CEL after deceleration from highway speed to idle

First ever post. I searched and couldn't find anything that matched my circumstances. I am the original owner of a 2004 C4S cab. It has 70K really fun miles on it. On seemingly random days, after driving 10+ mins on the freeway (@ 75-85 mph, it's southern Calif...) when I exit on the offramp and and am idling at the stoplight, the RPMs drop below 1000 (800-900) and sputter, followed by the CEL. The engine figures it out after a few moments and the CEL goes away. No stalls. Months ago, I took the car to a local indie shop and they were unable to recreate the symptoms and said there were no error codes on the OBDII monitor. Of course, the car randomly continued to do the same thing. I finally got a OBDII reader and checked it today. I haven't gotten the CEL for several days. The only fault logged is P0304 Cylinder 4 Misfire Detected. My plan is to interrogate the monitor immediately when it happens again and see if anything else comes up. But, I'm impatient... Has anybody had or heard of this type of problem? Thanks, Barry San Diego, CA

It will be a lot easier if you wait till the CEL comes on and retrieve the code(s). The simplest case would be plug and/or coil. Have the plugs and coils been replaced? In the meantime, what are the short term and long term fuel trims?

What Ahsai said. ^^^^ If you have not done the plugs and coils yet it should have been done at 60K miles. Most likely you have cracked coil that catches some moisture and misfires every now and then. I would start from there.

Sounds like the first thing I would do is read the codes - or if you don't have the cable and software - get someone to do this for you or buy a cheap reader from an auto parts store. Get the codes and then do a Google search on the codes. From that info it may point you in the right direction. From the description of the odd idle after highway speeds, I might check if the MAF is working properly or if you have a minor air leak in the air box/filter housing area which can affect the MAF readings leading the DME astray. Plugs and coils are dumb items and i mean that from a technology standpoint. They either work or work poorly in a steady state. Things like vacuum leaks can also be an area to look into. But the bottom line if you have a CEL on even for a short period, you will have codes stored in memory. Some codes are in there even if you don't have a CEL as the threshold condition may not have been reached to throw the CEL on. Of course never drive a car with a flashing CEL - pull over where safe to do so and turn car off and call for a tow as you can damage the cats or other hardware. Good luck.

It's unanimous. Plugs and coils probably need replacing. I'm just waiting for it to happen again so I can pull the codes. Ahsai, I'm not sure the inexpensive bluetooth OBDII reader I got will give me the fuel trims but will look into it. Thanks for all the support everybody.

I have a similar problem when coming off the highway, I started a thread on it a few months ago. Turned out to be the SAI kicking in, I would see the voltage drop for a couple seconds, and the engine would sputter (just a bit). My mechanic changed the vacuum reservoir and some vacuum lines in an attempt to fix it, but it still happens when the SAI kicks in. I don't get a CEL so I'm just going to live with it for now. Here's my original posting and video: Hey guys, After driving at speed (i.e. highway) and coming to a stop, I depress the clutch and the voltage drops a couple seconds later and then goes back to normal. I can also feel the engine stutter/revs drop a bit. Anyone have any idea what might be causing this? Anything to worry about? Thanks, Chris 2002 Targa, 82k miles Porsche 996 Voltage/Rev Drop When Coming to a Stop - YouTube " target="_blank">Video Link: Porsche 996 Voltage/Rev Drop When Coming to a Stop - YouTube

Yes, it will. The PIDs are called short term fuel trim bank1, long term fuel trim bank1, etc.

I can't watch the video right now but it sounds like your SAI pump is drawing too much current which puts excessive load on the idling engine. Will need to measure the actual current it sucks when turned on. Shouldbe around 20A or less.

When over the course of a couple of days on a road trip half way across the country my Boxster did something quite similar it proved to a bad AOS. On the 3rd day when I started the engine first thing in the morning the thing smoked like a coal fired power plant. However, the difference was the CEL would come on and stay on and I'd read the code and while I can't remember the code it strongly suggested to me the MAF was at fault. The code was related to air/fuel mixture/adaptation. 'course, it was the AOS allowing unmetered air into the intake that was affecting the mixture. With misfire error codes though you have to suspect -- as others have already pointed out -- coils and plugs.

If the plugs and coil don't do it, look at the MAF as well, a coil problem should show up at more than idle but then again not necessarily.

For what is worth ... I have a 2004 C4S for which the CEL started coming on 2 days after I bought it When I checked for codes I was able to retrieve P0491 and P0492. I could clear the codes and they would stay off for awhile but then the CEL would come on - usually when slowing down from hiway speeds. I went through the recommended diagnostics checking my SAI and looking for vacuum leaks. I did find a leak in my sport exhaust solenoid but when I corrected that, I continued to get the fault codes. One of the other potential causes for these codes are bad CATs. Last summer I replaced my oem sport exhaust with some Fabspeed maxflos because my PPI identified a possible loose baffle in one of my cans. When I removed the oem exhaust - they were solid with no rattle. It then appeared that the rattle was coming from one of my CATs, and this was likely the source of my CEL. Yesterday I installed a set of used fabspeed CAT deletes that I had gotten pretty cheap. (They setup sounds great by the way, but maybe more appropriate for someone young than me - 48.) It turns out that the insert in one of my CATs was loose and was pushing up against the O2 sensor. Below are 2 pictures my CAT looking in through the O2 orifice. The first pictures shows the insert with a divot in it where it was likely rubbing against the 02 sensor. The second picture is with the inset pushed back to where it should be. Steve

Quotes source post: 11359428

Interesting, it seems P0491 and P0492 can be triggered for a wide variety of faults! To add one more to the knowledge-base, I got a check engine light after slowing down from ~75mph to idle fairly quickly. The engine became rough for a brief time, idle dropped below 700 and the CEL was illuminated. Durametric showed the codes but in my case the issue was that I had dislodged a vacuum pipe while checking the dipstick. Durametric also allowed me to start up the SAI to exclude this as a possibility. Anyway, hope this helps someone else. -------------------------------------------------------------------------------------- P0491 Secondary Air Injection System, Bank 1 - Insufficient Flow P0492 Secondary Air Injection System, Bank 2 - Insufficient Flow Possible cause of fault - Electrical fault in power supply or line between relay and secondary air injection pump - Secondary air injection pump relay mechanically faulty - Mechanical fault in electrical secondary air valve - Pneumatic secondary air valve faulty/sluggish - Electrical fault in secondary air injection pump - Mechanical fault in secondary air injection pump or secondary air injection pump blocked - Vacuum system leaking - Air hose to secondary air injection pump slipped off or constricted --------------------------------------------------------------------------------------