1985 L98 - Since the rebuild I have not managed to get the AFR down from 17,16 or 15 to 1. It starts cold at about 12:1; then as it warms up, the AFR goes to 14:1 (great!) but, as it gets hotter, as the oil temp comes up to normal, the AFR shoots up to 15-17:1. Everything is new except the MAF sensor (have one on order) and injectors. I've played with the chip programming but it only makes it extremely rich. With the mods I have, I shouldn't even need to touch the chip. As is, the supercharger kit is not installed. Fuel pressure is good; timing is good. Any input appreciated.


[Modified by GRX, 7:39 AM 5/24/2004]

 

So are you saying that the ECM cannot maintain stoch A/F ratio once the engine is warmed up?

If so, what are the BLM cell & INT values; are they within adjustment range?

 

How's the O2 sensor? Do you have LT headers? If so you might need a heated O2.

 

Actally it runs in the 14:1 range right after I have the battery disconnected (then connect battery, and start test). I seems that as the BLMs learn, that the mixture is turning lean?

 

New O2 sensor running in stock factory headers.

 

How are you measuring AFR? Do you have a Wideband O2?

 

Yep. Got a wide band (techedge) Currently I'm measuring in the rightside exhaust. I guess I need to install it next to the stock O2 and see what I get....

 

It takes 5 minutes for the ecm to go into closed loop after startup so the 14.1 is probably the mixture with preset values. Closed loop uses the MAF, O2 sensors to maintain mixture and this is where you see the change. What are the MAF reading at idle and do they change if you tap the MAF?

 

Where are you getting the A/F ratio numbers from: Diacom, ScanTool, AutoXRay, etc.?

Jake

 

My wide band gauge.

 

I think I've tried tapping the MAF but nothing significant happened. Does that mean the MAF is defective?

 

Here's how I think it works:

The ECM commands the A/F ratios based on input from several different sensors, like throttle position, manifold air temperature, coolant temperature, engine run time, etc.

Those ratios are programmed into the PROM and you'd have to re-burn the PROM in order to change them. The ECM then makes the required adjustments to fuel and spark (within the range that it can) to achieve the A/F ratio that is burned in the PROM.

For example, say you raise the fuel pressure in order to try to richen the mixture. The 02 will detect this, signal the ECM and the ECM will then reduce injector on-time to compensate. So you'd be pretty much right back where you started.

According to TPIS and others, higher fuel pressure (within limits of course) is desirable since the injectors give a better spray pattern. The actual commanded A/F ratio won't change, but a more complete burn will occur so throttle response and, perhaps gas mileage will improve.

It's different at WOT, though, since in that Power Enrichment (PE) mode, the ECM ignores input from the 02 and uses programmed maps for fuel and spark. So in PE mode, higher fuel pressure will give a richer mixture and a numerically lower A/F ratio.

At least, that's how I understand it. I don't know whether there is an actual difference between what the PROM has burned in it and what the wide band is showing. Or, if there is a difference, based on the wide band readings, is it as great as it appears.

You might want to post your question on the Scan and Tune board to get the input from those guys who are the gurus on A/F ratios and wide band 02s.

BTW, with the engine idling, if you tap the base of the MAF with something like the handle of a screwdriver and the engine stumbles, the MAF is bad.

Jake

 

I switched the O2 wide band sensor to the left side of the exhaust and the AFR was 14:1!!! Now, there is one thing though....when I removed it from the right ride bung, it was not very tight. Could this cause a lean reading??? I also experimented by unplugging an injector at a time and it caused the AFR to shoot up to 17-18:1.

 

[SNIP]....when I removed it from the right ride bung, it was not very tight. Could this cause a lean reading??? I also experimented by unplugging an injector at a time and it caused the AFR to shoot up to 17-18:1.[/SNIP]

Sure, if outside air was being introduced to the sensor the reading would show a lean condition.

Jake

 

That's puzzling. The exhaust is under pressure so I can't figure out how air gets in. :confused:

 

Where you purchased the WB O2 sensor? how much?
Thanks
-beppe-

 

Mine came from here: http://www.techedge.com.au/ ...works great. Got it a few years ago. Price, I think was around 250.00USD for the unit and about 250.00USD for the sensor (found it online cheap back then).
Its an indespensible tool.


[Modified by GRX, 6:04 AM 5/26/2004]

 

Actually in the exhaust system, the flow moves back and forth, not just in one (the out) direction. It's not like water flowing from a garden hose. It doesn't take much to throw off the 02 readings.

As an example, with long duration camshafts which have early intake valve opening and late exhaust valve closing, some of the exhaust actually gets sucked back into the combustion chamber and even into the intake port. Reversion.

An exhaust leak, which is essentially what you'd have if the 02 connection is leaking, upstream or even downstrean of the 02 will skew the 02 readings. The closer to the 02 that the leak is the more the readings will be effected.

Jake

 

:iagree: :iagree:

 

The hot air rushing by the hole to the cooler outside air sucks it in. I had an exhaust leak last year on the dyno and it should 17:1 on the meter! :eek: The thing was though that the car smelt rich and was dead nuts equal on the chart to a dyno pull I had done a few months earlier on the same exact dyno. Later found leaks at the collector behind the O2 sensor. So... the car was fine, but the wideband sensor shoved up the tailpipe thought I was lean.