P0300, P0420, P0440

[nghtshd88]

Long story short a family members car failed inspection so I ended up with it. Have a random misfire P0300 and the back upstream o2 sensor throwing P0420. The gas cap P0440 is pretty self explanatory so saving that for last I dont see that being related at all.

Before I got to it, the GMC dealer had it, they claim spark and fuel is good, there was a vacuum leak and compression on cylinder #4 was 120 compared to #3 at 170 and #4 was at 90% leak down. 100 miles before the next due oil change when I got it and there was little oil left, the light actually just came on.


I can see the super thin plastic vacuum hose had broken and they fixed it with some black RTV. I sprayed around and could not find any leaks LIM UIM gaskets hoses etc.

I tested the coils and ICM with a voltage meter and found that the back coil pack had no resistance so i changed it as well as the wires and plugs, NGK, #4 was oil fouled. All coils are sparking strong except #1 which has a very weak spark. Not sure if thats a big issue?

Anyway, if I have P0420 I should assume my issue is on the back 3 cylinders which contains #4?

Lastly if cylinder #4 is at 90% leak down am I just wasting my time? Never incurred this before so not sure how bad of a misfire that can cause.

Its shaking up at idle a bit and when under load up an incline its stumbling but definitely not stalling or dieing on me.

Waiting on an obd2 scanner this wknd so I can get some real time info.

[96 SSEi]

Anyway, if I have P0420 I should assume my issue is on the back 3 cylinders which contains #4?

no, 420 can have many causes............look here...http://www.easterncatalytic.com/educati ... 420-codes/

[MKMike]

Compression between cylinders should be roughly equal--or within 15-20% of each other.
Cylinder #4s compression is too low. This can be from bad rings or a valve problem.
I'd be surprised if GM did not state the cause of the low compression and poor cylinder sealing.

GM could very easily have isolated the cause during the leakdown test simply by listening to where the air was escaping from.
An intake valve issue causes hissing air to escape at the air cleaner.
An exhaust valve causes the hissing to occur from the tailpipe.
Bad rings or a hole in a piston cause hissing to be heard from the oil dipstick.

P0420 is not an oxygen sensor code. It means that the catalytic converter is operating below the efficiency threshold that GM has set.
While a faulty downstream O2 sensor is sometimes the cause, the catalyst gets ruined when it becomes coated with oil or coolant and the catalyst melts when it gets gas fouled.

Since the GMC dealer gave you misinformation about the spark being good---I'd be inclined to test the compression myself---at least in cylinder #4.
Here's how to test compression:
http://troubleshootmyvehicle.com/gm/3.8 ... ion-test-1
Compression testers are not very expensive.

Info on leakdown testing http://www.gregsengine.com/cylinder-lea ... sting.html

[nghtshd88]

They told me the plug was fouled, and that the rings are bad. Im just wondering if its worth it at 122k miles.

[garagerog]

They told me the plug was fouled, and that the rings are bad. Im just wondering if its worth it at 122k miles.



A very tough decision that every car owner faces from time to time. General rule of thumb is to dump it if it costs more to fix it than it's worth book value or at trade in. However there are many variables starting with how well you like the car, is it going to continue to be a money pit, i.e. needing new tires, suspension work, etc. A lot depends upon your current financial situation, can you afford to spend enough on another car that's new enough not to put you back in the repair cycle almost before the ink is dry on the title change? My 04 SLE is still chugging along after 158k miles although it has cosmetic issues that has my wife longing for that new car smell. The shivers down my spine when I think of the sales tax, increased excise license tax and insurance costs dampen my enthusiasm for that in a heartbeat.

[MKMike]

It would probably make more sense to have a good used engine put in, if you'd prefer to keep the car.
An engine in need of rings most likely has a history of poor maintenance and may end up with other internal failures in the not too distant future.

If the PO was in the habit of driving around with little oil or not bothering to change the oil, it's not surprising that the rings wore out.
What also should be considered, as garagerog said, is the overall condition of the car.
I'd lean towards an engine shop rather than the dealer for whichever you choose.
Like garagerog, the myriad costs associated with a newer vehicle deter me from becoming too eager to get one.

[nghtshd88]

I was able to eliminate P0420 by replacing the sensor and it ran better. Cleared codes got random misfire again, then cyclinder 5 misfire now 1 and 2.. Took it to be checked out and was told the PCM is shot. They replaced the ICM with a new one to test it and its still missing all over like crazy. Now its just in limp mode. I found online that I can get a rebuilt PCM for $200 already programmed is this true? Worst case I have 14 days to return..

[MKMike]

I was able to eliminate P0420 by replacing the sensor and it ran better. Cleared codes got random misfire again, then cyclinder 5 misfire now 1 and 2.. Took it to be checked out and was told the PCM is shot. They replaced the ICM with a new one to test it and its still missing all over like crazy. Now its just in limp mode. I found online that I can get a rebuilt PCM for $200 already programmed is this true? Worst case I have 14 days to return..

Your mechanic told you the PCM was bad and then he replaced the ICM?
This makes no sense at all.
Our cars are older, so it makes sense to check the wiring to components.
Grounds can become somewhat oxidized over the years, leading to problems that mimic bad sensors and other components--such as PCMs.

It would help others to know whether your engine is supercharged or if it's "naturally aspirated".

As you've discovered, you can replace some parts and get the engine to run better than it has been.
The P0420 can be caused by a bad downstream oxygen sensor but it isn't always the case, as previously mentioned.

The problem with poor compression, especially if it's causing an oil fouled plug---if those things are actually true---is that the car will never idle smoothly and probably won't pass an emissions test--if your state inspection requires that.
If you just want to improve the way the engine runs, then you can keep checking things and replacing parts but the caveat above still stands.

Testing compression for yourself would be a good first step.
Doing a wet compression test, after a regular compression test will let you know if the dealer's mechanic was "full of it" or not.
Bad rings, after some oil is added to the cylinder, will seal better and compression will increase temporarily.

Random misfires can be caused by any of a number of things; Too lean a fuel mixture due to vacuum leaks, a weak/failing fuel pump or severely clogged fuel filter, bad oxygen sensor(s),worn spark plugs, bad ignition wires, clogged fuel injectors and of course bad ICM or PCM, as well as wiring issues.

When you remove each plug have a good look at it.
Are the plugs worn, so that they are causing misfires?
Are any of them oil fouled (wet brown slick deposits) or carbon fouled (black sooty deposits)
http://www.aa1car.com/library/spark_plug_fouling.htm has a chart that you can click on to help you identify plug problems

Spark plug wires, as they age, lose their ability to insulate and so the spark can travel to nearby metal, instead of getting to the spark plug.
Often, this is more apparent when it's very humid out or had been raining.
You can use a spray bottle filled with tap water to mist the wires and see if there are sparks arcing anywhere.
Doing this after dark will make it easier to see any stray sparks.

If the fuel filter hasn't been replaced in a long time, then replace it. It is cheap enough and relatively simple.
See the Common Problems "sticky post" and the Tech Info section has much good advice.
http://www.pontiacbonnevilleclub.com/fo ... d-feeding/ changing the fuel filter is the same from 2000 and up.

A stuck open PCV valve will act like a vacuum leak. Again, cheap and easy to replace.

These are the things I would do before replacing expensive parts.

[nghtshd88]

No SC, plug #4 was badly oil fouled and worn so I believe the compression info.

They swapped in a new ICM to test it only and that wasnt it. Didnt believe the PCM was bad really because it finally threw me a code for 1 cylinder and it wasnt throwing anything random. Found out myself a plug was defective, I assume cracked, found it by following it around on 3 cylinders.

Replaced fuel filter added seafoam, changed upstream o2 with a Delphi sensor (in my experience changing the o2 always gets rid of p0420 unless its running like sh*t or the downstream sensor is bad then its usually time for a new cat), ran it without the sensor in and no difference so dont believe its clogged yet. It is the original cat it looks like.

Complete tune up, wires, good plugs, new coils, pvc, air filter, tested for vacuum leaks, cleaned maf etc. I have no codes at all now.

And yes unfortunately inspection requires emissions.

I still have a slight hesitation at idle and uphill. I assume its just cyclinder #4 and will never run straight, but its pretty decent now cant complain. Hope it passes.

Other then that I need to replace the fuel line O rings in the back of the engine. Seems like a common issue? A/C is off to with good pressure so not leaks.

[MKMike]

Glad the upstream O2 sensor did the trick with code P0420.

You might get it to pass by replacing the plug immediately before the inspection.
Replacing the injector o rings might also be necessary to pass, since vacuum leaks do affect emissions.
After 15 or more years of engine heat and freezing temps, seals do tend to get brittle and/or shrink.

I'm not sure how you determined that the AC is fully charged but if it truly is, then have a look at HVAC and AC fuses & relays in both the underhood and the under the rear seat fuse boxes.
Our cars have fuses for numerous things that many cars don't have fuses for.

Underhood
Fuse #26 and Relay #32 both are for the AC Clutch
Relay #33 controls the HVAC solenoid

Rear Seat Fusebox
Fuse #2 is for the HVAC Blower
Fuse #11 is Ventilation solenoid
Fuse #33 HVAC
Fuse #37 HVAC battery

There may be others tied in to the HVAC system.
The HVAC battery is most likely just to retain memory of the settings you made.

After checking fuses and relays, go to the basics, such as whether or not the clutch is being energized.

There are also modules which sometimes fail affecting the AC and the HVAC control unit itself can go bad.
These can only be checked with a Tech 2, if I'm not mistaken.
I haven't delved deeply into the AC system yet, so others can provide better details on them.