How & what sensors should be cleaned periodically?

YUKON 4 JT · November 20, 2003

Hi All,

I own a 98 GMC Yukon that has 96k miles on it. A couple months ago my cat converters went bad. Had those replaced, but lately the truck has been rough. It lacks the horsepower and seems to be sluggish. Injectors were cleaned about 1 years ago by the dealer.

I was curious as to what sensor can be cleaned & with what chemicals/cleaning sprays. I realize some sensors are more sensitive than others, so they may require cleaning with specific chemicals.

By the way can the EGR Valve on my 98 Yukon be cleaned? Or would I simply take it off and remove any carbon build up by hand?

Thanks!

JT

Shaners · November 20, 2003

I know the MAFS (mass airflow sensor) can be cleaned with contact cleaner or brake cleanz. It's important to clean it with something that doesn't leave a residue.

I don't know if an O2 sensor could be cleaned or not, but they are often the cause for a poorly running engine.

Taaaaaaahoe? · November 21, 2003

I know the MAFS (mass airflow sensor) can be cleaned with contact cleaner or brake cleanz. It's important to clean it with something that doesn't leave a residue.

I don't know if an O2 sensor could be cleaned or not, but they are often the cause for a poorly running engine.

Im not so sure I would use brake cleaner. I think it would destroy the plastic. Id stick with the electronic cleaner.

mike

JayMan · November 21, 2003

Uh-oh, why did the cat's go bad??

Check if there is an O2 sensor before the cat and after the cat. To meet OBD2 regulations (Your 1998 is OBD2), the PCM has to be able to determine if a cat is going bad. When the upstream and downstream O2 sensors read the same, the cat is bad, and the Check Engine comes on.

If you have a bad O2, especially upstream, the PCM will use a completely wrong fuel trim and your cat's go into meltdown. As I'm sure you know, that's really $$$.

Did they replace the O2 sensors when the replaced the cat's?? If not, WHY NOT?? I've always changed my O2 sensors every 60,000 miles or so, as they eventually get clogged.

Before they really bugger up, an aging or failing O2 sensor will cause decreased mileage, lower power, and a nasty rotten egg smell from the exhaust. Since cat's are usually way more expensive than O2 sensors, it's probably good preventative maintenance.

I don't think you can clean an O2 sensor. My Helm manual has page after page warning against cleaning ANY sensor, including O2, MAF, and TPS.

You don't have to buy from the dealer / stealer. Most places like NAPA and Autozone carry aftermarket that should work, at much lower cost. I've had no problems with the Neihoff brand. Most O2 sensors are made by Bosch anyway, and those folks invented O2 sensors.

The new EGR valves have electric (Stepper motor) actuation. I don't think it's a good idea to get too much carbon cleaner on those, as you could ruin the stepper motor. I would check the EGR passages as those may well be carboned up at 96kmiles.

Wouldn't hurt to check the throttle body and the IAC (Idle Air Control) while you're at it. Driveability problems, especially hard cold starting and rough idle, can be caused by carbon buildup on the throttle body and IAC passages.

If you live in a cold climate like I do, short trips at -30 blow a lot of gunk and moisture into the intake manifold. The IAC passages clog a lot faster. I like to quickly spray out the TB and IAC with 3M Throttle Body Cleaner. 3M claims it's safe for sensors and IAC's. Though anything made by 3M is $$$.

BTW: did the Check Engine come on, or flash, while this was going on??

FYI: My Helm shop manual recommends the throttle body clean at LEAST every 50kmiles.

YUKON 4 JT · November 21, 2003

Thanks everyone for your feedback!

Jay Man - Veyr helpful and complete information. Thanks!

This is the order of sequence of problems that I encountered prior to my Cat's going bad:

*Service Engine light comes on at about 90k miles - Dealer replaced injectors under a warranty. They also replaced one O2 sensor that showed as being bad.

*Service Engine Light comes on about 4,000 miles after the injectors & O2 sensor is replaced. Dealer diagnosed it as a bad Cat & the O2 sensor that they replaced was also faulty. Had both Cat's replaced by a different shop. Went back and had the dealer replaced the O2 Sensor that they initially installed. Service Engine Light no longer on.

*At this point the truck runs rough. Other sympton is a odor coming out from the exhaust. Not a rotten egg smell but seems like more of a rich unburned chemical odor. Maybe all four O2's should be replaced?

k1500chevy97 · November 21, 2003

d**n , I guess I should replace my o2 sensors then

My mileage has taken a big dip and i have gone through 2 cats within the past two year

118k and still runnin original o2 sensors

JayMan · November 21, 2003

I'm leery that two different shops have worked on your Tahoe and now it runs rough. I would demand a printout of the codes found and have them show you the diagnostic section of the shop manual to prove that is what was really wrong.

The stealer claims to have replaced the fuel injectors. All 8 were bad?? What could have caused that?? Something just does NOT add up here!

If the intake manifold is the new style composite (Plastic) one, you shouldn't have to remove the manifold to change the injectors. You just remove the fuel rails and they unplug from the manifold.

Here is a warning from my Helm shop manual:

"LOWER O-RING REMOVAL CAUTION: Caution Verify that the lower (small) O-ring of each injector does not remain in the lower manifold ... If the O-ring is not removed with the injector, the replacement injector with new O-rings will not seat properly in the injector socket ..."

Well, it goes on for quite a well. You can have a fuel leak out of the manifold, a vacuum leak, that would really confuse the PCM.

Check for vacuum leaks, check the fuel pressure, and I would even have a smog test done. The tailpipe test is actually a very good way to see how the motor is running and can pick up problems with rich/lean conditions.

After that sort of work, you usually have to do some sort of Idle Learn procedure. Each PCM is a little different, so please reference the shop manual. For my 2000 Vortec 5.3, I have to warm the motor up, put it in drive with my foot on the brake for 5 minutes, put it in Park for 5 minutes, shut it off for 30 seconds, then restart it.

Then I have to see the Bone Bender to fix my right foot

K1500Chevy97: yeah, I would never replace cats and leave the original O2 sensors in. I actually heard that good advice in the early 90's on Shadetree Mechanic, on TNN. Those guys now host Two Guys Garage on Speed Channel, but they seem to have left behind all those handy tips. Too bad

MyBigToy · November 22, 2003

You need to consider the fuel filter and fuel pump also. I had a fuel filter that came apart internally one time (only a couple of months after putting it in) that caused the engine to cut out and lose power periodically at highway speeds. I decided the cheapest thing to do was to try replacing the filter. That's when I discovered the internals had come apart!

Something could have fouled the plugs, there could be bad plug wires also. Just throwing out a few additional ideas.....presumably the dealer or shop would have detected these simple, easily checked items.

JayMan · November 22, 2003

MyBigToy:

Right on about the fuel filter or fuel pump!

Since the fuel pressure is SO easy to check on GM/Chevy trucks (Schraeder valve on the fuel rail) you will quickly find out if low fuel pressure is the problem. Should be 55-64 psi on the newer high-pressure SMFI.

I got my pressure gauge from NAPA and check the fuel pressure several times a year just to satisfy myself. Even more important, there is a bleed-down valve on the gauge, so I can safely depressurize the fuel system before changing the fuel filter. Have only had the filter spray gas all over me once, and that was enough.

Rough running to the point of misfire will turn on the Check Engine, as OBD2-compliant PCM's have misfire detection. They also keep track of per-cylinder misfire history and total misfire.

Though it wouldn't hurt to pull out one plug and examine it. Same as pulling off one plug wire and doing a quick ohm check (5000 ohm max).

Though you wonder if the stealer ever does that stuff, right? I'm convinced most of them want to nickle-n-dime you to the point you trade off.