Low CRX HF Mileage
 

Didn't know quite where else to put this. I have a 1990 CRX HF. I purchased it knowing it needed a little work. I have replaced nearly all suspension bushings, front wheel bearings, EGR system has been cleaned, PCV parts all replaced. Valve stem seals replaced. Found a PO had put a Si ECU in the car. And, have since replaced it with the proper HF ECU. Car has a header/no cat. Has passenger side mirror. Also, it has oversized tires(185/60-14) where as it should have 155/70-13 I think. The electric load detector (ELD) is bad according to the ECU. That is the only code it is giving me. I am working on replacing it now. My mileage is all over the place. I have a 2001 BMW 325xi wagon that I can get over the rated milage on driving the same roads as the CRX. I do not expect the CRX to get the 50 or so that I have heard tales of. But I was hoping a consistant mid to upper 40's. I just cannot believe that the tires could cause that huge of a drop. I am getting mid to upper 30's. I drive the car very gingerly. Always aware of the MPG and my goal of the mid 40's. Last fill up was one of the best at about 45 mpg but I drove it last week and it has dropped. Where as I was getting 150 or more out of a half tank I am now at 110 at a half tank. I know how to check mileage. I drive to about 1/4 tank left, fill it up and divide mileage since last fill up by the amount of gallons needed to fill it up. The is getting very frustrating. I bought the CRX because I have about a 150 mile round trip commute. The BMW is real nice and of course much nicer to drive. But could not afford to drive it everyday @ 23 mpg. Can the ELD make much of a difference? I was thinking it might as it might cause the alternator to be switched "on" by the ECU when it really is not needed. Thus causing more load on the engine at highway speeds. Tires I thought might make a 3-5 mpg difference. I just bought the tires. I keep them inflated to about 40psi. They are not LRR tires. The PO had put 14" wheels on the CRX. So, I stuck with them when buying tires. That may be my big mistake. I would just expect mileage to be consistant. If it were consistant then I could start to knock down issues and figure out where I need to go next. The CRX runs great. Any ideas?

Presumably without the ELD, you will not get the leanest mixture, so that will directly affect cruising MPG.

Header & no cat... is there an O2 sensor in there somewhere?

Also: how far off is your odometer with the non-stock tire size? Oversize tires will cause the speedo/odometer to under report, so your "MPG" calcs will be lower than actual.

Yea, o2 sensor is there. Put a new on on. I kept getting o2 sensor errors with the Si ECU. That was before I knew the ECU was wrong. I didn't have the up shift light that the HF is supposed to have. Plus no check engine light when starting. Traced down the o2 issue to the car running rich and no up shift light because of the Si ECU. And, the lack of the CEL light due to a burnt bulb. The car would load up when coasting in gear...it was loading up because of too much fuel. HF ECU fixed all of that. The tire calculators say they are off 2.9% larger/greater circumference. I refigured my last tank and that would give me about 47 mpg. So, I feel better. I was actually trying to figure out if I would be lower or higher when I got your post. I was thinking it would calculate more miles than read. Good to have confirmation on that. I still have an inconsistency issue though. But info from you has helped. Thing about old civics and CRXs. So many parts can be interchanged. To buy a used one you could get quite a mixed bag as I have learned. Add Acura Integras in to the mix and it could be a real challenge to get something running correctly.

Glad you figured out the ECU issues. An si ecu on an hf would suck mighty hard :p

Well, get that ELD fixed. Seems like the last thing on the list.

If it's preventing lean-burn, that would explain a lot.

(I didn't realize the HF CRX had lean burn.)

ELD would also reduce electrical load on the engine, so it's actually a double improvement you could be missing out on.

I do not believe the CRX does have lean burn. If one of the indications is a multiwire o2 sensor then it does not. It has a single wire o2 sensor. I agree the ELD could cause an increase is resistance thus harm MPG a little.

The header and over sized exhaust will drop your mileage as the stock exhaust pipe is around 1 1/4", you have a small engine that is tuned to run at low RPM's.
Check that you have the stock transmision, at highway speed you should see the engine spinning at 2,000 RPM or so.
You might also look in to getting stock HF wheels or Civic VX or Civic HX wheels as even the HF steel wheels are narrower and lighter then any other steel Honda wheels.
It sounds like your CRX has spent to much time in the hands of a highschool kid, they might of even done a "mini me" head swap, it's hard to say, figuring out what you have and what is missing needs to be done if you want to see the mileage that your car should be getting.

It is a HF. I turn about 1900rpm @ 65. With my tire size issue that is probably why I am a little under 2000rpm. I do not understand how the exhaust might hurt me. It should breathe(exhale) extremely freely. The exhaust appears to be stock size. The cat was probably bad and the header was the cheaper route for the PO. The head appears to be correct. It is a 8 valve head. The springs are very weak. I can just about push the valves down with my hand. I know BMW put extremely weak springs in the 325e engine. The "e" indicated economy. They can get 35 mpg. If I correct my distance for the bigger tires my last tank is 47mpg. I fixed the EGR valve previous to the 47 mpg run. It was not sensing the movement. And, the EGR ports were completely clogged. It just seams that my current tank is not giving the mileage of the last tank. I am driving the Das Wagon this week. Will go back to the CRX next week.

exhaust fallows some weird fluid dynamic laws, because it's pulses not a steady flow, it's true that if it was a steady flow then larger would be better, but gas engines rely on scavenging to operate correctly, that is that the one exhaust pules helps to pull the next pules, just like a musical insterment, you get a guy with little lungs on a tuba and he can't keep the flow rate up to produce a tone, a tone that is produced at the lips by pulsating the air.
If you would like to see an exhaust calculator you can look at Autolounge.net | Calculators | Exhaust Pipe Sizing you just put in the engine size and speed that you want it to run at, same rules for high speed high power engines as for low speed high mileage engines, after all it's just math.