Number of cylinders?

Aberk · 09-01-2009, 01:18 PM

Is there a reliable way to determine the number of cylinders in a car using only OBD II data? I am thinking of using the number of O2 sensors, but that doesn't seem reliable on some older models as they may not have as many.

p2psmurf · 09-01-2009, 02:17 PM

4-cylinder BMW's have 2 O2-sensors in front of the cat.
Older 6-cylinder BMW's only have 1 O2-sensor in front of the cat.
Some V12's have 2 O2-sensors in front of the cat, others have 4.
How do you think you can determine the number of cylinder's?

Diesels (4, 6, 8 cyl) have none, 1 or 2 O2 sensors.
Again, how do you think you can determine the number of cylinder's?

Aberk · 09-01-2009, 03:04 PM

Quote: Originally Posted by p2psmurf

4-cylinder BMW's have 2 O2-sensors in front of the cat.
Older 6-cylinder BMW's only have 1 O2-sensor in front of the cat.
Some V12's have 2 O2-sensors in front of the cat, others have 4.
How do you think you can determine the number of cylinder's?

Diesels (4, 6, 8 cyl) have none, 1 or 2 O2 sensors.
Again, how do you think you can determine the number of cylinder's?

I thought this would be the case with O2 sensors. That's why I asked if there was any other thoughts.

Curiosity · 09-01-2009, 04:47 PM

The best method would be decoding the VIN, but it's manufacturer specific so would take a bit of work. Fuel trim could give you a hint but it's a lot like the O2 issue.

DarquePervert · 09-01-2009, 08:46 PM

Why don't you just open the hood and count the number of spark plugs or spark plug wires?

Tidder · 09-01-2009, 08:51 PM

Seems the most logical to me.

bratnetwork · 09-01-2009, 10:13 PM

use the streaming obdII data to see how many injectors are firing, or how many cylinders it is monitoring for misfire. you will obviously need obd software that can monitor live data and not just read codes.

soundman98 · 09-02-2009, 10:05 AM

Quote: Originally Posted by DarquePervert

Why don't you just open the hood and count the number of spark plugs or spark plug wires?

this method has worked flawlessly for me, though, i think desiels use glow plugs(no spark plug wire?), so you might need to count fuel lines, or something.

DarquePervert · 09-02-2009, 10:10 AM

Quote: Originally Posted by soundman98

this method has worked flawlessly for me, though, i think desiels use glow plugs(no spark plug wire?), so you might need to count fuel lines, or something.

Good point about the diesel. I didn't think about that at all, obviously....

Of course my method of counting plug wires would be inaccurate if a motor happens to be missing a plug or plug wire for some reason, such as removing them due to a problem in a certain cylinder, possibly.

soundman98 · 09-02-2009, 11:13 AM

Quote: Originally Posted by DarquePervert

Good point about the diesel. I didn't think about that at all, obviously....

Of course my method of counting plug wires would be inaccurate if a motor happens to be missing a plug or plug wire for some reason, such as removing them due to a problem in a certain cylinder, possibly.

after you said that, i remembered that in at least the mid 90's--i am not a engine guy, so it could have been earlier/later, ford had a inline 4 cylinder motor that actually had 8 spark plugs, so counting the wires might not always be the best option either...

i guess the only reliable way would be to decode the vin, or, in case the engine was swapped to a different one at some point, learn your motor good enough to just know what it has.

DarquePervert · 09-02-2009, 11:52 AM

Decoding the VIN is probably best, I agree.....

As long as the vehicle has the original engine. A Mazda Miata/MX5 or Ford Ranger with a 302 conversion, for example, would have a different engine than the result of the VIN decoding would reveal.
And there are a lot of vehicle/engine combinations that make swapping to a bigger motor with more horsepower (or to a smaller more fuel-efficient motor) a relatively easy operation for a shadetree mechanic.

Superduck · 09-02-2009, 10:33 PM

Quote: Originally Posted by DarquePervert

Decoding the VIN is probably best, I agree.....

As long as the vehicle has the original engine. A Mazda Miata/MX5 or Ford Ranger with a 302 conversion, for example, would have a different engine than the result of the VIN decoding would reveal.
And there are a lot of vehicle/engine combinations that make swapping to a bigger motor with more horsepower (or to a smaller more fuel-efficient motor) a relatively easy operation for a shadetree mechanic.

However, since we're talking OBDII, it'd have to be a modern engine, and to do such a swap, you'd require the PCM matching the engine, therefore it'd still report that VIN (I'm only assuming the OBDII reports the VIN) that matches the engine, therefore it'd say the proper # of cylinders. I've never done any OBDII decoding, so I couldn't be sure. There are codes for Cylinder #x misfire, etc. Though the only codes I know are the actual trouble codes, therefore will only show up when there's a problem.

Good luck with that. I'm sure someone that's done some OBD programming would know. Maybe check the Megasquirt forums?

Cheers,

Kris

DarquePervert · 09-03-2009, 06:54 AM

But that's an idea....
If an OBD reader would report a misfire on cylinder #X, you could, in theory, remove a plug wire and get the trouble code for the misfire and you have a cylinder number.
However there is still the potential that you would have to remove each plug wire to get to the highest cylinder number.

Besides, if you're opening the hood to remove a plug wire, then you can just count them and be done, too.... Unless, of course you have a diesel, was pointed out previously.

p2psmurf · 09-03-2009, 10:10 AM

If you want to damage the catalyst, then be my guest and remove spark plug wires.
Catalyst have a problem with fuel burning in the exhaust.

User Name		Remember Me?
Password