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And the diagnosis is....


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And it's back....
MarcW - 2 weeks ago
The CEL with the P1128 error code.

<sigh> I'm now 1 for 3 when it comes to correctly diagnosing a CEL/engine symptom as being due to a bad MAF.

Frankly and honestly I was a bit surprised the CEL came back. It was not my imagination the engine perked up from the new MAF. And since the new MAF was installed the engine has continued to show marked improvement. It wasn't just a flash in the pan. Gas mileage has improved a bit too. Last mileage check -- all driving having been done after the MAF was installed -- had the MPG up to 21.4 which is a MPG better than the car has been doing for a long time. With the improvement in the engine's performance -- and of course the absence of the CEL -- I was sure my diagnosis was right.

Anyhow, the CEL came on this AM during the drive into the office. As it has the times before the CEL just comes on with no drama. There is no indication from the engine anything is wrong. Today when I checked the freeze frame data the coolant temperature (205F), RPMs (680), intake air temperature (102F), were all about where they were all the previous times. I did note the long term fuel trims were 0.0 for one side and -0.8 for the other side. These are really close and not at all any real cause for concern. The short term fuel trims likewise looked quite normal. Before I recalled the long and sometimes the short term fuel trims were quite a bit away from zero, either close to -10% or +10% at least on one bank.

At the office I decided to add two bottles of Swepco 502 oil improver. The oil level -- after the oil service some weeks and over 1000 miles ago -- was down enough I could add the 2 bottles without any concern of overfilling the engine with "oil".

Before I add the fluid with the engine idling I checked the oil filler tube cap. It was quite secure. It is not that old having been replaced I think sometime in the last 12 months. Anyhow, I was unable to move the cap about.

Then I removed the cap and of course the engine reacted.

Quickly I added the two bottles of Swepco oil improver and then put the oil filler tube cap back on and as a number of times before when I've done this the engine quickly resumed its sewing machine smooth idle.

But then it struck me... the oil dipstick. I checked the dipstick and while it was fully inserted it had quite a bit of lateral play. I pulled the dipstick out enough to check and the o-ring is intact and when I installed the dipstick there was still considerable "play". I listened carefully to the engine but didn't hear it react though. But the leak is not severe. Kind of like the leaks from the oil filler tube caps. Just a bit of an leak at low RPMs, at idle.

Anyhow, I have not used the dipstick in years and frankly do not remember how it felt before. I don't recall it being loose. Everything about the car has a solid feel and this includes the dipstick, or at least used to.

Will have to stop by the dealer time permitting Thursday AM and have a tech check the dipstick to see if it feels loose to him. (Another way I could possibly make a diagnosis is to observe short term fuel trims with my OBD2 code reader/data viewer while I move the dipstick about.) May be premature (and hopeful thinking on my part) but after having gone through 3 AOS's, two oil filler tube caps, one gas cap, it would not surprise me after 315K+ miles the oil dipstick proved to be a "wear item", too.
21 MPG is really low. I only see that with all city driving. As soon as I get out on open roads it will go up to between 25 and 27 MPG.

Also, if you can't remove the oil filler cap when the engine is idling you have a bad AOS. The vacuum is way too high. The best way to test the AOS is to have a spare cap with a vacuum gauge attached. I don't remember the exact crank vacuum that you should get with a good AOS, but a search should pop that up right away.
Quote
Anker
21 MPG is really low. I only see that with all city driving. As soon as I get out on open roads it will go up to between 25 and 27 MPG.

Also, if you can't remove the oil filler cap when the engine is idling you have a bad AOS. The vacuum is way too high. The best way to test the AOS is to have a spare cap with a vacuum gauge attached. I don't remember the exact crank vacuum that you should get with a good AOS, but a search should pop that up right away.

My usage doesn't involve pure freeway driving any. While I commute 30 miles to work and maybe 90% of those miles are freeway miles traffic does not move at freeway speeds. Most of the time the driving more closely resembles city driving.

But the 21mpg is a mpg better than what the car has been averaging.

The oil filler tube cap comes off just fine. The AOS is ok. It was replaced back in 2014 at I don't recall the mileage now but it was probably at around 300K miles.
Not the dipstick. Tech checked it today and saId it is not loose, the o-ring's intact, and he's never seen one leak.

To recap most of the the time the tech told me the cause of a P1128 error is a dirty MAF, dirty because of a failure of the air filter -- the early Boxster air filter element is notoriously a difficult item to replace and often it gets torn/damaged when installed. There's no way to check it once in place. The bad filter lets dust through and this over time settles on the MAF and its hot film surface. This surface is because of its size relatively immune to some contamination but at some point the bad filter and the dust it lets through takes its toll.

However, when I replaced the MAF the other day the old MAF looked just fine. No signs of any dust build up.

Another possible explanation can be when the old engine air filter is removed some trash that has collected on top of the filter falls down into the air box and if not removed can end up against some plastic screen/mesh located further down the intake just ahead of the MAF. This trash -- most likely a leaf -- can disturb the air flow and affect the MAF's performance which can trigger a P1128.

I am well aware of the trash falling off the filter as this has happened almost every time I have changed the air filter. The filter element top surface collects a lot of trash. Last time the top had a solid layer of leaves on it. What a mess. But I was careful to remove all the stuff that fell into the air box.

What the tech said then if none of the above that a common problem is the intake develops a leak. He said the intake o-rings dry out and leak. He also mentioned there are a number of vacuum hoses and once in a while one of these cracks/splits and leaks.

Every time the P1128 has appeared it was not accompanied by any other error code. Also, I always check for any pending codes and there haven't been any. I've never gotten an error code related to the gas tank ventilation or vapor recovery system or the secondary air injection system, so I sort of kind of think the hoses are ok.

The tech said what is done is a smoke test is used to determine where the leaks are and then that determines what is done. He said often the intake manifold ends up getting removed and re-sealed -- new o-rings installed. 'course, if the leak is from a hose the hose and whatever else may be contributing the leak is replaced.

While I was hoping it was the dipstick that appears to be not the case.

I kind of like the intake o-rings for the reason. I know the spark plug tube o-rings failed -- they just deteriorated due to age -- and had to be replaced. So it would not surprise me at least one intake o-ring was leaking due to old age.

So, I have tentatively made an appointment to bring the car in next Thursday and have this looked into. I was thinking of waiting until the car's next service but with the lower number of miles I'm putting on the car now -- just 10K a year -- the next oil service is 4/5 months out. So next Thursday is as soon as the service department can get the car in.

Since after leaving the dealer on the way to work the CEL came on again today I am most assuredly going to keep the appointment. And after I get home tonight after work probably park the car and use the other car in the meantime.
1. very small intake leak
2. marginal/bad MAF (defective, IM)

I really doubt the leaves or other debris....

Grant gee-lenahan-at-vee-eff-email-dot-net
IIRC intake leak is at the top of the list of possible causes. I fell prey to the tendency to blame the MAF.

The leaves and dirt the tech told me are things he and his co-workers have encountered over the years. It wasn't a hard diagnosis just something to check. I was aware of stuff falling off the filter into the air box when the filter is removed. I've changed the filter enough times and this I recall has happened every time. 'course, I am careful to remove the trash.

I didn't think the air filter was damaged. I'm careful when I instal a new air filter and I've never seen an old one come out damaged in any way.

While I don't look forward to the bill to reseal the intake I like the intake o-rings for this. At 315K+ miles those o-rings have been in service a long time. All the heat up cool down cycles and what have you have to have taken a toll. I am looking forward to getting the car back with this leak be it from the intake seals or something else (hose?) and seeing if the engine runs any better. It runs pretty good and the new MAF helped as I have mentioned, but I do notice the idle speed is a bit low at times -- the idle speed at the time the CEL is tripped is 680 and that's low. The nominal idle speed should be 740 +/- 40 and 680 is "low". Last night after the drive home the radiator fans came on just as I pulled into the parking space and I let the egnine idle while the fans ran to bring the coolant temperature down and the idle speed was low and the engine felt like it was laboring a bit. Don't recall it doing this before, in the years past, but it has been doing this of late.
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