Stumble @1600-1800 RPM w/SWong chip but not OEM chip. Ideas?

[Bob Prosser]

Friends, I know this low RPM bucking topic has been discussed before. Some even call it the “parking lot buckaroo,” but a remedy often remains unsolved. Now I have started to experience it as well, though I have an interesting twist that has not been mentioned in other threads. It has to do with DME chips. With the help of tuning experts, this clue might point to the remedy (at least in my case and maybe for others).

Background: a very well maintained 1987 Carrera with ~200K miles. The engine was rebuilt 10K ago, including the addition of a 964 cam, new catalytic converter, Fabspeed exhaust and a Steve Wong chip programmed for the 964 cam.

For the first 8K miles after rebuilding the car ran beautifully in all RPM ranges up to redline. Then about 2K miles ago, I started to notice a stumble that appeared under very light throttle in the 1600-1800 RPM range (negotiating parking lots, stop-and-go traffic, etc). This stumble did not happen 100% of the time but it was often. Other than that, the car still runs perfectly all the way to redline.

I started to investigate the typical culprits. This is what has been done with no appreciable change:

1. Checked that the idle switch "clicks" the instant the throttle lever moves -- it does
2. Replaced the throttle body with a newer one (the action felt rough)
3. Re-tracked the original AFM
4. Checked / torqued the intake manifold bolts to 18 lb/ft
5. Changed the DME relay
6. Replaced coil with a new Bosh unit.
7. Had the DME gone through by Steve Wong
8. Engine out: Smoke inspection. Replaced vacuum lines and connections behind engine near the fuel pressure regulator. Replaced the seal between throttle body and manifold. No other leaks found, including injectors and intake manifold. Plugs looked good as did the distributor cap.
9. Engine in: stumble was still there. Swapped-out another AFM as a test with no change.

Before getting out-in-the-weeds any further, I decided to remove the SWong chip and put the OEM back in. The stumble was completely gone! Okay, that narrows it down. Three Porsche experts I told about this said there must be a problem with the chip.

I called Steve Wong. He said it wasn’t his chip and the OEM chip is covering up for a lean surge. I sent the chip back to get the chip tested and/or programmed more like the OEM chip only below 2K RPM for better drivability. A week later, I put the newly customized SWong chip back in. It stumbles at 1600-1800 RPM just like before. Dang it!

Tuning experts: since the car runs beautifully with the OEM chip, and still stumbles with the modified SWong chip, where do I go next? I’m stumped.

Thanks for your input.

[tschall]

Bob, I have a near identical problem. Differences: 3.2 New rebuild 1200 miles. Swapped US P&C's for Mahle 10.3 &c's, FabSpeed Pre Muffler, SW rebuilt DEM (Motronic), SW Chipped for RoW/no Cat Converter, SW rebuilt AFM. '86 Cab 3.2, 63K original mile. By the by, the carbon track is very good...
I have been dicking with AFM, over 15 times in/put of car vs a loaner(better, but no real top end).
Lean and leaner issues. SW suggested pulling chip and R&R with factory. I'm not sure if it is "factory". I feel I am getting zeroed in slowly and methodically. Just a PIA plus to do so many swaps and resets. Wiper arm first, clock spring second. Fine tune with CO screw. I'm still on Wiper arm mostly and a little clock spring. At least I'm learning alot.
The point of the above is, today I ran a set up with silky smooth throttle response throughout the tach. I mean silky smooth. Below 4000 rpm all you could want. Above 4000 rpm NOTHING you would want. Extremely rich @ WOT, think high 10's on AFR. Tomorrow I move Wiper arm back down a couple of millimeters and loosen a couple of teeth on clock spring. Repeat. I have advanced the Wiper to max out on Carbon track (minus 1 Millimeter), and now will go less. Trial and error ad nauseum. Keep us posted please!

[proporsche]

Bob ,did you set the comp ECU under your seat to what position?there are 7 different ...
Also i would try to get better gasoline- like 10G of racing gas and see what it does.The gas is crap lately....
Ivan

[Arne2]

Contact Sal (Pelican user Pelican Parts Forums - View Profile: scarceller ) for advice.

FWIW, I had the same issue on my '84 with a Steve Wong chip, and my car was otherwise 100% stock. I gave up on the chip and went back to stock. Much happier that way.

[cabmandone]

When you're talking to Steve, you're talking to a "tuning expert". Was there something simple you missed when installing the new chip like a FQS setting or a jumper that needed to be removed or o2 sensor unplugged? Seems odd that the old chip works well but the new tune doesn't.

[Bob Prosser]

Originally Posted by cabmando

Was there something simple you missed when installing the new chip like a FQS setting or a jumper that needed to be removed or o2 sensor unplugged? Seems odd that the old chip works well but the new tune doesn't.

Seems odd indeed. My original post stated: "For the first 8K miles after rebuilding the car ran beautifully in all RPM ranges up to redline. Then about 2K miles ago, I started to notice a stumble that appeared under very light throttle in the 1600-1800 RPM range (negotiating parking lots, stop-and-go traffic, etc).

[cabmandone]

Originally Posted by Bob Prosser

Seems odd indeed. My original post stated: "For the first 8K miles after rebuilding the car ran beautifully in all RPM ranges up to redline. Then about 2K miles ago, I started to notice a stumble that appeared under very light throttle in the 1600-1800 RPM range (negotiating parking lots, stop-and-go traffic, etc).

Yeah I went back and read your OP again and saw that it was a developing issue so that rules out jumpers etc. The issue you're having is in partly why I have an onboard AFR with logging. I can take the car out and log a few runs and share that data with the engine tuner.

[mysocal911]

Originally Posted by cabmando

When you're talking to Steve, you're talking to a "tuning expert". Was there something simple you missed when installing the new chip like a FQS setting or a jumper that needed to be removed or o2 sensor unplugged? Seems odd that the old chip works well but the new tune doesn't.

Performance Tuning - The Myth -

[Bob Prosser]

Originally Posted by mysocal911

Performance Tuning - The Myth -

Care to elaborate on your comment?

[mysocal911]

Originally Posted by Bob Prosser

Before getting out-in-the-weeds any further, I decided to remove the SWong chip and put the OEM back in. The stumble was completely gone! Okay, that narrows it down.

Sounds like you've solved your problem! What's the point in wasting more time and money, e.g. "DME gone thru", throttle body, ignition coil - hopefully not silver?
What benefit have you found in not using the stock chip? You have converted to the stock 28 pin EPROM, right?

[mysocal911]

Most knowledgeable DME ECM techs would attempt to localize this problem by just varying one parameter, i.e. injection time or timing advance,
at a time from stock. As an example, maybe the peek&hold injectors are being pulsed too long for an RPM, or the coil dwell time is nearing coil saturation,
given the timing advance. This is easily done in most "tuning" software packages. It's unfortunate you don't have access to the typical PC based
software (~$200) and an EPROM eraser/programmer (<$100). You've probably spent more money replacing parts you've thought were bad.
As you'll find with the above mentioned equipment, chip "tuning" is very simple, but problematic when a lack of understanding of the overall engine
effects that may occur with the "tuning" tweak.

As an example, making changes in the processor firmware (not software), e.g. ignition timing or injection time, may have effects in other parts
of the DME ECM firmware, which result in running problems. As an example, there maybe a firmware instruction test, e.g. if timing (@ RPM = X) > Y
then injector time = Z * A, resulting in two modifications of the firmware versus just a simple timing change. So making arbitrary mods to ignition timing
or injection pulse widths, may cause unknown running problems.

[cabmandone]

Originally Posted by Bob Prosser

Care to elaborate on your comment?

I wouldn't go there. It will yield no results that are beneficial in figuring out why the performance tuned chip is behaving the way it is.

I'd continue to work with Steve to see if there's any data you can provide that might help explain why this problem has developed.

[Classic11]

I have chased a lean above 5000rpm issue for month and seriously done anything but take the engine out again and start over. I am also running a SW chick and experimented with it and stock DME. I don't want to talk about my issue but agree with @Arne2 1000% in contacting SalCarceller for advise and upgrading to his system. He is THE EXPERT hands down and was beyond supportive in helping the trouble shooting.

[Bob Prosser]

Friends, thank you for your contributions and thoughts.

proporsche: Ivan, the ECU setting has remained untouched. Perhaps better octane might make a difference, but that’s not a sustainable solution.

Arne2: Thanks for the tip. I understand about going back to the OE chip, but that’s my last option. Thing is, doing so makes me feel like this problem beat me and I capitulated.

cabmando: I need someone in my area (north San Diego County) that can diagnose with AFR logging, etc. I don’t feel like messing with that myself.

mysocal911: Yeah, I know I found the solution, but this problem BUGS me. Yes, I bought a silver Bosh 944-602-115-00 M14 from our host. Can’t notice any difference. I kept the original black one though. The chip on the motherboard has 24 pins. On top the OE chip reads 1267355302. That’s early 1987 I believe. So, no it has not been upgraded to 28. The rest of your suggestions / input are beyond my scope of competence with this issue.

Classic11: Thanks for your advice. I will reach out to SalCarceller.

[proporsche]

Bob..if you put the S.W.chip there is a procedure he advices to do on the ECU .....Plus the betters gas should take care of it...
here Bob
911Chips.com - Fuel Quality Switch

btw if you like the 28 pin chip i have one and you can have it...

[cabmandone]

Originally Posted by Bob Prosser

Friends, thank you for your contributions and thoughts.

proporsche: Ivan, the ECU setting has remained untouched. Perhaps better octane might make a difference, but that’s not a sustainable solution.

Arne2: Thanks for the tip. I understand about going back to the OE chip, but that’s my last option. Thing is, doing so makes me feel like this problem beat me and I capitulated.

cabmando: I need someone in my area (north San Diego County) that can diagnose with AFR logging, etc. I don’t feel like messing with that myself.

mysocal911: Yeah, I know I found the solution, but this problem BUGS me. Yes, I bought a silver Bosh 944-602-115-00 M14 from our host. Can’t notice any difference. I kept the original black one though. The chip on the motherboard has 24 pins. On top the OE chip reads 1267355302. That’s early 1987 I believe. So, no it has not been upgraded to 28. The rest of your suggestions / input are beyond my scope of competence with this issue.

Classic11: Thanks for your advice. I will reach out to SalCarceller.

It's not really all that difficult. Just get an AFR with logging like an Innovate LM2. Get the exhaust tip clamp for the wideband 02 (not ideal but it works). And for $250 you're logging AFR's at a given rpm. Cost is around $250. If you have an available bung in the exhaust, you can just twist the wideband o2 in, route the wires and you're off to the races!


When you talk to Sal he'll get you squared away on an AFR gauge. Sal is the person I went to when I was in the market for a performance tune. He upgraded my board to the 89 spec 28 pin, provided a chip with an 89 tune and installed his chip with his performance tune. The best part about working with Sal is he gives you a checklist to report back to him with. He'll give you test procedures to make sure everything is working the way it should before you, and here's the catch, send your ECU to him for the chip install. Sal likes to make sure the boards are in good shape and the solder joints are all solid, or at least that's how he operated when I worked with him. Sal is a great guy to work with and I'd highly recommend him.

[mysocal911]

Originally Posted by Bob Prosser

Yes, I bought a silver Bosh 944-602-115-00 M14 from our host.

Although not related to your present problem, you are aware that the silver Bosch coils are not reliable, right?

Originally Posted by Bob Prosser

The chip on the motherboard has 24 pins. On top the OE chip reads 1267355302. That’s early 1987 I believe. So, no it has not been upgraded to 28.

Foremost in any consideration of a chip change, upgrading to the stock 28 pin chip should've taken priority over chip tuning,
given the overall 28 pin engine improvements.

Originally Posted by Bob Prosser

The rest of your suggestions / input are beyond my scope of competence with this issue.

My suggestion relating to the problematic chip was to have the "tuner" use two stock 302 chips and just tweak only one parameter at a time;

Step 1. Only change the timing to the values used in your present problematic non-stock chip,
AFRs at stock, and then have you do a test drive for an evaluation.
Step 2. Only change the AFR to values used in your present problematic non-stock chip, timing at stock,
and then have you do a test drive for evaluation and feedback.

Using that methodology, you can potentially localize whether it's the timing changes or the AFR changes that contribute to your driving issues.

It's not that complicated!

[mysocal911]

Originally Posted by cabmando

Sal is a great guy to work with and I'd highly recommend him.

Yes, he's the "tuning expert", given that he has disassembled the 911 3.2 EPROM firmware code, and is aware of potential problematic "tuning" issues.

[Bob Prosser]

Update: with a newly acquired air/fuel ratio meter, I was able to get the AFR numbers to where I needed them. They now are:

• AFR at idle: 14.4± (desired 14.4)
• AFR at 3K RPM in 3rd gear: 14.2 - 14.6 (desired 14.2 - 14.7)
• AFR at 3K RPM in 4th gear: 14.2 - 14.7 (desired 14.2 - 14.7)

Apparently, the car was running a bit too lean before, but unfortunately the stumble was still there from time to time. Dang it.

On a hunch, I pulled the distributor cap to inspect the distributor itself, having been rebuilt two years ago. When I removed the rotor, I noticed it wasn't very snug on the shaft meaning there was a lot of extra movement going counterclockwise when pushed. (This video will demonstrate: https://youtu.be/PzhR354Ij4c.) I measured the movement. It could move a full 10° CCW.

I bought a new rotor from Porsche (PN 930-602-902-01) and compared it side-by-side to the old one. All the numbers were the same, except the new one had a "2.3" embossed underneath, whereas the used one had "2.4." See pix below. Anyone know why there's a difference? The new rotor fit nice and snug, and had almost none of the movement you see in the video.

I buttoned everything up, started the car and drove it for over 30 miles until fully warm in every conceivable condition, including stop-and-go traffic. The car drove perfectly! ZERO stumble at low RPM as before. I write this in hopes others who have been chasing this problem (and there are many) might give this a try. It's a cheap & easy fix. Thanks.

[17yrsWith930]

Bob,
Fantastic that you found your easy solution. Please confirm in a while that it still works. I understand you already drove 30 miles in varying conditions.
I was very curious because I've used a SW chip successfully in the past on a car I used to own and I intend on doing it again in my current 911.
BTW: I doubt it has anything to do with the 2.3 vs. 2.4, I believe it is simply the old rotor was defective or got loose from wear for some reason. I checked mine and I have about a third of what you have, so I look forward to a new rotor also. By the way, mine says 2.3 on it.
Great job not giving up.
By the way, do you also have a Pagoda?
Enjoy !!
Mark