HELP: Porsche 911 sc (1980) Fuel consumption 9 MPG

[athosp89]

Hello,

I recently purchased a 911 SC coupe (1980) and fixed the oil leak and also replaced all the head studs from the one side of the engine because there were 3 head studs broken. Changed the oil and performed a general service.

The car does an average of 9-10 MPG by driving 90% at max 3000rpm.

This consumption is mostly city based but I don't think its normal because i think it should have an average of 22-25 mpg.

Do you know what's wrong?

Also some gear changes are a bit stiff and I 've been told to install a racing short shift. Will that help?

PLEASE HELP

Thank you

[Dave Colangelo]

The first thing to check here is your mixture, you may simply be running rich.

The second thing you need to do is check your fuel system for leaks, these will generally show them selves in the form of a strong gas smell. And by strong I mean really strong.

Im no expert in this matter so I will let some other people weigh in more heavily on this matter.

As for the transmission, a short shift kit is NOT the solution to your problem. Your issue is caused by a few things.

1) these transmissions (915) take some time to heat up. My 78 SC shifts stiff for the first 20 minutes in the summer and the first hour or so in the winter. There is really not much you can do here its just a property of the transmission.

2) Transmission fluid, flush out what ever is in there and put in some SWEPCO you will be happy you did.

3) Change your shift bushings. This does not have much to do with the stiffness but chances are they are worn out and will help with the smoothness.

A short shift kit does not fix issues with the transmission. It simply moves the pivot point on the stick so that you do not have to move the stick quite as far to shift gears. The 911 has a notoriously long throw on the shifter and the short shift was created to aid in quicker shifts (less distance to cover). The issue that this causes is an increase in force on the transmission. Since you moved the pivot point you have increased the torque applied to the shift mechanism by the other end of the shifter. Many have noted that if used wrong this can increase the wear on a transmission. That being said there are many fine cars out there with short shift kits but you should know what you are getting into before installing one.

Regards
Dave

[NYNick]

Frozen caliper?

[tirwin]

First, congratulations! You should post a pic. We all like to relish in newly acquired cars. Think of it as a rite of passage.

Agree with checking the integrity of the fuel lines. If nothing else, it is a personal safety issue. If the fuel lines are original, it is a matter of when, not if, they need to be replaced.

Agree that it sounds like the car could be running rich. Having the car checked by a shop with an exhaust gas analyzer would be the way to go. If you find it is running rich, I would suspect that the overly rich condition is because someone previously adjusted the mixture to compensate for another problem. This is very, very common with these CIS fuel injection cars. You will find a wealth of information about it on this forum. Incredibly short version is that CIS relies on proper fuel pressures and NO vacuum leaks present. A lot of people monkey with other parts because they either don't know how or don't take the time to track down the real problem.

You could also have less than ideal spark, meaning you're getting an inefficient fuel burn. Timing should be checked, as well as the condition of plugs, plug wires, distributor cap and rotor.

Frozen caliper is a possibility and easily checked. Bottom line is that you need a methodical plan of attacking the problem before you start swapping parts. Your wallet will thank you.

2 questions:
1. Did you get a PPI (paid private inspection) done before you bought the car? If so, what did they note? Do you have any other history on the car?

2. Are you the kind of person that wants to do the work yourself or take it to a shop? Not trying to be judgmental -- just want to know how to frame responses to be the most helpful.

PS - short shift kit is not going to help with sloppy shifting. I would focus on one problem at at time.

[Targalid]

You should be getting as much as 20 mpg driving in traffic and 20 to 25 mpg on the highway. Your 9 mpg indicates a problem. Drive a few miles , then feel each wheel hub for heat. If you find a hot wheel, this indicates a sticking brake or other problem such as a failed bearing that is preventing free movement. Otherwise there is likely some inefficiency in the engine. Your mixture may be too rich. Ignition timing may be improperly set. You may have a blockage in the exhaust such as a plugged catalytic converter. Finding the problem simply requires methodical diagnosis which is covered in this forum very well. Search teh forum for each suggestion above and you will find the solution. Also, if you are driving your car at low engine speeds, below 3000 rpm as indicated in your post, this may be part of the problem. Peak torque, horsepower and efficiency are found at higher rpm with these cars. It sounds as if you have spent a fair amount getting your car repaired. You have come to the right place for help and it should not cost much for you to resolve your low mileage.

[SC-targa]

I'd look into the O2 sensor feedback system. Look for a bad relay under the passenger seat or corroroded contacts or a failed computer or frequency valve.

[blucille]

Do the interior lights work? Same fuse as the o2 sensor system

[athosp89]

Originally Posted by sonett43

Do the interior lights work? Same fuse as the o2 sensor system

Yes they work I had them fixed!

Another problem that occurred is that the headlight switch is probably gone as the high beam headlights work at the position where the low beam headlights should have worked and I have to hold the switch back in order for the low beam headlights to work.

Do i have to replace the switch or can this be fixed?

[athosp89]

Originally Posted by tirwin

First, congratulations! You should post a pic. We all like to relish in newly acquired cars. Think of it as a rite of passage.

Agree with checking the integrity of the fuel lines. If nothing else, it is a personal safety issue. If the fuel lines are original, it is a matter of when, not if, they need to be replaced.

Agree that it sounds like the car could be running rich. Having the car checked by a shop with an exhaust gas analyzer would be the way to go. If you find it is running rich, I would suspect that the overly rich condition is because someone previously adjusted the mixture to compensate for another problem. This is very, very common with these CIS fuel injection cars. You will find a wealth of information about it on this forum. Incredibly short version is that CIS relies on proper fuel pressures and NO vacuum leaks present. A lot of people monkey with other parts because they either don't know how or don't take the time to track down the real problem.

You could also have less than ideal spark, meaning you're getting an inefficient fuel burn. Timing should be checked, as well as the condition of plugs, plug wires, distributor cap and rotor.

Frozen caliper is a possibility and easily checked. Bottom line is that you need a methodical plan of attacking the problem before you start swapping parts. Your wallet will thank you.

2 questions:
1. Did you get a PPI (paid private inspection) done before you bought the car? If so, what did they note? Do you have any other history on the car?

2. Are you the kind of person that wants to do the work yourself or take it to a shop? Not trying to be judgmental -- just want to know how to frame responses to be the most helpful.

PS - short shift kit is not going to help with sloppy shifting. I would focus on one problem at at time.

First of all thank you for your response.

Yes I have done a PPI and there was no accident on the car, no rust underneath just had an oil leak. After the mechanic tried to fixed the oil leak he found that the 3 head studs were broken so I replaced all of them from the right part of the engine. The car is originaly from Singapure and it is from 1983 and onwards in Cyprus! (hot weather!) I will post a picture soon!

I am sorry to say that as much as i love cars and especially 911s I dont know much about the technical part of it. :/

[athosp89]

Thank you all very much for your responses! I really appreciate it!

[athosp89]

Originally Posted by Targalid

You should be getting as much as 20 mpg driving in traffic and 20 to 25 mpg on the highway. Your 9 mpg indicates a problem. Drive a few miles , then feel each wheel hub for heat. If you find a hot wheel, this indicates a sticking brake or other problem such as a failed bearing that is preventing free movement. Otherwise there is likely some inefficiency in the engine. Your mixture may be too rich. Ignition timing may be improperly set. You may have a blockage in the exhaust such as a plugged catalytic converter. Finding the problem simply requires methodical diagnosis which is covered in this forum very well. Search teh forum for each suggestion above and you will find the solution. Also, if you are driving your car at low engine speeds, below 3000 rpm as indicated in your post, this may be part of the problem. Peak torque, horsepower and efficiency are found at higher rpm with these cars. It sounds as if you have spent a fair amount getting your car repaired. You have come to the right place for help and it should not cost much for you to resolve your low mileage.

Thank you for your response. So what is the optimal way to drive the car in order to achieve a normal fuel consumption? around 4000 rpm? Also |I have been told it is good to leave the car to run at idle for 1 minute before turning off the engine, is that correct?

Thank you

[Bill Douglas]

Originally Posted by athosp89

Do i have to replace the switch or can this be fixed?

I don't know about the fuel problem - very weird. My SC is actually very fuel efficent.

It's easy to fix the switch. I bend the little brass thingies inside until high and low both work. Give them a bit of a clean while you are at it. Leaving it wired in you can see the headlamps against the garage wall so you get an idea of what's happening as you tweek the brass bits. If that doesn't work Pelican sells them - good luck with both isues

[tirwin]

Originally Posted by athosp89

First of all thank you for your response.

Yes I have done a PPI and there was no accident on the car, no rust underneath just had an oil leak. After the mechanic tried to fixed the oil leak he found that the 3 head studs were broken so I replaced all of them from the right part of the engine. The car is originaly from Singapure and it is from 1983 and onwards in Cyprus! (hot weather!) I will post a picture soon!

I am sorry to say that as much as i love cars and especially 911s I dont know much about the technical part of it. :/

Given the history, we need to establish which engine is in the car. In cars produced from August 1980, the first few characters of the VIN number will tell you:

Character position/meaning:
1-3 / WPO (WPO is the code for Porsche)
4 / body code for US & Canada (A=coupe,E=Targa,J=Turbo) OR Z for Rest of World
5 / engine version in US

On cars produced before August 1980, you'll have to look at the engine serial number. If you don't have a VIN that starts with WPO, can you find and post the engine serial number so we know what we're dealing with?

PS - Tough luck on the head studs! That would've been extremely valuable to know in negotiating price.

[javadog]

Originally Posted by athosp89

Thank you for your response. So what is the optimal way to drive the car in order to achieve a normal fuel consumption? around 4000 rpm? Also |I have been told it is good to leave the car to run at idle for 1 minute before turning off the engine, is that correct?

Thank you

Shift at 3000 in normal driving in town on flat ground.

Idling the car for one minute before shutting it off serves no purpose.

JR

[Targalid]

Just to clarify, I shift to a lower gear whenever the rpm fall below 3000 rpm, especially when under load as, for example, going uphill. The car will run below that engine speed but I try to keep it at 3000 or higher. I have experimented with driving style and mileage and have learned that running the engine below 3000 rpm leads my car to use more fuel per mile driven. Your mechanic may have advised you to not over rev the engine after the recent repair, however, lugging the engine is not good for it either.

[athosp89]

Originally Posted by tirwin

Given the history, we need to establish which engine is in the car. In cars produced from August 1980, the first few characters of the VIN number will tell you:

Character position/meaning:
1-3 / WPO (WPO is the code for Porsche)
4 / body code for US & Canada (A=coupe,E=Targa,J=Turbo) OR Z for Rest of World
5 / engine version in US

On cars produced before August 1980, you'll have to look at the engine serial number. If you don't have a VIN that starts with WPO, can you find and post the engine serial number so we know what we're dealing with?

PS - Tough luck on the head studs! That would've been extremely valuable to know in negotiating price.

Yes unfortunately the head studs were a big hit!
This is the engine serial number is 930 101 103 4R. I will post the VIN later!

So...what are we dealing with?

[tirwin]

The engine serial number should probably be 6310001-on. Maybe there is a different number nearby where you found the number that starts with 930. That number indicates yours is a 930/10. The chassis serial number should be something like WPOZZZ91ZBS1xxxxx. If I'm correct, you have a RoW car. There are differences in the RoW and US cars and we just need to know that -- for example, US cars have some emissions related bits that the RoW cars don't have.

[Reiver]

1980 should be a 930/09 if it is a ROW...lower compression than the 930/10.

[tirwin]

Reiver, according to the red book, the 930/09 was in production until July of 1980 and then it switched to 930/10 in August. When the switch happened they introduced the international VIN.

[timmy2]

Engine serial number is on the right side of the fan housing near the oil pressure sender written sideways down the housing. The engine type should be stamped below it on the flat area.