Bad gas milage on 79 OB
 

Guys,
I have moved and have been driving my shark on the commute. (40 miles each way) Some times this can take an hour or more. It turns out that i am getting just 3 days commute per tank. That is crap gas millage. The car seems to run well even if it seems to accelerate a little slow. I mentioned this on earlier threads but folks said that was the automatic. I mentioned this gas mileage on another thread and a couple of folks said something is wrong. I am considering buying a commuter car that would get better mileage but would rather spend the $$ on the shark. What would your suggestions be? I have done the R&R on the fuel system. TB/WP, air pump removal, new vac lines, new WUR, new plugs, wires, cap etc... I had the guy at Eurowerks set the air/fuel mixture. My thought is that the timing could be off one notch. Could this cause the symptoms? (Car runs great but bad mileage) Oh, the tail pipe is black as the ace of spades and i have to clean the black sutt off the back of the car ever so often.

Thanks in advance for any wisdom you might offer:)

Thats where your gas mileage is going...out the tail pipe in soot.Its running to rich.CO needs to be set to .75 to 1.00 max.This should get at least 15mpg around town in traffic.Should get 22 to 25 mpg on the highway with cruise speeds in the 65 to 70 range

Thanks 928Fixer I had it adjusted by a pro at Eurowerks but it is still running too rich. Should i start suspecting the fuel dist?

Do you have a cat?

When my ol Euro S was running way rich when I first bought it, anything behind the tail pipe would be covered in black soot when I revved it up.

Frank-- Did your 'pro' set the mixture using an exhaust analyser looking at CO and HC, with the sample taken upstream of the cats? If so, your fuel system is fine. That CO test will very quickly help you figure out if the CIS is OK, at least at idle and low load. The HC looks at combustion quality.

Soot out the tailpipe usually means poor combustion. Sometimes that's from too much fuel in the intake, but more oftem is a sign of weak ignition. How do your plugs look? Are they wet or oily when you remove them immediately after a hard run? What color are they? Is that color consistent on all plugs?

Consider how long you've driven since the last major ignition tune-up too. Caps/rotors/wires give out with mileage for sure. Wires also deteriorate just sitting on the car, albeit very sowly if the car isn't driven. Copper plugs can benefit from regular replacement, with intervals dependednt on the engine and how you drive. I found that 50k or three years was the limit on wire life in my Ford, 20-25k payback on plug replacement.

You can check the cam timing after pulling the two belt covers. Do it just like you would following a TB change, where you rotate the engine to TDC on the #1 compression stroke and look for the marks on the cam gears.

Ignition timing is affected by a few things. The basic mechanical timing setting is important, but there is also vacuum and mechanical/centrifugal advance to verify. Using a timing light, you can see the effect of removing the vacuum hoses at the distributor to verify that the hoses are connected at the other end and aren't leaking. With the two hoses disconnected and plugged, you can verify that the mechanical advance is working by watching the timing change as you rev the engine. Some folks recommend adding a few degrees of advance beyond the original factory recommendation, using the distributor. It supposedly wakes the car up a bit. I have no personal experience doing this to a 928, at least with verifying the 'wake-up' part.

A partially-plugged cat will wreak havoc on the engine and performance, including fuel mileage. A rich mixture or an ignition miss will trash a catalytic converter darn quickly. If the honeycomb substrate in the cat starts to melt at one end or it gets fouled by soot, your performance and mileage will suffer dramatically, especially at higher RPM's and loads. You can diagnose this with a vacuum gauge on the intake in a few minutes, by the way.

Thanks guys for the great suggestions. The car does not have a cat nor does it have an air pump. It is just running rich. I have replaced everything i can think of to no avail. I will mess with the co2 again. The pro didn't use the exhaust sniffer. His computer was down that day. I guess Rixter can set me up with someone in Raleigh that can help.

Thanks,
Frank

Ok Guys, I went out and pulled the left cam cover. (Left facing the engine from the front of the car) I lined the tdc mark on the crank exact. Now the mark on the left cam is exact, right on the money, them mark on the right is about 1/2 tooth to far right. IIRC it should be off to the left (inside) instead of the right.
Now to change it, can i just remove both upper covers, line the crank at tdc, loosen the adjuster and move the cams?

Thanks,

Frank, You don't know that the cam is out of time. You're just guessing that it should be half tooth advanced and not half tooth retarded. I wouldn't go messing with the cams at this point.

Do the checks called out by DrBob. Check your plugs and then check your spark advance with a timing light. On your CIS module there is a mixture(CO) adjustment that can be tweaked by using either a sniffer or by guess method. The guess method isn't very accurate, and the CIS adjustment is very sensitive.

The control pressures are what make the system work, and the pressures are counter-intuitive. Meaning, that low pressure results in rich mixure. The warm up regulator has adjustments as well, and the whole thing needs to be calibrated with a compound pressure gauge and fittings.

Here are the pressure readings for the 928 according to Bosch:

Cold Control pressure; 17PSI @ 50F, 30PSI @ 75F
system pressure; 65-75PSI
Warm control pressure; 41-46PSI (note that this is leaner tahn cold setting)
Residual pressure; 25PSI for 25 minutes.

Go back to Euroworks, and if they didn't check these settings it's a waste of time to change anything. The adjustment of the CO mixture is only valid when the pressure is set up right. I'm fighting this very problem on a different car, but suffering the same thing you are with too rich mixture. Most likely is the Warm Up regulator has a small pinhole in the diaphram and that is letting the system pressure bleed off too low, and causing the rich mixture situation.

Doc

I would sure love to see an older CIS car get this kind of mileage. My 78 when it was near new never did any better than 16 on the road and more like 12 around town. After 40 MPG with a 914 and almost unlimited reliability the frequent and expensive rapairs and poor gas mileage came as a shock.

Besides the WUR which is the real heart of the system check the cold start valve. If it is dripping then you will run rich no matter how many adjustments you make.

As I understand it the WUR is adjustable. There is a 911 post somewhere on the net that shows how to put an adjustment screw into the WUR. The actual factory approved method of adjusting the WUR is hitting it in the proper spot with a hammer. As I understand it WURs right out of the box can be bad or poorly adjusted too.

Good luck with this one'

Dan the Pod Guy

Thanks Guys,

I think at this point i will just admit that i have this thing adjusted as well as can be without the proper tools. I will contact Rixter and get a good wrench here in Raleigh to do it correctly.

Frank,
European Performance on Hillsborough... it's about 5 mins from your office
the owner/head mechanic Jimmy Ellison is a genius w/928s
919-851-5084

Frank,

I would not get too concerned about your cam timing. Based on your explanation, the timing marks are normal. The WSM states that when the driver's side cam is aligned with the mark, the passanger side cam may be off a bit (about half a tooth or so before 12-o-clock).

Take care of the easy wins as the other guys have already mentioned - give the OB a tune up.

Additionally, I would not be too concerned with the wur or the cold start valve if your shark starts and idles good. If so, you mixture should just need a tweek to the lean side. I have found that when your mixture is close, an adjustment as small as an eight of a turn makes a big difference.

I'm with podguy, if I ever see over 19-20 mpg I would be suprised.

My 79 only got around 7 mpg when I first got it.I could not beleave that a 4.5 litre 230 hp low performance V8 would get that poor a mileage.Hell... my other driver at that time was a Pontiac GP with a 400cid that got 16 to 18 back and forth to work and 25 mpg on highway trips up the east coast and it weighed over 4000 lbs.Thats when I started to go to work on the CIS system.Found that the CO was set at over 6%.I started to lean that out,first to the factory setting around 2 to 2.5.Car ran good,even felt stronger when run hard.Now...fast foward to the next morning to start car up to go to work.Now that was fun.start stall,start stall,and on an on that went.Thats why they had set CO at 6% ...to cover up other problems with the CIS system.In the end after sorting out all the other issues I have CO set at 1.0 to 1.25% at warm idle,Cold 30 degree control pressure set at .75 bar and fully warmed up control pressure set at 4.2 bar.I also added in the quest to get better fuel mileage,especially on the frequent highway trips north,a dual diaphram dist vacuum unit that has ignition advance and retard.I forgot what year it was for but I still have the box somewhere as it has the orginal vacuum unit stored in it.

Hey 928Fixer,
Sounds like you have yours sorted out pretty well. I think i already have the dual diaphragm dist. It has two vacuum lines going to it. If not i would love to hear more about yours. I would love to get this car getting good enough mileage to continue to use it as a commuter.

Thanks,