Nate.Evans

Some background to give a clear picture of what’s happened and what was done. I noticed when I would get to a light, once in a blue moon, the idle would drop from the ~800 rpm down to ~400 for a split second, then come back up and be fine.

Read a lot about the ignition switch causing phantom problems, which I was getting like door/dome lights coming on randomly. Replaced the ignition switch, all that stuff went away. Seems like it fixed the once in a blue moon dip in rpms. It did…for about 2-3 days.

No check engine light, so I assumed needs new plugs and coils. On the way to the Indy to get the coils and plugs replaced, it dips, then the check engine light comes on. Perfect, no need to explain anything in detail, surely that’s related.

Went to pick up the car and said “I guess that took care of the CEL.” Well, I was shown the CEL was for no signal to the SAI pump. No signal from SAI pump, and both banks lower than they should be. Car definitely runs better either way as the coils had small cracks and needed to be replaced anyways.

Seems like after some reading, that was my issue with the dip in rpms given I know the coils and plugs are now new. Replaced the SAI pump with a generic one with good reviews and it idled perfect, test drove perfect….for 2 days. Dip came back. CEL came on, this time just low bank values. Cleared the codes to see if it was fluke, got a dip this morning with it cold, but no CEL came on.

So…what causes a dip in RPMS like that that doesn’t always throw a code? Did I get a bad SAI pump? Car runs phenomenal otherwise. Pulls really strong. No smoke or other weird hesitations anywhere. I only use Shell or BP premium, so it’s not the gas (too sporadic to be that anyways). Air filter is clean. It’s not throwing codes, so not sure where to go next. Vacuum leak somewhere?

theprf

Does the SAI pump actually run? If it seizes up - very common btw - it draws so much power from the alternator the engine bogs down for an instant.

Nate.Evans

The new SAI pump I think is…it’s quiet and there’s no code. I wouldn’t think a new pump would seize up though???

theprf

Unplug the SAI blower and see if it clears up.

Nate.Evans

No change in anything. Replaced the air filter, went with a different brand. Got a Carquest Premium paper filter from Advance...fixed the issue....for about a week. It came back. No codes until tonight where the air was 29*F. Codes were "insufficient volume in bank 1 and 2".

It's got to be a faulty SAI pump right? It's the only thing that could make it throw an insufficient volume code right?

DBJoe996

What year is your car? The secondary air injection (SAI) pump only pumps air into the exhaust system to help light off the cats, and has nothing to do with engine air intake. The SAI system will throw specific codes when faulty, however it does not affect engine running. What specific P codes are you getting? Have you cleaned or replaced the MAF sensor? I ask the year because if it is an early car, idle is most affected by the Idle Control Valve (ICV) on the throttle body.

Nate.Evans

It’s a 2004 so it’s not an ICV.
the codes I got the first time it came on were:


Not sure what the clutch thing was..but after replacing the SAI pump, I no longer get the P0418-04 code. If it does give me a CEL, I get the bottom two SAI codes.

i haven’t cleaned or replaced the MAF sensor, but wouldn’t that give an MAF code?

De Jeeper

U may want to swap or test the change over valve. Its the gray box thingy with an electrical and vacuum connection. If u r just getting the bottom 2 codes then its seeing the sai pump but the motor sensors r not getting the desired effect from it turning on. The change over valve must work and open the system for the sai air to get in.

Nate.Evans

Looks like both of em need to be changed then. I’ll give that a shot an report back.

De Jeeper

I think there is only 1 change over valve for the system. I think u r getting 2 codes because it is reading the result of a sensor on each bank. I just think the sai air in not making into the exhaust to lean it out during start up. Also inspect the rubber hose in both ends to make sure its connected and not leaking.

Nate.Evans

ahh, you’re right…I saw two changeover valves , but there’s only one for the SAI:

https://rennlist.com/forums/996-foru...omponents.html

thing is though…if that’s only used for circulation of exhaust gases, why would that cause the stutter/hesitation once in a blue moon?

De Jeeper

May have nothing to do with your shutter or it can be causing a vacuum.leak but i doubt it. U may have 2 different unrelated issues.

Sjdhsb

draws so much power from the alternator the engine bogs down for an instant.

Nate.Evans

seems too coincidental to unrelated…..about 60-70% of the time, after that brief stutter the CEL comes on with those error codes.

Outatimemk

I really want to post a thread with my experience on the Crank Position Sensor but there are so many. Most people will tell you that it causes a stall when warmed up/hot.

In my case, I started out with idle dips exactly like yours and would stabilize after a second or 2. It progressed to stalling only when hot when the AC was off, to stalling on cold start as soon as you’d turn the AC on (or if it was already on), to stalling when cold, to stalling ALL THE TIME. After replacing it today, the car runs perfect.

All this is to say, I suspected the wrong items based on just the idle problems such as yours.