I did the HO swap (kindof) a while ago; I've now driven 400 miles on my new engine and been through a couple tanks.
My first tank, perhaps due to zealous revving of my new engine, perhaps also do to a failsafe thermostat that had gotten itself stuck open for no good reason whatsoever, netted 10mpg: 150 miles, and 15 gallons to refill back to a full tank.
My second tank should have been a fairer indication. I played less, commuted more, and had a working thermostat.
12.5mpg.
My old lopo used to get 18mpg all day long. I think 15mpg was my worst tank, and 24mpg was my best.
12.5mpg is... something might be a little off.
There's another thread here about someone's '87 with a vacuum leak. That should not be my problem. (a) I replaced all the easy vacuum lines, anyway, in the engine bay. (b) until I corrected it by cleaning the IAC and resetting the tps voltage at idle, I was idling at 500rpm. Now it's 700rpm (hot). And vacuum at idle is a healthy 20" Hg. So I could do a smoke test... but improbable.
The engine is running smoothly. Brandy new injectors. No misses at all. So obvious rough-running horrible tuning issues for which fuel economy is the least of your worries are not present. Other than vapor out of the exhaust when the engine's cold on a dreary rainy day, no smoke. Nothing obvious manifesting.
What else can lead to poor fuel economy?
The reason I ask about exhaust leaks is because i never did retighten the header bolts after I installed them. Why not? Because some instructor talked me into putting header wrap on my headers, making installation a *bitch.* I've got the mustang headers on now, btw; with the independent flanges, which didn't help since they'd shifted a little and lining up the holes was hard. I did not use loctite, and there was no space for a lockwasher.
They were hard to tighten, though I DID tighten them, even when the engine was out of the car! Now I don't know how I'd get to half of them at all.
Also, while the oem exhaust has flat flanges, the mustang headers and the mustang h-pipe have a ball and socket. I did not find, and therefore did not use, a gasket for this socket: it seemed like maybe it should just be metal to metal tightened hard.
So I'm wondering (a) if that ball and socket should've had a gasket anyway... and (b) if heat cycles can loosten bolts? and (c) one coworker said he heard a little exhaust leak.
I'm wondering if I really should make the effort to tighten all the header bolts again, then. How tight do they need to be anyway? Just snug, like upper:lower intake manifold kindof tight?
I'm wondering if an exhaust leak upstream of the O2 sensors could let fresh air in, convince the O2 sensor it's lean, and cause the engine to run rich, wasting fuel economy?
I'm wondering if there's a way to really VERIFY an exhaust leak rather than just guessing at what's plausible... and then confirm if it's fixed after tightening bolts. I've never really diagnosed exhaust leaks before.
My first tank, perhaps due to zealous revving of my new engine, perhaps also do to a failsafe thermostat that had gotten itself stuck open for no good reason whatsoever, netted 10mpg: 150 miles, and 15 gallons to refill back to a full tank.
My second tank should have been a fairer indication. I played less, commuted more, and had a working thermostat.
12.5mpg.
My old lopo used to get 18mpg all day long. I think 15mpg was my worst tank, and 24mpg was my best.
12.5mpg is... something might be a little off.
There's another thread here about someone's '87 with a vacuum leak. That should not be my problem. (a) I replaced all the easy vacuum lines, anyway, in the engine bay. (b) until I corrected it by cleaning the IAC and resetting the tps voltage at idle, I was idling at 500rpm. Now it's 700rpm (hot). And vacuum at idle is a healthy 20" Hg. So I could do a smoke test... but improbable.
The engine is running smoothly. Brandy new injectors. No misses at all. So obvious rough-running horrible tuning issues for which fuel economy is the least of your worries are not present. Other than vapor out of the exhaust when the engine's cold on a dreary rainy day, no smoke. Nothing obvious manifesting.
What else can lead to poor fuel economy?
The reason I ask about exhaust leaks is because i never did retighten the header bolts after I installed them. Why not? Because some instructor talked me into putting header wrap on my headers, making installation a *bitch.* I've got the mustang headers on now, btw; with the independent flanges, which didn't help since they'd shifted a little and lining up the holes was hard. I did not use loctite, and there was no space for a lockwasher.
They were hard to tighten, though I DID tighten them, even when the engine was out of the car! Now I don't know how I'd get to half of them at all.
Also, while the oem exhaust has flat flanges, the mustang headers and the mustang h-pipe have a ball and socket. I did not find, and therefore did not use, a gasket for this socket: it seemed like maybe it should just be metal to metal tightened hard.
So I'm wondering (a) if that ball and socket should've had a gasket anyway... and (b) if heat cycles can loosten bolts? and (c) one coworker said he heard a little exhaust leak.
I'm wondering if I really should make the effort to tighten all the header bolts again, then. How tight do they need to be anyway? Just snug, like upper:lower intake manifold kindof tight?
I'm wondering if an exhaust leak upstream of the O2 sensors could let fresh air in, convince the O2 sensor it's lean, and cause the engine to run rich, wasting fuel economy?
I'm wondering if there's a way to really VERIFY an exhaust leak rather than just guessing at what's plausible... and then confirm if it's fixed after tightening bolts. I've never really diagnosed exhaust leaks before.
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