rskovira

I'm throwing a P1126 & P1133 code. My FRA is 1.31 & RKAT is -1.83 both banks. Already cleaned MAF twice. Air intake temp is 5-7deg C higher than ambient when cold engine.

Do I need a new MAF?

If I disconnect the MAF how far can I drive the car without doing damage?

Ron
2000 Boxster S
75k miles

Macster

Doesn't have to be from a bad MAF.

Could be from an intake leak. This always raises the possibility of an AOS. I note your car has 75K miles and is 13 years old. My 02's first AOS failed at around 78K miles though at just 2 years.

At hot idle can you unscrew and remove the oil filler tube cap? Even if you can this is not a foolproof sign the AOS is ok.

While you are there, be sure to check the oil filler tube cap for leaks too. Missing this had me replacing a perfectly good MAF instead of a $13 cap.

You can disconnect the MAF and clear the error codes to reset the adaptation values to their defaults and then drive the car normally.

The CEL might come on the error code related to the MAF being disconnected. My experience is you can drive the car without any damage. I would not say decide to take the car on a spirited mountain road drive, or try confirm its max. speed or track the car. Just drive the car around town.

While I do not think there's anything to worry about always if the engine starts acting up in any way shut the engine off first and ask questions later.

BTW, that difference in intake air temperature over ambient is normal in my experience. The Boxster engine air intake is located at a place that has it pulling in hot air from the radiator. (Also dirty air which accounts for some of the trash one finds on top of the air filter element.) The best I could do was around a 10F difference in intake air temperature vs. ambient, with 12F or 15F or even a bit higher the more usual difference.

rskovira

Macster- thanks for responding. Short story Long, I've been struggling with this CEL for a month plus. Started with a RKAT reading of +3.14 so I did replace the AOS, still had the codes so I replaced the oil filler tube which did have multiple holes in it. Now my KRAT is negative however my FRA is positive. Since my RKAT is -1.83 I thinking I don't have a vac leak anymore. I'm looking for suggestions on how to determine if my MAF is bad/marginal. I do have a Durametric reader and the MAF volts at engine off is 1.03v and at idle is 1.26-1.3v. Don't know if that is too far below 1.4v to cause my problem. ??????????

rskovira

If I disconnect the MAF, I know I will get a different code because of being disconnected, if P1126 & P 1133 do not come back does that prove the MAF is the cause? Does a disconnected MAF put it into a "limp" mode?

Macster

Sorry I do not recall the MAF voltage levels off hand.

And I'm one for two in the MAF problem diagnosis department. My first bad MAF diagnosis turned out to be a leaking oil filler tube cap.

But I redeemed myself a bit when the replacement MAF started acting up -- after I forget how many miles and how much time (but lots) -- I diagnosed the MAF was the culprit.

Unfortunately I do not remember the error codes now and I do not recall any symptoms but the error codes pointed to the MAF. I still had the original MAF in a box the replacement MAF came in so I removed the replacement MAF and installed the original MAF and the error codes went away.

I'm away from my OBD2 references for the Porsche error codes but IIRC one possible explanation for a P1126 is an air leak in the exhaust ahead of the O2 sensors.

Also, it can be due to too high a fuel pressure, a leaking injector, and of course a bad MAF.

To sort of kind of eliminate the MAF you can disconnect it at the electrical harness connector, clear the error codes which resets fuel trims to their defaults and then drive the car normally.

The CEL might come on but the error code could be related to the MAF being disconnected. I forget what error code or codes this might be but you can read it and look it up when (if) the CEL comes on. (I have done this once or twice with my Boxstser and the CEL never came on.)

(Do *not* erase the error codes as this then resets the fuel trims.)

What you are looking for are the same error codes as before which then suggests the problem does not lie with the MAF.

After the engine has gone through a warm up -- provided the error codes stay away -- reconnect the MAF, "erase" the error codes (to reset the fuel trims) then drive the car as before. What you want to see is the same error codes come back which then sort of points to the MAF.

rskovira

Disconnected the MAF and data logged for 28 miles. Only codes were P0102 & P0112 which are due to the MAF being disconnected. My FRA and RkAT remained the same at 0.99 & 0.0 which I guess is the open loop values. I suspect the 1.31 FRA value is what throw the CEL with the MAP connected. What I was wondering is since the FRA and RKAT is "fixed" with the MAF disconnected I will not see the FRA go to 1.31, so does that really mean my MAF is the root cause?

Macster

The odds are high if when the MAF is disconnected and the car driven a reasonable distance/amount of time and the error codes that can have a bad MAF at their source stay away and then when the MAF is reconnected and the error codes come back that the MAF is at the root of the problem.

The odds are high but the bet is not a sure thing. It depends upon what it takes to eliminate all doubt or how sensitive you are to say buying a new MAF when the problem may be found to be with something else.

I have my OBD2 error code reference handy: P1126, and P1133...

P1126 - Fuel/air mixture so lean the control is at the enrichment limit. Possible causes: Intake system leaking; fuel pressure too low; fuel injectors contaminated; volume of fuel supply is low.

Note there is nothing about a MAF in all of that. Note sure how MAF got into the discussion without going back and reading older posts which I do not have the time to do.

P1133 - My reference has nothing on this.

But with P1126 you need to look elsewhere for the source of the error code. MAF is not on the list.

I have to stress an intake air leak is at the top of the list and you need to be sure there is not one. As I think I mentioned before a leaking oil tube filler cap had me replacing a MAF when the MAF was just fine.

Furthermore, I was convinced because it was the MAF for that is the only way I would have spent nearly $400 on a new MAF when a $13 cap was what proved to be all that was required.

The leak can be that simple. Or it could be a -- sorry but I have to say it -- bad AOS install. A hose got bumped and due to its age split or cracked. A fitting could be cracked. The new AOS could be bad. There are lots of possible places an intake air leak could be.

Or it could be something else. Fuel pressure and supply is also mentioned.

Now in my car's case the fuel pump just up and quit. But they do not all fail so suddenly. Some die a protracted death.

There's the fuel filter too. My 02 has no serviceable filter. The filter consists of the fine mesh at base of the plastic fuel pump housing -- but IIRC your 2000 does have a fuel filter.

rskovira

Macster- Since my RKAT is a negative value so the ECU is cutting fuel at idle, correct? Would not that mean I probably don't have a vacuum leak since the ECU would be most sensitive to a leak at idle?

I could understand a bad fuel filter causing the FRA going positive even though the RKAT is negative. If I log O2 sensors at higher RPM I should see values tend on the lower voltage (.1-.48) side, correct? Or I'll pick up a new filter on the way home tomorrow. I can't remember when I changed it last.

I appreciate your suggestions.

Macster

RKAT and FRA numbers may be unreliable at this stage as the DME is striving to relearn adaptation values since the error codes were erased.

The times I have erased error codes and then observed the short term fuel trims the trims swing back and forth as the DME I guess experiments with too rich then too lean to find the center.

What are the error codes? Error codes tell you what the DME has found it didn't like.

Also, how does the engine run? What specific engine symptoms or error codes are you going after?

The #1 sensors should swing from under 0.1 volt to over 0.7 volt as the DME constantly cycles the fuel mixture from rich to lean and back again. At idle this cycle time can be on the order of 1 to 2 seconds.

At higher rpms the cycle time can be shortened to less than a second. Most scan tools can't query the sensor readings fast enough to see this but with a scope one can monitor the voltage levels and see this on the scope screen.

The #2 sensors should read above 0.6 to 0.7 volt as the oxygen is consumed in the converters when the exhaust gases are processed.

Under hard acceleration the sensor readings are ignored by the DME as it switches to open loop mode.

rskovira

Macster- On your suggestion I reinstalled my AOS with new o-rings and RTVed to insure I did not have any vac leaks. After resetting ECU I got 40 miles before my CEL can on with the same P1126 and P1133 codes. Engine runs fine and idles smooth.

My FRA is +1.29 and my RKAT is -0.89, both banks. Will these values throw the code? Why is one positive and the other negative?

I have a new MAF sensor on order as back up because I can't figure out what's throwing the code using the Durametric data.

2000 Boxster S w/75k miles

rskovira

After 125 miles FRA is +1.31 and RKAT is -2.25. Anybody have a suggestion what to look at???

ToSi

Depends how much effort you want to put into troubleshooting - you can continue verifying no vacuum / exhaust leaks, fuel pressure is good, etc.. to confirm the MAF needs replacing or just replace MAF & see what happens

Macster

Well, as I touched upon in an earlier post my resources do not list a MAF as a possible cause of the error codes you give.

Intake air leak. Fuel pressure too low. Fuel injectors contaminated. Volume supply of fuel pump too low.

Based on my info I could not in all good conscience advise you replace the MAF.

If you are sure the recently touched AOS is not leaking and is working ok that leaves fuel pressure, fuel supply or contaminated injectors.

IIRC you did disconnect the MAF, clear the error codes (mainly to reset the fuel trims) and then drove the car. While the car logged an error code or two directly related to the MAF being out of the picture so to speak the other error codes did not reappear.

You have reconnected the MAF and the error codes have reappeared.

This does suggest the MAF is at fault, which is I have to point out contrary to my sources of info. But they are I'm sure not all that complete or even error free.

You mentioned getting a new fuel filter. Have you received this yet and installed it?

The MAF is expensive but sometimes it is better in the long run to throw a part at a symptom rather than go further into areas which have you dealing with fuel flow/supply and pressure testing.

However, I have to point out if you replace the MAF and the error codes come back, you still have to deal with, eliminate the other possibilities.

So, look elsewhere or replace the MAF?

Your call.

Macster

RKAT and FRA are not terms I work with, have all that much experience with. I prefer short term and long term fuel trims. But RKAT and FRA are not quite the same thing as short or long term fuel trims. They are more (as best I can recall) adaptation readings.

None of the equipment I have reads the RKAT and FRA (or any other numbers) related to adaptation.

As best I recall, RKAT being negative is a sign the DME is subtracting fuel to compensate for an overly rich condition. This subtraction is taking place at idle speed.

FRA is positive which indicates the DME is adding fuel at some engine speed and this is above idle speed.

I do not know what is normal for these readings.

Now a search on the web did turn up a case where a negative RKAT and a positive FRA actually rules out an air leak and does in fact point to a MAF problem, at least in the opinion of the person posting this.

I have a bit of a problem with this but do not have the time to take to point out why. (Actually I do have some time if I'm brief: It is based on my experience with my car and its oil filler tube cap leak, which at times caused an error code from the DME reaching a rich adaptation threshold but at other times reaching a lean adaptation error code. These had me convinced the MAF was at fault when as I pointed out it was the oil filler tube cap. But you have eliminated the cap (or any leaks in the tube) I hope!)

Have you the mass air flow reading for say idle? IIRC -- it has been some time since I monitored this in my Boxster -- the reading at idle should be around 4.16 g/s with a variation of around +/- 0.5 g/s depending upon various factors.

mikefocke

– Incorrect signal from MAF sensor
– Intake air system leaking
– Fuel pressure too low or high
– Volume supply of fuel pump too low
– Fuel injectors fouled

and even exhaust system leaks.

First cause diagnosis

Connect voltmeter to pin III/23 and ground.
Switch on the ignition.
Display: 0.9 to 1.1 V
Start engine.
Display: approx. 1.4 V

Some info on replacement MAFs here.