Horrible gas mileage

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I'm only getting about 28 mpg down from the 36 I was averaging when I first swapped in the b6. I ran for about 1200 miles no problem, then in a weeks time I developed a miss and started throwing codes. I uploaded a vid awhile back if sparks coming off the ditsy cap at night. Swapped to a new cap changed the plugs and wires and the miss disappeared but I was still getting codes 15 and 17.

I went ahead and exchanged the Bosch o2 I had installed with the new engine for a denso 234-1000 unit, and last week when I was at Bryant's house be forced me to solder the connection lol.. The codes went away and haven't been back.

I'm still getting terrible mileage, and I have no idea why. The car runs fine, I can tell it's lost some power, from 70-90+ it doesn't want to accelerate as fast as it used to.

The temp gauge doesn't climb up to even 1/4. New thermostat with swap too, advance special I remember it said something about it goes wide open if the car tries to overheat but I do t remember details about it.

I've had a high idle this whole time, it's around 2k rpm.

If I'm in first or second gear and go slowly up to around 35/4000 rpm and let off the gas, the car will do a nosedive literally like I just slammed on the brakes and locked up the wheels. Only happens occasionally.

I don't know if any or all if that is related but it's about all I can think of that isn't right with it.

[FestYboy]

first get your coolent back up to temp, it sounds like you're stuck in open loop (coolent temp too low). if you have a safety-stat, toss that crap out and get a real t-stat, those safety units are JUNK.

[Rocketman]

Doesn't sound like the engine is warming up, and the ECU will run it pig rich. High idle is because the idle valve isn't closing as the coolant warms up

[FestYboy]

is there and echo!?!? :p

[Rocketman]

ECHO?!?! echooooooo

[Bert]

Plugged Cat can do that to you too. Has happened to me more than once; piece breaks off inside and restricts the flow. Fuel economy and power then drops like a stone.

[TorqueEffect]

first get your coolent back up to temp, it sounds like you're stuck in open loop (coolent temp too low). if you have a safety-stat, toss that crap out and get a real t-stat, those safety units are JUNK.

Great, and I have one of those stats too.....
But luckily it works good, my temp gauge in this recent cold DOES get up to between a quarter, and a third. But this is on a B3. Also I have a fan switch and have let it get a bit passed half way on the gauge, and it didn't stick open. I would change it, but that is a headache I don't want, since my housing and stat are JBwelded to the head. Long story short, sheered one of the two bolts, and there was no room to possibly drill and retap for a new bolt, so I took the cheap, and easy route.

But yeah, either change the stat, or throw some cardboard over the rad.

[1chrisapple]

Plugged Cat can do that to you too. Has happened to me more than once; piece breaks off inside and restricts the flow. Fuel economy and power then drops like a stone.

plugged cat usually accompanies overheating some tho not always tho

[Bert]

plugged cat usually accompanies overheating some tho not always tho

Never did for me. Only symptom was loss of power.

[bravekozak]

You already have the electric fuel pump. I have an extra set of twin carbs + intake adapter + choke cable + air cleaner + twin carb badge. All you have to do is jump the fuel pump relay and add a bypass regulator. Low idle (500 rpm) will work again, no more O2 sensors, no more codes, no more headaches, instant throttle response. Ask your friends in Ohio and Pennsylvannia to show you theirs.

[1chrisapple]

Keep fuel injection just fix it

[zoom zoom]

What's a catalytic converter?

Twin carb conversion sounds interesting, I'm not sure I want to do it with this car but I am tossing ideas around for next years project.

[Spike]

^^Haha. Catalytic converters are simply a very valuable piece of scrap metal to me, too. :p

O'Reilly's sells the correct t-stat you need, it's for a Mazda Protege, I believe.. the two-stager. It got me +2 MPG in the Auto.

[zoom zoom]

^ WHAT? +2 mpg?! I might check into that lol, I ended up buying a gates brand 195* thermostat. The old thermostat could be opened with one finger, but even with the new one in the temp gauge isn't reading hot. I swapped the thermo housing for the Festiva one and cleaned the sending unit with a wire brush while I had it out... Didn't do me any good.

On the bright side arty, my idle issue is fixed, occasionally dips down to 500 but it isn't stalling or anything. I netted 32.6 mpg on the trip to and from Ohiosteva and I was cruising 75+ on the way home, over 120 miles straight. Haven't seen those numbers in a long time it seems like. I'm gonna run this tank down and see what the mpg is like then swap over to stock size tires and rims up front to see if the hugemongous tires are responsible. Need to rule that out.

Also did two seafoam induction treatments back to back and put 1oz of seafoam in each cylinder for an overnight (11 hr) soak. The car smoked a little but not nearly as much as I thought it would.

I can't remember the digits exactly but I think it was 125 140 125 119 compression or something wasn't it? I'm gonna do another one tomorrow.

[WmWatt]

Isn't that compression low? My Haynes manual says standard is 204 PSI and minimum is 149 PSI.
Compression on my Festiva with 120 km last summer was 185,187,187,185 dry and 190,192,190,187 wet.
There's not supposed to be more than a 5% variation over all cyclinders.
Notice on mine the lowest is #4 as well. Wonder if there is some reason for that.