Carpokes Porsche Forum

A while back, I took over my dad's '79 450SL. Put a ton of work into bring it back to its former glory - pics below.

In the meantime, it failed CA smog testing pretty badly last week -- super rich -- so I'm going to dig in. Basic tune-up stuff probably doubled to available power from when I first got it (along with fixing the throttle linkage to the throttle now actually opens past half way....).

At any rate, with CIS and no computers, it's hard to 'tune' the AFRs on these cars without an exhaust gas analyzer. So....I'm going to put a wideband O2 sensor into the exhaust so I can finally see what's going on. I've also ordered fittings to check the CIS fuel pressures, to see if the WUR or something is clogged or malfunctioning. Something ain't right. It's possible I just need to adjust the mixture, but I'll find out soon enough.... Most vacuum leaks have been fixed, with the exception of a smallish leak in the intake gaskets. Figured I'd keep a running account of my antics here, as plenty of older Porsches use CIS and I know a couple of Carpokians have SL's hiding in their garages.





Here's where I'm thinking of putting the wideband. I figure in the cross-over pipe it will have a blend of exhaust from both sides of the motor, albeit skewed to that one side....

Pretty car!

That's a good spot for the sensor. Pull the pipe out and have a couple bungs put in, one on each side.

CIS is its own animal as far as tuning. You are on the right track with getting gauges and the oxy sensor placed. Fix the pressures (low control pressure = rich...) before you put the sensor in. Fix every vacuum leak. Make sure the vacuum hose to the warm-up regulator is intact. Original fuel hoses are a serious fire risk, but you knew that from working on 911 cars with CIS.

I have the Probst book on Bosch injection, including the CIS section that's one of the two 'bibles', and I think have a lot of that k-jet section scanned/archived so I can print pages and color on them while working. The early US and many later ROW 928's are k-jet cars, as are many other period European cars. There's nothing particularly special about the Merc V-8 version, except that there are individual flow-balancing screws on the fuel-distributor by the outlet ports. Resist the urge to adjust those...

LMK if you don't already have the book and I'll find a way to get at least those pages to you.

I'm an old school book kind of guy so have a couple hundred books on various automotive subjects, including these two I've been using. Is the blue one the one you mean?

I got the pipe out today, albeit with plenty of cutting. I'll be doing my own TIG welding, so if there's enough metal under all the rust, it should be easy enough to weld this back together. I had to cut the end off the pipe because the slip joint was never going to slip again.... Then had to slot that joint further to get the little end stub out of there. Should make for a good welding project to make it all work like new again (with O2 port).


Arrows point to where I had to cut...

Sandblasted all the parts today, and welded up the cuts. I left the re-constructed tail piece off the long pipe for now, as I ordered a new length of pipe that I'm hoping to use instead. Tomorrow, I'll put the O2 port in and weld the long pipe back together one way or another. Lots of rust pitting, but the pipes were thick enough to survive with plenty of strength left over. I'll paint them in POR 15 high temp flat black. Not that I'm doing anything that affects smog, but I've learned to attract as little attention as possible for CA smog checks....

I opted to use a new section of 1.25" OD pipe. It's smooth and perfectly round, so should seal better in the slip joint.


New tail peace TIG'd in place and cut to length.


I had to grind down the welds on the tail piece to give the slip joint enough range to install the pipes. TIG'd the O2 port in place and then sand blasted the whole thing for paint.


Painted with Por 15 High Temp and cured for 2 hours at 400F. I have fairly low expectations for the Por 15, but figured it was better than nothing, maybe. Will put it back together tomorrow and hook up the wideband. I'm guessing it's cruising somewhere in 11 or 12 AFR range right now, but time will tell....

Here's the pipe all installed. Initial testing confirms it's running rich, although 'only' about 13.8:1. I didn't realize how sensitive CO% is to small changes in AFR. The car pretty much needs to be around 14.1+ to pass CA smog, and the CO% shoots up super fast as the AFR drops.
That H pipe is reversible in case I need to check closer to the other side.


13.8 equates to almost exactly what I saw on the smog test (CO% of 1.7-1.8%). Now I just need to test the fuel pressures and see if something is not working right, or if I just need to adjust the mixture.....

Fast forward a year or so and happy to report I just got the car to pass CA smog. The wideband is key. Without it, there's just no way to know how to set the mixture. I set it to 14:7:1 at idle (FULLY warmed up) and drove around to confirm that the AFR's seemed logical when driving -- leaner with less load, richer with more, but centering around 14.7.

The problem the car was having previously was due to an understandable mistake some mechanic made years ago. The mixtures are mostly controlled by the so-called Warm-Up Regulator (WUR) aka Control Pressure Regulator. Problem is, the vacuum lines to the WUR had been reversed for years, causing the car to run rich in general and actually lean out on heavy load -- causing predictable bad manners and bad mpg. I say it was an understandable mistake because the hoses on my car -- a CA-spec '79 -- are the opposite of virtually every diagram you see floating around. It was apparently a one-year only change, and only for the US market. I caught it when checking the control pressures and getting illogical results with and without the vacuum lines. If you seriously dig into the vintage documentation, there is a tiny note in the FSM about the hoses being backwards on this particular car. Too many mechanics in a hurry over the years I guess to actually check the pressures.

At any rate, between getting the hoses right, and setting the mixture right, the car has literally never run better since I've owned it. I had planned to tear about the intake and injection system in an effort to fix an off-idle stumble, but that is now entirely gone after probably 10-20 years and countless mechanics (most of whom recommended a new fuel distributor...). Anyway, the car has been sprung from smog jail so today is a good day.

Those Benz SLs are such wonderful cars, terrific for weekend drives.....classic lines. I had a 87 560SL....so nice to drive and admire! Keep up the good work....values keep going up and up! Another fine example of German Engineering!

Fixing dad's car and making it better than it ever was in his ownership is a great feeling, I can totally empathise with that. Well done Tom.

Wow, that was awesome work to get it to pass. I love those older SL's.

We have no emission test on a car over 25 years old. The problem here is to get them to run well (mostly overall as they bring little money at auctions).

Tom did a bunch of great tech to get a nice car to pass emissions. Most mechanical guys around here can't even work on older German cars.

In addition, the older great Porsche "old school" mechanics are retired. I had a 1969 Porsche 911 2.2L that had older Weber carbs. I couldn't make it work right. I finally got a guy in Texas that helped me to get it running via Zoom.