wiplash

My 1982 5speed manual 928 runs great but gets horrible gas mileage.I bought this car for $400.00 with a blown engine.The car had only 50,000 miles on it.The old engine had a very unfriendly 10 mm bolt in the number five cylinder(hence the cracked block ,bent valve, and a pecullier sight hole through the piston to the push rod). I replaced it with a 1984 4.7 liter with 51,000 miles on it from 928 international.I replaced all sensors with new ones and checked the ohms to make sure they are functioning properly, after I got the engine running.I have no idiot lights on any more after hours of chasing ghosts.I replaced all the vacum hoses.( how much vacum should I have?) Right now at idle I have 15 psi.)Of course I put in new spark plugs.The car already had a set of headers, and everyone who has looked at the catylitic converter says its aftermarket and do not change it.I did replace the mid muffler and muffler with a single magnaflow muffler that I could see through(it sounds great ).I have it timed at 8 degrees at idle without vacum advance, and about 26 degrees at 5000 rpm.(is this right for an 84 4.7 liter?)I also am not running the smog pump or the pulley fan.I have an extra electric fan on the inside of the radiator.This car never overheats. not even in the middle of summer in a traffic jam with the air conditioning on,(Yes it still works great).I do keep the belt in the car just in case.As far as the mixture screw goes I have turned it in all the way to get it to smooth out the idle.I constantly check the smell of the exhaust, the suet build up, and the color of the spark plugs.It apears to not be rich at all.(I do not have the right equipment to check it properly) but it gets between 8-12 miles to the gallon.I have put 12,000 miles on the car since the swap.This is my daily driver so I need the extra mileage.This car seems to run great. the only other 928 I have driven is Jim Baileys at sharktoberfest and my car is very comparible in the feel of power. We will find out for sure on the 27th as I am going to go racing with him and his friends at willowsprings. Any suggestions would be greatly appreciated, also Does anyone know a good mechanic in the orange county area who can do a final tune up on it with an oxygen sensor. You know someone you would trust with your daughter.This is a last resort because I have never taken a car to a mechanic before, and it is hard to trust someone else in these matters.
Thankyou
1984 4.7 liter
how much vacuum should it have at idle?
what is the right timing for it?
How do I check the richness of it?
What gas mileage should I get?
Who is the mechanic for the job?

ViribusUnits

I get around 20 mpg in highway situations, sometimes as high as 25 at times as low as 17. I run like a bat out of hell. Much of the time on the high side of 75. Illegal I know, so I will deny that at any future point.

Around town I get more like 15 mpg.

Your fuel mileage on the highway is almost certainly lower than mine. You final drive is lower than mine at 2.75, verse mine at 2.2.

I would say check your O2 sensor. Do a search on the site of how to check it. It's fairly easy. From there, you can check your mixture. It's very easy to do, but is not perfect accurate.

Also, check for a plugged cat, just to be sure. If the car has run rich for a period if time, it can plug that cat, and then you'll need a new one. Since your not running a smog pump, this is especially possible. The smog pump supplies the extra O2 to burn of the fuel used to enrich the engine when you’re on the throttle hard, or when the car is just started. W/o this O2, is possible to sort of coke the gasoline onto the cat until it's basically carbon, plugging it, and ruining it.

Oh, I did a search on the archives. And found this for the 4.7l engine. Btw, just about all of the L-jetronic engine's specs. Are the same, accost the board.

" Around 23 degrees at 3,000 RPM, 31 at 6,000"

Oh, I did a search on the archives. And found this for the 4.7l engine. Btw, just about all of the L-jetronic engine's specs. are the same, accrost the board.

" around 23 degrees at 3,000 RPM , 31 at 6,000"

wiplash

Thanks I am warming it up now to check the timing and O2 sensor. What seemed like a good idea may just be another way of figuring out how to spend more money on this car than I need to.This is the only time since moving to California that I wish I was back in Kansas so I could rip the cat and the smog pump out.Did I really just write that?

ViribusUnits

I severely dough your getting 15 psi of vacuum.

(Atmospheric pressure is a mere 14.7 most days at sea level.)

wiplash

Thats just what my $5.00 vacuum gauge said. Sorry about that.

SteveM928

If I had to guess, I'd say that it said 15" Hg, and not 15 psi.

SharkSkin

probably 15 inches of vacuum....

ViribusUnits

If it's 15" hg, thats rather low. If your idle is all right, I would be really looking at a pluged cat.