Klondike

Is there any evidence that aftermarket fuel pressure regulators are more accurate or track manifold vacuum changes better than stock ones? The reason I ask is, I can change the instant mileage while maintaining the same speed. I've noticed that after accelerating up to a speed and setting the cruise control, the instant mileage reading will level off at a certain point and hold fairly steady. If I add just a touch of throttle for a few seconds and then let off till the car settles back to that same set cruise speed the instant mileage will jump up from the increase in manifold vacuum and then settle into a higher number than it had after a steady pull up to that same speed. I'm wondering if the stock spring could be weak or the valve sticking because I can always get better mileage readings by backing down to a cruise speed than accelerating up to that same speed.

aDigitalPhantom

I don't know about the vacuum accuracy, but I doubt it makes much difference. You can get really high instant MPG, by costing down a hill.

I cant tell you what my instant MPG is, unless you believe it is 99 instant MPG, 0 average MPG, and 0 range. :P

Cliff Harris

I have seen 87 MPG when coasting down a steep hill at high speed.

I don't think it matters how accurate the fuel pressure regulator is. The ECM will use the O2 sensor to detect the AFR and adjust the injector pulse width as needed. That's one of the reasons an adjustable fuel pressure regulator is not a good choice. It messes up the AFR in open loop mode when the engine is cold and MIGHT give some advantage at WOT but it would be very small.

Klondike

I know coasting or going down hill will really increase the instant mileage, that's common knowledge and not really what I'm getting at here. I'm talking about long steady state flat land cruising at locked in speed.
If I accelerate up to 75 then click on the cruise, the instant mileage may hover around 24-25mpg. If I accelerate up to 80 and then let the speed drop back to 75 before I click on cruise, the mileage will settle in a couple of mpg higher, usually around 27mpg. Same speed on level ground but with a tad higher reading. I know it's nit picking, but I was wondering ,why the difference, if I click in on the way up or click in on the way down? Just an oddity I've noticed while playing with it on long trips.
(I'm always fiddling with stuff trying to figure out why things happen like that.)

Cliff Harris

The ECM sends the injector pulse width to the dash. The dash uses the injector pulse width and vehicle speed to calculate MPG. Why it's different "going up" vs. "going down" is a mystery.

Thinking about this a little... The ECM uses what is known as "throttle follower mode". What it does is open the IAC as the gas pedal is pressed down and vehicle speed increases. That is used to prevent the engine from dying when you take your foot off the gas and the throttle blades slam shut. Apparently the ECM is adding fuel to compensate for the extra air from the IAC, which affects the MPG calculation.

Klondike

I guess that makes as much sense as anything I can think of. It's not like I can't find something else that really needs to be fixed on this damn thing anyway.

C409

........ I believe what you are referring to is known in the ECM as highway mode ........ one of the factory ECM tuners would be able to explain this more readily than I can ...........

Klondike

Yes, this could just be a computer display anomaly. I have no way to confirm if the instant mileage display differences are real or not. This car always gets better mileage for a tank full than the computer ave. mileage display says anyway, so it's just a curiosity and not a problem.

Cliff Harris

Highway mode is disabled in my '86 ECM. I don't know if it was enabled in later years.

Once you get up to a certain speed the ECM leans out the mixture. I don't know offhand what parameters the ECM is looking at to enable highway mode. Probably some minimum coolant temperature and running time above some speed.