healey1968

OK, I haven't played around with the Durametric software too much - but I think I need some help.

Background: My 996 has had a stumbling issue for almost a year now. Used to happen rarely, typically the first stop light after prolonged cruising on the freeway. It got bad enough that I couldn't start from a dead stop without the engine cutting in and out. Makes for a bad drive.

This weekend I cleaned my MAF with some MAF cleaner and it's significantly better. But not completely fixed - still stumbled once on my drive to work this morning.

I figure I'm in for a new MAF, but I'd like some definitive proof first - especially because I already own a Durametric. Ideally I'd like to graph the MAF output to a file with my computer and the next time it stumbles, see some wacko reading come off of it.

Is this doable?

Ahsai

Don't know what year is you car but if it's not e-gas, you may want to clean the idle stabilizer. If it's e-gas, you may want to do a throttle body recalibration.

Yes, you can log the MAF reading in Durametric and store it in csv format and do post processing with excel afterwards.

Third-Reef

How is a throttle body recalibration done?

Thanks

Ahsai

Turn key to last position before cranking with foot off the gas. Wait for 30 seconds then turn key to off. You should hear the throttle body servo rotating with some high pitch electrical buzzing noise during the 30s.

healey1968

Ahsai,

It is a 2002 - so egas. I'll try recalibrating the throttle body first as I have disconnected the battery a few times over the past year.

Ahsai

I think it will be beneficial to log the FRA and RKAT values as well. Also check if the evap regeneration valve above the alternator is leaking since you said it happens after along highway drive, where the canister will have very little fuel left in it so any leak will become a vacuum leak. So maybe you (your assistant at the passenger seat) will see that from the RKAT.

Macster

You sure it is not 60 seconds?

Ahsai

30 seconds worked for me.

healey1968

Slight update here.

Did the throttle body recalibration last week and promptly left the car at the airport for a week. However it still gave me some hiccups on the drive there and on my commute to work today - so next I plan on logging the data via Durametric. Need to get the cable all setup so I can plug it in and go on my next drive.

george996

I agree with Ahsai, a stuck open evap valve could fit the symptoms described.

I assume there are no fault codes.

healey1968

Correct - no codes logged.

Umm - anybody got a DIY on how to replace the evap valve? Is this what I should be looking for?

Macster

You say the CEL is off and there are no active or pending codes?

You say you left the car parked at the airport a week? Do you do this often? You know where I'm going with this: stale gas. 'course, the symptom has been there a year. Unless you drive the car so seldom as to not add fresh gas for a year...

Speaking of year, hard to imagine a MAF acting up for a year without throwing any error codes, or more likely just failing outright.

In fact hard to imagine anything that would cause rough running for a year not to throw any error codes. I'm thinking plugs, coils, MAF, AOS, evap valve, anything with the vapor recovery system in fact.

Does the engine run ok at higher RPMs?

If so, I'm thinking an air leak. Had an air leak that triggered a CEL every once in a while. The error codes pointed to the MAF in that they were mixture related. I do not recall the error codes now. Finally convinced the MAF was at fault -- even though I never could spot anything amiss looking at the air flow reading in real time -- I replaced the original MAF with a new MAF but the error codes came back. Turned out to be a leaking oil tube filler cap. This leak was only significant at low engine RPMs, at idle. The engine seldom idled long enough to trip a CEL.

I'm not sure you're going to see much in the way of a smoking gun looking at the MAF output. Observe the short term fuel trims. These want to vary a bit on either side of 0 (zero) but not by much. If you see swings of 4, 5 or more percent when the engine is warmed up and the rough running appears that's a sign the MAF could be at fault. Readings close to or over 10% (+/-) will almost always cause the CEL to come one with a mixture related error code.

Had a failing AOS, well, what in hindsight proved to be a failing AOS, that would cause a bit of a stumble when I stopped at a light and the engine RPMs dropped to idle. This was after leaving the freeway after a long drive, leaving the freeway to fuel up or for the night. The CEL would come up about the same time the engine stumbled. Not a big stumble just a enough though to make my heart skip half a beat. Based on the behavior and codes I put this down to a bad MAF. But at the end of my 2K mile road trip over the span of a several days the next morning's code start had the engine smoking like crazy. AOS, obviously. Hard to believe an AOS in your car's case could be causing this rough idle and yet have not failed outright.

Given a MAF is so expensive I can't recommend you replace the MAF even if you see the short term fuel trims going wacko. There can be other explanations for this behavior.

You know to there is the poor man's MAF test and that is to disconnect the MAF at the wiring harness and clear the codes -- to reset all the adaptations to their defaults -- then drive the car as you were before and see if the behavior appears. If it doesn't that's about as good a smoking gun as you'll find without taking the car to a shop with all the Porsche diagnostic tools and training/experience to use them to their fullest.

healey1968

Yeah - I do leave it un driven for periods of time. I've had stale gas issues as well - but I've cycled 3 or 4 tanks in the last 2 months, driven the car 'often' for my standards. The trouble started when I was driving it alot over a year ago. I really really doubt stale gas.

I agree - no codes - I'm not thinking it's the MAF anymore, but I'd love proof.

I will admit, this problem started roughly around the time I changed my plugs a long time ago. Just thought of that. Wouldn't that eventually throw a code??

Engine runs great up to 6k. I don't take it up there much, but never a problem there. Only stumbles accelerating from a stop or <15mph in 1/2 gear.

Me too - I was hoping you guys could figure that out if I record some RKAT values. Heck, I don't even know what RKAT is.

Got it.

Car does smoke sometimes on startup, but I've attributed that either to stale gas in the past, or the 'cold' weather we've had here sometimes.

Thanks for the long reply Macster.

george996

RKAT is the mixture adaptation near "idle". FRAU is adaptation for the middle range and FRAO is for the upper range. Post your results and we probably figure it out.

Does it idle smooth after some period of time or always rough?

Macster

Based on what you posted I'm inclined to blame the plugs, or coils.

If the behavior appeared concurrent with plug change the plugs or coils have to be suspect. What plugs did you use? I ask because in at least one case a Cayman owner replaced the plugs in his car's engine, and used plugs that were according to one cross reference chart the "same" as the Porsche plugs, but which proved to not be the same to the point he had to remove the new plugs and install the real McCoy plugs.

I have to comment further on this plug business. The Porsche plug has an extended tip. The benefit of this type of plug, of this extended tip, is at low engine speed the tip is exposed to the heat of the combustion and thus runs hot which helps to keep the plug clean. But at higher engine speed the extended tip is exposed to the incoming charge and thus is kept cool.

Even with the right plugs installed there's always the possibility the plugs were not installed properly, or the coil/plug connection is not good. There is also the possibility a plug is just bad/marginally bad.

This coil/plug connection is extremely important. Years ago in my 1996 Mustang GT the engine out of nowhere and out in the middle of nowhere developed a miss at freeway cruising RPMs. I say miss but there was no check engine light. But the behavior was real enough. Long story short I traced the problem to a bad coil to plug wire connection. It appeared that when the plugs were changed the tech failed to secure this one coil wire to its plug tip. For a while this was ok but over time the plug tip eroded until the miss developed. But at this time the tip was eroded enough that the coil wire would not snap onto the plug tip like it was supposed to. My fix was to use pliers to squeeze the coil wire connector down so it would "snap" onto the plug tip. As a back up as soon as I could I bought a new set of coil wires but never used them.

So you have to be sure each coil is connected securely to its respective plug. And you have to be sure each coil is likewise securely connected to its respective wiring harness connector. Techs tell me that often a car comes in with a miss and it is the coil connection at the harness that is at fault.

Thus my advice would be before you looked too far afield for the cause of the engine behavior you should seriously consider revisiting the plugs/coils to ensure everything is secure. And if you strayed off the reservation regarding spark plugs to seriously consider replacing the existing plugs with genuine Porsche plugs. You don't have to buy these at the dealer parts department, you can buy the same plugs from another business I guess. The important point is to buy the right plugs. Oh and then install them correctly and of course this includes making sure each coil and plug connection is sound and each coil and wing harness connection is sound.

To kind of jump around and to address a possible intake leak, do not overlook the oil filler tube cap. With the engine warmed up and idling see if by just trying to move the cap you can cause the engine to react. You might be able to hear the air leaking past the cap. A co-worker of mine heard this which is how I learned the cap was leaking.