terminal_crazy

Injuneer, Hi
Just something I've noticed when previously checking injector voltages and dead times tuning a 1995 LT1 whilst idling.
PCM was displaying Input voltage 13.6V
Injectors measured 13.8V
Battery measured 14.4V
so I'm not 100% sure the PCM logged voltage is an accurate measurement.

Mitch

wbacall

Just to add to the confusion I was verifying the egr valve is correct and found 17115059 on the car. I just replaced it with 17092160. There is lots of confusion about correct egr valves. I also found c4 corvette uses other part number for A4. And same ( 17092160) for m6? Could I have a wrong valve or might there be a valve that would work better? Thank you.

Injuneer

The PCM logs the voltage on pin D3. The battery will always be the highest voltage in the system. Measured at each load, the voltage will be less than that measured at the battery due to resistive losses in the wiring, fuses, splices, connectors, corrosion on terminals, etc.. That includes losses on both the positive and ground sides of the circuits.

The PCM is happy with 8.0 volts. Below that level, it sets DTC 50. The injector offsets programmed in the PCM compensate for voltage changes, altering the injector pulse widths to supply the correct amount of fuel.

Many people ask why the voltage shown on the dash gauge is different than what is shown on a real time scanner. Same reason.... different circuits.

Injuneer

The EGR part # is confusing. In 1994, the M6 cars (like mine) sold in CA were 50-State legal. I believe this was true in 1995 M6 as well. The 1995 A4 cars had a CA emissions package the included dual cats. They had a different EGR part # as well. I can’t remember if the 1994 M6 and 1994 A4 CA cars used the same EGR. A quick look at an online GM parts dealer catalog shows two different EGR part #’s, and none of them match yours.

https://www.wholesalegmpartsonline.c...ssembly=379814

With reference to the 96/97, I should have said “a different” negative back pressure valve. Have you looked at Shoebox's writeup?

http://shbox.com/1/4th_gen_EGR.html

Problems with NOx on CA LT1’s seems to be a frequently discussed issue on various forums.

peterpar

I don't know if this will help, but I attached pages from the parts manual that shows the application requirements and the 17092160 number that was mentioned.

Injuneer

Did you clear/reset the PCM before you ran the data log linked above? At the end of the log, with the engine idling in Cell 16 the idle RPM is about 25-30 RPM higher than the target of 550 RPM. The PCM is closing down the IAC, dropping it 5 counts in less than 2 seconds, and the RPM is dropping slowly. But the log ends at that point, with 615 RPM actual vs. 550 RPM target..

Have you worked your way through the diagnostic chart for DTC 32 in the factory service manual? I know you don't have the code, but there are tests to determine if the intake passages are blocked. Have you downloaded a copy of the 1994 factory service manual?

It's kind of interesting to see in the description of the various tests that "Suction from shop exhaust hoses can alter backpressure and may affect the operation of the EGR valve during in stall testing." How large is the room in your emissions test location, and is there evidence of significant ventilation system operation?

wbacall

The shop does not use an exhaust suction hose. Typical California shop- row of about 8 bays about 20 feet deep.
I have performed all the workshop diagnosis re: DTC 32. All tests passed. Lots of vacuum and exhaust at the valve. I just checked vacuum drop with egr valve open.

Idle vacuum 20"
Egr open 5"

2k rpm 20"
Egr open 16"

looks like very good egr flow.
I did reset it recently. I hope that doesn’t make a difference? Should I do another scan?
I have read shoebox’s info. Also found some interesting info from Tomco and corvette central.

I found this very interesting especially the last couple of paragraphs regarding valve function and system back pressure.

https://tech.corvettecentral.com/201...egr-diagnosis/

Just a note: My exhaust system is all original except the magnaflow cat.

Thank you all for all the help!

Injuneer

To answer your PM, yes I do intend to continue looking at your data log. Just got busy with other stuff, including responding to others on this site. There is no one else who seems to come here every day, and if a newbie posts and doesn't get an answer right away, they tend not to come back. Then there's the large number of people who insist on posting tech problems in the wrong forums - "Site Problems and Suggestions", "New Member Intro", "CamaroZ28 Podcast", etc..

I found this quote while searching, which seems to cover your CO results:

"The lean fuel condition should be seen in the CO numbers on your VIR report. Anything between 0.00 and 0.03 percent CO is too lean and needs to be diagnosed."

I have not heard that previously, and do not know if it is valid.

https://www.smogtips.com/smog-questi...igh-NO-PPM-why

Yes, alcohols (ethanol) contain oxygen, and do have a different stoichiometry than gasoline. That could be leaning things out a bit.

Stoichiometry (E10 based on brief internet search):

Gasoline - 14.7:1
E10 - 14.1:1

In the post above, what vacuum point were you measuring when you got the ( Idle vacuum 20" - EGR open 5" ) result? Appears to be intake manifold vacuum. That would confirm substantial flow.

I guess the fact the PCM was reset would explain my first comment that the LTFT's are all as close to 128 as I have ever seen in a data log. You can redo the log, but at this point I'l keep looking at the first one. It takes a lot of effort to get things set up in a spreadsheet that is useful for me to manipulate and interpret. The log I have is already covered with color coding the cells that may have some relationship to the NOx problem, and that I need to look at further. That color coding requires scrolling through endless frames of data for each sensor or parameter to find the anomalies.

wbacall

Fred, thank you so much! I do suspect slightly lean do to E10. Although the lambda is 1.007. Also looking at gearhead forum. Hesitant to try modify pcm and end up with a brick. Do you know of an easy way to go slightly richer without pcm mod? Everything I have tried is compensated by O2 sensors.
Going to try pretest today. I’ll post the results. Thank you 🙏.

Injuneer

Is lambda set to E10? There should be a list of fuels to choose from in the setup.

wbacall

No sure what you are referring to? Where is this set up?

Injuneer

The wide band measures lambda. It has to be told what the stoichiometric ratio is, so the when it displays 1.00, it means it is at stoichiometric for that specific fuel.

Useful thread, but have to read it a few times to get things in perspective.

AFR Gasoline - E10 Wide Band 02 sensor tuning using Lambda

Depending on make/model, the programming should have a place to enter a scalar for lambda for various types of fuel, or even a table of fuel types that enters the correct scalar for you. My only experience with a wideband is helping a buddy install one a long time ago, and I don’t even remember the brand. And that may be just to allow a digital gauge to display the correct A/F ratio.

By my understanding, 1.007 would indicate you are close to 14.13:1, very slightly lean (14,22) for E10.

Innovate, for example, now sells “flex-fuel” setups that sho the fuel type selection on the face of the gauge, and allows you to change it with a push button.

Thoughts on changing the actual A/F ratio - use the narrow-band voltage emulator output from the wideband controller to feed the PCM, instead of the stockers. Adjust the lambda scalar until the LTFT's start to rise above 128. Would require a 2 channel controller and a sensor in each branch of the exhaust.

Getting confusing.

Injuneer

I tried looking at the change in MAP when the EGR valve opens, but it is very hard to find the EGR valve being commanded open when the throttle % is not changing, and the EVAP purge is not opening, both of which would decrease vacuum (increase MAP). As the EGR is commanded open, it typically starts at 12.5% (reads 13 in data log) and steps up to 100%. In some cases there is an initial increase of MAP by 1 or 2 kPA, a slight reduction, and then not much change as the valve opens to 100%. In another case, there is a slight increase in MAP as the EGR DC holds at 100%.. But again, slight changes in throttle % (less than 1 whole percentage unit) are masked by the fact that TPS-% is shown in whole percentage.

From the multiple cases of extended periods of steady TPS-% with the EGR opening during the steady %, I just can't see a pattern that says there is or is not a definite change in MAP with EGR flow. Where change does exist, the increase in MAP is typically less than 5 kPa. I don't know if that is a reasonable number or not.The factory manual indicates the PCM is looking for at least a 1 kPa change when it commands the EGR open.

I also looked at the effect of EVAP DC on the O2 sensors/STFT's, and there is no indication of a vacuum leak in the EVAP system.

The factory manual procedure for the EGR system (DTC 32) indicates if the engine RPM does not drop or stall when the valve is forced open, use this test to check for blocked intake passages - remove EGR valve, plug exhaust side hole, attempt to start engine. If engine runs as high as ~3,000 RPM or starts and stalls, passages are not blocked. Not easy to do, but have you tried that?

I'm running out of ideas.

wbacall

First I want to thank everyone for there help. I finally passed the smog test. I performed another top end clean with Seafoam, I cleaned the throttle body, I tried another EGR valve and lucked out with a cool rainy morning.
I still am convinced that the EGR system is not working as it should due to lower back pressure of replacement catalyst or the catalytic converters available today are not up to their task and the E10 fuel we have at the pump. Unfortunately without the availability of a new factory catalyst this is the best I can do.


Fred I really want to thank you for the time you took to analyze my data download and all the advice and recommendations you made.


Thank you all,
Walter

Injuneer

Glad to help.... not sure I managed to do much besides throw out ideas and guesses. Solving a puzzle is the part I enjoy, and “we” may have just got lucky.