Stuttering performance at high speeds

[Ishmot]

I have a 2005 Saturn Relay with about 120k miles that just recently started experiencing an odd problem. When I reach speeds above about 45 mph it starts to stutter. It almost feels like the transmission trying to repeatedly change gears. If I let off the gas, the stuttering will stop and I can more gradually take the van up to higher speeds. I don't think it's the transmission because the transmission is only a couple of months old. After researching the issue a little I'm wondering if it could be a vacuum problem or a problem with the Mass Airflow sensor or perhaps the O2 sensor? Does anyone have any ideas? I recently replaced the Engine Coolant Temp sensor and the thermostat. The check engine light has not come on yet so I do not know of any engine codes generated. Does anyone have any ideas? Thanks!

[fdryer]

While not familiar with Relays, engines and xmissions are the same wherever they're used. The trick is understanding EFI systems and how they fail with symptoms that are repeatable whether or not error codes are generated. Xmissions are commanded by either a separate xmission control module or directly from the engine control module since xmissions are slave units and cannot make shifts on their own. This tends to make troubleshooting xmissions easier since most problems are engine related that directly affect xmission operation.

In general, no check engine light means no error code(s) in memory. The error codes that turn on the check engine light are basically emissions related that would fail most state inspections requiring emissions checks.

Without any check engine light to tell you a stored error code is ready for decoding with a reader, some basic troubleshooting remains for problems computers aren't capable of detecting. One of these problems may be exhaust related - if a catalytic converter is partially or completely blocked, there are no error codes to detect this, requiring basic troubleshooting to detect a blocked exhaust systems. A pressure gauge is used by dealers while DIY testing is simpler - removing the front O2 sensors to allow an alternate exhaust path is the catcon is blocked. If the engine suddenly has renewed power the catcon is blocked. The two open holes will result in a LOUD exhaust and may have to be redirected to prevent burning parts of the car but a brief drive should reveal whether or not the catcon is blocked. A good muffler shop should be able to determine a worn out catcon too if you decide to have a pro examine your exhaust system.

[Ishmot]

Just an update on what the problem ended up being on this van. Turns out it was the Transmission. I had just had a new transmission put in it 6 months ago and it went bad already. Luckily it was still under warranty.

It took quite a while to eventually figure it out but it got so bad that it became obvious that it was the transmission.

My mechanic suspected the cat at one point and then the fuel pump. Luckily none of that got replaced before the transmission got so bad that it was obviously the problem.

Also, The catalytic converter has an O2 sensor above and below it, so if it was bad, the engine light would probably have come on.

[fdryer]

Nice of you to follow up with a solution. Many posting here are only interested in help and never post their solution. A faulty xmission is rare and fortunately for you, covered under a warranty.

BTW, there are many posts where check engine lights fail to inform owners of faulty catcons. Not everything will turn on a check engine light. That's what forums like Saturnfans are for, to share what dealers and repair shops won't or don't know.