OBD1 test question

[Mustang4]

Have to bring my 1991 Mustang in for E-test shortly. The only concern I have is that I have a Check Engine Light comes on after 10-15 minutes of driving. Codes it puts out are 41, 91 which refer to the LH and RH Oxygen sensors and running lean, however ZR checked it and the car is not running lean.

Codes only appeared after new Oxygen sensors and some emission sensors were put in including the charcoal canister.

Since the OBD1 testing doesn't hook up to the computer and is only a sniff test, assuming the test results are good, which they should be, will it pass or fail if my check engine light comes on during the test?

[ZR]

Yes and no, as I've been saying, for sure it's sometimes going lean for a moment or two during warm up but no clear answer as to why. My suggestion has always been to ditch the current MAF for a stocker or go with a calibrated MAF and the 24lb injectors.
Light started coming on after we removed the 24's and slot style MAF along with chip and returned to stock tune.
I'd still pull the bulb and go in for the test. May require timing knocked back and a slight bump in idle first.
Since it otherwise runs amazing with no glitches, if it passes the etest..............ignore it and be happy as few cammed 5.0's run as flawlessly as yours does.

[Slope]

What MAF is in this car?

[Mustang4]

Pro-M 19lb. Stock one got damaged.

[Slope]

Pro-M 19lb. Stock one got damaged.

Is it an early Pro-M?

[Mustang4]

Got it in 2010.
Calibrated for cold air in fenderwell, if that really matters, and 19 lb injectors.

[Slope]

Got it in 2010.
Calibrated for cold air in fenderwell, if that really matters, and 19 lb injectors.

As mentioned above, I'd be interested to try a known good stock MAF to see how the car behaves once it settles in.

[Mustang4]

I will try and locate one tomorrow and test it.

Any other thoughts?

[Slope]

Does the CEL come on at the exact same time every time you see it. Meaning, is it somewhat systemic when it occurs (time from start up, etc.)

[Mustang4]

Usually it seems to come on after 15 min or so of driving but once it was like 40 minutes.

[Mustang4]

Okay I put in good stock mass air that Jeff at Mustang Specialties loaned me to test with.

Results, car did not like it. Check engine light came on and stayed on the whole time, idle dropped down to 500 then surged up to 800 and would not stabilize the entire time I let it run.

Switched back to the the larger Pro-M and added a new screen to it (wasn't running one since it had been damaged during some earlier re+ re) and the car started and went to steady idle around 800 as before and no check engine light. Likely after 10 or 15 minutes it would have come on.

I guess with the E-cam, Edelbrock heads and GT-40 intake it likes more airflow. So I think I'll look at a new 24lb calibrated mass air and put my 24lb injectors in.

[Mustang4]

Passed the e-test with what I have, yeah. Codes not an issue, but I don't know if they even showed up since the test was pretty short.


Think the issue with the stock Mass Air Sensor I tested not working was likely the replacement connector for the mass air since it will not lock on the mass air. It loosened on current mass air and the car started exhibiting the same pulsing idle, pushed it on and idle smoothed out, so I zip-tied it.