Intermittent codes

[Andy J]

Got a very odd thing going on with my 96 LT1. I can get in and start it and drive it for as long as I want with no issue. However, if I stop for a short while (up to a few hours) and then get back in and start it it will illuminate the service now light after a short while. Fortunately the 96 has OBD2 so I can pull the codes. I get P0171 Lean bank 1 P0174 Lean bank 2 and very occasionally P441 Evaporative emissions control system incorrect purge flow.
Sometimes it will be just be one bank but mostly both. The P0441 code comes up only once in a while and never without the other 2.
If I drive the car without stopping and then leave it overnight it wont throw codes next time I drive so it seems to be only when the engine is started while warm. If I leave the car running and clear the codes they wont reappear unless I turn the car off and do another warm start so it must be something that happens then clears immediately.
Any thoughts on where to start looking? It's more an annoyance than a real issue.

 

[Oneball]

I’d have a look for a vacuum leak and/or a problem with the fuel vapour purge valve which is in the vacuum system somewhere.

Also next time you have a short stop take the fuel cap off and see if the tank is pressurised.

 

[teamzr1]

Got a very odd thing going on with my 96 LT1. I can get in and start it and drive it for as long as I want with no issue. However, if I stop for a short while (up to a few hours) and then get back in and start it it will illuminate the service now light after a short while. Fortunately the 96 has OBD2 so I can pull the codes. I get P0171 Lean bank 1 P0174 Lean bank 2 and very occasionally P441 Evaporative emissions control system incorrect purge flow.
Sometimes it will be just be one bank but mostly both. The P0441 code comes up only once in a while and never without the other 2.
If I drive the car without stopping and then leave it overnight it wont throw codes next time I drive so it seems to be only when the engine is started while warm. If I leave the car running and clear the codes they wont reappear unless I turn the car off and do another warm start so it must be something that happens then clears immediately.
Any thoughts on where to start looking? It's more an annoyance than a real issue.

Being my business since 1995 has been making GM vehicles like Corvettes perform I have testruns and analyzed over 6,000 vehicles, mostly Corvettes via OBD test tools
Class in session !

In short - the PCM ( engine controller) commands how the engine functions on a cold start and on warm start, that includes how long it commands the fuel injectors on

When engine is cold started the engine is colder so the fuel and its fuel system, so is the air into the intake and coolant
That then means the charge into the cylinders is also colder and that makes the fuel trims better (AFR)

Now when the engine has gone closed loop, i.e, coolant temps above 160 F deg, everything else heats up and now the charge to cylinders are hotter,
that makes the AFR leaner.
The O2 sensors are reading the exhaust makeup and reports that to the PCM, and it tries to make up for the leaner AFR

Now add, gas type, Ethanol leans up the AFR more, the higher percentage of Ethanol the AFR goes up again being leaner
Add hot weather and leaner more.
C4s were not designed to be using Ethanol

When the PCM trips the lean DTCs your seeing means the fuel trims are at least 12 % plus too lean, where zero would be perfect

Start with the easy things to do

1. Get Mass Airflow sensor (MAF) spray cleaner and clean it inside, also clean the throttlebody
2. Look very closely from the aircleaner to the TB that there is no airleaks, even small cracks to loose fittings
3. Check to make sure no leaks as to any vacuum hoses.
4. PCV valve is stuck, if so clean
5. Assure fuel injectors are clean, use a good injector cleaner into tank of gas
6. C4s fuel rail pressure should report no less than 43 PSI ( use a fuel pressure tester at testpoint of fuel rail)
7. 43 PSI should be seen at different engine loads, if not then
a.bad fuel filter
b. faulty fuel pressure regulator, worse case the fuel pump
c. fuel injectors just too old, might need to be replaced or send to a service that does heavy-duty cleaning and changing the filters in them
7. O2 sensors begin to degrade after about 40,000 miles and since they report how rich or lean AFR is
if in doubt, replace at lease the ones before the CAT(s)

 

[teamzr1]

Now being your C4 has OBD-II you really need to own or borrow a OBD-II scanner and with that could zoom into what the problems are

Here is a 1996 C4 I did a testrun on, analyzed the scanner did, and corrected what was needed before doing a PCM tune to gain 30 more HP from doing that
Notice the black arrows I inserted in the analyzed scanner results

TP make this a bit easier to follow , I superimposed 2 functions per graph
Look at the top one you see the red line is for engine RPMs and the green line is for vehicle speed
The green vertical line is at a certain point of engine running during the test drive, and then you see 2 values
4,171 RPMs and 116 MPH values
Each graph then reports 2 functions and their values shown for that moment of engine run time

Being there is fuel injectors, O2 sensors and fuel trims for each head to see if both sides are running the same is reason to superimpose
Then, which makes it easy to see if one head/side are functioning different

In this testcase we are looking at injectors, O2s MAF and fuel trims
The PCM adjusts in part for LON TERM fuel trim adjustments
Those values would dictate when the PCM trips the 2 lean DTCs you have
Perfect world the value would be zero, the higher the positive values are the leaner the trims/AFR is, and negative values mean rich
In the charts look at the 2nd graph, red is left head, green is right head

Where the green vertical line is your see RPMS was 4,171, MPH was 116,
Look closely you see B1 (left head) was 9.4% lean which B2(right head) was 4.7% so for whatever reason B1 is running leaner then B2

Now look at the results chart for the 2 front O2 sensors, and again they are superimposed, and both results should overlay each other perfectly
but they do not.
Notice they do not, one is lazier (oscillates slower) than the other, that means both of their results would confuse the PCM as to what the AFR/trims really are
O2s function from zero to only 1 volt, the results in that graphs are in mVolts.
For non WOT conditions the front O2s (before the CAT) would read about 0.450 mVolts, (that equals an AFR of 14.7:1)
the higher the engine RPMs and load is the richer the AFR needs to be so the values could read in the 0.900 mVolts

Lastly look at the graph of the fuel injectors, they do overlay pretty close to easy other so that means the PCM is commanding them on for the same length of time so that looks fine
OK, hope I did not waste my time explaining all this so if having questions,

ASK as the only dumb question is the one that is not asked

 

[Andy J]

Thanks for the concise reply. That gives me an idea of where to look. Thanks.
A question though. If it were an air leak, vacuum leak, dirty maf etc then surely it would have the fault all the time? If I clear it it doesn't return until the next warm start. I'm just at a bit of a loss as to why it throws the codes after a warm start and has been driven a short distance then whatever has caused the code then goes away?

 

[teamzr1]

Hi Andy

That is because the PCM' coprocessor is many times slower than even a cellphone
If I recall, the PCM for 1996 is only like an 8 or 16 Mhz CPU
With all the tasks it has to do, the PCM is not designed to constantly monitor and report DTCs
Add tasks like if there is an auto tranny is part of the PCM calibration

So most checks for any DTCs are done on a cold start
DTCs are given a rating, Some GM felt were really important and the DTC trips on the first test
Other DTCs/functions are lower rated and PCM allows the problem to happen like 2 times before DTC is tripped

Also, you might have some unmetered leak that only happens when a part gets hotter and expands, and then it leaks
but cools down and crack, etc shrinks enough to no leaking

Examples, hose for PCV, heats up and has slight crack, when hotter expands and then leaks
Also, I have seen cracked welds in exhaust
Weld is thicker than the pipe, when hot enough the 2 metals expand differently and crack only then leaks (why a smoker is useful)

In that case say a DTC tripped, the PCM checks and the DTC decays out, so DTC no longer trips
Then problem happens again and same DTC trips again, so can happen cycles on/off as if let's say leak comes and goes or

Something is at the margin of functioning window, so sometimes PCM says being within the operating window, no DTC
but then something goes just outside that window and trips DTC again

Really need to catch the problem(s) with a decent OBD-II scanner that can record and trap the problem to then analyze
or maybe need a smoker to find if a leak ( do after engine has heated up and something expands)

A lean DTC, in your case B2, right side of engine, the PCM does not trip that DTC until maybe about +12% lean,
When engine is colder that also makes the fuel trims cool enough, so it is below that +12% threshold so no DTC tripped stays
Lastly when the engine is cooler that means so is the exhaust content, that also means it is cooler which effects the O2 sensors
so it reports to PCM a cooler AFR and PCM then says OK, no lean fuel trims, so no DTC trips

You understand this, or did I make it too techie and confusing ?

 

[Oneball]

They’re may also be a cycle of operation that only occurs after a hot start. I wonder if fuel vapour purge is one of them?

 

[Andy J]

Hi Andy

That is because the PCM' coprocessor is many times slower than even a cellphone
If I recall, the PCM for 1996 is only like an 8 or 16 Mhz CPU
With all the tasks it has to do, the PCM is not designed to constantly monitor and report DTCs
Add tasks like if there is an auto tranny is part of the PCM calibration

So most checks for any DTCs are done on a cold start
DTCs are given a rating, Some GM felt were really important and the DTC trips on the first test
Other DTCs/functions are lower rated and PCM allows the problem to happen like 2 times before DTC is tripped

Also, you might have some unmetered leak that only happens when a part gets hotter and expands, and then it leaks
but cools down and crack, etc shrinks enough to no leaking

Examples, hose for PCV, heats up and has slight crack, when hotter expands and then leaks
Also, I have seen cracked welds in exhaust
Weld is thicker than the pipe, when hot enough the 2 metals expand differently and crack only then leaks (why a smoker is useful)

In that case say a DTC tripped, the PCM checks and the DTC decays out, so DTC no longer trips
Then problem happens again and same DTC trips again, so can happen cycles on/off as if let's say leak comes and goes or

Something is at the margin of functioning window, so sometimes PCM says being within the operating window, no DTC
but then something goes just outside that window and trips DTC again

Really need to catch the problem(s) with a decent OBD-II scanner that can record and trap the problem to then analyze
or maybe need a smoker to find if a leak ( do after engine has heated up and something expands)

A lean DTC, in your case B2, right side of engine, the PCM does not trip that DTC until maybe about +12% lean,
When engine is colder that also makes the fuel trims cool enough, so it is below that +12% threshold so no DTC tripped stays
Lastly when the engine is cooler that means so is the exhaust content, that also means it is cooler which effects the O2 sensors
so it reports to PCM a cooler AFR and PCM then says OK, no lean fuel trims, so no DTC trips

You understand this, or did I make it too techie and confusing ?

No i get what youre saying. I was also thinking along the lines of a crack in a line that only opens enough to cause an issue when heated sufficiently. When the car is stationary after being driven theres a lot of heat build up under the hood and i did wonder if something was being baked enough to cause the issue but once the car is moving again the air flow cools everything down just enough to stop it occurring again.

 

[teamzr1]

If a leak is suspected it would be real helpful to either make a smoker, borrow one or buy one for about $120 US
as the smoke or smell would find even small leaks that you just might not be able to see

This one has its own electric pump and uses car's battery

Smoker