Clutch and Tachometer

I drive a 1995 Saturn SL1. I was cruising around the other day and realized that when I depress the clutch without touching the gas, my tachometer spins upwards into around 2500 rpms and then slowly recedes back into idle. By “slowly” I mean, it takes maybe 10 full seconds for it to settle. Shouldn’t depressing the clutch make the tachometer spin downwards to idle speed immediately?

Are you saying this happens when you depress the clutch while the car is moving in gear? If you’re sitting in neutral with the engine running, does it rev up when you put the clutch in?

It only happens when I’m in gear and the car is in motion.

I agree this is not normal for an engine that is fully warmed up. It suggests some sensor is giving a false reading. Is your CEL lit? Temperature gauge reading normal? Any other symptoms?

I’ve noticed this phenomenon before in several cars. I was always told it had something to do with emissions. The RPM’s “hang” for a second or two when you engage your clutch, and then gradually fall off. On some cars a tweaks to the ECU can eliminate this, on other cars you have to modify certain valves and/or vacuum lines to get rid of it.

Well, the temperature gauge is normal, but the CEL is always on. This is because it has a 2002 engine in it and the connection for that sensor wasn’t on this new engine.

One more thing, when the tachometer spins up, it’s not just the indicator. I can actually hear and feel the engine speed up.

I can see problems trying to control a 2002 engine with a 1995 wiring harness and attachments. Has this problem existed since the engine swap?

Whoa! You have a 2002 engine in a 1995 car?! You’re lucky the thing is running at all!

Do you know which sensor it is that isn’t hooked up?

One of two things is wrong. Either you’re trying to run a 2002 engine using a 1995 program (ECU) (in which case, as Greasy said, you’re lucky it runs at all) or you installed the 2002 ECU with the 2002 engine and its program is operating one signal short. My guess is the latter.

Well, okay, there are more possibilities. The 2002 definitely has a temp sensor. If the ECU isn’t getting a temp sensor input it’ll have a missing equation variable and may not know that the engine is warm. It may be bypassing the oxygen sensor loop and running constantly rich. That might make it rev when you disconnect the clutch. And it definitely would be tripping the CEL light fantastic.

You might (I emphasize “MIGHT”) be able to determine which sensor is missing by comparing the fuel control system and emissions system schematics from 1995 and 2002 Haynes manuals.

Another possibility would be to look for a Saturn club forum and ask there.

I am not sure. This car was a gift from my grandpa. He got it from my dad and my dad had swapped the engine before grandpa got it. I’ve asked him which sensor isn’t connected and he said he couldn’t remember. He said that the connection just didn’t exist. Does this mean that I’m using an old wiring harness?