420A Weird idle issue

[michaelfun888]

Hi all!

I’ve got a 1996 Eclipse GS (420a motor) with 165k miles. I’m having a strange issue with the start/idle and I’m hoping someone could help. I should mention there is no check engine light but the battery light is on.

When I cold start the car, it really struggles to idle. It’ll idle for about 5 seconds at 200-500rpms, then suddenly jump up to around 2000rpms, where it’ll stay for about 3 seconds. Then it slowly (maybe over the course of 3 seconds) drops down to just above 1000 rpms. Sometimes it sort of oscillates above and below 1000rpms.

Once the engine is warm, the idle will be stable at ~1000 rpms. Then, when I’m driving and I go to put the clutch in to shift, the revs will stay where they were for about 3 seconds then slowly drop, maybe taking another 2-3 seconds to drop to the appropriate revs for the next gear.

Also, when I drive slowly (like in a parking lot in 1st or 2nd gear at low rpms), it’ll shoot up to 2000rpms when I clutch in, then stay there for numerous seconds and drop down slowly.

I can’t seem to figure out what’s going on. I’ve checked for vacuum leaks (found none), replaced the IAC valve, and cleaned the throttle body with throttle body cleaner.

Anybody else have a similar problem? What was the fix? Maybe an electrical/ECU problem?

Thanks!

 

[BLACK'98DSM]

How did you check for vacuum leaks? And how thoroughly? I'd assume one of the plastic vacuum lines has cracked or broken. It's probably time to tear those out and replace with rubber. If you're having trouble finding a suspected leak, get some starting fluid and spray around the possible problem areas with the engine running. Let it run until the idle settles down first, then listen as you spray. If a certain area raises the idle, you've found the leak.

A quick test for the IAC valve is to unplug it. If the valve is faulty, nothing will change. If the valve is functional, engine operation will be different.

I would also check the charging voltage from the alternator. Since you mentioned the battery light. That means system voltage is abnormal, and if the battery appears fine, perhaps the alternator is problematic. You should see ~13.7-14.3 volts with engine running at proper idle. Too much more or less needs to be addressed. And could cause engine operation issues. If you have a local AutoZone, they usually offer free battery and alternator testing.

 

[swassoni]

Did you figure out what the problem was?