I feel like I'm running rich, thoughts?

[Alex Mc]

Gentleman, I'll start by saying thank you for any input. I've just purchased my first M20-powered E30 since the late 90s and I'm quite rusty on what to check and what can be adjusted.

So it's a M20B25 that's been rebuilt with only about 5k miles on her. Stock build. Currently, I'm getting about 21mpg in a more-or-less combined cycle driven conservatively, and I'm fairly certain she should be doing much better than 21mpg. More like 24-26mpg on that kind of highway/around town cycle.

Let me add:

-Fair amount of soot accumulation on the rear bumper
-General smell of unburnt fuel when driven but not in a dangerous sort of way
-Car has no catalytic converter (if that should matter)
-Everything else is stock
-Sometimes it takes a while to crank (bad FPR?)
-Idle is less than smooth

What can I test/play with? Anyway to test the O2 sensor? How about AFM adjustment?

Thanks guys.

[jlevie]

21mpg for a mixed driving cycle isn't unusual and isn't far off the usual 22-23mpg.

Unless someone has twiddled with the bypass air adjustment or clock spring tension in the AFM, it is unlikely to be the cause. There are a number of possibilities for what you have observed:

Intake leaks will cause the DME learn to run a richer mixture
An O2 sensor at the end of it's service life (100k) will cause a rich condition
Invalid fuel pressure can cause a rich condition or partial misfires
An engine running cold or a bad ECT sensor will cause a rich condition
The wrong plugs, aged ignition wires, or distributor can cause partial misfires
Leaking or dirty injectors can cause a rich condition or partial misfires.
The DME could be the cause.

Decreased fuel mileage and/or carbon at the tail pipe can be from an engine that is running rich or it can be that unburned fuel from partial combustion is the cause.

[Alex Mc]

Thanks Jim,

I'll investigate a few of those variables this weekend and see what I find. I also just found out that the DME is running an aftermarket chip, so that is likely a factor. Will also identify that chip tomorrow.