Checking AFR on SSIs
 

Morning all ...

78 911SC 250k km

Normally I check my AFR periodically with a borrowed meter at the tail pipe and its pretty steady and i can make minor adjustments.

I now have added two or three variables.

1. I bought my own AFR meter ( a kit off Ali so it may be junk but looks good quality)
2. I rebuilt my engine ...
3. I added SSIs.

so i checked it after phase one of rebuilding engine and all was good. That was with the borrowed one and i was able to screw the O2 sensor right into my test pipe which originally replaced the cat and all was good .
I have since put on the SSIs and a 2 in 1 out muffler and all was good. But at that time did not check AFR

So Im planning a long trip and was doing a tune up to make sure everything was okay , replaced plugs, ( did cap and rotor just about 5K ago ) did timing , set it exact at 5 BTDC. Although i did get a little shock from the wires which i thought odd. Im not sure if that is normal as i am providing a pretty good ground path.

Anyway so i went to check the afr with my new meter.. the SSIs have a bung on the drivers bank so i put my O2 sensor in there.. i realize i am only measuring 3 cylinders.

After the small shock i took the wires off the cap as i thought the rubber seals could actually be pushing the wires off the cap , i lightly rubbed dielectric grease on the cap so the rubber boots would push right on all the way and make sure wires were solid,, didnt help much as i could still feel small zaps if i touched it, enough to pull my hand away.

Anyway the AFR was oscillating wildly from 14.00-18.00, so something is wrong, but the plug i took out to insert the O2 sensor was black and i can see where the SSI mates to my exhaust tiny tell tall signs of a leak as they are black with soot. but its after the O2 obviously. All that to say it tells me its rich. so none of this makes sense..

So i am stuck with several possibilities that i would ask an opinion on..

1. Is the shock when i touch the wires down where they meet the the cap while running at all normal ? or should this be 100 % touchable ?
2. Is checking the AFR and where i put the O2 sensor causing erratic readings or should I build a contraption to hang the O2 right at the tailpipe ? There is some sort of test pipe connection in my exhaust but doesnt look like it would fit an O2 sensor.
3. Could my AFR meter just be cheap junk and doesnt work ( thats up to me to figure out.

It should be noted that car starts and runs fine. I only found this through routine checking.

Also I put new Beru wires with ground shield on when i rebuilt the engine about 5 years ago but car only has about 50K km since, if that .

Any and all thoughts appreciated..

10K km per year is pretty good with Ontario winters!

I've been shocked by spark plug wires before, and the car has run fine. It shouldn't be that way, though.

Had this problem for 3 years now ever since I put the SSI’s on my 83. AFR at warm idle is 16.

Can you set mixture with the frequency valve and compare to AFR?

Would be interesting to get data points from others setting AFR with SSI.

My guess is that you have both an exhaust leak and a mediocre AFR sensor.

If you have an exhaust leak on the driver's side SSI, then any reading you take for AFR will be affected by that leak. At High RPM you won't see that variation in AFR but at idle and low RPM driving (where the individual exhaust pulses are more pronounced and not smushed together) you'll see a spike or jump in the AFR number as the gas is exiting the leak and not making it to the sensor.

interesting info ...

so some additional info ..
78 has no frequency valve of any kind.
There is no exhaust leak upstream of the O2 sensor of my afr meter. Based on the tiny traces of black i saw where the muffler mates to the SSi i would expect to see that at the heads and that is all good and sealed. Then there is the O2 sensor so it appears like no leak.
dodgy AFR meter is definitely possible so i might find another car to test it on.

i cant set anything while it is wildly swinging like this. i need precision before i can set accuracy.

Yeah i get my car out first of April and drive it pretty good until the end of October.. I have 250k, ( km) on it now.

edit .. so a somewhat related question.. if i wanted to just test the meter so see what it is doing , could i just pop the O2 sensor out of one of my other cars and stick this in and see how it reads ?? Sure it would pop some codes etc, but i really dont care about that .. would I put it in the precat O2 on a conventional car or the post cat O2 ? ( pre cat on my Audi is super easy to get to . )

What sensor is it? They usually have a defined method to bench check if they’re good- heat them up, check voltage, etc… same with the controller- they usually have some way of checking if they’re good- a startup routine, etc…

My guess is that the one you got is not great and that may be the problem. But any exhaust leak is also a problem so don’t discount that.


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I use the AEI AFM gauge on my car with 1980 RoW CIS SC engine, with SSI's. I have bungs welded in both left and right heat exchangers 1" aft of the collector. I only have the sensor in the driver's side (I now wish I had bought a meter that uses and displays two sensors). Measuring O2 at the bung at the collector is the best place to take measurements. The tailpipe method has more opportunity for leaks and air dilution to affect the readings.

When at steady cruise, my AFR fluctuates a little, but within .05. E.g., at cruise it will read from 14.40 to 14.45 or so, sometimes narrower range than that. It should not jump around unless you have loose exhaust manifold gaskets, or the engine is missing on one (or more) cylinders. NEVER buy cheap measurement tools--you will always doubt their accuracy.

Shocks from the spark plug boots: You should not get shocked. The quality of the Beru plug wire sets started going downhill about 5 years ago. There's lots of discussion on these threads about that. Likely you have a cracked plug connector. If so, throw way the Beru set and buy something like the Clewetts, or at least Bosch. You do not need the braided/shileded wires--those are just a pain in the ass and don't reduce ignition interference or crossfire appreciably.

below is the sensor i got.. not sure if link will work .. ( the price is for 10 but i got 1 )
https://www.aliexpress.com/item/1005008714559538.html?pdp_npi=4%40dis%21CAD%21C%24 %20701.38%21C%24%20701.38%21%21%21495.92%21495.92% 21%40212f0bc017484568756836778da600%21120000463605 90684%21sh%21CA%21860381941%21X&spm=a2g0o.store_pc _home.productList_9841707.1005008714559538

good info .. i bought a set of wires that i used for a very short amount of time and then bought these to look more original. maybe I will throw those on for a test .

Interesting comment about the BERU as they were quite expensive.
But as you say , if they are leaking then what is the point. I will work on this first .

There is a bung built into my SSIs as they are for a 3.2 so i have jst been using that, but maybe idle is not the ideal time to be checking, maybe i want a more cruising steady state measurement. I do have a 964 cam grind in it , so it is a little more lumpy and maybe that is impacting things too.

The 964 cam is a factory emissions-compliant cam. It should not make your AFR jump around.

got it

Bosch LSU 4.9 spec sheet:

https://www.bosch-motorsport.com/content/downloads/Raceparts/Resources/pdf/Data%20Sheet_69034379_Lambda_Sensor_LSU_4.9.pdf

You can test the sensor to see if it's performing within spec. I highly doubt you bought an authentic Bosch LSU 4.9 with the gauge etc... for $90.

Before going too deep- check the sensor to make sure it's not bad. I've had multiple sensors arrive DOA.

1. (just a side note – I wouldn't trust any cheap Aliexpress Chinese stuff - that offer from Aliexpress above never ever comes with a genuine LSU sensor from BOSCH).
2. ALWAYS check for proper ground potential, means best is to lead the lambda/AFR controllers GND wire directly to the vehicles battery, same with the 12v wire, via Relay.
3. When using SSIs the lambda sensor at the position of SSIs factory welded bung is resulting colder as it gots more distance to the heads, so the readings at idle –where the exhaust flow is at coldest state– could be off / result different.
4. Some AFR/Lambda controllers give wrong results at Lambda 1. Innovate LC-2 at factory state i.E. does actually result in 2.55v at Lambda 1 and not at 2.45v. Several tests with different LC-2 controllers and different used LSU4.9 sensors proofed that.

wow . i am learning a ton from all this. I will definitely do more testing and keep this thread up to date. I just got the kit to play with an an experiment , not becasue im having issues with my car, but i will definitely keep everyone updated..

it is a LONG way to go to get to the front of the car for battery and ground, but noted.