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Your text says idle CO was .44%, but the notes on the report sheets say 'idle', and show the 1.86%/2.09%/4.03% numbers...
Something is sending the mixture too rich, way beyond what a cat could be expected to clean up.
Time for some real diagnostic work... Might be time to take it to a pro?
Something is making it erratically rich at idle.
Check fuel pressure
Check ECU temp sender, different circuit than dash gauge. 951 engine harnesses are notorious... open circuit or high resistance makes it richer. Does it run worse when you unplug it? Does it run better when you jump the pins together? That'll tell you if it's doing anything, but not whether the sender is giving the appropriate resistance value. There's a spec somewhere, but I don't know it.
Check the AFM. has the plastic cover been off? Maybe someone was in there 'improving' it. Does the flapper move freely? Does it move a little TOO freely?
Just some easy things to check. The factory manual is loaded with info and has really nice wiring diagrams...
Something is making it erratically rich at idle.
Check fuel pressure
Check ECU temp sender, different circuit than dash gauge. 951 engine harnesses are notorious... open circuit or high resistance makes it richer. Does it run worse when you unplug it? Does it run better when you jump the pins together? That'll tell you if it's doing anything, but not whether the sender is giving the appropriate resistance value. There's a spec somewhere, but I don't know it.
Check the AFM. has the plastic cover been off? Maybe someone was in there 'improving' it. Does the flapper move freely? Does it move a little TOO freely?
Just some easy things to check. The factory manual is loaded with info and has really nice wiring diagrams...
There is some good advice here, but it is not entirely accurate. More on that in a minute.
But first things first: how does the engine run with the O2 sensor disconnected? That is the first thing I do on an unfamiliar '80s/early '90s Porsche with a suspected air/fuel mixture issue. This will eliminate the O2 sensor itself as the cause of the issue, and it will determine if the base mixture is rich or lean.
Otherwise, the advice posted here has been mostly good. Because the mixture only seems to be lean at idle and is much better at 2500 RPM, I feel that most of the usual suspects that cause an overall rich mixture (fuel pressure too high, larger unknown injectors, etc) probably do not apply---this would cause the mixture to be rich across the board. If you still have the rich mixture at idle, I would remove the AFM and check/clean the idle bypass passage. This is what the AFM idle mixture screw controls, and over years/mileage this passage can fill with carbon and cause an overly rich mixture, even with the idle screw turned all the way out.
As for the DME coolant temp sensor, this can cause issues, but if it were at fault, again the mixture would be rich across the RPM range, not only at idle. Furthermore, an open circuit OR high resistance does not make the mixture richer as stated above. As clearly stated in the factory manual, open circuit/higher resistance=rich, short to ground/lower resistance=lean.
In addition to any underlying issues, I would also suspect that the catalytic converter might be weak, no matter how "new" it looks. Most aftermarket cats are crap and only last a few years. Perhaps your friendly smog shop could do a manual emissions check immediately after a long drive and with a hot catalytic converter and see if the idle readings are any better . . . .
Good point on the Idle Vs. Cruise mixture discrepancy, Droops.
Thinking about it, fuel pressure seems less likely to be the culprit, it might have just as large an effect at cruise.
In Motronic strategy, the temp sender value is more likely to be a 'pulse width adder' than a PW multiplier... causing less of an issue at cruise.
Most of my Motronic experience was on BMW, and that seemed to fit the symptoms of bad temp sensor (circuits...) I ran across. It was actually a pretty common occurance. A factory mod for better cold running involved soldering a resistor inline in the circuit. Broken harnesses started appearing after a few years.
I'd be surprised if Porsche Motronic operated differently.
As for the AFM... God only knows what might have been done to that over the years!
Well the old cat was gutted or disintegrated :-). That would contribute to failing emissions at idle. I’m quite impressed it passed emissions at 2500rpm with no cat (or maybe that’s normal). I’m wondering if that means I have an extremely healthy engine.
My biggest concern is that the CO levels are still too high even with a new cat and there is a deeper issue. Perhaps related to a carbon buildup around the head gasket location like Dan Martinic had: https://rennlist.com/forums/944-turb...failure-6.html. My car has 180,000 miles too.
I will still
-refresh injectors
-change spark plugs
-inspect TPS
-inspect AFM
-run compression test
-replace DME temp sensor
-install fuel pressure gauge on fuel rail
-disconnected o2 sensor and see how she runs
Gutted or disintegrated from age?
-
Last edited by Chapman951; 06-26-2019 at 10:00 PM.
Useful info finding the cat was hollow. Tailpipe readings are actual engine readings, so whatever issues you have only manifest themselves at idle. Minor richness at cruise can be compensated for by the O2 sensor.
Consider the cat like an emissions band-aid.
At idle, your CO should fluctuate slightly, somewhere between .5% and .8% if the O2 sensor feedback system is working.
If it was below (maybe!) 2%, the catalytic converter band-aid might be enough first aid to get it through smog.
Your's is 3.5%+?
Band-aid won't help, it needs a tourniquet....
Don't overlook the temp sender wiring while you're swapping out parts!
...also, iirc, Euro cars without CATs have instruction to adjust the AFM bypass to 1% CO. This means a properly running engine should achieve this without the CAT
...also, iirc, Euro cars without CATs have instruction to adjust the AFM bypass to 1% CO. This means a properly running engine should achieve this without the CAT
Yes...
In fact, the O2 sensor feedback system commands it leaner than that when the car is operating properly.
Should be <1% (fully warm) without the O2 sensor connected.
all correct, however, if this was run on a Dyno for smog. (enhanced area / more populated areas of California) it would fail both low speed (15mph) and high speed (25mph) for HC , CO and NOx with that lovely cat you have.. (one of the first things I pointed to in my earlier reply based on C02 and 02 results)
i had a new California legal catalytic converter installed. Have not gone for a smog yet
nowanker - I disconnected my brand new o2 sensor and the powerband between 2500rpm and 3000rpm gets smoothed out. This is while driving the car. Not idling. What does this mean?
I inspected the AFM and it does not appear to be tampered with. The barn door opens and closes smoothly. The silicone does not appeared to be tampered with.
I fussed with the idle screw on the afm and it functions just fine... I am idling slightly below the 1000rpm line.
If the engine runs differently with/without the O2 sensor, it shows that the system is functioning. As designed, it targets toward the lower end of an acceptably lean mixture.
Whatever malady is causing your rich condition seems to be worst at idle, but is probably still present to some degree at cruise. From your readings, it looks like the O2 sensor (Lambda) system has enough range to compensate.
If you disconnect the sensor, it reverts back to something richer, good for response, but not for emissions or mileage.
Or at least that's what the situation seems to be from here. All the symptoms fit this scenario.
I fussed with the idle screw on the afm and it functions just fine... I am idling slightly below the 1000rpm line.
If I remember correctly, you set the idle speed using the bypass screw on the throttle body housing, and you use the adjustment on the air flow sensor for setting idle CO level. Take a look at section 24 of the workshop manual for instructions. In a nutshell, turn the AFM screw counterclockwise to make the mixture leaner, clockwise for richer. Idle speed may change as you adjust the mixture - use the screw on the throttle body to compensate. I think you need to jumper the idle air valve and disconnect the o2 sensor before you start. If you don’t have a CO meter, I would use a wideband AFR meter to see what’s happening as you adjust the idle mixture.
If the engine runs differently with/without the O2 sensor, it shows that the system is functioning. As designed, it targets toward the lower end of an acceptably lean mixture.
Whatever malady is causing your rich condition seems to be worst at idle, but is probably still present to some degree at cruise. From your readings, it looks like the O2 sensor (Lambda) system has enough range to compensate.
If you disconnect the sensor, it reverts back to something richer, good for response, but not for emissions or mileage.
Or at least that's what the situation seems to be from here. All the symptoms fit this scenario.
Well that’s good news :-). I did a good job with my electrical work. That will build confidence with other projects coming up.
Thank you!
Originally Posted by Jim O.
If I remember correctly, you set the idle speed using the bypass screw on the throttle body housing, and you use the adjustment on the air flow sensor for setting idle CO level. Take a look at section 24 of the workshop manual for instructions. In a nutshell, turn the AFM screw counterclockwise to make the mixture leaner, clockwise for richer. Idle speed may change as you adjust the mixture - use the screw on the throttle body to compensate. I think you need to jumper the idle air valve and disconnect the o2 sensor before you start. If you don’t have a CO meter, I would use a wideband AFR meter to see what’s happening as you adjust the idle mixture.
ahh sweet I will read up on this. This is mega thank you! Now I need to figure out which wideband to go with.
06-21-2019, 09:36 PM
). I will start checking all these things out.
