CRX VX Conversion Completed - Some Questions
 

Well, I finished a VX conversion on my father's 91 CRX HF. After troubleshooting some check engine lights, distributor related, we got the car running great. Everything functions as it should, and there are no codes. The questions I have is this...

After running two full tanks of gas through the car, my father is only getting 45MPG. This is the exact same mileage he got with the original HF engine in the car. It is summer time, 90-100 degrees, and he is using the air conditioning consistently.

Would using the AC cause the car to get 45MPG, as opposed to the expected 55-60MPG?

Can the LAF sensor be "bad" without causing the Check Engine Light to illuminate? He says he can "feel" the slight stumbling at cruise from what I assume is the lean burn kicking in.

The enigne is very healthy, runs excellent, has had a full tuneup, timing is dead on, spark plugs are new OEM NGKs, fuel filter is new, air filter is new, etc. The car runs as if it were a very well maintained VX Civic.


When I had my HF, I got between 49-55MPG, my father never crossed the 51MPG mark, usually aroun 48-49MPG without AC. I tell him he needs to drive gingerly to get the great fuel economy numbers, but he says "I'm not going to drive like grandma", and "sometimes I need to use the throttle to avoid being a hazard pulling out in traffic."

I almost want to ask him to trade vehicles for the week to see what kind of numbers I can pull out of it... just as a sanity check. Suggestions?

Pics or it didn't happen!

AC will definitely make a difference, especially in the high heat where the compressor will be engaged most of the time. I've measured as much as a 20% drop in fuel economy from running the AC in the summer on my old Civics. The compressor eats up a good bit of power- which the Z1 engine doesn't have much to spare. I live in Texas though, so the AC is on all the time.

Pedal behavior makes a HUGE difference too. I just took a long road trip in the family Mazda 5- trying to catch up to my in-laws, I got only 20mpg on the highway. With the car set to cruise at 70mph the whole way, I got 30mpg. Big difference.

I think you already know the answers, judging from your post. Running the AC and leaning on the throttle, dad is keeping it out of lean burn and using a lot of power for accessories. To be sure, either he can learn to drive hypermile or you could swap cars for a week, as you're suggesting. And post pics, man!

I agree with Cali, your father is keeping it from reaching its lean burn. I also agree with you and swap cars for a week, show him what mileage it can really get!!

I can not confirm the 10 MPG drop on my old VX (sold) but the 2002 Insight (sold) mileage hit was severe with AC us, even minimal AC use cost 10 MPG.

regards
Mech

Are you using the HF or VX transmission? if I remember right the HF tranny will give you slightly better gearing in 5th gear and altho that will not help with the A/C using more gas, it will help the rest of the year.
I would say that if the A/C is running as much as it is, make sure that it is in perfect working order, that the belt is not to tight, that it is cooling properly, that the coils are clean and that the fan under the hood is working so that it is not running more then it needs to run, if you can, it might be worth figuring out how to reduce the cabin area by putting a clear partition behind the drivers seat so you are not cooling the huge greenhouse-like back portion of the car, that would allow you to reduce the load on the A/C and thus improve your mileage without reducing comfort.

How is the rest of the car? do you have light weight HF wheels and properly inflated tires? is the alinement right? stock CRX HF exhaust?

That's a very helpful response, thank you.


I really believe it's the A/C. It's frustrating because he's disappointed in the results, since some here have seen MPG in the 60+ with this conversion. His mileage is no worse than the old D15B6, and he now has a newer engine with half the miles of the old one and 20 more HP when needed. Even with the same mileage the overall improvement of the vehicle is there. I know the real gains won't be seen unless he changes his habits, but old habits die hard... ya know?

The next time he's over I'll snap some pics. It's not much to see... at least to me anyway.

Once we have a forcast week in the high 80's to low 90's I will. I'm a shirt and tie guy at my job and I can't drive to work in this heat without AC and remain presentable by the time I get there in 100 degree weather. Haha!

That's great info to have, thanks!

HF Federal Transmission. It turns out the relay for the secondary AC fan was bad, so we fixed that last week. We'll see if it helps.

The rest of the car is in good shape, it was a "loaded" HF, meaning it has bot side-view mirrors, a clock, and A/C... haha. The alignment is good, and I told him to have the toe set to as close to 0 as they could get it to minimize rolling resistance. It is on stock 13" steelies with stock size tires, so no rubber overdrive there. It has stock exhaust, as a matter of fact we used the HF manifold and Cat as well during the conversion since the VX motor we got didn't have either. Stock HF intake pipe/airbox. My dad and I are good with keeping up on maintenance.





I'm going to propose the vehicle swap after his next couple tanks if his mileage doesn't improve. If it's still crappy with me behind the wheel, we'll look into a new LAF/O2 sensor.

Sounds like a reasonable and patient plan. I'm sure you'll figure this out more easily than I could, but good luck.

+1 ^^