coach928

I recently bought my dream car since I was about 5 years old. 1986 Porsche 928 5speed, Black on Black. It is a blast!

The car runs great, but like many others, it failed smog test for marginally high NOx. HC is average, CO is way below average.

15mph HC measured: 38, CO measured: .02, max NO 1106, measured: 1098
25mph HC measured: 22, CO measured: .02, max NO 936, measured: 1178

lean mixture/high temps cause high NOx.

I've checked many of the usual suspects in other posts:
https://rennlist.com/forums/928-foru...smog-test.html
intake/vacuum smoke test, no leaks found
intake cleaning with BG solutions
Checked output of O2 sensor:


Starts at .5V during warm up, then seems to fluctuate, but low voltage is lower than .2V

Then I came upon this thread which suggested that an adjustment to the pot on the MAF could lower NOx.
https://rennlist.com/forums/928-foru...-high-nox.html

I checked the resistance of the MAF pot per this thread:
https://rennlist.com/forums/928-foru...-85-86-us.html

and found the resistance to be 2ohms. Factory nominal is 380 ohms. Sounds like a likely culprit for lean mixture...
So I went ahead and built myself a PorKen Blinkr and adjustment tool per the instructions.

I temporarily adjusted MAF pot to 380 ohms as a starting point.
At 380 ohms car has wild startup from cold. 2000+ rpm down to almost 0 rpm, then usually stalls, but if doesn't stall will fluctuate wildly until O2 sensor warms up. I had to go all the way back down to 2 ohms to get Blinkr to blink. At 2 ohms, cold startup is normal small fluctuations in rpm.

Questions:
1. Is MAF suspect for failing if the set point is at 2 ohms (Range is 0-1000 ohms)?
2. Is O2 sensor suspect for failing if low voltage drops < 0.1V?
3. Is it feasible to pass smog (lower NOx) by adjusting MAF pot higher ~300 ohms even though Blinkr suggests that is the wrong set point?
4. Will replacing the CAT fix high NOx? Seems like replacing the CAT does not get to the root of the lean/high temp cause of high NOx.
5. Other suggestions?

It's getting down to the wire. I have 1 week left to figure this out!

Previous Owner installed PorKen chips, new fuel pressure regulator (set at 55psi), new 24lb fuel injectors 7k miles/8 years ago.
O2 sensor output .08V - .8V

V2Rocket

High NOx is hot chamber but a decent cat should knock those tailpipe numbers way, way down.

If mixture was so lean then the HC number would be low, not "average".

What were the CO2 and O2 % ?

coach928

Test | RPM | CO2% | O2% | HCmax |HCmeas | CO%max|COmeas | NOmax | NOmeas
15mph | 1891 | 14.78 | 0.33 | 128 | 38 | 0.8 | 0.02 | 1106 | 1098
25mph | 2157 | 14.89 | 0.20 | 103 | 23 | 0.6 | 0.02 | 936 | 1178

Car has high flow cats, but they only have 5k miles on them. Seem like a shame to switch them out.

Snipe656

When I still had my 928 it did something similar to this one year. For me it ultimately was a slight misfire that we really just stumbled upon via noticing something in the passenger side seat vibrating a little more than it should have while at a stop light. We nailed it down to I think one or two injectors but ultimately what we did is pulled all the injectors off and sent them off to be cleaned/flowed. That fixed the issue, the problem was the car had sat for a decently long time and fuel was still in the injectors when it was sitting, the stuff turned into "snot" as the injector cleaning guy described it.

With that said though, I have a non Porsche doing some readings similar to yours here recently and narrowed it down to two bad spark plug wires causing a slight misfire.

In both cases the cars seemed to drive around just fine and only reason even discovered the issues was because failed State sniffer tests. But after fixed they picked up a good bit of power and drove even better.

V2Rocket

All those numbers are pretty good (indicating good state of tune).
I'd say its your cats causing the problem..."high flow cats" are a relic of the 1980s when cats posed some real restriction to an exhaust system.

Regular production catalytic converters nowadays are just as flow-friendly as the "high flow" type, but actually work to cut down NOx.

Put the right cats back on...if you are in CA, the new cat(s) must be CARB-approved but they are easy to find and inexpensive.

Here is the Walker PN for a 928 CARB-legal cat.
80830

https://www.amazon.com/Walker-80830-...=1&*entries*=0

zekgb

FYI Bill Ball had a similar failure with a low mileage high flow Cat documented in this thread. Replacing with a used Porsche Cat solved his issue.

That said it's almost a certainty that a rebuilt MAF will have the car running better.

PorKen

Sounds like you could have a combination of mal-adjustments and/or faulty parts.

Were you able to adjust the idle to ~700 rpm with the throttle body screw when the diag plug was jumpered B->C?

Is the FPR an adjustable type? (I don't like those myself - too many variables.) Should be 58-52 psi with the engine off and the relay jumpered. 55 is the nominal when the engine and fuel are warm. Pressure is higher when cold, lower hot.

dr bob

Review of facts about what causes the various emissions readings:

CO is a direct result of the mixture during combustion. More fuel = more CO. It's independent of HC, but is a diagnostic for HC and NOx problems.

HC is unburned fuel, pure and simple. High HC is caused by ignition problems, and by lean misfire. So low CO and high HC say mixture, where high HC alone says ignition.

NOx is the result of high combustion temperatures. Somewhere around a flame temp of 2200º F, NOx starts to form during combustion. This can be caused by lean mixture, ignition advance, detonation.

The OP's symptom of HC OK, CO low, NOx high, all point to a lean mixture problem. Meanwhile, the oxygen sensor responds to changes in the AVERAGE oxygen, so having high NOx with low CO in this case, without lean misfire, suggests that there are some low-flowing injectors, possibly with vacuum leaks. The consistent high NOx on both tests narrows it down to injectors, since a vacuum leak would be a lower percent of total mass through the engine at the higher 25 MPH load.


Remove and clean or replace the injectors.

coach928

I don't have RPM meter, but idle was good at ~700 rpm based on tachometer. FPR is '87 FPR: 928 110 198 02.
Very satisfying to see LED blinking when finding correct POT position.

Thanks. I understand this is was likely going to happen, but I wanted to exhaust all other possibilities first. I'm somewhat surprised that these are listed as 928 compatible when there doesn't seem to be an air port??

Thank you for the thorough analysis. I suspected the same after reading Red Shark's ultimate solution to his smog problem.
https://rennlist.com/forums/928-foru...og-test-6.html
I tried to get a in-car fuel injection cleaning performed with BG 210. However at least 3 BG-supplied shops I called don't do the fuel injection cleaning with BG210. They only do the air intake cleaning with BG 206 and BG 44K in the gas tank. So I had that done at least.

As a starting point I planning on another in-car injection cleaning solution next week before retest. The injectors only have 7k miles/8 years on them, but the car may have been sitting for long periods of time unused which may have gummed them up. I'll post an update next week.

Snipe656

7k in 8 years sure implies a lot of just sitting. I just realized not only did this happen on my old 928 but also a 944 we had too. My memory on the 944 is vague but I think we tried using an cleaning solution that did not require removing injectors but it did not solve things. I know on both the 944 and 928 we ultimately send the injectors off to be cleaned.

If it sat too much hopefully fuel did not gum up elsewhere. I made the mistake of leaving fuel sitting in lines on my old race car. Discovered some gum up fuel in one of the fuel feed lines and ultimately had to flush the entire system multiple times and get the injectors cleaned/flowed.

coach928

I want to close the loop on this thread I started. I learned a few things.

1. CA DMV (or AAA in my case) will give you day 3 passes at a time to use the car for smog related purposes only if you don't pass within the allotted time window.

2. I found a shop to do some diagnostic testing for me to test the earlier attempts to lower NOx (intake cleaning with BG solution, in-tank fuel injector cleaning with BG 44K. I don't have the numbers, but NO PASS.

3. The shop identified that my engine fan was lazy: demonstrated by stopping the fan with fingers. (Don't try this at home.) So I followed Rennlist instructions on refilling the silicone fan clutch fluid. The dash engine temperature indicator previously hovered just above last white indicator line (before red area). After refilling fan clutch, engine temperature hovered just above the previous white indicator line (lower than half). I had little confidence this would make a significant difference in emissions...

4. Passed Smog Test:
Test......RPM.....CO2......O2......HCmax...HCmeas...COmax...COmeas...NOm ax...NOmeas
15mph..1988....14.77....0.10....128........59...........0.80.......0.10. ........1106.......227
25mph..2174....14.55....0.04....103........50...........0.6.........0.58 .........936.........185

Just barely passed CO at 25mph after it was originally very low.

I don't understand, but I'm not going to argue.

I still don't believe that the lazy fan was responsible for my original fail, but I don't have any other data.

Thank you for all the advice. This has been a terrific resource.

Speedtoys

Being there is really no such thing, and cheap cats have almost no catalyst in them (wonder why theyre so cheap?)...

You need cats, real ones.

Speedtoys

Temperature matters.

dr bob

I'd still go after getting the injectors cleaned. Barely passing with NOx means it's still lean, just a little less than before. As shared previously, NOX forms when combustion temps get too high. Echoing Jeff's post above, temperature matters when it comes to NOx. But that 'fix' still points to lean cylinder(s). Grab your IR temp gauge and shoot the exhaust pipes cylinder by cylinder, and find the hotter ones. Hot combustion gasses move to heat the header, so you should be able to get a good idea which is/are hotter. Rem,ember that the oxy sensor responds to the -average- of all the cylinders, so a weak cylinder or two obviously won't boost injector duty cycle enough to solve the individual weak ones. NOx generation spikes hard when weak mixture causes high combustion temps, almost like turning a switch on.

Clean the injectors, even if you have to remove them all for the service. Trust me.