Low power and CEL when warm...

[paanta]

Sorry for this long post. It's...complicated.

On a >60F degree day when the car is warmed up, it has slightly low power throughout the power band, the CEL is intermittently on (and comes on solidly once it's really hot) and there's a bit of hesitation just off idle. The CEL light seems more likely to come on at idle and just off idle at a slow cruise. The engine codes say bad lambda control value and bad o2 sensor (1221 and 1222). The car idles a little rough when the CEL is on, but it's not awful.

It's getting worse now that it's warm. When it was cooler the car ran perfectly.

When I got the car, it wouldn't rev past 5600 rpm. It was also hesitating badly just after a warm start. Those problems were solved with the new AFM, but I did other stuff at the time: Fuel pressure checked out. Temp sensors read right. Car has new CPS, FPR, plug wires, fuel filter, vacuum lines, O2 sensor. Swapping coils did nothing. Distributor+rotor look fine.

The O2 sensor values fluctuate from ~.05 to .8 volts, which is lower than the .2 to .8 you're supposed to see. Back before I fixed the initial problems, with the old O2 sensor values were more like .2 to .9 with it going _over_ 1.0 when the car was cold and starting up, but a lot has been replaced since. And again, the weather was colder.

The car idles pretty steady at 850 rpm, but it runs exactly the same whether or not the ICV is plugged in. Unplug the harness and there's no change in the idle AT ALL. It's buzzing weakly if I listen with a stethoscope. I've soaked it in carb cleaner, but it still seems sticky. It rattles a little if I shake it, but not if I just slowly invert it back and forth. I don't remember how freely it's supposed to rattle, but I suspect it's shot. Still, I don't see how this could cause a CEL for mixture, since it's metered air when it gets to the ICV.

When it's hot and the CEL is on, I can pull the oil filler cap and it doesn't change the idle at all, nor does pulling the ICV away from the intake boot. The idle stays smooth. That brings me to vacuum leaks.

This problem just SCREAMS vacuum leak to me...but I can't find anything. Not with carb cleaner. Not by pressurizing with compressed air and putting soap water on everything. Nothing. It's hard to do the latter test with the car hot, though, since the soap water just evaporates away.

So possibilities:
1. Could a bad ICV cause any of this?
2. Or did I get a bad O2 sensor when I replaced it?
3. Do I have bad intake manifold or throttle body gaskets that show up on a hot car? Previous owner did the intake manifold gaskets when the head was done a few thousand miles ago...maybe something worked loose?

I'm leaning towards #3, but I can't get the idle to budge by spraying carb cleaner around the intake.

[Rus]

The reason you're seeing the CEL come on as the engine warms up is because the ECU transitions to closed loop operation and starts using the O2 sensor. It sees an incorrect response from the sensor and gives you the light. If you want to check your ICV, pull it out and keep it plugged in on a running engine. The wheel inside should spin when the engine is running. It does sound like you have a massive vacuum leak somewhere based on your description. Check your oil dipstick too. It should seal in the tube and on my car it causes the idle to get shaky if pulled on a running engine...so it does have to hold back vacuum. Check the line going to the FPR. It runs under the intake and you won't see anything if its broken below the manifold. The PCV breather hose vacuum line connects around the same area below the intake manifold and can also be broken. I've replaced all of these lines on my engine and the old hoses felt like rock when I removed them. Old age isn't kind to rubber hoses. Inspect the connection to the brake booster too. That's a pretty big hose and could easily cause a huge vacuum leak if its cracked or broken. Those are just about all the vacuum connections that the M30 has. My money is on one of those hoses

[BennyM]

Hey paanta,
did you ever figure out the solution to your idle problem re: http://www.bimmernut.com/forum/showt...23697#poststop

I'm trying to diagnose the same issue, but with an M50TU, and I'd be really curious to see how you solved it.

Thanks.
BennyM

[shogun]

if it would be an M30, I would first check these hoses, because no one sees them and they are even not shown on the parts catalog
http://s71.photobucket.com/albums/i1...=whichhose.jpg