90 c1500 305 - failed emmisions High CO

[richardca]

My truck runs great and displays no error codes yet it failed emmisions test due to high co (double minimum at idle and under load). Passed all other emmision tests. Last year emmisions tests were so low that tech commented about how good they were. Since then, I have replaced O2 sensor and fuel pump. After the test, I noticed some blowback at the pcv valve grommet and the speed sensor has been acting up intermittently for a while (Speedometer quits registering). Tommorow, I will replace the gromet and test fuel pressure and egr valve and take a look at the chacoal cannister. Water temp is 165-170ish after it warms up.

What do you think might be causing the high co readings and what test do you recommend to determine the problem?

BTW, this is my first post here and the information I have found here is the best I've seen in all the forums I have visited for the c1500.

[horse482]

Welcome to the forum. Do you have a chip in your truck? If not I would see why your truck is not warming up to 195. If you truck is staying in closed loop then it will put more fuel, there for causing it to run rich and fail the test. If your t-stat is sticking or if you are running a lower temp t-stat, replace it with a 195. If you have a chip, try the t-stat, but change it back after the test. You may also look for leaking injectors. Hope this helps.

[richardca]

Thanks. I was suspect about the temperature. I did put in a a lower temp t-stat but several years back. I've had a cracked radiator for over a year that I kept trying to JB weld. When I had the radiator rebuilt, I noticed the temperatur drop. Probably because of the amount of water.

Can you visually see leaking injectors? Is there a simple test? Will injector additive usually clear up the problem?

[richardca]

I did the fuel test and it rises to pressure fine but immeadiately drops to 0. Since I am running rich I'm guessing it is the fuel pressure regulator.

Does this make sense and if so is it located in the intake manifold?

[maxwedge]

Yes and the regulator is in the throttle body, if this is TBI, make sure an injector is not leaking.

[richardca]

Thanks for your suggestions. Rebuilt the fuel pressure regulator but the fuel pressure is still 15 during run and drops very quicly to 0 after shutdown. The assembly was pretty dirty with buildup, looked like it may have been leaking a little bit, and most of the bolts were pretty loose. Installed a 190 degree tstat and bought some sea foam.

Took the distributor cap off and it looked pretty coroded at each of the pins but they were not torn up. Will clean out the breather holes if I can find them. Pulled all the plugs on the passenger side. They looked fine but were not tight.

Somehow I managed to burn up my coil so will replace that tomorrow.

I ordered a Bosch O2 sensor to replace the aftermarket sensor I put in last year after reading all the bad info about after market O2 sensors.

Plan on checking the coolant temp sensor and the throttle posistion sensor tomorrow.

Are there any negative effects from a bad check valve in the fuel pump besides long start time? Any other sugestions? Could a fouled CAT be causing the engine to run rich?

[richardca]

P.S. The injectors were not leaking

[2000CAYukon]

Originally Posted by richardca

I did the fuel test and it rises to pressure fine but immeadiately drops to 0. Since I am running rich I'm guessing it is the fuel pressure regulator.

Does this make sense and if so is it located in the intake manifold?

Did it drop to zero when you turned the ignition off? If so, that is normal for TBI engines.

The Fuel Pressure Regulator, is located in the TBI. It is basically a diaphram and spring. You need to take the top of the TBI off to get at it.

//2000CAYukon

[richardca]

I didn't know that was normal. Figured that was my problem. Learn something new everyday. Well, now I have a rebuilt fuel pressure regulator.

[richardca]

Just wanted to follow up for the next guy. Took it in and had it reinspected today. It passed and CO fell from 4.2 down to .32 on high speed test.

In retrospect, I'm not exactly sure which fix cured the problem but my guess is the corrosion in the distributor cap. Probably changing the new no name O2 sensor to an AC Delco didn't hurt either. I also think upping the t-stat to 195 was a good thing too.

Hope this helps the next guy.