1993C4LT1

I hate to keep asking you guys for help, belive me. My car is a 93 LT1 A4. But I get like 10 or 11 MPG in the city. I do one WOT run most of the time I drive it. I doubt that's enough to bring the mileage that low. Most drives are in the city, below 10 miles, under 45 mph. I use to get worse, 8 mpg. But take it on the highway for a short trip, I get 23 to 25 MPG. 93 car, so no MAF.

No vacuum leaks, car idles at 550 rpm with a warm engine, no AC on. 650 rpm with AC on. That is perfect, according to the FSM. I even blocked off the TB in July 2016. The car stalled, meaning no vacuum leaks. It runs great, no kidding.

Fuel pressure is 38 PSI running.

Haven't data logged since like August or September. But o2's go from rich to lean very fast. Coolant sensor agrees with what I see on the car's digital readout. Nothing has changed on the car. Just a new opti and coil pack conversion. It is not throwing any codes. If we can't figure it out, oh well. I'll enjoy it like I always have.

Before I give up on finding the cause, where do I start looking?

1993C4LT1

Also, don't take it the wrong way. I feel I ask you guys a lot of questions.

JrRifleCoach

O2 sensor status? Last replaced?

1993C4LT1

May 2015. And last time I with data log, it went from rich to lean quickly in closed loop. I can check again later today to verify.

Tom400CFI

I'd be looking at my driving cycle and habits. If you're getting 23-25 highway, that's a little low, but I'm guessing your highway isn't for a full tank, uninterrupted driving. Sounds like it's in the realm of "normal". How you're driving in "city driving" dramatically affects your ave MPG. Idling at a light, you're getting 0 mpg. That hurts your average in a big way. Starting/stopping, every time you're using the brakes you're turning gasoline into heat, not forward motion. Last, the early LT1 is batchfire and idle/low speed driving are where it is most inefficient (compared to later sequential cars). My "around town" driving in Park city consists of some stop lights, but mostly open roads with speeds around 40-60 mph....I average ~20-22 in these conditions with my early LT1. When I road trip it, I get 28-32, so slowing it down "in town" really hurts the average even though MY "in town" isn't very representative of "city driving", IMO.

ddahlgren

Everyone's 'city' is different. Frequent long stop lights and 25 mph speed limits you are lucky to get what you do.

antfarmer2

O2s are normal maintance and will pay for themselves in gas.

Tom400CFI

...IF they're not working.

All of my cars have their original 02's. Their age and gas mileage is as follows:
'96 Silverado x-cab 4x4; 293,000 miles, 20+hwy, 17-19 around town
'92'Vette; 180k, 28-32 hwy, 20-22 around town
'05 CTS-V; 145k, 23-28 mpg

As soon as one of my O2's fails...I'll replace it.

1993C4LT1

My o2 sensors work great. They do cycle rich to lean fast. If bad, one, or both would stay in a certain position. Thank you all for the responses! Tom, didn't know you have that many miles

JrRifleCoach

If one has the tools and knowledge.
Some need to have a shop do this for them before throwing parts.

antfarmer2

If you say so mine looked good too but changed them and what a differance.

1993C4LT1

Thanks ant. But yes, that is you tell if the are good or bad when using a scan tool. As stated, I changed them May 2015.

antfarmer2

Missed that are your injectors original?

DGXR

IMO this is the core of your MPG concern. Based on this one statement, it sounds like your car spends equal time in open loop and closed loop. This will hammer your fuel economy. I'll also go out on a limb and say the oil probably doesn't see full temp very often, which can become a problem if it goes on for a long time, but this is separate from your MPG question.

I bet your "city MPG" would improve if your trips were 20 miles instead of 10... and possibly replacing the O2 sensors (which I might do as well after reading this thread).

C409

..... Are you actually calculating your fuel mileage or reading it off the dash ? .....

1993C4LT1

Off the dash. But I would assume it's correct because my injector bank pulse widths are high. FSM states, Inj. Pulse Width bank 1 and 2 should be between 1 and 4 milliseconds. I don't know if that is per injector, or bank.

antfarmer2

Call Jon at FIC great guy and knows his stuff. I got mine for my 94 for $169 on sale.

Make sure your o2 wires are not melted.

1993C4LT1

Injectors we're changed with injectors from him in September 2015. Also put a new fuel filter before doing so. Last time I did a fuel pressure test, it went from 41 psi to 34 psi in twenty minutes. Always 41 or 42 psi at key on.

antfarmer2

You sure you tucked the o2 wires where they did not melt? Is your digital temp gauge working right that sensor on the water pump can do some strange things.

Tom400CFI

I don't think the guy has a problem. He's driving in the city at low speeds. He has lots of cold starts. He's gonna get bad gas mileage driving that way.