87 CRX HF fuel mileage

[mrbillhf]

What makes it tick? Not "make noise", but what makes it perform? I am amazed that if I average 53-55 mph and run 2400-2500rpm I get 54-55mpg. But if I go to 60-62mph and run 2900-3000rpm my mileage drops to 45-47mpg. First, I think someoneone has swapped my transmission cause I read I should be turning much slower at those speeds. But my question is: what is the feedback loop? Is there a transducer output from the speedometer? Is the speed sensor the signal interpretter? Does that feed the ecu? What is the logic? Need to know... can't sleep. thanks.

[GasSavers_BEEF]

one thing that you haven't mentioned is the aerodynamic advantage at 55 mph vs those higher speeds. your drag goes up slightly as you speed up to around 55 (I have heard 52) and then it really dog legs upwards after that. your drag is significantly higer at higer speeds.

I would think that would be at least a small factor in it.

[GasSavers_Erik]

Yes- someone swapped the transmission- the hf trans should only be turning 2000 rpm in 5th gear at 60 mph.

There is an electronic vehicle speed sensor built onto the back of the speedometer.

The ecu is under the passengers seat. The feedback loop is the ECU getting info from the oxygen sensor and other sensors about the load and other conditions and then leaning out the air/fuel mixture and advancing timing as much as it is programed to do for those conditions.

[mrbillhf]

Originally Posted by Erik

Yes- someone swapped the transmission- the hf trans should only be turning 2000 rpm in 5th gear at 60 mph.

There is an electronic vehicle speed sensor built onto the back of the speedometer.

The ecu is under the passengers seat. The feedback loop is the ECU getting info from the oxygen sensor and other sensors about the load and other conditions and then leaning out the air/fuel mixture and advancing timing as much as it is programed to do for those conditions.

Thanks Erik, the same day I posted I also ordered an HF transmission. UPS delivered it Saturday as I was leaving. It's now Sunday afternoon and I am ready to unpack it and confirm that it is an HF Fed final drive. As for the feedback loop, do you know of any documentation that might explain the different inputs to the ecu? You mentioned "load sensing", how would that be performed? And do you know does the carburetor model have a knock sensor? Again, if you can point me to documentation/technical literature it would be greatly appreciated.

[GasSavers_Erik]

I was at my public library and found a detailed auto manual that had the wiring and emissions diagrams for the 87 CRX hf. It was one of a set of 7-8 manuals that were written for professional mechanics and were really detailed.

In terms of the ecu knowing the load, this car has an egr valve position sensor as well as a manifold vacuum sensor. A 5th gear switch in the transmission (that tells the ecu when the car in 5th) and the speed sensor on the back of the speedometer. I'm not sure about the knock sensor.

[mrbillhf]

Good info Erik... keep it coming However, I just got an 87 HF Fed trans. Haven't installed yet. It has two switches on it. I put a multi-meter on them. One is a reverse light indicator sw. The other is a neutral safety switch. No way I see to tell the ecu when it is in 5th. Could that have been a CA model feature?
I do see the EGR position sensor piggy-backing the EGR valve. Not sure of the manifold vacuum sensor yet.
And would you happen to have the title, author, etc for that library book? Was that a Honda manual or other?

[mrbillhf]

I would agree it is a factor, definitely, but something tells me there's more going on here. I don't know enough about this little car yet... just got it in Feb. I can never have enough info so if you think of other things please share. Thanks

[GasSavers_Erik]

My mistake on the trans switch- I assumed it was a 5th gear switch.

Not sure on the author. I'll try to get more info.

[mrbillhf]

I have an intermittent problem, 220 miles into this tank it reared its ugly head again. Here is how it typically goes: As I am motoring along the car starts to have a slight miss. Not a steady hard dropped cylinder but a stuttering/fluttering. When this miss starts, the car won't idle, and the engine seems to be very lean.
I carry my tools with me cause I never know when this is gonna start again and I want to try to figure it out. So I pulled over and turned up the idle screw so I could keep it running. Float level was good. The timing was ok and spark was good on each cylinder. And if I slightly closed the choke butterfly I could rev the engine without the hesitation. So I popped the top of the carb and pulled the primary low speed jet and cleaned it. I also blew out all the other jets, re-assembled it, and it ran like a dream.
Maybe you're asking, "so what's the problem"? Problem is I'm 'poking at straws'. Sometimes this procedure works. Sometimes I can just park the car overnight and the next day it cures itself. I don't know if I'm doing anything or not. And I may go another tank before it happens again. Even so, I got 46.1mpg on that last tank. But I want 55.

[GasSavers_Erik]

Here's a shot of the various data inputs for the 87 CRX HF ECU. I still don't recall what the source was.