We have a speed density system set up in a street rod but cannot get the mileage better than 14mpg. Operating in closed loop with BLM's pretty reasonable. Changed the O2 sensor to a new Delco heated one. Everything in the scan data seems normal and the engine runs just fine except for the mileage. I was sure that changing the O2 sensor would fix it but no change. Any ideas?

Steve

 

timing tables along eith the VE massively afferct mileage if you play too far the wrong way.

another biggie, make sure and I mean absolutely sure, that your headers do not leak before the 02 or directly after it. this will destroy any chances you may have of gettingn a good tune and mileage.

This of course only applies if the mechanical systems are in good working order and that all ign/fueling components are good as well.

later
Jeremy

 

Can you please explain why? (My headers gaskets are toast and are leaking)

 

Exhaust leaking out and air leaking in will chew up the O2 readings. The ECM can't accurately control fuel without good O2 readings.

 

Quote:

Can you please explain why? (My headers gaskets are toast and are leaking)

Fresh O2 enters the exhaust stream at the leak, the O2 sensor detects this fresh O2 from the leak as a lean mixture and the ECM adds fuel to correct what it thinks is a lean mixture but in reality is not, hence causing poor milage. Pull your spark plugs, theyll probably show rich.

 

Thanks guys, we'll check for exhaust leaks but I'm not confident there are any there. It's a show type vehicle with all new parts and lots of continuous maintenance. If the exhaust is good then it seems it is narrowed down to fueling or spark. I've tweaked the VE tables and BLM's are in the 120's and 130's. As I understand it, if they are not hitting the stop then the ecm is adjusting and we should be pretty close to 14.7. That leaves timing and I'm a bit stumped there. All I have done there is copy the timing tables from the super aujp posted here. It seems to run fine but how do I tell if I have too much or too little in any particular place?

Steve

 

Too much timing is easy to find....just check your datalogs for knock counts and adjust appropriately. Too little timing is a bit more of a gray area. Some guys will just keep bumping the timing a couple degrees at a time until they hit knock, and then back down a step. It seems to me there should be a better way to go about it, but that's the only actual "method" I know of.

Are you using the decel enleanment and highway mode features in the programming? They can provide pretty fair gains in mileage from what I've read. Also keep in mind that you ARE limited by the parts you're running. I.E. if the cam has a lot of overlap it's going to dump fuel out the tailpipes at idle no matter HOW you tune it.

 

If you use the spark table from Super aujup, remember that the Spark Tables includes PE Spark advance .
So be sure that PE Spark advance be zeroed.
-Beppe-

 

Probably too much timing.

The O2 sensor detects unburnt O2 in the exhaust.
Running alot of timing means having to add a bunch of fuel to keep the AFR reporting correctly, so your running too rich to make up for too much timing. But, leaner mixtures take longer to burn and they also need more timing. So you need to experiment slowly and accurately to find out what your car really likes.

Take lots of notes, and being methodical will lead you to the best results. There are several different ways to get there, and you'll need to experiment to kind out your cars sweet spot.