on here and had an idea of what it was.

I'd be surprised if its a timing chain issue. Runs perfect at speed and gets normal fuel mileage. Nothing really indicates timing chain issue. And this is a totally different engine than the 4.0. I don't believe it's prone to timing chain issues.

 

[tubeless]

I didn't see any engine specs.
Glad it's not the chains.

 

[ponkotsu]

At this point I have to say there is no significant vacuum leak. There are only 3 vacuum ports on the intake and I systematically pinched each off with no effect on engine performance. Even removing a small vacuum line entirely and creating a leak does not have as much effect on the long term fuel trim. I do have tons of vacuum so I don't suspect a worn out engine or timing chain issue. I'm just starting to think that up to a 10% fuel trim is normal for this vehicle. Ford's spec is +/-20%, which seems really high. Still the EGR is a bit questionable, but I'm not going to start throwing expensive parts at this without some confirmation.