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'02 Impala 3.4L: Diagnose This!

Without first getting into a discussion of the problem, please review these three data captures and tell me if you see anything that would point to a problem. The Captures were taken on the drive home from work yesterday. 1st was coming out of work, heading toward highway. 2nd was ON the highway. 3rd was off the highway heading home.

Note: I just bought this OBD Scanner and I literally pulled it out of the box, plugged it in, and hit “Record Data”, so while I think these Captures are legit, who knows??

BTW, the code is the familiar P0442, small EVAP leak. It had gone away for a few thousand miles after smearing a bit of Petroleum Jelly on the seal, but apparently it’s back…

IMO, that data isn’t particularly useful

Just off the top of my head, I believe the problem could be a purge valve that isn’t able to pull vacuum on the evap system . . . blown diaphragm, or perhaps manifold vacuum isn’t getting TO the purge valve, for example

As for that cap, sure we know you smeared petroleum jelly on the seal, but is the seal pliable, not hard as a rock and split?

Is the corresponding surface on the filler neck in decent shape?

Since I saw your post on the website, please answer my questions . . . in the “general discussion” section, because I started a new thread, there

Am I to understand there is just 1 problem, the P0442?
If so, then I agree!

Here’s where I’d be looking:



Why does the VSS (mph), capture #1 show 0 MPH if you’re driving from work to the highway?


The only thing I see a little unusual is the long term fuel trim for capture 2. Seems maybe a little on the high side. But that could be normal for your vehicle too. Usually if it is out of bounds by 10% or more, that’s when to be concerned about long term fuel trim readings. But if you are having air/fuel mixture issues, that might be a clue. That number means the computer (based on the pre-cat o2 sensor reading) is having to inject more (or less, can’t recall which) gas than it thinks it should need to based on the other sensor readings, the maf, iat, map, rpm, etc. For example, if an evap purge valve was allowing fuel to get sucked into the engine from the canister, and the computer didn’t know about it, that could cause a deviation from normal fuel trim readings.

db4690, Common_Sense_Answer :

Yeah - I’m not sure about this Scan Tool. Just got it. It’s a little weird in that I can do the 3 Data Captures while the vehicle’s operating, but then I can’t view the data offline (inside the house). When I try that - even though the 9V battery is installed to retain the data, it shows the 3 Captures, but all 3 have the same data. It’s not until I go back out to the vehicle and hook it up again that I can see the 3 distinct sets of data. I could also hook it up to the computer and download the data that way, but I think the 9V battery is supposed to retain the data AND allow you to view it offline - but maybe not on the unit’s screen - which, if true, is kind of weird!

Anyway, as for the petroleum jelly, I just put a thin film on the gas cap seal. It definitely made a difference, holding off the code for much longer than it had been happening. It was kind of an experiment to see if I really needed a new cap. The cap seal looks perfectly fine and so is the filler neck. No problems with either, but maybe the seal is just hard and not pliable - which is why the Jelly “fixed” it (for a while).

BTW: the P0442, I believe is just a side-show here. The real problem is an intermittent weakening of the idle at a stop which can work its way into an increasing stumble, and then … POOF … a stall. Never illuminates the CEL. Never happens in the morning on the 30 mile drive to work, and it won’t happen on the ride home if I use the Interstate, either - because the engine is going full throttle most of the way. It happens if I go through town, with all the lights. As I get closer to home, say 3/4 of the way or more (~18 miles), stopping at a light can be “iffy”. First few stops are no problem, then I stop at a light and I can start to feel the faint stumbling idle starting its death spiral (but not always). So it’s related to the vehicle getting hotter, or running longer, but if I just use the Interstate the entire way … no problem.

However - I have noticed numerous “stumbles” or apparent mis-fires while running at 70 mph on the Interstate. But never any MIL lamp illuminated. That’s why I bought this Scan Tool. Thought maybe I could trap a buried code that just wasn’t setting the MIL lamp ON.

As for the “General Discussion” comment, wasn’t sure what you wanted there at first, but now I see, so I’ll post over there, too.



I think the VSS shows 0 MPH because I activated it just as I was pulling out of my parking spot. It takes about a minute or so for it to Capture all its data, and maybe it’s stupidly capturing the VSS right at the beginning and then ignoring any uptick? I mean, I had to go through (2) automated gates, so I wasn’t able to get up to more than 25 mph, and only for a very short time. You’d think it would’ve shown some non-zero value, though. In general, I’ve noticed the speed Captures have been on the low side. Doing 70+ on the Interstate today only showed up in the 60’s … ???


Yeah - I haven’t really tried to digest the data yet - that’s why I went ahead and posted it first. Just trying to figure out if I like this Scan Tool or not. Initial reaction is … I don’t think so …

Here’s todays Captures: all contiguous, #1 from my driveway, #2 from a Stop Light about 10 miles away, #3 On the Interstate again … going 70+ (but the Capture doesn’t show that)

You certainly took the long route to get to " the engine stalls at idle". Replace the idle air control motor and clean the throttle body.

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I guess I did, but it wasn’t by accident…

The stalling could well be a dirty throttle body/idle motor as posted above. What is the warm-engine idle rpm set at? Automatic transmission problems can cause stalling too sometimes. If not either of those, I’m sort of suspecting the maf sensor might still be giving you some problems, based on the long term fuel trims. I note you cleaned it recently, what was your motivation for doing that?


The recent MAF sensor cleanings have origin all the way back to March 2012.

In March 2012, 154,9k miles, I got two codes: P0401 (EGR Flow Insufficient), and P0135 (O2 Circuit Heater Circuit Malfunction (Bank 1 Sensor 1). So I cleared both codes

@157.2k miles, both codes returned.

So I replaced the O2 sensor @ 157.5k miles (Denso , $43)

Immediately afterward (157.6k miles), I now had a P0102 (Mass or Volume Air Flow Circuit Low Input)

So I cleared that code.

But it would reappear in 2013, 2014, 2015, until I finally got around to cleaning the MAF at 189.5k, and then again 194.7k.

But it came back again at 197.7k.

So maybe I need to replace the darn MAF sensor, heh?

I did clean the throttle body after the P0401 code at 157,2k miles, but I probably need to do it again.

So maybe:
1.) New MAF sensor
2.) New gas cap
3.) Clean Throttle Body

How can I definitively tell if the idle air control motor (IAC ?) needs replacing?

That’s sure a bevy of codes you are dealing with there Colt … for an 2002 Impala, seems a heck of a lot of code and sensor drama going on. Unless this engine has been underwater or something and many of the electrical connectors are corroded, or there’s been a lot of deferred maintenance, I’m sort of thinking you may have a ground problem there. Suggest to make sure the ground for the sensors and the ecu is securely attached to the car’s chassis or body like it should. On my Corolla all those ground wires come into a big bundle and attach at a single point, on the firewall. If you can find something like that on your Impala, remove the wires temporarily and clean that attachment point so you know it is making excellent electrical contact. The do the same where the battery negative attaches to the chassis or body. If I had all those codes happening on my Corolla, that’s what I’d do first anyway. If you have a ground problem it won’t matter what else you do, you can replace every single sensor, you’ll never be able to solve it. Until the ground problem is fixed. Problems with the battery and alternator voltages can cause this type of problems too.

For the IAC, measure the rpm at idle, both with the transmission in neutral, and in D with the brakes on. What does it measure in both cases, warm engine?

You have an old car with a restricted ERG passage, a failing mass air flow sensor and a stuck IAC motor. Don’t get side tracked with “deferred maintenance” talk.

P0102 MAF sensor circuit low frequency . . . This DTC detects a continous short to low or open circuit in either the signal circuit or the MAF sensor

Doesn’t sound like the kind of code you’d get from a dirty MAF sensor, IMO

P0401 EGR insufficient flow detected . . . This diagnostic will determine if there is a reduction in EGR flow

IMO, cleaning the throttle body will not get rid of this code, or your other codes, for that matter. Maybe the egr passages and/or egr pipe are plugged

See post below.

Nevada seems to think there’s a problem with the MAP. One thing that’s easy enough to do is test it w/a hand-held vacuumed pump to make sure the diaphragm isn’t leaking. It should hold vacuum to 20 in Hg. However, MAP failure is a very uncommon thing reported here. Note to OP: MAF’s can sometimes be successfully cleaned, MAP’s can’t.

I meant to write mass air flow sensor, corrected it.

Well, this past weekend, took the Impala out on the highway, got it up to temp, and took some MAF sensor readings with my Harbor Fake scanner.

MAF was consistently 0.03 across three tests. That’s too low, right?

Next, detached the MAF sensor completely from its housing and sprayed the hell out of it while it sat in a plastic container (so as to even get some backspray cleaning). Flipped it a few times, too, so I got both sides washed really well.

Then emptied 2/3 can of Carb Cleaner into throttle body until the outflow ran mostly clear (took a while).

Yesterday on the way to work, car ran perfectly fine. Super smooth, and seemed to have more pep.

But I took another round of MAF readings and now the sensor is consistently ZERO … even though 12 volts was verified at the plug.

SO - I’m guessing the MAF sensor is finally history, but how is the vehicle running so well now? Is the computer inferring the MAF from another sensor (or set of sensors)?

Maybe, but I’d guess that it would turn the check engine light then. Is it on?

If not, maybe by cleaning the MAF you’ve fixed the problem but your scanner for some reason isn’t able to communicate with it to read what it is actually telling the engine computer.

The way the MAF works usually, there’s wires directly in the engine intake air path that are heated to a certain temperature by the engine computer running the exact amount current through them to hold at that controlled temperature. The control temperature is much hotter than outside air temp, like 170 degrees maybe. Anyway, ambient airflow across the wires then cools them, requiring more current to keep them at 170 degrees. The amount of current in the wires is proportional to the airflow. But if those wires get dirt on them, it acts like a coat, and prevents the airflow from cooling the wires as much as it should.

So I think you’ve probably fixed the problem. As long as the CEL remains off, you got some braggin’ rights now.


I’m not bragging about anything because I know what’ll happen next (LOL!)

The Check Engine Light is back ON, but I haven’t read the code. It’s very likely the P0102 again (because the MAF appears to be dead), and it might also be the gas cap code again (pretty sure I need to replace that, too).

But here’s something interesting: When I was running those data captures with the Harbor Fake scanner, I decided to clear the CEL at 70 mph on I-95.


Anyway, the engine very noticeably skipped a beat. It was a controlled engine ‘miss’, which I thought was strange. I did it once again and it did the same thing, so it wasn’t a coincidence.

Why would clearing the code with the engine running cause that skip?? That value should be stored in a separate memory cell for display purposes only. This seems like a programming bug in the ECM to me…

I expect what’s happening is the engine computer has to take a little break to communicate w/your scanner and clean up some memory cells and all that takes enough time, that the delay causes it to fail to fire the ignition pulses to the spark plugs like it normally would for a crankshaft revolution or two. When it is done doing its housework, it starts the ignition system back up again.

Folks sometimes hook up their scanner full time to monitor stuff while they drive, and this happens too. The engineers who wrote engine computer software didn’t design it to be used simultaneously with driving. It’s like if you have too many programs open all at once on your computer, it slows down and even crashes sometimes.

Car ran great all week. No missing on the highway, no sputtering idle … but I’d betcha that MAF sensor is still showing ZERO on the scanner. I was just about to order a new MAF sensor - just the sensor itself from RockAuto (Spectra?), but now I’m thinking maybe I don’t need it (or a MAF sensor at all … LOL!)