chadk

Hello, I am trying to wrap my head around the data read from my car. The data from my ECU/PCM (what have you) says the engine is running lean. I say this because the LTFT (long term fuel trim) counts are 10% high. When I check my exhaust, spark plugs and O2 sensor they all have soot on them. I get terrible gas mileage. To me this indicates the engine is running RICH. I have read and read all about this stuff but I am still confused. Lets say I want to make a change to the tune. Do I add more fuel or take away more fuel? That is my question and what I am struggling with trying to understand. Thanks
Chad

MrWillys

What are your LTFT's running at? You have a PCM because it controls the transmission.

http://carputing.com/

chadk

thanks for Link.

On average:
Left = 142 and Right = 136

I could not find vacuum leak and O2's look fine to me.

chadk

is 35 PSI for fuel pressure bad? 38 PSI with regulator vacuum unhooked.

New regulator by the way.

Also, the MAP sensor indicates in the data log like 10 inHG at idle but when checked with vacuum gauge it is reading 19. WTF! Does this mean the MAP sensor is bad? is the vacuum port at the MAP going to give different readings than that of the brake vacuum booster port?

Chad

Cliff Harris

There are several ways to measure vacuum and you are trying to compare two different methods. That's why they don't agree.

MAP is Manifold Absolute Pressure. Vacuum gauges compare the measurement to atmospheric pressure.

MrWillys

Increase fuel pressure until the LTFT comes in at 128. Always measure fuel pressure without vacuum attached. 19 inch lbs would be expected of a stock engine and 10 would be such a radical cam it may not operate correctly.

Jimib92

Increase the fuel pressure like other people have said.

93Rubie

Your data suggests the engine is running lean and being commanded rich. Hence the higher LTFT numbers. Your evidence on the 02's etc...suggests otherwise so a false lean is occurring.

Are the 02 sensors contaminated? Did you test them for response? What was the data on the 02?
https://www.youtube.com/playlist?lis...fOSbxBy4ciZrDv

What year and engine Corvette are we talking? Those numbers are not stupid rich of a command.

On a stock LT1 any how 35psi with the vac. line on the regulator at idle is not a bad reading at all.

chadk

@93Rubie

the right side o2 is brand new. I could not get the old one off when I swapped to headers. The Left side has less than 5k miles on it. I did have to wipe it down when reinstalling it. It may be fubard.

I have not tested them for response. I will look at the link you posted tonight. Thank you for that.

From memory the o2's seemed responsive.

The engine is 1995.

I just put a new fuel regulator on it last year. I wish I would have purchased an adjustable one. I would increase the pressure like others have mentioned.

I read that one could adjust the injector constant in the PCM to change AFR. This was a quick or "lazy" fix. I understand it isn't the correct method. I will have to log on other PC to find that link.

My point being with this is I don't know for sure what has been done to this car in the past. I am the 7th owner. The car and engine look totally stock, it runs good etc. Maybe someone changed the injectors to different rate? I cant tell. Maybe they changed the tune? I compared bin files to what I think is a stock car and it looked identical. I don't know for sure because there are so many variables and I got bored looking at all of them.

All I know is the LTFT are high and I see black exhaust soot on the O2 and plugs. Maybe I am being to picky but that's me. I want the most from the engine.

Thanks for putting up with me. LOL. (wish my wife would tell me that) LOL

Cliff Harris

What do you mean by "wipe it down"? The O2 sensing element in protected by a cage of metal with holes in it. You can't touch the element without taking it apart.

chadk

I meant that i wiped the "cage" as you call it down. There was back soot all over it. I did not spray anything on or distrust the element.

C4vettrn

Quick question related to above: When you say come in at 128 is this at idle or WOT ? Thanks!

MrWillys

The BLM (block learn memory) or now called LTFT (long term fuel trim) is constantly adjusting but does not function at WOT. So you're adjusting for idle and part throttle cruise at varying speeds. What is happening is the ECM or PCM is adjusting to an air fuel ratio of 14.7. The BLM can adjust from 108 to 160 with 128 being the sweet spot. When you get it close to that the car runs at its best. it also allows the computer to make its best adjustments for extremes such as hot temps or cold.

C4vettrn

Copy, Thanks!

Pete919

OP any update on this?

1993C4LT1

Your BLM number are almost identical to mine. I'm also leaner on the left. But right bank for me is like 118-124. Have you figured this out?