Where to measure exhaust back pressure 5.4L E-350

[lashidalgo]

I'm suspecting my cat is clogged so I got this:

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1


Now, where do I plug it to take the measurements?

[regis101]

Why do you think that?

Another thing to check is the throttle body is opening all the way if you suspect power levels are low

[lashidalgo]

Originally Posted by regis101

Why do you think that?

Another thing to check is the throttle body is opening all the way if you suspect power levels are low

A while ago I had a coolant leak in the intake manifold getting into the spark plugs causing cylinder 7-8 to misfire, as I was remote I had to drive the van up some sketchy terrain, a bad smell was coming out of the escape after a long slow climb...

I went to the dealership which replaced the intake manifold and many other parts (sparks, coils, gasket ....) after $2k the van was running normal, no misfire.

When they delivered it to me, they told me it feels like it doesn't have power and they suspected that the cat was clogged out of all the driving I did and the symptoms.

Since they wanted to charge me a lot more money just to get it back pressure tested, I researched a little bit and realized it wasn't rocket science if you have the right tool, so I decided to get away from that money hole and DIY.


So I've been driving it all of these days, after around 10k miles I can tell that even though the van has good low-speed power, when going on the highway @65 and hitting a climb it starts losing momentum, downshifting and trying to get to higher RPMs, this did not happen before the misfire. I'm not towing either have a heavy rig. I'm currently running 245 tires which is the max you can put in a stock.

I want to start with the low hanging fruit first, then go and check other parts since my mechanical knowledge is limited. Hopefully, I'll hit the jackpot.

[regis101]

Ok. Thanks for the added info.
My remark about the TB was from a YouTube vid about a guy who claimed a 20 hp increase due to cable slack. Well yeah

[carringb]

Easiest place to measure is the O2 sensor port. Just unplug the o2 sensor. It'll run fine, since it's only there to make adjustments to the fuel trim, but the actual A/F ratio is dictated by the MAF sensor. It doesn't matter which one, since the pipes go into a Y.

[lashidalgo]

Originally Posted by carringb

Easiest place to measure is the O2 sensor port. Just unplug the o2 sensor. It'll run fine, since it's only there to make adjustments to the fuel trim, but the actual A/F ratio is dictated by the MAF sensor. It doesn't matter which one, since the pipes go into a Y.

Do you know what are the ranges I should look at? From here I see:
https://www.aa1car.com/library/exhaust_backpressure.htm

<1.5psi at idle
< 3 psi @2000rpm

[carringb]

Originally Posted by lashidalgo

Do you know what are the ranges I should look at? From here I see:
https://www.aa1car.com/library/exhaust_backpressure.htm

<1.5psi at idle
< 3 psi @2000rpm

I have no idea. When I had some leaky collectors, it would actually suck in air most of the time, except under high throttle at low RPM (like yanking a trailer up a steep driveway).

[BUFFALO]

I have tested backpressure before and typically it is done at the before cat o2 sensor hole. Specs are likely different between vehicles but the numbers you have above look like they could be correct.
Also you can try banging on the Cat and see if it rattles. Then you know there is some breakup that can cause blockage.

[lashidalgo]

The cat was not clogged it marked 0 at idle then like 1psi at 2000k, Now I'll start checking other things, starting by the TB.


UNLESS is leaking the emissions through the pipes and there's no backpressure. Is backpressure needed at all? I've heard some engines need it to create a vacuum effect...

[boywonder]

Originally Posted by lashidalgo

Is backpressure needed at all? I've heard some engines need it to create a vacuum effect...

Gasoline engines create vacuum in the intake manifold when the throttle is closed from the pistons and intake valves sucking on the intake stroke across a closed throttle.

That's why vacuum decreases with throttle position all the way to WOT (wide open throttle) when the intake manifold is roughly at ambient pressure.

Some related trivia: Volumetric efficiency for a gas engine (thermodynamic cycle=Otto cycle) the maximum volumetric efficiency (100%) is at WOT (wide open throttle) when the intake manifold pressure is at 0 psig.

..so turbochargers and superchargers produce positive (greater than zero) manifold pressure (boost psi) and those engine's volumetric efficiency is over 100%....of course it takes energy to spin a turbo or supercharger but the net effect is more hp.

Diesels don't have a throttle (and use a different thermodynamic cycle, the diesel cycle) so they don't have the same losses in volumetric efficiency.