Volumetric Efficiency

I have a 2006 Scion tC with 2.4L. It runs ok, but idles a little rough without any DTC's. I have a cheap scan tool that will collect live data but to really look at the data, I have to enter it manually into a spreadsheet. Today I took it out at lunch and while recording I hit full throttle in first through second gears. When I got home I entered the data and calculated volumetric efficiency. I've done this twice but I made sure to use full throttle this time. One question I have is at idle, the VE is horrible and there are rythmic little humps every few seconds. Are these humps telling me something I need to investigate further?

My two full throttle runs show the VE hitting 108% and 139% which I am convinced is good breathing.

I'm not familiar with the software your using but I'm pretty sure there is a issue in capture rate which is leading you to have incorrect VE readings . If your issue is at idle only and runs good at all other throttle positions VE is not the problem. Also higher than expected VE could be caused by a misreporting MAF or incorrect data entered (such as elevation/baro and air temperature). Is the complaint surging, misfire or something else?

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SheetsTech wrote:

My two full throttle runs show the VE hitting 108% and 139% which I am convinced is good breathing.

139%? :woohoo: That's moving some air! No offense, but are you positive you've got the calculation set up correctly? Because I don't think I've ever seen a 2AZ have that kind of volumetric efficiency. I'd expect more in the 85-90% range. Is this tC supercharged?

One question I have is at idle, the VE is horrible and there are rythmic little humps every few seconds. Are these humps telling me something I need to investigate further

The terrible VE at idle is pretty normal. :silly: At idle, the engine is breathing a minimum amount of air, and the VE numbers reflect that.

The regular humps in the VE line... Is the A/C on? Defroster? That's about the right frequency for the A/C compressor clutch to come on and off in low ambient temperatures. Any kind of engine load changes will cause the throttle plate/IAC valve to change the idle airflow, which will show up in your VE calculation.

Capture rate? I wondered about that too because I pushed it hard toward red line which I cant see in daylight and the max RPM I recorded is about 1000 lower than what I know I hit. I checked the formula I'm using against another formula and both gave the same answers. I'm using VE = MAF / (RPM * D * (2.4L/120)) .
MAF = grams/second
P = barometric pressure in pascals
IAT = intake temp in kelvin
D = P / (287 * IAT)


The only issue I have with how it runs is the rough idle. Maybe a smoke test for vacuum leaks?

Wow, yeah I had the AC on with the temp turned up. So, that's what I'm seeing. I was thinking bad valve maybe, but simplest explanations first. For what it's worth I got my formula from this video.


And then I checked my results against this calculator.
atgtraining.com/atg-volumetric-efficiency-calculator/

They check against each other. I got my atmospheric pressure from the airport website.

I thought 139% was ridiculous too and no supercharger. Crappy data collector?

Thanks all.

Ben wrote:

I'm not familiar with the software your using but I'm pretty sure there is a issue in capture rate which is leading you to have incorrect VE readings .

That sure looks to be at least part of the issue with those VE numbers. OBD-II can take a second or more to loop thru' the PIDs required for a VE calculation. Way too long when RPM is increasing by over 1,000 RPM per second. Best to do a VE pull in a high gear.

SheetsTech wrote:

I thought 139% was ridiculous too and no supercharger. Crappy data collector?

Partly that? The scan data rate from this era of Toyota product wasn't screaming fast, either. :silly:

For giggles, I stuck some known good values into the ATG calculator:



Then I reduced the RPM by 2,000 and left all the other values the same:

Interesting about the data capture rate not keeping up. I found a good course handout from Bosch on VE and they say to do the WOT and then use the max values in a calculator. My method assumes all the relevant PID's are concurrent. I took my max values for RPM and MAF within a few seconds of each other and put them in the calculator and got ~85%. That's realistic even to my experience level. I'll run the test again for longer period and higher gear and see how that works with my quasi real time method.

I only have my lunch time to do these tests, so only ran one test today and picked one I havent done yet. Bosch lists a few tests to do if the VE is low, such as test the MAF, record at 250 rpm increments up to 2500 rpm. The MAF response should be linear. Arrrggggh, I'm not sure if I'm seeing the same data recording bottle neck or if I'm seeing non linear MAF.

I got some data today. I WOT in third gear and it seems like it worked. I hit max VE at max RPM and got 89% which seems good to me.

I'm not quiet sure why you are chasing VE right off the hop, mind you it is great to learn nonetheless.

What are your fuel trims doing more importantly?

The Total Fuel Trim (TFT) ( Short term + Long term) should be within -10% to 10% on a good engine throughout all engine RPM ranges.Here is a photo of what good total trim looks like in MS Excel:


And here is a bad MAF (you wouldn't know this unless you did some VE testing, but in any case it shows a trim issue for a reason)

Here we can see at low engine speed we have negative trim, and as engine speed increases we go to positive trims. In this particular engines case the MAF was over-reporting at low engine speed and under-reporting at high engine speed.

I did the VE test because I had read in the Bosch paper on VE that it's a good initial test because it can determine if it's fuel, air, exhaust or mechanical and it's fairly easy to do. I didn't have to tear into anything to do it, just collect some data. Now, interpreting what I collected is a different story. My TFT and RPM is all over the place with a few TFT excursions beyond +/- 10%. I added the poly trendlines to RPM and TFT and total trim looks steady that way.

Nice Graph!!!!

I set the right scale to secondary so it's display of RPM.

Then once I have the TFT in data table, I create a table and sort everything from low to high in the rpm. That's the trick.

So I can see from your graph you have good trim control from low to high engine rpm. On average you are +4 Percent according to your polyline.

I would say your MAF is reading Correctly, You will also note your load pids will max out @WOT. CALC LD and ABS LD.

But you have to rework your graph, because those -15 might be at low rpm.

Here is an example of a data table

PM me an email Addy and I'll send you a spreadsheet to play with.

You can add your load pids too that makes it even more apparent whether it's an Air issue.

Nice. I sorted some of my data on RPM low to high. I see why that is a good way to visualize the data. The TFT or whatever PID at a certain RPM is still paired with that RPM data point when it occurred.

I revisited this data I collected one more time before I test my fuel pump or swap a MAF sensor and gather new data. My assesment of the TFT is that it is elevated slightly across all RPM. Here's the chart comparing load, TFT, RPM.

Fuel trim in that graph looks great to me, there is excellent fuel control across all RPM ranges.

From the graph the highest I see is ~12 ish and the lowest is ~ -12ish.

What is the average TFT, use your spreadsheet and get an average for that column, sometimes this can be a good indiction.

Also, be mindfull of scaling, it can cause the eye to be mislead.

If I was just interested in the TFT vs.RPM with this data set I would scale the TFT +50% to -50%.

This same data set with the Load pids +100% to -100%.

Nice work!

Thanks for all the informative replies. I've learned something from every single one.

I started out by doing the volumetric efficiency test with a slow scanner. I've since learned that it records a new data point about every 5 seconds. That's like an eternity, so I've adapted. The most recent test I did is the Bosch MAF test by capturing RPM and MAF data between 1000 and 2500 RPM at 250 rpm increments. To make sure my scanner grabbed accurate data, I held each increment for 5 Mississippi and then increased throttle to the next step on up to 2500. The conclusion is my Denso MAF sensor is responding linearly like Bosch says the test will show if its good. After all that, I'm convinced the MAF is good. I still have a slightly rough idle which prompted me to dive in and learn something.

Is there a YouTube video or a tutorial you guys recommend for making awesome charts out of Excel data? :lol: Mostly can't figure out how to have two different vertical scales.

Couln't resist here is what ChatGPT says to do!!!



Tyler I googled videos and learned Excel on my own, it took time, more time than learning to use my Pico, uuuggghhh.

But this gal knows her excel stuff, she's very thorough.

www.youtube.com/@LeilaGharani

Feel free to pick my brain every once in a while