RPM WS6

iTrader: (3)

Having a strange issue, just wanted to run a theory past someone that knows the EVAP system better than myself.....

Is EVAP ever supposed to operate at idle? Meaning, assuming the system is working properly, would the PCM ever command the purge solenoid to open when the throttle angle is at 0.0% and TPS is at 0.6v or less?

Basically, can anyone tell me under what conditions the EVAP system is supposed to operate? Idle? Cruise? Decel? All of the above?

biggb1783

GENERAL - The Evaporative Emission (EVAP) Control system used on all vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the vapors when the vehicle is not operating.

When the engine is operating, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.



MORE IN DEPTH - The EVAP purge solenoid valve allows manifold vacuum to purge the canister. The Powertrain Control Module (PCM) supplies a ground to energize the EVAP purge solenoid valve (purge on). The EVAP purge solenoid control is Pulse Width Modulated (PWM) or turned on and off several times a second .

The EVAP canister purge PWM duty cycle varies according to the operating conditions determined by the mass air flow, the fuel trim, the engine coolant temperature, and the intake air temperature. For certain EVAP tests, the diagnostic will be disabled if the TP angle increases to above 75 percent . The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring for vacuum decay.

The fuel level sensor input to the PCM is used to determine if the fuel level in the tank is correct to run the EVAP diagnostic tests. To ensure sufficient volume in the tank to begin the various diagnostic tests, the fuel level must be between 15 and 85 percent .

The PCM monitors the fuel tank pressure/vacuum level via the fuel tank pressure sensor input.

g/l.

joshp14

iTrader: (5)

Purge can and will operate at idle but will be a very low percent purge commanded...too much and the engine will die. Purge occurs under normal driving, accel, decel, most of the time once certain conditions are met.

RPM WS6

iTrader: (3)

Thanks guys....

This is basically what I was looking for in general, now what would really be great is if you happen to know what a normal duty cycle percent would be for the purge solenoid on an LS1 at stock idle rpms?

Even more in-depth, at what point above the desired/commanded percent purge will the PCM trigger a DTC?

Basically, I'm wondering if it's possible for either:

A) enough fresh outside air to enter the EVAP system, via a system vacuum leak, during normal purge to have a minor effect on idle quality without said leak being great enough to set a DTC

or,

B) for the purge solenoid to exceed PCM commanded duty cycle percent enough to have a minor effect on idle quality without setting a DTC.

Hopefully that all makes sense.

Formula98ramair

iTrader: (1)

So why does my allways say not ready when I have it checked ?

RPM WS6

iTrader: (3)

For whatever reason, your PCM is not detecting the necessary parameters to run the diagnostic test for that system. Usually, the EVAP system test will take longer to complete than the others.

eseibel67

iTrader: (10)

Couple of items:

I checked through the EVAP DTC's, most of them are set by vacuum problems, not incorrect purge pulses.

EVAP typically does not purge at idle.

A small vacuum leak may not show up at all because the IAC will compensate by lowering the counts.

Send me your E mail address, I will send you the diagnostic parameters so you know what the thresholds are for setting codes. EVAP codes are P0440 - P0453 on the chart.

RPM WS6

iTrader: (3)

Thanks.

Before you take the time to gather and send all that info though, allow me to explain a bit more about what I'm looking for. I am not setting any EVAP DTCs at all. My EVAP system tests are complete and they pass with no pending or stored/active codes.

What I'm looking for is a very small vacuum leak that *intermittently* has a slight effect on idle speed. This intermittent effect can be instantly corrected by shutting off and restarting the engine. This leads me to beleive that somewhere there is an electric valve which is passing more air to the engine than it should, and once the key is cycled this valve is reset and the condition goes away. It's not the IAC, because I have replaced that valve twice and cleaned the bore to no avail (PCV has also been replaced twice, but it's not eletronic so I doubt that would have helped anyway). This leaves only the EVAP and EGR valves.

EGR should never be active at idle under any conditon, so I would assume any "leak" at idle would, a) be a mechanical fault in the valve that would not be cleared by a simple restart, and b) set a DTC since the valve should be fully shut at idle.

But, if there is any situation where EVAP would normally be active at idle, then it stands to reason that if the purge solenoid were to allow slightly more vacuum draw than commanded there would be a negative effect on idle speed (this also assumes that the purge solenoid is more than just an "on/off" switch, and is able to be commanded to open at specific rates under specific conditions...is that correct?). So I guess what I'm really looking for is the threshold for setting a DTC in this situation (meaning purge valve is open more than commanded during a given cycle). I beleive the DTC that would/should be set would be P1441. Do you have any info on the threshold for setting that DTC?

eseibel67

iTrader: (10)

I'm not sure if the EVAP has some possible action at idle under some conditions.

A small vacuum leak will not affect idle speed since the IAC will compensate. The only way it would not is if it's bottomed or maxed out.

The purge solenoid is not an on-off switch, it opens by %age duty cycle. And yes, P1441 if purge valve is leaking. Criterea is:

purge valve closed
TP between 0 and 99.6%
intake vacuum > 10Kpa
tank vacuum > 12 in. H2O for 2 seconds within < 37.5 seconds after 30 second delay.

I can easily send you the diagnostic info. It will tell you what conditions will set codes, so you can check if you are below them. It's a 329kb PDF file.

RPM WS6

iTrader: (3)

Whatever is happening, it is more than the IAC can compensate for. The condition that develops is an idle speed that is about 150rpm too high (just below the 200rpm threshold for setting a P0507) with a ~50rpm surge in P/N. During the surge, I can see the IAC counts bouncing between 0 and about 12-14 or so. If I free-rev the motor, rpms are very slow to come back down, etc. All the stuff you'd expect from a vac leak. The leak must be small, but enough that the IAC can't fully compensate.

Again, this issue is intermittent, only happens after a long drive (usually after being on the expressway), and can be totally 100% cleared by a simple restart of the engine (as if some valve/electronic control device is "corrected/reset" by a key cycle). I've been over all the basics in checking for a hard vac leak (intake bolts, all vac connection points, etc.) and there is nothing. Have not tried a smoke machine, but I don't have easy access to one and I really don't beleive there is any hard leak.

In your opinion, is it possibile for the purge valve to leak enough at idle to boost rpms by ~150, slightly more than the IAC can compensate for, yet not ever set an EVAP system DTC? Looking at the threshold above for P1441, it seems that this code could only be set when the purge valve is commanded shut. But what if a purge valve is opening when it's supposed to, but opening further (higher percent duty cycle) than commanded, is there even a DTC for that? This would also suggest that there would in fact be a situation where the valve would be commanded open (even slightly) at idle.

Thanks for any insight you have. This is a tough one, I'm looking to gather any thoughts even if it's just a theory.....

eseibel67

iTrader: (10)

I'm not an expert in EVAP, however i just had a similar problem in the wife's Olds 88 that was driving me nuts (her problem turned out to be the PCV valve BTW).

It is possible for the PCM to command the purge valve closed, and it's still open. It's also possible for the valve to have a leak that's dependant on temp. From what you describe, I think you may be onto something with the valve being a dud. Is it possible for you to borrow a known good valve and retest? Or, bypass the valve completely with an auxillary vacuum connector, and see if your IAC counts go up after the engine is hot?

I send you the diagnostic chart.

Good luck.

RPM WS6

iTrader: (3)

Thank you very much for the file, that's a lot of great information.

Looking at the threshold for P1441, it seems that throttle position has to be greater than 0%. So this test would probably never run at idle. In that case, it's at least possibile for the valve to leak a bit at idle without setting this DTC. When I have a bit more time, I'll review all the EVAP DTCs and see if any ever run at 0% TP that would catch a system leak. If not, then this would be a possibile cause for my issue. The leak would have to be at the purge valve in my case though, since a leak anywhere else in the system would just cause the tank to vent to the atmoshpere but not into the intake.

I will have to bypass or replace this valve as you've suggested and see what happens. I think that's the next most logical step.

One more question.... you said in your case the issue was a PCV valve. When the problem was happening, were you able to clear it with an immediate restart of the engine? I have replaced the PCV valve once already with no effect, but as cheap as they are I could try another. Just seems like a quick key cycle wouldn't be able to correct a fault with a mechanical valve like that.

RPM WS6

iTrader: (3)

I will continue to update this thread as I gather new info, in case someone searches this topic later my results will be here for all to see.

I did extensive driving with both of my LS1 cars in the last two days trying to gather more info. The "problem" car in this case is my '98. The '02 is not having the issue. Here is what I've learned about EVAP operation:

EVAP is definitely active at idle, though only to a small degree. Both cars showed the purge valve duty cycle at 18 to 18.8% once fully warm at idle (in P/N or D). I only have one field to view this data, and I am not sure if that percentage is the PCM commanded amount or the actual valve position as determined by feedback to the PCM. Either way, I have been monitoring the fuel tank pressure sensor in conjunction with the EVAP duty cycle percentage to better understand just how much vacuum draw EVAP is placing on the fuel tank.

I have not yet been able to duplicate the irregular idle condition with the '98, so my testing is inconclusive at this point, however I do have some data worth noting:

Using my '02 car as the control sample, it seems that on average the fuel pressure sensor shows greater tank vacuum on the '98 car at the same purge valve duty cycles as the '02 car. Meaning it's possible that the purge valve on the '98 is in fact opening further than commanded and/or for longer than commanded leading to increased tank vacuum. My theory is that, intermittently, this condition may worsen to the point of effecting idle until the valve is reset by a key cycle. One point of interest, I noted MUCH higher tank vacuum while steady-speed cruising on the expressway with the '98 over the '02. Funny, because it's usually after an expressway drive that the issue will develop, when it does develop.

One thing to bear in mind though, the '98 car uses the older 15.5 gallon metal gas tank vs the 16.x gallon plastic tank in the '02. I do not know how much effect that might have on tank pressure readings. I've tried to keep an equal fuel level in both cars to remove that variable.

I am also monitoring EGR valve position data, but as the '02 does not have EGR I don't have a control sample. But I will use the data from drives where the issue does not occur as the control sample.

I have not yet tried a new purge valve (or even swapping valves) because I was hoping to catch the issue again and log more data. I will update when I have new info, which may be a while since I don't drive the car very much. In the mean time, if anyone has any ideas or comments to add, feel free.

RPM WS6

iTrader: (3)

UPDATE:

I was able to duplicate the issue today, this time I had my scanner ready. What I noted was, in synch with the 40-50rpm surge, the EVAP purge duty cycle was moving from 18% to nearly 21% at idle. I am not sure if this was the cause of the slightly high/surging idle, or just a reation to increased idle rpm from another airflow source. Also, I am not sure if the percentage duty cycle shown on my laptop is the PCM commanded amount or the actual position based on voltage feedback. Some other note worthy info: TPS was at 0.6v and TP angle was at 0.0% throughout the idle hang and surge period. Also, I noted the STFTs showing more positive than usual, which eventually caused my LTFTs to go higher.

I was unable to notice anything else out of the norm. IAC counts would bounce in synch with the surge, then when the surging stopped and the idle just stayed about 100rpm too high, the IAC would stay a 0. EGR pintle position was at 0% as it should be, and the EGR valve felt relatively cool to the touch so I don't think it was leaking. Went over all my usual checks again, fiddling and looking for a hard leak and found nothing. Then, after about 30 mins of watching various data on the scanner and messing with the engine bay, I did the usual fix; shut the engine off and within about 2 seconds time I restarted it. Instantly it was fixed, as usual. Looked at all the same data, the only things that changed were the IAC counts and the EVAP purge duty cycle...both were back to normal. So once again, whatever this problem is, the cure is a simple restart. One thing that I did notice on the scanner after the restart, the STFTs went more negative than usual and started to correct the elevated LTFTs right away. So it seems that the leak was in fact "sealed" by the restart.

I then shut it back down, and installed a new purge valve solenoid. I figured, why not? The part only cost $24 and it's the only thing I haven't tried. Of course, the issue did not return after that, but as this had been an intermittent issue I'll just have to wait and see what happens. Hopefully it's fixed.

joecar

That would be the commanded EVAP duty cycle... (EVAP solenoid does not have position reporting signal like the EGR valve has).

IAC was trying to shut all the way... interesting... sounds like the EVAP solenoid was flopping open too much... the EVAP readiness test should have picked this up, if you could get it to run.

Thanks for sharing...

RPM WS6

iTrader: (3)

Thanks for clearing that up. This is one thing I was unsure of.

That's the strangest part of this whole thing. My EVAP system test is complete, and there are no stored, active, or pending DTCs. If it was the purge valve, I guess whatever extra flow it was allowing was slight enough to go unnoticed by the PCM. The fact that I didn't notice any significant difference in fuel tank pressure sensor readings during the issue also points to it being very small.

I'll come back and update this again if the issue returns, including whatever else I might try to fix it. Hopefully this might help someone else in the future.

LS1WS6dreamer

iTrader: (1)

Just wondering if your problem ever came back, because I am having similiar issues as you were. I am getting P0507 code. Idle surges, high idle. Running out of options. Seems like it might be a cheaper fix than other options.

NBM-LS1

iTrader: (7)

Idk how it is on fbodies because I ripped all of my EVAP **** out but on newer vehicles the computer will pretty much find the smallest leak imaginable by pulling a vacuum on the entire system while the car is off.

perf0rmance

iTrader: (5)

raising the dead here.... rather than creating a new thread of course.

I'm trying to determine whether or not the 98-02 camaro's have any part of the EVAP system incorporated with the fuel tank. I believe I read somewhere that the GTO's have an EVAP system that is incorporated in the fuel tank.

RPM WS6

iTrader: (3)

Sorry my reply is about 1.5 years too late.....haha, I didn't catch this until now.

So far, the issue has never returned since replacing the EVAP purge valve. It's been two years since I replaced the valve, but I've only driven the car maybe ~400 miles in that time. Having said that, before I replaced the valve I had learned exactly how I needed to drive the car to get the issue to occur with the old valve, and I drove that same routine several times with the new valve and never had the problem....so I think it's OK now. I'll be getting this car out of storage in the next few weeks and I'll go through the same routine again just to make sure that the issue is still fixed, but I feel pretty confident that it will in fact be fine.

I hope that you were able to get yours fixed at some point since posting this.

The vent valve, purge valve and canister are all located outside of the fuel tank for the F-bodies.