Good post. Another way to look at an exhaust leak is that the o2 sensors sample the exhaust gases. For accuracy of the sample you'd want the entire sample and not lose part of it to a leak. Also, in a non-leaking system the sensor knows which cylinder the exhaust gas is from as it passes by. With a leak in the system this could lead to inaccurate monitoring of a cylinder or CEL's that try to pinpoint a particular cylinder.

 

The purpose of exhaust systems is to route exhaust out of motor......efficently.

One of the ways exhaust systems do this is vacuum. For instance headers try to optimize exhaust pulses to create more vacuum in the non-active tubes. Among other things. This in turn helps "suck" or pull exhaust away from exhaust valve.

It's basic dynamics of flow. You can see the basic principal of physics in a devise that is a air pressure driven vacuum pump.

Granted the OEM manifolds are not optimized like headers, but they follow physical laws. As air passes an opening it creates some vacuum. If my memory serves it has to do with friction between molecules of air. An extreme example is the wing of an airplane. Pressure on under and front of a wing will create a vacuum on upper and rear of wing. Or just stick your hand out the window as you go down the highway. You will notice pressure on 1 side and vacuum on another. Another extreme example is a tornado that will suck farm animals up into it. Fast moving air causing a vacuum that sucks things into it.

So, just as the pistons push exhaust out of the cylinder very fast creating pressure in that exhaust tube, that pressure is directly corrulated to velocity of that pulse. That pulse moves past several closed exhaust valves which creates vacuum pulses somewhat porportnal and opposite the pressure. This is why manifolds are built to alow exhaust to flow in 1 direction easily. If they were not the exhaust pulse would dead head on the next exhaust valve and create excessive turbulent flow reducing efficency as no vacuum would be created. Even turbo chargers can cavitate which is vacuum in a pressurized system.

I'm not even considering timing and if the exhaust valve is still open for a micro second as the piston starts moving down and sucking from exhaust.

So, I do think vacuum is produced in the exhaust manifolds/headers, the exhaust pipe is a different animal and the pipe will be under a constant pressure.....that is until it passes another opening like a necked down wye pipe.

Turbochargers are different as they create substantial resistance to flow that can negate vacuum pulses due to excessive pressures, but that a whole different discussion.

 

Your concept of this subject is flawed because exhaust does not flow in a steadily pressurized stream like a water hose. As each exhaust valve opens there is a pulse of exhaust pressure.

Yes, it is a fact that a leaky exhaust manifold gasket or a cracked manifold can affect the O2 sensor readings. Like the saying goes, science doesn't care if you believe in it or not. For proof positive, let your vehicle idle then go out back and place your hand over the tail pipe.

 

In short… I’m not sure. I do feel like the answer is yes but it’s certainly not severe. I wish I had a second truck to compare it to.

 

Good points guys and definitely thought provoking. I agree it’s more complicated than I made it sound. Pulses and cavitation and sensors not reading the correct cylinder are things I wasn’t considering. So fair enough I’m backing down a bit.

But not completely. Tornadoes and airplane wings - decent examples but they are in open air. But we’re talking about a cast iron (stainless steel?) log with openings that bolt up to a cylinder head. we’re talking about an engine which is computer programmed to use as little energy as possible while producing the desired result. Not barometric pressure building up to form a funnel cloud.

Nor is our discussion comparable to a turbocharger which spins at 80,000+ rpm with the goal of doubling or tripling airflow.

All the science theory aside, (because none of us actually know) I drove my 07 E350 v10 more than 200,000 miles with increasingly leaky manifolds and the only time it ran bad was with the cat(s) plugged.
Now if you folks want to theorize that the broken studs and bad gaskets caused the bad cat(s) then I suppose that’s a bit of a gear change for the conversation.

The OP came hear hear talking about steady pinging at highway speeds. I’ve had that. I’ve also had leaking manifolds. The FIRST DRIVE after removing the cat(s) removed the pinging and low power. Go check my post history from 2015/2016; undoubtedly I posted details when it happened.

But now the OP has scheduled his truck for a $600-$800 repair for his exhaust leak, and he’s STILL GOING TO HAVE THE PINGING when complete. Because of the advice he got here for a pressurized gas tube that’s somehow magically sucking outside air from a leak which makes noise when the opposite thing happens.

OP; tell your repair shop to inspect the cat(s) while they have your truck. Use a borescope (when cold) if possible. Or fire it up and check the inlet and outlet temps of the pipes at each cat. Or just hit it (somewhat gently) with your fist or a rubber hammer and listen for catalyst rattling. Or do that vacuum test mentioned by the other guy in this thread.

 

I needed to replace the manifold anyway, so it’s not a total waste. It would be nice if both need to be done to do them at the same time, probably some labor savings in doing so. I may be able to spend some time messing with the truck this weekend. It’s an occasional use vehicle for me, so it’s easy to defer maintenance! But I also want it to be dependable.

 

Ok, I will not argue furather I will just comment on a couple things.

1) the O2 sensors cannot detect individual cylinder a/f ratios. Each sensor is responsible for a bank. With exception of a 3rd sensor if equipped that is after the cat to determine if cat is working ok.

2) odds are that a verity if issues are are responsible for the pinging. Top of the list is high % ethanol in fuel. Next is a vacuum leak. Then poor injector function. Than leaking exhaust manifold.....which would actually help a tad if cat was plugged. Then I would look at the MAF and just put a new one in. They are cheap, very important items. They tell pcm the amount of air coming in and how hot or cold it is. If IAT sensor is bad on it than goofy things can happen.

3) Could a correctly functioning exhaust manifold fix this issue.....yes. probability is 30% in my opinion. Will a good exhaust manifold improve engine performance.....yes. probability 100%.

I always say to preform maintenance on motor prior to trying to trouble shoot. I have been slapped around for suggesting this so I don't anymore. This leads to alot of guessing, from everyone, as any number of issues could be the cause.

A simple vacuum guage hooked to intake manifold will tell you alot. Including if cat is plugged.

New factory cat is about $2000, small aftermarket ones are cheaper but may not work as well. So, confirming the cat is bad 1st is recommended.

 

Ok this is great.

Here is what I would recommend, depending on what you can afford.

1) new Denso coils and new spark plugs. Fix any coil clips that are broken.
2) remove injectors and put refurbished and tested ones in. Fix and injector clips that are broken.
3) put a new MAF in
4) put a new IAC in
5) new air, and fuel filters
6) new vacuum hoses and pcv
7) load test battery and alternator replace as needed
8) put a new accessory belt on.
9) change oil and filter
10) pressure test coolant system for leaks. Repair as needed.
11) make sure thermostat is proper one. 185 degree for 2V and 195 degree for 3V. This is important because of MAF to pcm air temp corrections.
12) clean the throttle body
13) check for vacuum leaks, expecially at brake booster.


Once done if ping is not fixed, look at knock sensor and wiring. This sensor is supposed to detect knock and retard motor so it will not ping. It may be having issues.

 

with all due respect...that list is insane! No way would I throw parts at it like that. Maybe plugs or a fuel filter, but definitely not the coils oeninjectors unless they test bad.

 

I knew that I would get beat up over my post.
But it's my belief that it's not throwing parts at it. It's maintenance. Just like an oil change. I change my oil every 3 to 5000 miles. I don't test it to see if it's bad before I do.

As OP asked and stated he wants a dependable vehicle and the list I gave was in my opinion the maintenance needed for that, if it hasn't already been done.

Plugs and coils recommended change out is 100,000 miles.

I try not to wait for things to fail and then fix. That could damage motor or engine parts. For example a shorted coil can take a pcm out. This will strand the vehicle require time and alot of $$ to fix.

I realize this is not most people's opinion but its what I do and it's what I recommend if possible.

 

Ya know I don't automatically dismiss your list of pre-repair procedures at all Rock---if that works for you and you're comfortable with it that's the only opinion that really matters. Too often we've seen problems that were or could be cured with basic maintenance so there's a lot of wisdom there.

Like you my oil & filter is changed every 3-5K miles. I'm too old to believe even the "best" motor oils in gasoline engines is good enough to leave in longer than that.

I do disagree about plug changing though---I prefer 50K miles along with thorough inspection of the boots, COP's and chassis wiring harness connectors. Again I'm too old to think plugs don't lose enough efficiency at 50K to not offset the minor expense that is.

Good posts here---interesting conversation!

 

You need to stop using the term vacuum and instead use the term pressure differential. It makes more sense that way.

 

But not entirely correct. There is a pressure differential but some are negative pressures, which are vacuums.


Below is from:
How Headers Work

The inertia from the escaping exhaust gases moving through the header pipes creates energy pulses that form vacuums, pulling additional exhaust out of the combustion chamber so that the maximum amount of fresh fuel and air can be drawn in. The more fresh fuel and air you can pack into the cylinder, the more power the engine can make. The less energy the piston has to use pushing exhaust out of the cylinder, the more it can use for something fun, like moving you down the road.

 

I've often heard the term "scavaging" to describe the idea behind headers and how they're supposedly more efficient over the cast iron exhaust manifolds most of us are accustomed to seeing and dealing with.

Sure that's just one more term we have to learn but in this case its a bit more descriptive in relation to exhaust flow out of the combustion chambers.

 

Yah, I know but the main reason for this conversation is to help OP with an issue on pinging. It was posted that exhaust manifold was always under pressure and could not suck outside air into system and screw up O2 readings.

Then it was posted that it's just fluxuations of high and low positive presures, which is better but not 100% accurate for manifold. I agree this could be the case in the pipes.

Scavenging is a verb describing what vacuum a noun is doing. I guess I'm just poor at conveying my thoughts. Scavenging happens in our OEM cast "STAINLESS" manifolds too. Just not as well as headers.....kinda. Headers tend to be more efficent at higher rpm hp situations. Our stainless OEM manifolds may be more effective at lower rpm situations, I'm not sure.