samana76

Hello everyone,

I have written a few posts regarding my 951S in the past. I have had the car for about two years and did some mild performance upgrades to it about one and a half years ago.

I have had issues with it burning oil after a head rebuild that was done July of last year and, after about a year of trouble-shooting (I put it aside for a while as I couldn't deal with it), my mechanic has finally discovered and corrected the problem - a small crack in head just under a valve guide. This has been professionally welded, the head rebuilt again and reinstalled.

The engine now purrs and has great power that is easily and smoothly accessed with my right foot.

Fabulous! So why am I writing this post? There is still an old issue that is driving me nuts and no one, not even the mechanics at a specialized Porsche garage, can figure out. The problem is that at idle the engine is running rich at around 12.6-13 A/F ratio. At light acceleration/cruising speed it is close to ideal at a ratio of 14.3 A/F and at Wide Open Throttle the ratio is 11 A/F. I do not know when this started happening, maybe I bought the car with this issue and did not know it, but since I installed the A/F gauge over a year ago this has always been a problem. I have brought it to several garages, wrote about it here on Rennlist (see post here), but nobody seems to be able to pinpoint the culprit. As I want to finally solve this problem, especially now that the engine has been gone through so thoroughly, I am writing yet another post to see if anyone can help me with this.

I will detail all the upgrades and symptoms here to give as much information as possible to help the gurus out there come up with any thoughts.

The upgrades I did are the following:

- 3.0" Fabspeed performance exhaust package
- Tial 38 dual port waste gate
- Rogue's A-Tune Package
- Lindsey Racing manual boost controller
- Autometer Sport Comp Boost gauge
- Innovate Motorsports Air Fuel Ratio gauge with wideband controller and oxygen sensor (correctly installed after the turbo and not in the factory spot)
- Lindsey racing adjustable Fuel Pressure Regulator

With my setup above it is suggested that the fuel pressure regulator be set at 3 bars, however when I did this the engine was running excessively rich, both during idle and at cruising speed. By bringing it back down to 2.5 bar the numbers decreased to what I wrote above, which are:

idle: 12.6
cruising: 14.3
WOT: 11

I also set the FQS switch to the second position reducing the fuel by -3.1% (which I saw an appreciable difference on the A/F gauge).

To see if it had anything to do with the mapping of the A-Tune chip I also swapped the original Porsche chip back into the DME, however this had no effect – the numbers remained the same as listed above. So it cannot be the A-Tune chip or map. As instructed in the A-Tune installation procedure, I did cut a little resistor in the DME and am now wondering if this could have anything to do with it? Maybe RogueAnt can chip in here. In any case, please continue reading the rest of this post, as there is more information that can help come up with a plausible theory.

What is also curious is that when the engine is cold, for instance when I first start it up in the morning, as it begins to warm up (when the temperature needle is around the top of the first thick line in the temperature gauge) the A/F ratio increases rapidly from 11 to 14. Then, as the engine continues to warm up to regular operating temperature (around the first thin line in the temperature gauge) the A/F needle slowly comes back down to 12.6 where it stays indefinitely (during idle). So for a short moment during the warm-up process the engine idles at close to the correct air fuel ratio. Why does it change as the engine temperature increases?

In the past people have suggested a vacuum leak somewhere, however as the engine has been thoroughly checked (and partly rebuilt) and as the boost/vacuum needle is close to 20 when idling, there are no vacuum leaks. Therefore, this cannot be it either.

Now, with this wealth of information, can anyone here come up with a plausible explanation on what is going on and how I can solve this issue?

Sorry for the long-winded post but I am determined to get to the bottom of this and solve it. After many months of having the car worked on by a number of mechanics and having a number of issues solved, it is close to being in nearly perfect running condition (and it looks great too!). This is one of the remaining issues I REALY want to solve.

Thank you all for your attention and help.
Igor

Rogue_Ant

Igor,
When you set fuel pressure, did you use this method:

http://www.clarks-garage.com/shop-manual/fuel-01.htm

If you set it with the vacuum line on, and engine running, then the fuel pressure is probably still a little bit too high.

It looks like your setup is still slightly rich. I would adjust fuel pressure down a little bit to get WOT closer to mid/upper 11s.

Regarding the resistor in the DME, this resistor is for the normally unused circuit in the DME. The factory software does not use this circuit at all. The A/M-Tune takes advantage of this open circuit for use with the MAP sensor. The resistor adds a +5volt bias to the circuit that is unwanted, since it would skew the MAP sensor reading. Removing the resistor allows the DME to properly read the MAP sensor, and does not affect any other system.

What is the condition of your factory-position O2 sensor?

When the motor is cold (<150°F), there is extra fuel added to help the motor warm-up, and overcome the poor atomizes properties of fuel in a cool motor. As the engine warms, this extra fueling is reduced.
At 150°F there is no extra warm-up fueling added. However, as engine temps continue to rise, the fuel atomizes easier/better. This effectively makes the AFRs report richer, even though fueling might not have changed.

This is most likely the reason you see the AFRs go from rich -> stoich -> rich:

At first start, the engine has extra warm-up fueling - result: rich.
As the engine warms, the extra warm-up fueling is reduced - result: AFRs enlean and approach stoich.
When the engine gets to full temp, the fuel atomizes better - result: AFRs enrichen.


Usually this wouldn't be an issue, since the DME goes into closed-loop once the engine warm-up fueling is removed. However, if the engine is running a little rich (or a little) lean to begin with, then the limited closed-loop fueling control might not be enough to pull it back to stoich.
Additionally, if your O2 sensor is bad/dying, they typically start having trouble when there is less exhaust heat, such as at idle. The typical failure method of a NBO2 is to report that the car is lean, which will cause the DME closed-loop to add fuel...

Finally, the AFM could have drifted too far from factory calibration, producing a situation beyond the DME's closed-loop ability to fully correct.

Dougs951S

Being a touch rich at idle isnt an issue, when I tune idle i tune for lowest manifold pressure. This will likely not be stoich, and will depend on a lot of factors. My particular motor gets its best/lowest idle map of 20" hg @ 1000 rpm at ~12.5:1 afr. Dont chase a perfect 14.7:1

samana76

Thank you Josha for such a detailed and clear explanation. Thank you also Dougs951S for your input as well.

This morning, before my commute to work, I fiddled with the Fuel Pressure Regulator and lowered the the pressure to 30 PSI on the gauge (while the engine was running, at idle after full warm-up). This had the effect of raising the A/F ratio to 13.6 during idle, however it had no effect whatsoever to the A/F ration during cruising, but had a devastating effect on WOT bringing it to nearly 14. So I will increase pressure this evening for my drive back home, trying to get WOT to mid-high 11s.

Decreasing the fuel pressure did have a positive effect on a slight idle hunting issue I've had for a very long time now (see video in post here). After the second head rebuild this issue was reduced significantly but was still present. By reducing the fuel pressure this morning this had the effect of eliminating this issue nearly completely and making the car more driveable in city traffic.

I will look into the AFM and the factory O2 sensor as suggested. Actually, I'm planning to upgrade to the M-Tune in the not to distant future. Would I need to bypass the factory O2 sensor if I complete this upgrade?

Thanks again for all the valuable input.
Igor

Rogue_Ant

Have you reset the idle according to:

http://www.clarks-garage.com/shop-manual/fuel-08.htm

The M-Tune will use the factory O2 sensor, so no reason to bypass it.

MCharbe

Have you found what the problem was? I would like to know because I have the same issue with the AFR on idle.

Thanks,
Mathieu

JacRyann

As indicated in previous posts, AFM may be out of spec. Or adjusted improperly in the past. Check the following:

1. With warmed up engine, idle vacuum should be 20-25 in.Hg at the FPR, dampener and MAP sensor.

2. AFM voltage should be 0.69-0.80 volts for 860-940rpm idle

3. ELECTRONICALLY verify TPS idle-switch at DME plug. Not good enough to just hear the click, the switch or wiring may be bad as well.

These variables will select the proper injector duty-cycle for idle. Otherwise, you'll be looking up the wrong data-cell on the chip maps.

MCharbe

FYI - my issue with running rich on idle was solved. I had a corroded spark plug wire connection.