Macster

Generally a backfire through the intake is a lean condition. Possible causes can be: Intake air leak; fuel under pressure; intake air temperature reading higher than actual; coolant temperature reading higher that actual.

A backfire from the exhaust is a rich condition. Possible causes: Leaking injector; fuel over pressure; intake air temperature reading lower than actual; coolant temperature reading lower than actual.

There can be mechanical causes for the backfire too, from improper valve timing, variable lift actuation faulty, burned valves and so on though thankfully these are rare.

Sincerely,

Macster.

Wanderer

Have not been able to do anything as the car is still buried in snow. But I did remember and wanted to mention I get about 20mpg on the car. 90% of my drive is highway. I thought the mpg should be around 23-26.

Macster

MPG varies depending upon many factors, not the least of which is the driver.

But 20mpg with 90% highway driving (assuming you are trying to drive the car in such a way that delivers good gas mileage) does seem a bit on the low end of normal. Based on my experience, 23-26mpg is about right.

If the backfiring is coming from the exhaust then this does suggest an overrich condition.

Would be nice to know what the long term fuel trims are. What the intake and coolant and true ambient temperatures are at cold start. Would be nice to know what the coolant and intake air temperatures do as the engine warms up and what they are when the backfiring occurs.

Sincerely,

Macster.

Wanderer

I was finally able to get the car out of the snow tonight.

I connected the scanner and got a few readings from it. I am still trying to understand all of them.

I noticed a TPS% that ended up being the throttle %. When I pressed the accelerator pedal to the floor with the car off I could only get 86% and the same when the car was running. So I can never get 100% throttle???

Before Start
LTFT B1 4.6
LTFT B2 6.2

After the car warmed up
LTFT B1 6.2
LTFT B2 6.2

STFT B1 was consistently about 1.5 off.
STFT B1 0
STFT B2 1.5

Warmed up
ECT 199
IAT 64

O2 sensors looks identical

A few other indicators I can read if needed are:
MAF /lb
EQ Ratio
Load
RPM
SparkAdv

Hope this helps.

Macster

Numbers don't look too bad. Nothing jumps out at me.

The long term fuel trims are rich but in cold weather that can be expected. The engine controller is having to add more fuel because the air is colder and denser and drier.

Short fuel trims can vary a bit from every second at idle to several times a second at higher rpms. The DME constantly varies the amount of fuel it injects per cylinder bank from a bit more than ideal to a bit less than ideal.

The short term fuel trims constantly cycle between a bit rich to a bit lean. Not all of this cycling will be centered at zero. For instance in cold weather the cycling may be rich to richer and if this continues long enough the long term fuel trims are adjusted (richer) and the short term fuel trims then are used to deal with the short term fuel needs of the engine. At this point the variation of a bit rich then a bit lean can be centered around the zero point.

But you did mention one bank's reading consistently off (+1.5). This is the DME adding extra fuel (stretching the fuel injector pulse widths) to that bank vs. the other bank. The bank with the consistent +1.5 reading is probably the 'good' bank, in that I would expect the trims to be biased to the rich end due to the cold weather and cold, cool engine. Thus the bank with the consistent 0 reading becomes suspect. But beyond this I have nothing. Let me think some more about it...

Regarding that TPS% reading: 86% sounds about right. I don't have the E-Gas throttle details handy but IIRC the reading won't be 100% (it won't be 0% either). Something to do with the device working between 0.5 and 4.5 volts with a reference voltage of 5 volts (but don't quote me I'm working from memory). But don't worry. When the engine is running the throttle will open all the way.

That everything looks ok may mean you just didn't happen to catch the car at the time that whatever is causing the excessive backfire was acting up. It took me some time -- with an OBD 2 code reader/data viewer connected and handy -- in my Boxster to catch an O2 sensor at just the right time. Before the check engine light would come on and some time later -- sometimes after shutting off the engine then restarting it -- when I checked the reading and compared it to the other bank's sensor it looked ok. But over time I got a feel for just how long I'd drive before the check engine light came on and I'd start watching the sensor readings before I reached this threshold. When I did this I clearly saw the suspected sensor acting up, remaining very low and flat for several cycle periods (based on the other sensor's alternating reading of above 0.7 volts and below 0.1 volts).

If possible you need to record the various readings/derived values *at* the time during which the backfiring occurs and doesn't occur. Then compare the readings of the various sensors/derived values.

I've forgotten -- maybe I should go back and re-read the beginnging of this thread? -- if I've suggested this before but have you tried disconnecting the MAF at the wiring harness connector (at the MAF) and seeing if the behavior still exists with the MAF out of the loop?

Sincerely,

Macster.

Wanderer

You did mention to disconnect the MAF but being that I do not have a garage and its still sleeting outside I opted for the scanner with the heated seat on :-) I will try this as soon as the weather warms a little.

My scanner does not have data recording ability. Can you recommend an excellent unit that will log data?

Wanderer

Just an update. As the weather has warmed most of the backfiring is gone.

cvazquez

Check entire exhaust system for leaks especially the headers.

Wanderer

Interesting you mention exhaust. I will take a look at it.

I remember changing the oil before and noticing one of the headers does not have the heat shield welded on it.

I still need to do what Macster suggested as well.

Macster

Well, with the symptom gone now you can't disconnect the MAF and see if this helps/hurts.

For an OBD code reader/data viewer I bought and use an Actron. I forget the model number but it reads/clears codes, and displays and records (but not for more than about 60 seconds) OBD data. It has other features too like reading the readiness monitor status and allowing me to control some OBD tests for proper O2 function, etc.

If you want a more sophisticated (and more expensive) unit check into the Durametric model.

Sincerely,

Macster.

Wanderer

Backfire is definitely from the intake and happening frequently. It was quite an "explosion" yesterday when rev-matching that was clearly from the intake.