New 928 Owner - Poor Gas Mileage - Please Help !!

[drola]

i have been a 911 fanatic for many years and still have a few.
i just picked up a relatively clean 1980 928 US version 5 speed with 77k miles
car is in pretty good overall condition
but i am only getting 9 mpg's !!!!
i noticed the black smoke under acceleration so obviously running rich
things i've already checked:
vacuum lines all look good
green wire in good shape
the o2 counter was unplugged - not sure whether to leave it alone or plug it back in
vacuum lines to distributor look good
all injectors were replaced 2 years ago
maf looks ok but i have not tried unplugging it to see if any change
I turned the mixture screw on the maf 2 full turns counterclockwise (which is supposed to lean the mixture) with no change in the way it runs or black smoke under load
maf to throttle body has good, tight fit (o-ring)
is there a way to check the o2 sensor?
i'm pretty sure the hvac system has some vacuum leaks. would this affect consumption?
any suggestions would really be appreciated.
and since i am well aware of forum etiquette, here she (or he, haven't decided yet) is:

[qdac]

I have a 1981 US 5-speed. When I bought it 10 years ago, it was getting 8 MPG on the highway (trip from Houston to Dallas.) This was the trip from point of purchase to my home.

After *much* work, I have plateaued at 14MPG city.

Most likely, you don't have a MAF, you have a AFM (Air Flow Meter). The screw (actually a hex thingy) only affects idle mixture. You probably don't want to mess with it.

My car had disconnected vacuum lines, no lines connected to the distributor, a broken vacuum valve, bad vacuum lines, timing not set properly. In retrospect, I am surprised that it ran so well on the ride home to Dallas after the initial purchase.

I replaced all vacuum lines, replaced the broken valve, insured that the lines were all connected properly, corrected the timing, replace Temp II sensor, replaced the AFM with a rebuilt unit, replaced plugs, replaced wires, replaced green wire. About that time, the OXS light came on & so I reset the counter (you say yours is disconnected) and replaced the oxygen sensor.

I also identified and cleaned the connections around the coil and related resistors.

I bought a set of injectors off of e-bay, sent them to Witchhunter for cleaning. I installed the new injectors along with replacing all fuel lines.

The fuel pressure dampers and regulators have been replaced.

Lots of engine cleaning, replaced the gaskets between the intake pipes and the heads, and replace the rubber seals between the plenum and the pipes, and the rubber seal between the throttle body and the plenum.

I bought this car to work on and make it the best that I can. I have greatly enjoyed the work and the accomplishment, but I will be surprised if I ever get over 16MPG in city driving.

Have fun!

[JK McDonald]

Hey Drola - There can be any one of an assortment of problems with a 35 year old 928 fuel system. When trying to sort out an L-Jetronic mpg issue the 1st and easiest place to start is probably the Temp II (Temperature Sensor Number 2) located in the upper water bridge on the top front of the motor. This sensor tells the computer (located in the passenger foot well right kick panel) to lengthen the injector pulse during a cold start - warm up. If either of it's two wires are disconnected or if the internal resistance of the temp sensor does not decrease as the engine warms up - the injected fuel will always be too rich.

The other temp sensor nearby in the water bridge is for the temp gauge and has no effect on the fuel injection system operation.

QDAC - has lined out an excellent list of in depth troubleshooting steps you should follow. It is just that the L-Jet Computer relies heavily on the Temp II sensor and can cause all kinds of fueling problems. It's resistance should be roughly 2K - 3K ohms when the engine is cold and around 200 ohms when at operating temp. Here is a little more information in the Temp II sensor. In this parts break down it is item #6.

https://www.google.com/search?q=1980+porsche+928+temp+II+sensor&tbm=isch&imgil=tOVlN332SlPndM%253A%253B2vajewjNr6zf7M%253Bh ttp%25253A%25252F%25252Frennlist.com%25252Fforums% 25252F928-forum%25252F97570-temp-ii-testing-and-results.html&source=iu&pf=m&fir=tOVlN332SlPndM%253A%252C2vajewjNr6zf7M%252C_&usg=___BL15uKRz6mTHHbJFAmtP_KXFLs%3D&biw=1252&bih=610&ved=0ahUKEwiEvIDbgqHQAhVqi1QKHfPvCbcQyjcIaw&ei=EuQlWITrMeqW0gLz36e4Cw#imgrc=tOVlN332SlPndM%3A


Keep us posted, Michael

[drola]

guys, i appreciate the comments
qdac, you are correct, my car has an afm not maf. duh, i should know this. i don't think mine is in as bad of shape as yours was to begin with so i will stick to the simpler options before declaring war on my car lol. that being said, i do agree with you that sometimes it makes more sense to just go over the entire system than trying to find a problem piece by piece. i use this approach with my cis 911's. i find it easier to tear the whole thing down and start from the beginning. but then again i find the 911 cis much simpler than that of the 928 even thought they are basically the same thing with a different layout.
michael, thanks for the insight on the temp sensor. i wouldn't have thought about this as my temp gauge works fine, so now i will definitely check on that.
all that being said, i checked the fuse panel and found the O2 sensor disconnected. i am left wondering why. was it bad? was a previous owner trying to accomplish something, or the shop who did repairs trying to mask something? i don't know but i plugged it back in and will see what happens. unfortunately, the guy i bought the car from had zero mechanical ability and asking him would be like asking my dog so i'm left to my own devices.
consumption aside, the car runs great !

[drola]

Originally Posted by JK McDonald

It's resistance should be roughly 2K - 3K ohms when the engine is cold and around 200 ohms when at operating temp.

Is this resistance measured between the two pins or each pin to ground? i was researching this and found different opinions. my car is a 1980. apparently later cars had a different temp 2 sensor

[JK McDonald]

The earlier L-Jetronic Temp II resistance from 1980 to 1984, is read between the two pins. It's internal resistor is physically mounted between the pins and isolated from the case.

The LH-Jetronic from 1985 - on the Temp II resistance is read between each pin and the case ground. There are two separate resistors in series internally. One end of each resistor is connected to a pin with their common center point connected to the sensor case. In this system the LH-Jetronic vehicle has two computers. One handles the ignition and the other the fuel injection. Each of these computers uses 1/2 of the Temp II sensor for it's own separate temp reference.

Good Luck, Michael

[qdac]

FYI:

The 928 uses separate temperature sensors for each distinct function. On my 1981 US 5-Speed L-Jet:

Temp II is L-Jet engine (water) temperature and is on the top of the water bridge.
Temp I is L-Jet intake air temperature and is a component of the AFM. It is not separately replaceable.

The engine temperature gauge is a separate sensor, also on the water bridge.

There is a Thermo-Time switch, on the front of the water bridge, that controls the cold start valve.

The A/C system has two temperature sensors, one in a front wheel well (outside air temp) and one in the dash (cabin temp).

That's all the temperature sensors that I have found so far in this car.

Only Temp I and II will affect fuel management. If Temp II is disconnected (loose) then the L-Jet will run your engine so rich that it will not start. I have heard horror stories of mechanics adding a small resistor to this sensor to make the engine run a bit richer - look for this.

If any of the connections to any of the injectors are loose or corroded, then that injector won't fire and your engine will run poorly. I had to fix several of mine.

Look for burned or corroded wires in the L-Jet harness.

The L-Jet injectors ground to the passenger (US) side engine cam tower in the middle where you can't see it. There is also a ground at the front of the engine on the cam tower - make sure you find both of them.

From the horror stories that I see on the boards about CIS, you should find the L-Jet trivial to work on!

[drola]

Again i appreciate the input.
I will look over all these things bext week. Heading to daytona in the 964 for the hsr historics race this weekend so i won't be able to look at it till next week.
From what i gather, coming from my 911 background, the injection on my 1980 seems to be a mix between the cis and the motronic systems of the 911's. Both of which i have a pretty good understanding of. So i am hopeful that i WILL get to the bottom of this.
On another note, after plugging in the O2 sensor last night and driving the car today, the car feels better. Hard to really describe it, and it may just be psychological, but it feels a little smoother, more crisp, more sensitive to throttle input. I have to run through tanks to calculate mileage so it will take some time to verify improvements.
Meanwhile, i'll just drive it and drive it :-)

[drola]

I checked the temp II sensor and got 2100 ohms cold and 230 ohms warm so seems like that is not the problem

[stepson]

Can't speak to an L-Jet car, but on the later cars with LH and MAF, disconnecting the O2 sensor by passes some of the fuel maps, and increases the fuel that is distributed to the various cylinders to a point (a maximum point). I had an issue with my '89 that would continue to add fuel to the cylinders until there was so much fuel the engine would not run. I could reset it by turning off the key, but it would continue to add more and more fuel until reset. I was able to get home by disconnecting the O2 sensor which apparently limited the amount of fuel that the computer could add.

Reconnecting and/or replacing the O2 sensor might help with your fuel consumption.

[drola]

Last tank, 8.3 mpg
I bought a new o2 sensor. The old one was cut at the top so it was disconnected. Thing is, there is not enough room between the exhaust pipe and torque tube to fit the sensor which is why the sensor had the wire sheared off. Am i missing something here ???