Carrera 3.2 stutter/misfire/surging on car with MAF conversion
 

I did have a good look around the forum and have read a bunch of threads about these not uncommon issues with the C3.2 so I think I have the beginnings of a handle on how to try and diagnose the issues. That said, as a bit of a curveball the car has an Auto Authority MAF conversion, SSIs and a dual exit back box (I think it's a Dansk, but I'm not convinced it wasn't a homebrew dual exit conversion), because there is nothing like adding a few more variables into the mix, right?

The symptoms

When I got the car back in January, it had a polite "misfire" at idle - it would occasionally drop a cylinder, with a little fart. It also had a stutter at around 1800rpm-2200rpm on light throttle openings, but that would clear up at higher revs.

I haven't had much wrenching time since, but managed to get around to doing a minor tuneup (plugs, cap, rotor, fuel filter) and both plugs and fuel filter were well by its use-by date. The cap and rotor wasn't far behind. I was planning to swap out the plug leads but the aftermarket ones I got from the forum sponsor didn't quite fit as well as I had hope so I left them off for now. The car had Autolite Platinum plugs in it that were severely worn (gap around 1.1mm) so I hoped that those would be part of the issue.

Starting it up after the tune up, it idled a bit better - the "cylinder drop" went from about once 10-15s to maybe 20-25s between drops, but it was still there. Not as pronounced. I had the battery disconnected for probably a couple of months so I expected that the car was running on the baseline tune (assuming the Motronic does have any self learning capacity).

I did end up having to reseat the distributor cap (oops, my bad) and it ran fine when luke warm, with only a minor stutter around 2k +/-250 rpm at light throttle openings. Took the car out for a test drive and to thoroughly warm it up. During that time, I could easily drive around the 2k stutter by applying a little more throttle, and it would pull nicely to about 4500-5000.

About 20 miles I noticed that the RPM range of the stutter/miss was expanding, it now started at lower rpms and went up to almost 3k. It also started backfiring on downshift blips about every third or fourth time. Again, nothing massive, but an audible pop. Still could drive around the issue by stepping on the right pedal bit more.

After about 40 miles that, I noticed that I got an occasional hesitation even as far up as 4500 rpm under load so I backed off a bit. The overall problem did definitely get worse the more thorough the engine was warmed up - I've never driven the car when it was hot outside, so I didn't notice this behaviour until now.

No problem starting hot or lukewarm, the polite pop at idle is still there but didn't get worse.

Diagnostic plan

Based on reading the forum here, I suspect the car is suffering from one or more of the typical issues:

• Old plug wires. I'll chuck on the set I already have and see if there's any improvement.
• Possibly a bad coil. The coil looks OEM and I'd guess is at least 12-13 years old as I don't see anything in the maintenance history of the car that shows it's been changed.
• Battery isn't at its best, but was fully charged (it's usually on a battery tender) and holds a decent charge.
• Intake gaskets. I'll have to do the starter fluid/brake fluid check to see if I can find anything. I did already get a set of gaskets with my order of tuneup parts. If I'm pulling the intakes I'm also going to send the injectors off to get tested and cleaned.
• Sensors going bad. I haven't checked yet what kind of CHT sensor it has, nor did I check the O2 sensor or the crank sensor. Connectors in the engine bay look decent.
• Electronics issue re the MAF conversion - I have no documentation on the Autothority swap. My understanding is that it needs a new chip for the Motronic, but I have no idea what's in there and if it's been tuned to accommodate the SSIs and exhaust. That also prevents me from swapping in known good parts unless someone has a spare MAF and Motronic with the chip lying around :).

Does anybody see anything that's missing on the list above that I should check? Compression test might be in order, although I don't think any of my testers have the necessary adapters. Checking and setting valve clearances is probably also a good idea.

Last, if anybody has documentation on the Auto Authority conversion (especially around general setup, if the same ignition base timing still applies etc) and would be able to share a copy, I'd be very grateful.

I think you may have answered you own questions regarding which way to go or what to check.

- You have a pretty good list of what to check etc.. so I'm assuming you know of a couple of ways to check the coil.. so check it.

- Make sure you have a good battery, don't keep fooling around with the maypop and recharging.

- If you have one, put the stock airflow meter back on the car properly connected and see how it runs or if the issue persists.

- Get a correct set of (non-Platinum) Bosch or NGK plugs.. in my opinion nothing else seems to work right.

- No baseline tune and self learning here. Motronic relies on simple sensors and its DME.. the only parameters you'll really change will be with the use of the chip in the DME and/or that autothority unit.

Typically, the Autothority Mass flow conversion was used with a custom chip..
- you need to check the DME to see if it at least looks like its been opened.. if so. Open and check to see if it has an Autothority chip installed.. if it does, make sure it is properly seated. If not, this could be part of the problem and Steve Wong (the chip dude) should be able to help you.

Thanks!

Will check the coil if it's still within spec. I suspect the failure is heat related which might be harder to track down, though.

The main need for the Battery Tender is because someone installed an aftermarket sound system and an alarm. Which of course drain the battery when the car is turned off :mad:.

Fixing the electrical mess in the car is one of the more long-term goals. I'm pretty decent with electrons, but first I've got to get it running properly.

Unfortunately I don't. Based from what I could piece together (PO is deceased, I purchased the car from his widow), the PO bought it with the MAF conversion and SSIs in place. I assume that's also why I don't have any documentation on the MAF conversion at all.

Already fitted a set of Bosch OEM spec plugs :).

OK, time to pull the DME and check. I'll probably fit a new DME relay while I'm at it, too.

The starter fluid method of diagnosing vacuum leaks is pretty unreliable. I'd recommend using a smoker from ebay to smoke the intake tract and exhaust. Also can you tell from plugs or using an IR gun which cylinder is dropping?

All the plugs looked very similarly worn so based on that I wasn't able to figure out if it's dropping only one cylinder or a random cylinder at any time.

tim -- don't have a Autothority MAF, but do have a similar one and ran into a similar problem a while back -- turns out that on mine, the MAF's hot wire became dirty and the dirt compromised its ability to accurately detect/report the quantity of air flowing across it. Was able to use some MAF cleaner to bring it (and my carrera's performance) back to stock

not sure what my mechanic used, but here's a link to a MAF-specific cleaner -- https://www.amazon.com/CRC-05110-Mass-Sensor-Cleaner/dp/B000J19XSA/ref=sr_1_3?keywords=maf+cleaner&qid=1565644518&s=g ateway&sr=8-3

Are you using a k&m air filter? understand that over-oiling one of these can result in premature crud buildup on the MAF hot wire.

Might want to check that out first?

"The main need for the Battery Tender is because someone installed an aftermarket sound system and an alarm. Which of course drain the battery when the car is turned off."

All the more reason to get a fresh battery in there and remove that variable from the equation. Sounds like you should likely go with the Group 49 option. I had a Porsche branded battery go bad in less than 18 months and it wasn't saddled with all the extra stuff it too was either on a tender or disconnected during that time.

Yank all that aftermarket crap as soon as practicable (I should probably put a viewer discretion warning on these pics)...

http://forums.pelicanparts.com/uploa...1565694017.jpg

http://forums.pelicanparts.com/uploa...1565693704.jpg

http://forums.pelicanparts.com/uploa...1565693704.jpg

Yeah, removing and redoing the add-on electronics is pretty high on the todo list. It's just a matter of picking my battles right now. Hear you regarding the battery. I hope it's not as bad as my last '89, that one had three(!) alarm systems fitted that were all engaging in competitive battery drainage.

As to the MAF, I already have some MAF cleaner somewhere (we recently moved house so "somewhere" is still a bit of a fuzzy geographic location) so I'll try to clean it while I'm in there. Can't hurt. The filter does look like the regular Autothority filter that came with the kit, and it does look like it might be an K&N style filter. It also looks like it hasn't seen filter oil in quite some time. I probably have to pull it anyway to get better access to the sound deadening mat that's falling apart, too:

http://www.unix-consult.com/car/911-build-3.jpg

Diagnostic plan has a few more items added to it:

• Check fuel pressure
• Pull the DME as suggested and while I'm in there, replace the DME relay.
• Clean the contacts for the relays and fusebox connectors, plus the usual checks and cleaning on electrical contacts, ground straps etc.

Check inside the distributor cap for evidence of carbon tracking. Explains why cars no longer have distributors.

The Path Of Least Resistance

tim -- on the battery drain front, also confirm that the driver's door interior light switch works -- this switch also triggers a relay that disables the power window circuit after you open the door (the power window circuit remains powered after you turn off the key so you can roll up your windows, and the relay turns off the circuit when you open the door).

When my door switch failed, my battery would drain down in around a week.

Occams Razor, the MAF system is the most likely culprit coupled with the DME chip. Close second is the intake manifold gaskets/intake leaks. The alarm/stereo crap is incidental, but necessary to clean up eventually.

Agree! The maf setup in the OP's photo is quite similar to the one I have and, when mine suffered from a nearly identical problem, cleaning the gunk that had deposited on the hot wire brought my car back to normal. Per the OP's explanations, sounds like it may have been quite a few miles (if ever) since the Hotwire was cleaned -- mine's cleaned (and found to be dirty) at every valve adjustment

Thanks, will check - I wasn't aware of this feature.

I'm pretty sure it's been more than a few miles since it's been last cleaned, so that makes a lot of sense. I also don't have a ton of faith in the installation of the MAF, but I'm guessing it must've worked properly at some point.

Looks like I have a non-working driver's door interior light switch. That might explain a thing or two.

If your passenger door switch works, it too should disable the power window circuit and let you determine if the drain is gone

I pulled the MAF out last night. I'm a bit surprised by the setup - this is the unit with the filter removed:

http://www.unix-consult.com/car/911-build-13.jpg

The odd thing is that the "outlet" of the MAF is just this tiny gap in the center of the picture. I thought these things needed more airflow than that. Only way I noticed the gap as being "open" was because that was where the MAF cleaner dribbled out after I blasted the sensor with it.

http://www.unix-consult.com/car/911-build-8.jpg

The filter was dirty, but fortunately no oil on the filter - it specifically states on the filter that you're not supposed to oil it. Washed the filter and let it dry for 24h.

When I pulled out the sound deadening mat that had come loose as they always do, I found someone had stuffed a filter on the breather hose that would normally go into the airbox and chucked it behind the other lines in the back (well, front) of the engine bay. Class, but a quick zip tie sorted that out:

http://www.unix-consult.com/car/911-build-7.jpg

I also checked the coil with a multimeter - the input side of the coil came in at 0.8Ohms (Bentley manual says 0.4-0.6), output side at 6.5kOhm, which is in spec. I'm guessing the coil has aged but doesn't seem to terribly out of spec. Not great, but at least it's the correct coil.

Put the whole shebang back together, but unfortunately it's still got the same symptoms. I may kid myself into them being a little less pronounced but they're still there.

It also came up with something new - when I held the throttle open at about 2000rpm, I noticed that it started smoking a bit out of the heater connector hose (the one that is hooked into the electric fan in the engine bay). Keep in mind this car has the old-style heater setup as it's got SSIs on there. It does have me a tad worried because it would suggest that something managed to get into the heat exchangers - I'm pretty sure that it's not supposed to be oily in there. I guess I can now add "pull headers" to the growing list. Still not quite sure how I would diagnose this problem.

I checked both doors and it doesn't look like either one triggers the interior light. Am I correct in assuming that if I turned off the ignition, pulled the key and opened/closed the doors, the electric windows should be turned off? If that's the case I think I have a bad relay as they stay on. I also noticed that the voltage at the battery appears to be only about 13.5V at idle, which is a tad low for my liking.

This thread has the Autothority documents, as well as some other info:
http://forums.pelicanparts.com/porsche-911-technical-forum/448897-autothority-mfs.html

Other info here:
http://forums.pelicanparts.com/porsche-911-technical-forum/110703-autothority-maf-lean-run-condition.html

I have this unit on my car, along with other mods, for many years now. Took some back and forth with Steve Wong (remotely) and some dyno sessions to collect data in order to get the tuning right, but it runs very well now.

PM if you have any questions.
Cheers, Mike.

Thanks Mike, appreciate it!

To clean the MAF, I think it's best to remove the two screws that hold the sensor into the aluminum housing, and pull out the assembly, which includes the sensor wires.
It allows a proper inspection and cleaning.
Be careful with the fine wire coils.

Spraying into the aluminum housing might not get the fluid to where it is needed.
btw, the MAF works by measuring the air flowing through the small tube, then extrapolating that to figure out the air flowing through the main bore.

Autothority MAF in the round file. Old technology. Put that airflow meter back on.