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I am currently working on a 1998 GMC Sonoma, 4.3X, 4x4 and I can't get the emissions monitors to run for the EGR, Evap and catalyst. I have driven the truck over 250 miles trying to get the drive cycle right. I have never had this much trouble before. The vehicle has not set any codes in this amount of time.
A little bit of background is due here. The truck was bought knowing that the engine smoked almost immediately after start up and only has 114,000 miles on it. The previous owner was told he needed a head gasket replaced. My first inclination was intake gaskets but the guy didn't want to invest any more money into it. I drove 10.5 hours south (from upstate NY) to get it and when I started it to load it on the tow dolly I witnessed an amazingly massive blue cloud coming out the exhaust and the engine was shaking. When I got the truck ready to go I checked the oil and it was 2/3 of the way up the stick. My father and I contemplated the root cause and left it to the fact that we would look at the oil more closely when we got it home. After 21 hours on the road and laying in the snow when I got it home I drained the oil to find no water/ coolant contamination, but it did have 13 full quarts of oil in it. I added the necessary 4.5 quarts and ran it for about a half hour before the blue fog cleared. That's when I also realized that it was no longer shaking. I did change the intake gaskets as well and it's a good thing I did. I finished it with a tune up.
Now on to the issue at hand. In order to get it inspected I need to have no more than 2 readiness monitors incomplete. As I said in the opening statement I have put a ton of miles and a ton of gas through this truck and the monitors will not complete. I replaced the downstream O2 sensor and it didn't help me. The catalytic converter is 2 years old. I wonder if the oil fouled it. I have checked the operation of the EGR valve and the timing is running at -7 degrees at idle. I can activate the evap system and it controls the vacuum to the charcoal canister. My outside temp is below 20 degrees so I don't think it will run the evap monitor test at all. Would this block the other two monitors from running? Doesn't seem like it should.
jconiber wrote: My outside temp is below 20 degrees so I don't think it will run the evap monitor test at all. Would this block the other two monitors from running? Doesn't seem like it should.
Looks like EVAP needs IAT of 39* or higher on a cold start so you ain't gonna get that one to run.
Looking at code setting criteria for the other two they share a couple things in common. ECT and BARO. Slightly different minimums but for both monitors to run ECT has to be 167* and BARO 73 kPa.
Thank You for the response. I had replaced the thermostat when I had the intake off so I wasn't too concerned about the engine temp being high enough. I had looked at it very closely though.
What i ended up doing is going into the computer in Mode 6 and digging into all of the sensors. What i ended up finding was the fuel tank pressure sensor was reading a constant -30.68mm of vacuum. It didn't changed at all when I ran the evap service bay tests. When I unplugged the sensor it went to +14mm. I had an old sending unit under my catchall bench (we all have one,right?) that happened to have the same part number sensor in it. I tested the sensor operation with a vacuum pump and it worked. After I replaced the sensor and tried the service bay test again it showed proper operation. I test drove the truck and had the EGR monitor completed within the first three miles. The catalyst was set in about 8 miles. Still don't have the evap but I know that won't run due to the outside temp. I filled the gas tank so it couldn't try the evap test and cause a check engine light if it failed the test before my inspection appointment. I should be good to go now. Ironically, my father had a friend stop in with a check engine light on and it was a code for the gas tank pressure sensor being out of range. Two in two days? That's weird. Thanks again for the input.
