Car goes open loop/rich at 72 MPH

[H-Man]

Due to the issues I've been having with my dad's car, I drove my 1994 geo prizm home with a multimeter watching the Ox (oxygen sensor) pin of the diagnostic connector to check how long it takes for an oxygen sensor to warm up and to check how fast the sensor is. I noticed on a 1.5% uphill grade I would go open loop/rich at 70-72 MPH (estimated power requirement: 25HP, RPM: 3500-3600.) I think it is triggered by throttle position, manifold pressure, RPM or potentially the EGR valve shutting (it stayed closed loop until at least 4000 RPM in 2nd gear so I know that it isn't purely RPM triggered.) The 4A-FE does not have a knock sensor so the ECU likely has very conservative settings. I did this so that I could simulate accelerating at high throttle at 3000-4000 RPM long enough for me to get an oxygen sensor readout.
Is there any way to get the engine to run closed loop at a higher power output so that I can keep the engine closer to the BSFC valley (BSFC valley for this engine is approx. 3500-4000 RPM)? As it is, the engine doesn't lend itself well to P&G.


I did find something useful: The 1994 Geo Prizm and the 1994 Toyota corolla with 1.6L 4A-FE (AKA ae101 on some forums,) stays in DFCO until the idle speed control kicks in, even with AC running it reliably stays in DFCO until this point.

The power steering idle up valve causes the engine to go openloop/rich. I'll see what happens when I plug that vacuum line (when I have the opportunity, I'll hook the pump up to a cordless drill to see if it provides enough assist for what I want.)

I need to put together a serial to TDCL cable (to read the OBD data stream that updates once a second) and an arduino oscilloscope (to watch the O2, EGT, and potentially the MAP sensor) so that I can get better data.

[Superfuelgero]

The egr is vacuum controlled, so a vacuum leak will affect that and the map sensor's load reading. I'd start there and check the ignition timing (unlikely, but never hurts).

[Baltothewolf]

I know perfect solution, don't drive 72mph .

[bestclimb]

does it hit at the same RPM in other gears?

[H-Man]

Originally Posted by xntrx

The egr is vacuum controlled, so a vacuum leak will affect that and the map sensor's load reading. I'd start there and check the ignition timing (unlikely, but never hurts).

I bought the car with it retarded 6 degrees (timing was retarded to pass smog I think, oxygen sensor was slow before I replaced it so it would flip flop between high NOx and high HC on the smog test,) when I got my hands on an inductive timing gun I advanced it by 9 degrees for a 3 degree advance.

Originally Posted by Baltothewolf

I know perfect solution, don't drive 72mph .

72 MPH is to simulate heavy loads like seen in P&G. I'm not going to watch my multimeter while accelerating, doing it like this gives me more time to determine where the engine switches over to a burnt in A:F map so that I can keep the car at the very limit of running closed loop.

Originally Posted by bestclimb

does it hit at the same RPM in other gears?

No. 72MPH is 3600 RPM or so, it stays closed loop until at least 4000 RPM in other gears when under lighter load.

I need my data cable, but I lost my high wattage soldering iron.

[jcp123]

Can you read fuel trims on this car?

[H-Man]

Originally Posted by jcp123

Can you read fuel trims on this car?

Without the TDCL cable? No. With it I can tell only slightly more.