just fixed the O2 sensor issue, see post "independent says bad ME1 module and though all was good (except for now knowing cats are gutted.)

Now have new problem(s) -

1. Hot start takes several tries and seems like engine is getting no fuel. Cold start is OK.

2. Warm idle is rough. Started out intermittent but is getting more and more constant. engine seems to miss up to 2000 rpm or so. Cold idle is OK.

3. Exhaust backfires slightly during freeway deceleration (foot off pedal, throttle fully closed) and can sometimes get it to do it at rest if I rev it and then take foot off pedal.

My question is this, could the lack of cats cause these symptoms? If the ECU is trying to lean out the mixture to compensate for the inefficiency (not there) of the cats, how lean will it make it? Will it go so far that driveability is compromised?

Also have a question about the cats. The code I get now is for main cats performance below specs, and the only way the computer knows cat performance is from the O2 sensors. It looks at the readings ahead of and behind the cat and evaluates the performance, I assume.

On many posts I have looked at, people refer to the cats near the exhaust manifold as "PRE-CATS". The ECU though is saying the MAIN cats are bad, not the pre cats. Based on where the O2 sensors are, the main cats are the ones closest to the engine, where the heat is, and where they can be the most efficient. The other ones farther back must be a "post" cat, maybe used to further clean up what the main cats may have missed.

Am I wrong in my thinking here? If the rears are the mains, how would the ECU know they were bad if there are no sensors behind them?

I searched for hours but there is little info on the newer cars (tons on the W124 though.)

Any help will be appreciated - Thanks in advance.

I can not answer your vehicle specific questions but, I can tell you a little about pre-cat and post cat O2 sensors.
The pre-cat O2 feeds information ( oxygen content ) to the ECU, based on which the ECU feeds more or less fuel into the engine.
The post-cat O2 measures oxygen content again and feeds that information to the ECU as well.
The difference is the cross-counts ( rich/lean transitions) going to the ECU.
While the transitions are very rapid on the pre-cat O2, the post-cat transitions are much slower.
Makes sense, fuelmixture changes take effect instantly, converter changes take effect in a much more " lazy" manner.
Confused yet ?

__________________
2007 C 230 Sport.

Are the cats closest to the engine the main cats? I have seen so may posts where they are referred to as the "pre cats", but I think they are actually the main cats. What is the function of the cats farther back? Are they the same internally?

Back on the subject of the problem (idle, backfire, etc.) I think I have found the problem. At the end of the fuel rail there is a device with a vacuum hose on it (fuel pressure regulator?) With the car idling (rough) I pulled the hose and the idle smoothed out almost completely. I revved the engine a little and gas started coming out of the hose - I don't think it should be doing this, should it? When reconnected, the idle got rough again.

What has happened inside this item to let gas out into the engine?

To answer your first question.
Pre-cats are there to " light off " the maincats, on a cold engine.
Secondly, the fuelpressure regulators job is to increase fuelpressure proportianatly to engine load, i.e. for every 2" of manifold vacuum drop ( increased load on the engine ), most fuelpressure regulators will increase railpressure by 1 lb.
The fuelpressure regulator receives this vacuum signal, acting on a diaphragm.
It should NEVER leak fuel past this diaphragm!
Apply vacuum to the regulator, with the engine shut off.
If you get a mouth full of gasoline, when you suck on the regulator, you need to get a new one.
Oh, and rinse your mouth out with some good canadian whiskey.

__________________
2007 C 230 Sport.

New fuel pressure regulator installed and idle is now rock solid at just over 500 rpm (in D). There is also a substantial performance improvement from 3000 rpm all the way to the full throttle shift point. I didn't realize how much of an effect fuel pressure has.

Now that most everything is fixed, anybody in the market for a 1997 brilliant silver E420 sport (M119) with some AMG bits? It has 107K miles, 17" front, 18" back wheels (AMG style w/2-piece look) good tires (fronts brand new), l/r multi-contour seats, headlight washers, nice window tint. Car looks sharp and is basically clean and in good shape but does have its share of small door dings. It does need new cats (mains) if you want it to be legal. I'd like to get $ 16,000 for it. My cell number is 206-769-7283 (Seattle, WA)

The reason for selling is I am picking up a 1997 BMW 540i 6-speed this weekend. Blasphemy I know (and I'm sure to regret it), but I think the stick will be fun for a while. Anyway, I'll still have my ML430 for my MB V-8 fix.

Glad to see you got your problem fixed for very little $$$.
Now hit the road you BMW loving, lowlife.

__________________
2007 C 230 Sport.

I'm flying to Chicago and driving the 540 back so by the time I get to Seattle, maybe I'll be so sick of the BMW that IT will get the FOR SALE sign.

Valentine V1, No-Doz, and a good co-driver - WHEEEEE!

"The reason for selling is I am picking up a 1997 BMW 540i 6-speed this weekend. Blasphemy I know (and I'm sure to regret it), but I think the stick will be fun for a while. Anyway, I'll still have my ML430 for my MB V-8 fix"

hpinfo....You will not regret it. I myself love MB but BMW is something to be desired. It is beyond comparison.

__________________
2004 BMW 325 xiT Wagon 40miles
1999 BMW 528i
1995 e320GTO Wagon 137K
GTO means Grocery Transport Only
I LOVE MERCEDES BUT I PREFER HAVING SEX WITH BMW.

NO CHANCE of selling the 540i now - what an incredible car! I had forgotten just how much performance the slush box sucks out of an engine. The six speed gives the car an acceleration edge that an automatic just cannot touch. And talk about torque - WOW!

Suprisingly, the car is actually much quieter during hard acceleration than the E420. You have to keep an eye on the speedo because your ears will fool you in this car.

BTW - I'll sell the E420 sport for $ 15,000, just mention this ad when you call.

Its listed on Autotrader now for $ 15,800.00.