Wanderer

I tend to rev-match when I drive. Lately the car is backfiring (loud pop) when rev-matching about 50% of time. Any ideas on what would cause this?

2008 Cayman S
31,000 miles on it

racer

This is a rev match on a Downshift? Is it possible you are over-revving the engine? I'd consider having codes pulled to see if there is anything being tripped.

Wanderer

Yes, rev match on downshift. Not over-reving as I rev match in normal driving. So between 2k-3k as an example of this backfiring.

drcollie

Backfires are caused by a rich condition in the exhaust, which means you want to check the air/fuel ratio. Its pretty common if you installed an aftermarket exhaust because the change in backpressure means more unburned gasses in the muffler. If you're on a stock exhaust and it just started happening, then something is causing your car to run rich. Air Mass Sensor may be the culprit....

Wanderer

I will check that. Thanks. Could a clogged air filter cause it? Its been 15k since I changed the air filter.

drcollie

Certainly an easy maintenance item that doesn't require a trip to the dealer. Most the time a clogged air filter manifests itself as an engine that is down on power and doesn't want to run properly. I doubt that would cause a rich condition on the fuel/air mix but stranger things have happened.

Wanderer

Ok. I'll hook up the scanner and see if there are any codes. No CEL.

Wanderer

Never thought about it until yesterday. I did install Wevo engine and trans mounts. I have heard people mentioning the possibility of these throwing off knock sensors. Any possible connection?

Byprodriver

Yes that is very possible

Macster

The knock sensors are essentially vibration sensors tuned to whatever vibration frequency Porsche engineers have determined is a sign of impending detonation. These sensors are mounted fairly securely to the engine to ensure they receive these vibrations with no interference. These engines are intended to operate just short of detonation so it is critical the engine that the knock sensors quickly/accurately detect the onset of detonation, time and time again.

Thus it is hard to imagine a motor/]transmission mount change that could affect the knock sensors' vibration frequency tune/sensitivity.

To carry this further, *if* the mounts were affecting the knock sensors they would either be subjected to vibrations that interfered with their ability to recognize the vibrations that signaled the onset of detonation thus the engine would enter detonation state and backfiring would be the least of your worries.

Or spurious vibrations would appear at the sensors that would signal the onset of detonation when in fact none was occurring.

In this case the DME would retard ignition timing by some predetermined amount/curve until the detonation signals from the knock sensors stopped (or reverted to the absence of detonation signals).

Now if the DME reached its ignition retard limit I would expectd it to set the CEL with an error code that pointed to one or both knock sensors being bad. That is one or both are signaling detonation onset when there should be none. Again working knock sensors critical to engine health so any signs of trouble with these sensors would be detected.

In short, I can't see how backfiring could arise from the knock sensors being affected by motor/transmission mounts.

Generally a backfire is the burning of gasoline in the exhaust system, so a condition that introduced gasoline into the exhaust system (a leaking fuel injector or possibly a burned exhaust valve) or a condition that introduced oxygen into the exhaust system, causes the backfire.

My WAG would be that in changing the mounts something was done that has caused the backfiring.

Not sure what is required to change the mounts but if the exhaust system touched (or even put under some stress as the engine was lifted/lowered a bit to remove/install the mounts) I would suspect an exhaust system leak. Either at a joint/connection or possibly even a crack.

Next on the list would be a possible coil or even plug problem, arising from the activity to change the mounts.

A leaking injector should show up as one bank having an overly lean long term fuel trim as the engine controller attempts to cut back on the amount of excessive fuel in that cylinder bank's exhaust gases by dialing back the fuel injector pulse widths for all injectors on that bank. But the leaking one is less affected by this since, well, it leaks, even when it is closed/off.

Sincerely,

Macster.

Wanderer

Wow, thanks for taking the time to write that Macster.

I did lower the engine in the front about 1 inch. I tried to make sure there was clearance on everything but you never know. It is very fair to say I missed something. It did not show up right away. It has taken several months of daily driving for this to occur. It has always occurred since I bought the car with 2200 miles on it. But the occurrence has been about 15% of the time.

I have an Equus Innova scanner that does show some realtime data. Any idea if I can glean anything from it? What would I look for?

Macster

Your post above is a bit confusing though the fault lies with me.

Was the backfiring symptom always present? Before you went with the aftermarket the engine and transmission mounts?

Or did the symptom only appear (rare at first) after you went with the aftermarket the engine and transmission mounts?

Generally the last thing touched is the first thing suspected so you really should consider double-checking for any exhaust leaks.

And that the engine/transmission mounts are installed properly. It is a good idea to before the stock mounts are removed, before anything is unbolted, to measure/make drawings, take pics, somehow get a reliable feel for where everything is and then after installing the aftermarket mounts double check to ensure everything ended back where it should be.

The mounts may be installed incorrectly, or they may be the wrong parts, or it is possible Porsche made a change and the aftermarket parts are not (yet) available for your car. (My Porsche parts manager contacts all tell me to order parts for my Porsches using a VIN. Porsche ties part numbers to VIN numbers and at some point Porsche might make a change that requires a different part number. The same part (with a different part number) for a car with an older (or newer) VIN may not work right.)

I'm not familiar with the Equus Innova scan tool but what you want to look at is to make sure the intake air temperature and coolant temp readings are reasonable. Not only from a cold start but after the engine has warmed up.

It might be a coolant sensor is dropping out and sending over a lower than actual temperature and the engine controller is adding fuel to a 'cold' engine and this of course is excess fuel (cause the engine is not 'cold') and this excess fuel is responsible for the backfire.

Be careful: The sensor problem may be intermittent. It may only happen when the engine is being subjected to a heavy load then the load removed, which sort of fits in with when the symptom appears: during a shift.

The engine twists about and this causes the coolant sensor wiring/connector to move about and the sensor signal drops out momentarily then reappears. The coolant wiring/connector (or even the sensor) might have been disturbed during the engine/transmission mount R&R.

Another thing you can look at is bank 1 and 2 long term fuel trims. They should be nearly equal, a bit negative or positive. When (if) any reading gets near +/- 10% that's a sign of some kind of trouble.

Short term fuel trims swing between +/- every second or so at idle and more often at higher engine speeds, though the scanner may not be able to read the trim values often enough to see this.

But the swings should not be that large. A few percent, though on occasion the swings can be higher, nearly +/- 10% as the engine controller adapts to a cold engine heating up for instance.

You can get a feel for this by with the engine warm and idling removing the oil filler tube cap. This is a big intake air leak. The short term trims should state swinging to the rich (+) side as the engine controller adds more fuel to bring the oxgyen readings at the O2 sensors back into line. Then after a moment or two install the cap and watch the readings go the other way as the engine controller corrects for a now too rich condition.

Or clear the DTCs (even if there aren't any) which clears learned fueling. The engine controller will strive to establish the right trims but at first they may swing about quite a bit.

If the backfiring from a fueling problem I think you'd see long term fuel trims that are unbalanced, unless the fueling problem is a common failure. Maybe.

Example: An over pressure condition at times. The engine controller expects a rather constant fuel pressure cause all it can vary is the time the injectors are open so if the fuel pressure varies the injector open times are wrong. The engine controller will 'see' this and take corrective action but if the over pressure a transient condition the engine controller won't have the time before the over pressure condition goes away and the trims are once again valid. In short they may not move that much cause the engine controller doesn't experience the over pressure condition long enough to move the trims.

Anyhow, if one side looks 'normal' and the other differs then the fuel problem is probably confined to the bank that has the unbalanced trims.

(Though as I think I mentioned one IIRC Boxster owner reported misfires (though I know we'er talking backfires and not misfires) on one bank only that turned out to be a bad MAF.)

(Now another source of unbalanced trims is for instance when a VarioCam actuator solenoid on my Boxster started acting up not only did the engine idle rough but the short term fuel trims were quite unbalanced (the 'bad' bank was richer, IIRC) cause the engine controller was trying to compensate with fuel the failure of the solenoid and the effect this had on intake valve timing.)

If you want you can disconnect the MAF at the wiring harness, use the scanner to clear DTCs (to signal the engine controller to 'unlearn' its fueling learned with the MAF active) and then drive the car normally as you do when the backfiring occurs. If it is gone.. maybe MAF. Since these are expensive you would want I think more evidence.

If the backfiring still present, most likely not the MAF.

A plus is you have two banks to look at and two sets of sensors. Unless the failure is a common failure (common to both banks like say a coolant sensor or MAF) one bank's long/short term fuel trims, O2 sensor readings will be normal, and the other bank's abnormal.

Sincerely,

Macster.

Wanderer

Thanks Macster.

Yes. The backfiring was seldom but there. I think it was gone for a while and has really come back strong in the last two months. I installed the mounts this past fall.

These mounts are from WEVO. I am emailing back and forth with Hayden right now who is helping me work through it as well. I wanted to get his thoughts on it as he makes/sells these.

I think I get a good bit of what you suggested to look for from the Equus scanner. I will play with it to see. As soon as it warms up a little I will get under the car to check over everything as well.

You said disconnet the MAF at the wiring harness. Just to make sure I am clear, is this the MAF side or the ECU (DME side). I never knew this about forcing the ECU to re-learn.

So when looking at the reading from the banks in the scanner the results of each set of sensors should be close if they are functioning correctly or at least if there is a large difference I have narrowed the problem down some?

Macster

If you want to disconnect the MAF as a test to see if the misfiring is possibly MAF related, disconnect the MAF from the wiring harness at the MAF. I just checked a pic of my Boxster's engine top (to refresh my memory) and the connector is right at the MAF.

The connector's a bit tricky and if the MAF is original the connector can be a bit reluctant to unlatch and then pull free. Take your time. The MAF may be ok but if you break the connector latch you buy a new MAF anyway.

And don't drop it. And unless filthy don't clean it and if you must clean it do so with the proper cleaner.

(I cleaned my Boxster's MAF at around 80K miles and didn't get enough dirt off the thing to even see. The stock air filter is a wonderful filter and the MAF just doesn't get dirty.)

Remember too you are dealing with a rather sensitive electronic component so mind static electrical discharges.

At the MAF but at the other half of the connector as well. This eventually leads to the DME and you do *not* want to static discharge to that device. ($)

I'll say it now: Stay the heck away from the DME. There can be an exception which I'll touch up but which probably won't pertain to your situation. In a few cases, rare cases, accessing the DME and disconnecting it from its connectors and then reconnecting can correct odd engine behavior. A connector pin/socket gets a bit of surface corrosion and this affects the electrical signal (some pins carry a digital signal while others carry an analog signal). But before you go to the DME there are many other more likely causes for the symptoms.

Both banks are almost entirely endependent. They are both fed by a common main air intake, air monitored/measured by a common MAF, so they are not completely independent.

But they are independent enough -- and the engine controller in many ways treats them as separate and equal entities -- that generally one can use the sensor readings (like O2 sensors) and derived readings (like fuel trims) to note differences. The differences can be significant and informative.

Do not know if I mentioned this earlier but when my 02 Boxster's passenger side VarioCam actuator solenoid went bad not only was there a CEL, and and error code (P1341 IIRC) to go with it, but the short term fuel trims between the two banks were quite unbalanced. The driver's side bank the trim varied just a bit, not more than +/- 1 or 2% on either side of 0%.

The other side, the passenger side's short term fuel trim read about +5 to +6% (rich) at a steady crusing of about 3K rpms. The Ecu was attempting to compensate with fuel (about all it has at its disposal) to bring the passenger's side O2 sensor readings into an acceptable range.

If one observed irrational readings from both banks this suggests a common failure: MAF; fuel pressure, coolant temp sensor; and so on.

Sincerely,

Macster.

Wanderer

I have not yet had a chance to connect the scanner. With the snow I wont be able to for several more days. I did speak to someone else about this and they asked if the backfire was on the intake side or exhaust side. I never thought about it but I think it may be on the intake side.