Euromagination

What are the "standards" of OBD I and OBD II? What exactly is the technological difference or advantage of OBD II over OBD I, if any? What can OBD II do for me that OBD I can't do? (Aside from just saying that OBD II is more elaborate and eloquent.)

In my current (and lengthy ) 911 search, I'm looking at the 993-series. After reading articles and threads here on Rennlist as well as other places, it seems that the 95 OBD I 993's inherently get less CEL's and have less problems due to emissions and diagnostics, etc. Is there truth to this?

Anyway, what will I be missing out on with an OBD I car? I've never owned a car older than a '96 before, and I haven't worked on many OBD I cars in recent times, so I really don't know. Will my car not be as "smart" as an OBD II car to discover and alert me of "real" problems (not just emissions stuff), or are both systems pretty much just as capable as one another to catch actual faults and problems with the car? Basically, what can OBD II do for me that OBD I can't do (other than cause more CELs due to tighter emissions standards on OBD II cars)???

Also, I have diagnostic software called "VAG-COM" for my Audi A6 and Passat 1.8T that I use to pull codes, do throttle body adaptations, et cetera, when one of them gives me trouble. The software works with any OBD II+ Audi or VW vehicle, so in addition to being fun to use and teaching me things about VAG products, the software has saved me a lot of money by allowing me to clear codes, etc. by myself. Is there such software for the 993? If so, is it only for the OBD II 993's?

Thnx in advance!
Harry

TheOtherEric

As a practical matter, there's no difference between OBDI and OBDII for the 993. Since there is no VAG-COM equivalent for a p-car, the codes in the computer are pretty much inaccessable and useless AFAIK. Two caveats to this statement:

1. The OBDII in the '96 and after will pick up clogged secondary air injection passages and will throw a CEL. A '95 (with OBDI) does not pick up on this, even tho the '95 might have the same problem. Sounds like a '95 is a better choice, right? BUT, the '95 didn't have varioram so you lose a lot of midrange torque and 13 hp. Food for thought.

2. You may be able to read some codes using a generic reader but you'll have to buy the service manual decoder book ($60?) to figure out what the codes mean. I've never heard of anyone doing this, but it's probably been done.

Euromagination

Eric,

Regarding your first courtesy-warning: If a '95 had clogged secondary air injection passages that OBD I didn't "notice", that wouldn't be detrimental to the motor since it sounds like it's just an emissions thing, would it? Maybe I'm associating the word "clogged" with something bad? Would it have to be fixed on a '95 or could it be ignored? If it's anything like the secondary air injection on Audis, I'm thinking that those passages are only used upon start-up to reduce emissions before the catalytic converters get up to op-temp. I may be wrong though as I'm no professional.

I guess since my experience with Audi/VW diagnostic software and OBD II has shown me that 80% of fault lights are emissions-related, I'm wary of having to deal with all that again in a 911 if I don't have to--especially if I don't have my own diagnostic software. OTOH, I don't want my car to be "less smart" than another one that is a year younger in regards to how OBD picks up on non-emissions problems with the 993.

The extra 13 hp picked up by having VarioRam is not a huge factor (for me) since I think I saw some aftermarket software that is available for the '95 993 to bump HP up enough to cover the difference between a VR and non-VR 993. I may be mistaken though.

Sorry I'm hitting you with so many questions!

TheOtherEric

I can only say that it IS ignored on a '95 since owners of '95 will never know if they have clogged SAI ports. I honestly don't understand SAI ports but your description sounds about right.

My understanding is that chipping a 95 won't quite cover the difference between non-vram and varioram. How close is it? Dunno. I've asked the same question before.

Father of 3

Yes, you are correct, purely emissions. OBDI will save you a lot of headaches but no varioram. Choice is up to you.

TheOtherEric

Can you expand on "lot of headaches" because all I know of is the SAI issue, and this only occurs in a modest fraction of cars AFAIK.

Father of 3

You know what, you are probably right. There is also a "flag readiness" issue when the battery is disconnected resulting in the need to go through a series of drive cycles to reset. There could be nothing wrong with your emissions or OBDII computer but you will no pass smog unless reset to readiness state. Some have been successful going to smog referee stations and getting passed if this is the only issue. Apparently this plagues all OBDII computers, not just on Porsches. What I don't know is whether this issue impacts OBDI cars as well. My guess is yes.

mborkow

"Would (clogged SAI ports) have to be fixed on a '95 or could it be ignored?"
they can be ignored. this only affects emissions. i have a '96 but if i were to get another 993 at some point in the future, i would probably look for a '95 so as to avoid the sai problem again (which cost me $6k)

Lorenfb

Basics:

ODBI -

1. CEL (check engine light) set by simple defined faults, e.g. O2 disconnected
2. non-standard access port
3. non-standard fault codes
4. non-stanard electrical interface
5. volatile fault memory

OBDII -

1. CEL set by much more that simple faults, e.g. O2 sensor out of range
2. standard access port
3. standard electrical interface
4. standard basic codes
5. continuous monitoring of the emission system (readiness)
6. non-volatile fault memory

Bottomline: OBDII is very problematic on the early cars, e.g. '96-'98 993s.
The 996 much less problematic than the 993 when dealing with emissions tests.
Also, OBDII repairs are much more costly, e.g. SAI repairs.
As a car owner, no real benefits to having an OBDII vs OBDI car.

Father of 3

other than more hp due to varioram

Ray Calvo

Considering I have a '95 with almost 90K miles on it, gotta say OBD I system has not given me a lick of trouble. RE the supposed 13 HP additional of the '96 and up cars, I have been on a race track with '96 and up cars, and they do not have appreciably faster acceleration than my car (and I have done NO engine/tranny mods or car lightening).

If you think the later cars would be faster, try to drive both and make up your own mind.

Any more, considering these cars are approaching 6-10 years old, a well maintained car is probably a higher consideration than OBD I vs. OBD II.

Oh yeah - VAG-COM won't work on the 993 (even OBD II) and I don't know how well the generic OBD II readers will work. There is nothing for the '95 OBD I cars besides the high 4-figure factory diagnostic reader.

Euromagination

Thanks for the answers, everyone! Much appreciated!

Lorenfb

"other than more hp due to varioram"

Not provided by OBDII!

TheOtherEric

Good point; I've read that here as well. But I have drained my battery a couple times and replaced the battery without doing the reset procedure. I passed emissions (just the OBDII hookup since I have a C4) just fine. So I can't say I really understand the issue.

Lorenfb

"But I have drained my battery a couple times and replaced the battery without doing the reset procedure."

The readiness codes get RESET any time the battery is removed/goes dead or the
fault codes in the DME unit are reset. The readiness codes are SET during basically
normal driving after two driving trips. The readiness codes are part of OBDII
and are present in ALL '96 and later U.S. vehicles. The issue is that the '96-'98 993s
have more difficulty in SETTING the readiness codes than other vehicles.

Two of the readiness codes can be "forced" via the Porsche PST2 tester
(SAI & tank venting). All the others must "occur" (hopefully) while driving.