I think something a little odd is going on with my engine when I start it for the past couple of days. When I start it it sort of runs slow 350-400ish rpms, then about a second later, it "idles up" to its normal idling rpm. The oil pressure seems kinda "bouncy" at idle. It's like the oil pressure gauge is having a seizure. I was wondering if it is possible for the ecu to throw a code without the cel going on. And no, my cel bulb isn't burnt out. It's only been doing this for the past couple days. It used to "stumble" on start, which I've now learned is probably the injectors leaking. By the low idle, then speed up, and the oil pressure thing is new too. It also vibrates a little more than usual at idle, it's not a constant vibration, it last probably 3-4 seconds, then stops for a second, then does it again.
Thanks,
Paul

 

Yes, it is entirely possible. There are "hard" codes (trip CEL/MIL) and "soft" codes (no CEL/MIL.)

This is why everyone should own an interrogator, if they have OBD-II (or an ELM327 and get Torque, if you have OBD-II. If you're OBD-I, you just need an interrogator.) Look for codes whenever you suspect anything, or even as part of an annual inspection.

Hard codes are codes that affect driveability or emissions output, and will trip the CEL/MIL.

Soft codes are "minor malfunctions" that have no significant effect at the time, and will NOT trip the CEL/MIL (unless & until the fault worsens into an emissions or driveability concern.)

I can see the CEL/MIL being tripped for a driveability reason, or even a major fuel efficiency/economy issue, but just for an emissions fault? Methinks that's trying too hard, but I've never quite agreed with the practise of the various Smog programmes (the idea - sure. The execution? Not so much)

 

From what I know all cars 96 and newer have obd2 only some 95's have it then some have obd1. So would a regular obd2 scanner pick something up?

 

The comm protocols are different - as are the interrogator plugs. And OBD-II reader will not physically connect to any OBD-I system - and it wouldn't understand anyhow.

SAE sez that the OBD-II plug needs to be under the IP, within 30cm of the vertical plane of the centerline of the steering wheel. Exception - some automakers put it behind the ashtray in the middle.

OBD-I plugs could be anywhere - under the IP or under the hood.