I just recently picked up a 1993 Honda Civic VX. It currently has 191k miles, pretty good shape all around. The previous owner did some things I'm not fond of, which consists of:
- DX exhaust manifold
- 2.5" exhaust piping after the catalytic converter to a loud Greddy muffler
- Aftermarket intake
- DX transmission (much shorter final drive ratio)
I am currently working to get it back to stock-like condition. Here's what I'm working on:
- 1 3/4" piping from the catalytic converter back to a stock-like muffler (or may go smaller? couldn't find a straight answer about what stock exhaust diameter is)
- HX transmission for longer gearing (got a good deal on it and couldn't find any VX transmissions locally, it is pretty close to VX gearing however)
- Smaller diameter intake pipe
- Tune up (cap, rotor, plugs, currently has NGK wires)
- New IACV from Honda (will occasionally try to die when slowing down, traced it to a code 14)
- Genuine Honda PCV valve which is the correct "one-way" type design, had an aftermarket PCV which I could blow air through both ways
- Put a bottle of Redline Fuel System cleaner before fill up
- I work at a shop and upon the first oil change, I put 5W-30 into the engine, ran for a few minutes, then repeated that two more times. Oil went from being drastically black to a normal golden color
Any other tips to try to get my mileage to improve? Currently on my first tank, almost at 290 miles and gas gauge is right around 1/3, but I notice it jumps around a lot, especially after making a turn, it will raise or lower significantly until it settles again. The gas gauge hardly moved for the first 150 miles and then drastically declined, I've read this is fairly common though?
Welcome. Nice project, there are a fair few Civic fans here of whom I sure can help with your questions. As for the fuel gauge, if you fill the tank right up to the filler, it will always take a long time to move. This is a common perk on most cars, not unique to Civics. Mine will do an epic 340 miles before the gauge moves, and then drops a whole quarter in just a few miles, so you see it happens on new cars too!
Yes, the fuel gauge seems to peg at a level below what the tank fills to on most cars.
Is it a federal or CARB VX? CARB models have restricted, or simply don't have, the lean burn ability of the engine. If it is CARB, you could swap in a federal ECM to get lean burn. It will result in higher emissions from the car, and there may be legal issues involved because of that.
Do you plan to swap the exhaust manifold for a VX one? I admit to not knowing for sure, but I think that is more important than the exhaust pipe size after the cat for the operation of the engine.
I'm fairly certain it is federal, but is there a way to confirm?
As far as the manifold goes, I could certainly try to obtain one, however I feel that it wouldn't be worth the price of the manifold and catalytic converter as it seems to be quite pricey. There actually appears to be a CX manifold in my area at a local yard, I believe the VX and CX share the same manifold?
After doing some searching, it would probably take $300+ to convert the exhaust back to stock form.
So after looking into some things, I recently Seafoam'd the car and saw a leak between the manifold and the front pipe so I will be replacing that gasket.
And initially I believed my IACV to be bad, but upon re-checking codes I am no longer getting a code 14, or any code for that matter. I will be replacing the PCV grommet that goes to the breather box and see if that helps at all with mileage and idle.
Decided to bite the bullet and buy all stock components for rebuilding the exhaust. Cost me around $375 for everything from the exhaust manifold to the muffler. This should bring me back to the mileage I'm hoping for.
Replaced some more parts, idle air control valve, fuel filter. Also replaced ALL the vacuum lines in the car. Unfortunately this did not change my idle issue. I proceeded to adjust the idle according to the service manual, which shows it as a two step process. I'd highly recommend doing it this way if you haven't already. I used an external tachometer tool to help me get it dialed in just right.
It helped a bit but I found that it would still die occasionally when I would put the clutch in to slow down. So after doing some research I found that some people had luck with replacing the MAP sensor o-ring. I did that by getting a $3 o-ring from my local dealer (had to be special ordered). However, the issues are still apparent. Tonight I tried another thing, removed the throttle body and cleaned out the intake manifold really well with throttle body cleaner. Then removed the backup fuse for 10 seconds and re-learned the idle. We will see tomorrow if that helps.
Also, stock replacement exhaust parts are here and will be put on tomorrow or Saturday.