I need to know things to check, it is a series 3 engine with the 4t65ehd, it doesn't have evap or egr, when I removed the evap line, this is is in an 88, I cut the wrong line on the expansion tank, the bottom one, so I ran the other line back to where it is hooked up the way I think it meant to be, the bottomline comes off the tank and the top line is now a breather.

I need to know what to look for onbad mileage, it hasnew plugs and tstat, I have two comps programed the same and I don't think that is the problem, so if you have ideas of what to look for, I got 19 mpg last tank and most of itwas highway and I am at 80 miles to the half tank.

Chris

Got to be something to help you?

May be the O2 sensor or a vacuum leak

The o2 sensor is brand new, and I know that doesn't mean that it is good, but it is still new.

I am pretty sure there isn't a vacuum leak, but i haven't done anything to check, but I will get to it when I go to a friends that has a inline tester.


THanks, and all I am looking for is ideas of what might cause it to do the bad mileage that I can look into.


Chris

any codes showing?

he SES light isn't on, but I haven't scanned it, I will go up to oreillys tomorrow and scan it to see though, but from what I can tell no, and I have had the light on before so I am pretty sure it works, but not 100% sure.


Chris

Bad alignment, dragging brakes, worn wheel bearings, low tire pressure.

What speed do you travel at? What rpms is the engine at at that speed? How much is at a steady speed? Is there a lot of traffic that causes speed varation?

If the engine is running smoothly and not throwing any codes it is most likely some thing else that is the problem. One way to get an idea of what is going on in the engine is to have a scan done as mentioned but pulling the spark plugs and inspecting them will also help.
Put the car is neatral on level ground and push it back and forth it should move easily and smoothly if not inspect the brakes for a sticky caliper. Get the car aff the ground and on jack stands and spinn each wheel by hand again there should be little resistance on the front wheels and the back wheels should turn fairly easily. Dan

Hey Chris
I'm also running 3800SC Series III with DBW setup, Around town driving like old granny, I'm getting any where between 17 and 19 mpg. Getting on it alittle the mpg drops like a rock around 14 to 16 On the highway between 70 and 75, I'm getting approx 29 to 30 mpg. I'm running 235x15 on the front and 245x15 on the rear. Running the same trans that you are. A heavy foot equals minus mpg

what would cause bad gas mileage in a 3800sc?

A bad case of heavy footitus.

quote Originally posted by 30+mpg: what would cause bad gas mileage in a 3800sc? A bad case of heavy footitus.

The right foot was my thought too.

[This message has been edited by FieroMaster88 (edited 04-05-2008).]

Maybe... I even told myself this tank I was gonna go easy on it and drive gas friendly, but i didn't think it would be as bad as it is


Chris

If I beat on my fiero, I get about 20....

I just did a highway calculation tonight, it was near 30. It was only half of my trip though..

I did a bunch of beating on it also tonight, I did about half beating, half highway and I got 26.

Since it is a Series III, get a ScanGauge II. It can read trouble codes plus function as digital gauges and a trip computer to give average & instantaneous mpg.

Well it's being run as a series II, basically it is just a series II swap with a gen V blower...


I will do a highway calculation next fill up I do.


Chris

i just did a long trip on the highway at 75-80 and i only got 22mpg. I know last year it seemed to be closer to 30. mine is uding the getrag. the car runs fine and i was taking it easy on it. what should i look for?

One thing to think about is the additives that are in the fuel right now. There is a differance in the summer/winter blend of fuel and with the winter blend, fuel mileage goes down overall. My gas mileage dropped a few months back and thought I had an issue also so I started looking into it. I can my car all the time when driving and nothing changed to much on the scans except my LTFT when it got colder. I swapped out my O2 sensor thinking that was the problem being its one of the common gas killers. After replacing the sensor, gas mielage stayed the same. Started reading around and found alot of these type Threads that popped up asking about fuel mileage. From reading, the gas companies add aditives to the fuel in the winter time to help with emmissions standards that the goverment set. Thinking one of the additives it Ethonal which would explain lose in mileage with the wintr blend of fuel. This may not be the cause but just throwing it out there.

any updates about this? my car is still getting around 20 if i baby it. last summer it was much better. i have a scanner and what should i look for when i am scanning. anyone have any ideas on what some of the specs should be for ltft, o2, ect? also is there anything else i should check? is anyone else not getting that great of mileage with their swap?

maybe try a commpression check on the clys.
i heard if you have a cracked cly head or block. you will have bad mpg. but im not to sure if thats true or not.

what is the avrage mpg that the series 3 sc engines get?

This same exact thing is happening to me with my 3400 getrag swap. I am starting to think that maybe it just is the gas. Last summer i could get over 35 mpg, now i am only getting about 30 on an all highway trip. I think i am going to go get some shell gas next fill up (i always get cheap arco or safeway) see if it makes a difference.

Possibly the throttle position sensor? If it turns out that it is the throttle position sensor, I have a new never installed 1 for a 99 and up sitting here I will sell for $35.00 shipped.