Steady low vacuum

2006 Honda Odyssey
3.5 (j35a7)
233k miles
No C.E.L.

Feels like a misfire at idle
Has a torn rear mount
Vacuum reads steady 10”hg
Valve timing marks are right on.

I noticed the muffler was crushed at the front and back

Vehicle drives well and seems to have lots of power

My question is what could be causing this?
If a cylinder is dead will it pull vacuum down to a steady 10”hg?

( I have not performed a compression or leak down test, but it sounds fine when it cranks)

That is very low vacuum.

I would suggest performing a exhaust back pressure test. ScannerDanner has several examples.

But, I think on Hondas you'll have to run test via the pre-cat A/F sensor. You know, you'll have to remove it, run engine, and get back pressure reading at port.

What are your fuel trims?

Oh. Wait..

I missed the line where you said vehicle has lots of power.

I would check my fuel trims and see if they determine there is a vacuum leak at idle. I would also test my MAF.

I agree, fuel trims at idle would be good to know. I'd also check the MAP at idle with no loads, see if it agrees with the vacuum gauge reading. Good engines will do 19-20 inHg, or less than 1.00V.

Valve adjustment is suspect, especially at over 200K miles. That'd account for the misfire and the low vacuum at idle. If the misfire is noticeable at idle, then you might be able to catch it on a relative compression waveform. That'd be strong evidence for pulling the intake and valve covers to inspect valve lash.

Fuel trims look fine all across the board.

My question is if the timing marks are right on why would the vacuum be holding at 10”hg?

I did a back pressure test on the manifold converter and it was fine.

Was wondering if the intake butterflies would restrict the vacuum and if anyone has ever dealt with these before.

Really scratching my head here as to why the vacuum gauge is indicating late valve timing?

One other thought!

This vehicle has vcm which I think is cylinder deactivation mode. If this is malfunctioning and causing some cylinders to shut down then engine vacuum would be low correct?

On the 2006 model year VCM essentially disables one bank of the V6 at idle, it is known to cause engine mount and vibration issues. Idle manifold vacuum is essentially am upside down measure of how much air the engine needs to maintain RPM so less vacuum (higher inlet manifold pressure) makes sense if 3 cylinders are cut out at idle and the remaining active cylinders are doing the work of 6 even so 10" hg would seem low --- but I haven't encountered this system as UK models don't have it fitted.

From what I gather the VCM only becomes active when the coolant temperature is fully warm based on the coolant temperature it reads from coolant temperature sensor ECT1 the system can be rendered inactive by inserting a resistor in series into circuit to drop the temperature read by the ECU by around 15 degrees. This of course may not be street legal where you live.

A Haywood84 wrote: One other thought!

This vehicle has vcm which I think is cylinder deactivation mode. If this is malfunctioning and causing some cylinders to shut down then engine vacuum would be low correct?

I'm fairly sure you'd have some P3XXX codes if that were the case. You could confirm this with a cylinder drop test across all six cylinders. If there are any weak cylinders, they'll show up that way.

At 10 inHg, this thing should be barely idling. :ohmy: A stuck open EGR could cause that, but you'd also have rich trims and a P14XX code. How/where are you measuring intake vacuum?

I've never know the intake manifold runner control systems to fail on these engines, but I guess it's possible. Most IMRC systems like this won't cause driveability issues when they fail.

IVE taken vacuum readings at every port on the intake manifold.
Thanks for the replies and I’ll just drive it and see what I find then post a solution soon hopefully.