Engine Surging at low speeds '01 Seville SLS

[Seville44]

The check engine light for my cat came on, I had it replaced, it was still on, I had them try another one, but it still set the check engine light off. I drove it for a few thousand KM's with no problem, running as smooth as ever. Recently when driving at low speeds, the speedometer will start bouncing up and down, and the engine will be surging; normal power speed/drop come back drop, this stops the faster I go. The weird thing is the revs stay normal, odd I know. I took it in, and they said its the 02 sensor in the cat. Is this most likely the cause of the problem? If so does anyone have any experience in replacing their 02 sensor? Anything helps thanks.

[submariner409]

Go through this forum and Northstar Performance - look for threads which refer to the intake plenum, that rubber coupler between the throttlebody and intake manifold. Remove the engine cover and start the engine - squirt a tad of choke cleaner down under the steel screw head you'll see at the bottom of the coupling. Any speed change ? Cracked plenum, not uncommon, $40 part at dealer or Lindsay Cadillac >>>>> only.

Did you pull your DTC's by using the sticky thread at the top of this main page - How to pull codes.........That will give you a base to start from......

..........the O2 sensors react to fuel mixtures, rich or lean, and vacuum leaks (plenum, PCV, or throttlebody line connectors) will set the CEL and set lean codes.

[Mark C]

Speedometer bouncing, sounds like a voltage issue. Hows your alternator and battery condition? What the voltage display say on the instrument cluster, especially when this is happening?

[65kotse78]

This cat problem for your caddy is missing something: when you replaced the cat assy, the post (after the cat) 02 sensor must be replaced too to save you the trouble. This is probably your problem. To change the 02 sensor, buy a new one from the dealer and when you see it, you will know what size of wrench you need to replace the sensor. Once done, you can reset the code by using a scanner. If the other error lights show up in your dash again, let me know the code so I can send you the fix but it will be probably another engine problem not related with this cat code.

Rich

[submariner409]

He doesn't need a scanner. The entire scan and code management system is already built into the car.

[65kotse78]

Submarine 409 is correct about your car having it's own on board diagnostics. But before you could use and study how to use and start pushing buttons to make it work, you are better of hooking up an OBD 2 scanner and find the source of the fault code. My 2 cents anyway. (GM techs recommends using Tech 2 scanner).

[Krashed989]

http://www.cadillacforums.com/forums...egr-valve.html (How To Clean The EGR Valve)

yours may be somewhat different than what's pictured in the thread, but it should be in the same area.

[submariner409]

The GM Tech-II Scan Tool is neat and fun, and all GM dealers have one, plus all the updates. The trouble is that the GM dealer charges labor time for diagnostic readouts, and a Tech-II costs $3,500 +.

The code reader/reset system in his car takes 30 seconds to learn and use and it's free. The lists of fault codes are hundreds of items long, and every code has an explanation in the link.

No brainer.

[NHRATA01]

I wouldn't blow all that money on a tech 2, but you can get a program such as AutoTap for about $199, and you can view and clear all of your codes, view sensor readings, and datalog numerous parameters. It is very helpful with troubleshooting and I highly recommend it for us backyard mechanics.

[Seville44]

Thank you all for your inputs, I sent the car in, and had the 02 sensor replaced and had them turn off the code/ check engine light. however, this has not stopped my problem. Now I'm really starting to worry that it is the transmission. I'll try to explain this to the best of my ability: I'll get in start it up, begin driving, and I'll be at say 30km/h, the speedometer will drop to zero, and the revs will continue going up, but the car will just coast. It is as if it is going into neutral, so under 45-50km/h this will happen, the speedometer will be bouncing and the as long as i give gas it the revs will continue but the car will coast. When I give it more gas and it speeds up, this stops happening. The transmission fluid is fine, theres no transmission codes or messages coming up. Voltage is reading normal on the dash. Could it be a sticky egr valve? Thanks again.

[spiff95]

Try checking for a faulty/intermittent Vehicle Speed Sensor code. My wife's '92 Deville did the same thing, and after checking the codes, I replaced the VSS; back to normal.

I'm not saying this is a failed VSS, but the circuit and sensor should be checked if there is a code set for it.