Coils & plugs replaced - now big power flat spots

As I said in my previous thread I have replaced all 6 coils & plugs to try and cure a misfire and refusal to rev to the red line. I cleared the codes with my reader. All 6 old coils had splits and cracks in the plastic cases.

It doesn't misfire now but there is a big yawning power flat spot when you hold your foot down. If you lift slightly the revs rise and the car surges forward then the power seems to cut in and out. You can get it to do this at medium and high revs.

Should I now suspect the MAF sensor, or could another part of the ignition system be to blame? The warning light doesn't come on and no error codes are produced.

I was so dissapointed when I drove it tonight I could cry! Help Pedro!



Edited 1 time(s). Last edit at 04/12/2011 04:12PM by marty. (view changes)

learning the engine controller did with possibly bad data from the suspected MAF) and drive the car and see if the behavior is still present.

Sincerely,

MarcW.

Unplug the battery so that the car will relearn the ignition mapping. If your car is pre-2003 you will need to have your radio code to get the radio working after you reconnect the battery. The car might run poorly for a minute or two while it learns the new parameters.

Marty,

Sorry to hear you are having trouble, the teen years are approaching and can be a difficult and trying time with your 2.5L and more maintenance than normal is probably expected. I have a few questions for you and your little friend:

1. Is the 5500 RPM ceiling the same with load Vs no-load, i.e. with clutch in / neutral revving Vs in 3rd gear WAO? If the two situations have different symptoms please list the deltas in detail.
2. Are you handy with a multimeter, if so please test the MAF output voltage at idle and revving. This can be done on a 99 easily because it has a mechanical throttle you can physically grab in the engine bay and rev, while looking at the meter on the MAF. I have seen MAFs fail where the output voltage is >5v. You should see a voltage between 0-5v on the signal line. If you need the wire color, let me know. MAFs fail in other ways than over voltage, so swapping out for a known good MAF from a friends Boxster or the like is the easiest test. A good OBDII reader will tell you the reading while you are driving, might be easier.
3. Have you rebooted your ECU? You can either remove the battery Positive for 5min (make sure you have your radio code), OR remove the ECU itself (4x 10mm) for 5min, then reinstall. Its easy as 123 to remove and reinstall, its in the trunk behind the carpet on the drivers (USA) side.
4. This could be fuel pressure related, but I need the answer to #1 before speculating much further. The fuel pump relay is cheap and they do fail. If you free rev OK (Question1), then check the fuel pressure under load. If it drops then replace the fuel pump relay first, then change the fuel filter second, then the fuel pressure regulator, lastly the fuel pump. As separate steps; change, test, change, test, etc.
5. Below 5500 RPM does it feel "normal"? If not whats different?

Suggestions:
S1. I have seen a batch of bad gas, bad = lots of water, cause a somewhat similar issue. So just as preventative maintenance, get some water remover from the auto parts store. It is cheap and wont hurt anything. Use a double dose.
S2. Check your intake for leaks. If you have a post MAF leak, this could fit your symptom. Lots of ways to look for leaks, start with a physical inspection, rev up the engine while your heads in the engine bay, listen for suction sounds, then you could move to more advanced methods like spraying propane over the intake pipes, intake manifold, etc looking for the idle to pickup. Do this OUTDOORS. Please dont blow yourself up.
S3. Check your air filter to make sure a colony of rats is not using it as Hotel Porsche, or other obstructions. Look for something big and obvious.

Most Likely:
The most likely issue is either an intake leak, MAF failure, or Fuel Pressure drop.

My science is shaky here, but going by the symptoms, could the air flow be close enough to spec at low rpms, even with with some leakage? Then at medium to high airflow vacuum causes the throttle body tube to progressively constrict and relax around the throttle body opening. At the higher speeds and the varying throttle openings that mimic acceleration the flow is more critical, and the flat spots more obvious. Ok, I'm winging it without a diagram or mental picture of the configuration, so perhaps I'm being too imaginative.

Thanks for the wonderful help! I will try to answer your questions to provide you with accurate symptoms.

1) When I drove this morning the low revs flat spot was gone. My journey was short, and by the time the temp. was up to 80deg I could only try to red line it in 1st gear. It reved out with no flat spots. Could the ECU have taken a while to alter settings yesterday once it detected the new coils/stronger sparks? I was concentrating on getting the car home this morning to check the MAF, so I didn't try it under proper load. I have removed the MAF sensor and are just about to clean it with electrical contact spray - no brush or rag.

1a) I will road test it later to see if it's cured, or if symptoms appear above 5000rpm under load/no load.

2) Please can you say which colour wires to measure on the MAF plug? I'm no electrician but I do have a multi meter.

3) I have not disconnected the battery, but I will when I refit the cleaned MAF sensor.

4) I will work through this if the afore mentioned fails.

5) Below 5000rpm this morning the car felt 100% normal.

I shall try your suggestions re. air leaks etc.

Thanks again. I will update when I have answers.

*Yes, dont red line your 2.5 until fully warmed up. You could cause a failure at worst and at best premature wear.

1. I usually clean my MAF with a flattened cue tip with clear alcohol. Whatever you use, just be careful. Its the flat film inside the tip of the MAF that needs to be cleaned. The element at the bottom is a thermister, clean it you can, but its not the critical part. The ECU does need to re-learn, but that is for closed-loop which is used mostly at idle, low revs, and higher revs under light loading. If you floor the throttle aka WAO (wide ss open), then the ECU switches from closed-loop to open-loop and the self tuning parameters are not used. Thus if you floor the car the self tuning items should not be used. BTW closed-loop is when the car sets its air-fiel mixture based on real time feedback from the oxygen sensors until it reaches its target air-fuel ratio. In open-loop it just uses a lookup table in the ECU which is pre determined air-fuel parameters from the factory. You cant close-loop at WAO, things change too fast to use O2 feedback. Especially from narrow band O2 sensors that are used on your Box.

2. MAF Output Signal : Pin 5 : White / Black. Measure the voltage between the signal wire [ White / Black ] and the ground reference wire [ Brown / White ].

From your comments, responses and your pother post, I am not confident you have fixed the issue. From my understanding you said that after changing 6 Coil Over Packs and 6 spark plugs that the issue remained, and since then you have changed nothing but the symptom is gone. I cant argue the symptoms are gone, so I suppose a quiet celebration is in order.

I have now cleaned and replaced the MAF. The voltage to the plug is 4.9V and doesn't fluctuate when you rev the engine. I disconnected the battery for an hour or so.

I will take the Box. for a test drive this evening and try red lining it under/off load when warmed up.

This may be of some consequence: Recently while replacing the alum. gaskets between the exhaust manifolds and the cat. flange, I had to use an exhaust shop to drive out one of the corroded bolts in the flange with an air tool. Could the sharp banging have damaged the cat sensor(s)?

When you go for your drive, you could notice funny idling and maybe even a stall or two, this will fix itself as the ECU re-learns the vehicle. This re-learn procedure will not affect your testing because your WAO pulls will be in open loop.

If the banging damaged your cat sensor [ from now on lets call that the narrow band O2 sensor ] then the closed loop learn cycle might not complete or might not complete correctly. Your OBDII should read the O2 values, just make sure the left and right bank O2 readings are "about" the same. narrow band O2 readings are all over the place at idle because the ECU is playing high - low on the air fuel, so the O2 reading will and should oscillate. Just make sure booth banks osc in the same range.

Could a failed O2 cause your issue at 5500 RPM, maybe, but not likely since you would be on open loop anyway at that point and the ECU should be ignoring the O2 readings.

Banging on the header with an Air Hammer, or Cat pipe that is bolted to the header, or anything bolted to the engine should always be avoided IMO.

The MAF voltage seems good, just remember that you have determined the voltage is in range and moving with throttle input, and that the MAF is clean; neither is a guarantee the MAF is working as expected.

Good luck and make sure you let us know how it tuns out.

the air cleaner is compromised I think cleaning the MAF is just an exercise in futility. My experience is even over 10s of thousands of miles they just don't get dirty. The new thin flim MAFs are quite an improvemetn over the old hot-wire MAFs.

Sincerely,

MarcW.

With an oiled conical filter you have to clean it regularly, or at least more often than the stock airbox/filter.

Many PPBBers have reported fixed symptoms after cleaning their MAF with and without conical filters. I do agree you have to be careful whichever way you clean your MAF. Since my MAF almost always has some oil on it, physical contact is needed. Interesting enough I have never heard of someone "destroying" their MAF from a cleaning, so maybe they are very durable, more so than we all think.

For the record I am not a mechanic, so what do I know...

and go back to the stock filter.

I don't have a MAF handy to take pics and post but the films are very small and located in a very small passage that loops around in a convoluted path.

I'm not sure a Q-Tip will even get in there, fit in there.

Wait, here's a link to a couple of pics of a Bosch hot-film air mass meter (sensor). The one at the top looks very similar to what I remember my Boxster's MAF looks like, down to the greenish o-ring.

[rb-kwin.bosch.com]

Sincerely

MarcW.

The first few minutes the low rev power had flat spots. This I put down to the ECU "learning" the cleaned MAF. I kept things gentle and as it warmed up and I gradually increased the revs. The flat spots went and it was smooth and normal.

Once the motor was warm I floored it in 3rd gear. Straight to and into the red line! Repeated tries in other gears were fine too. I held it at 5500rpm and there were no hiccups, or when when gently lifting off. Oh boy did that feel sweet!

From excellent description I understand how the MAF doesn't effect the wide open mixture settings. So I'm still in the dark why things seem sorted.

I have ordered a fuel pump relay and a fuel filter. The filter was changed by the previous owner at 52,000miles. As the car has now done 94,000miles I reckon it's ready to change anyway.

I have found a supplier here in the UK (www.worldcarparts.co.uk) who can supply an aftermarket MAF, with they say 12 months warranty, for just £70. They have real land address and web site. What's your opinion on using a non Bosch MAF?

I am sorry if I threw you off with my comments on the O2 sensor (in the exhaust system) and the MAF (in the intake system). The MAF + RPM is all the ECU has for WAO [ open loop ] calculations. The O2 + MAF + RPM is used for closed loop.

Closed-Loop : Idle, Light Load throttle, constant speeds, etc
================================================
ECU uses: O2 + MAF + RPM (and surely other inputs)

Open-Loop : Hard Acceleration, aka WAO, aka full throttle, or hard acceleration, etc
================================================
ECU uses: MAF + RPM
The O2 readings are changing too fast to be used to set Air/Fuel Ratios aka A/F or AFR, so the ECU uses RPM, MAF, and Load, where load is likely calculated from Throttle Position, and MAF or something like that. But not O2 readings.

Summary:
Your MAF is used for full throttle pulls, so the cleaning of the MAF could result in a better calculation of AFR during full throttle, thus what you are seeing makes sense.

I am very glad to hear your Boxster is running great. Should you replace your MAF (bosche or otherwise)? My opinion is no. Should you stick with the OEM Bosche part if you do replace the MAF, I say yes. You dont want to introduce another variable to save $50 IMO.

Ahhh! It now makes sense!

I shall take your advice re the non Bosch MAF. If (when...!) the flat spot/misfire returns I will get a genuine part.

Thanks ever-so-much for your help and advice. I love my Box. again now!

Marty.