: 2002 STS RPM Limited... Airflow Sensor Issue or other?

06-02-10, 10:12 PM
My 2002 Cadillac STS is exhibiting a strainge problem that is becoming more frequent. It now happens just about every time I drive for more than 15 min.

After the engine has warmed up, I'll be driving down the road, usually at highway speeds of around 50-65 MPH and 2,000 - 2,500 RPM. Then I will attempt to accelerate and the engine will not go above 2,500 RPM. I'll stop, and when I attempt to accelerate, the engine will get up around 3,000 RPM in the lower gears, but then max out at 2,000 to 2,500 again once it is in high gear. This happens just about every time I drive the car now.

No diagnostic lights. Have been through a couple tanks of gas and added some injector cleaner as well. Temp gauage is straight up as it's always been. Engine runs smoothly and quietly and there is no sign of any problem other than the computer apprenlty limiting the RPMs.

One time, I had an air cleaner that was not sealing properly and the car behaved this way. However, in that situation, it was persistent and was immediately resolved when I replaced the filter.

I have replaced the filter and subsequently checked to ensure that no air is leaking in prior to the air flow sensor that is right behind the filter. I'm guessing the problem lies with that sensor, the wires going to it, and / or a computer problem.

Am I on the right path or are there other things I should be checking out?

06-02-10, 10:18 PM
Throw away the resonator and do this.......and check all your MAF air duct coupling clamps in the process. (Remove the flex intake duct and use/trim a rattle can paint top to fit the resonator hole......) Use a proper size Philips machine screw for the discarded Torx screw in the PCM cover.

The spring clamp is there because I was seating a new PCM cover foam gasket.

Air leak problems show up after the MAF - between it and the throttlebody clamp.

06-02-10, 10:23 PM
Although it should set some lean trim codes, you might want to check to be sure that the plenum duct is not ripped as is somewhat common on the pre 2000's.

06-05-10, 06:52 PM
So far so good. Thanks for the tips guys. I plugged the hole like you did with a cap and went for a half hour drive with no problems. It must have been getting too much air in around where the resonator fit on. I didn't note any holes or leaks in the rubber anywhere and I ensured the clamps were tight.

I read some other threads which indicated that taking off the resonator made the car louder or added horsepower, but I noticed no difference after the resonator was off. My initial reaction was that it must be there for a reason, but apparently it serves little or no function? I'll keep it in the trunk for a while just in case.

Thanks again.

06-05-10, 08:30 PM
It serves little or no function except to cater to the crowd that believes a Cadillac performance car should float like a marshmallow, turn low 14's in the quarter mile, be dead quiet on the road, and get 28 mpg.

If you read the entire Cadillac Technical Archive up in the black bar ^^^, practice a few of the discussed WOT procedures and then do a WOT with the stick in 2 and your right foot through the firewall all the way to 2:redline you'll hear the difference with the resonator removed.

My resonator is on the garage shelf and has been there since early 2006.

06-06-10, 05:43 PM
Well, guess I'm not out of the woods yet. Everything was fine on my test drive, but today I went on a trip and started experiencing the problem again. About 15 min into my drive today, went to accelerate and RPMs were limited to 3,000 in low gears and 2500 in drive. I pulled over to the side of the road, turned off the ignition, turned it back on, and was fine for the next 30 miles, then it started doing it again.

I was at my destination for about 2 hours, got in and all was fine again, then about a half hour into my retun drive it did it again. This time check engine light came on.

When I got home, I chekced codes and the only active code was PCM P0106 - Manifold Absolute Pressure (MAP) System Performance...

So I'm guessing my problem is with the MAP and not the MAF sensor. So do I need to go to the Caddy dealer to get this figured out, or is there something else I can look into. I'm not mechanically inclined in the least. Should I try a different air filter? Do I risk damaging the engine if I run it in this situation? How do you tell when the car is running in the "limp-home" mode that I've read about. If it's running on 4 cylinders, I probably don't want to take it on a big trip, right?

06-06-10, 07:35 PM
"Limp" mode (electronic problems) defaults the ignition to a fixed advance (10 degrees BTDC) with no PCM advance curve.

"Camel" mode (gross overheating) drops pairs and quads of cylinders so as to lower the heat produced by combustion.

You will absolutely know when the engine goes into camel mode - the engine has practically no power.

If you have no mechanical experience, few tools, and no place to work then I suggest you let a shop do this one for you.....even changing the air cleaner (when was it done ?) has a few pitfalls.

Hmmmmmmmm......2002 STS........ Make very sure that kidney-shaped intake duct resonator is firmly seated in the hole in the flex intake pipe.

06-06-10, 08:38 PM
Thanks. Can you please define what "defaults the ignition to a fixed advance (10 degrees BTDC) with no PCM advance curve" means in laymans terms?

The resonator is gone per your previous post and the hole is firmly plugged with a cap like your pics show.

As for the cleaner, my old one cracked around the edge and was letting too much air in. That was a couple months ago, and at that time I had the same types of problems. Once I put the new (current) one in, the problem went away, until recently. Since the symptoms are the same, I just assumed the problem was the same... However, my new filter is nice and tight and I don't see anywhere that air could be leaking in and not going over the MAF sensor. I thought maybe the MAF sensor was bad until I got the MAP Sensor code tonight. Not sure how this is all related if at all...

06-06-10, 09:19 PM
"Limp" = something is screwed up in the engine controls electronics/ignition and the internal program(s) won't fully compensate for the problem, so the management system defaults to a fixed set of operating parameters which let you limp to a dealer and get a fix.

MAP = Manifold Absolute Pressure: the inverse of vacuum. Perfect vacuum is about 30" (of mercury) - atmospheric pressure of about 14.7 psi will support a column of mercury 30" high with a perfect vacuum acting on the top of the column. So MAP is used to determine engine load and thereby alter fuel/air ratios as engine power demands change.

The MAP sensor is that crab-looking widget on top of the TB casting - with a 3-wire (yellow ?) connector. You need to remove the throttle cable/fuel line bracket to get it out - a press fit held in by the tab on the bracket.

www.rockauto.com - your car, fuel/air, Airtex 5S2410 - probably an OEM DELPHI unit.