Cold Start Gremlin!

[High.miles.big.smiles]

EGT (Exhaust Gas Temperature) sensor, the way I understand it, is it does what its name implies; it monitors exhaust gas temperature, which is particularly important on a high performance engine, and back in he day, 100 hp per liter of engine displacement was the domain of cars with a prancing horse on its placard (Ferrari). The following is background on the EGT and function. Excessively hot exhaust gas is damaging to the catalytic converters, and on the M3, those are positioned right tight against the cylinder head’s exhaust port. Let’s take a plausible “what if” scenario on how the EGT comes into action; let’s say a driver is working the engine very hard on a twisty track. At high engine revs, the time for complete air + fuel combustion decreases, so some unburnt fuel (hydrocarbons) evacuates the head quickly and is routed into the catalytic converter.

A catalytic converter is where unburnt hydrocarbons go to get processed into water, carbon monoxide/dioxide and other gases. Remember, hydrocarbons are fuel, and if one dumps fuel on anything, it will get very hot quickly. Catalytic converters can only withstand so much heat, before it breaks down the converter’s structure, or in other words, melt. On a super high revving engine like a S54, let’s face it, it’s an efficient air pump, which allows it to make 100 hp per liter; this means an abundance of fuel Is required, and unspent fuel in hydrocarbons will stress the converters; to prevent catastrophic engine damage such as burnt exhaust valves and the myriad of things excessive heat can do to an aluminum head, an EGT is inserted into the system as a safeguard, so the converter’s core won’t become overheated, and melt.

The EGT is a thermocouple that sends a signal to the ECU. When the signal (voltage) falls out of the specified range the ECU expects to receive, a DTC code is triggered, and the ECU will make some adjustments (like slightly reduced engine power) because one of the engine safeguards is not operational.

The short answer is that the EGT does not affect open loop cold engine starting. Sorry for being verbose 😐, but some background is essential to describe the EGT’s function.

That was an awesome response and exactly what I like to read when I'm trying to find an issue. Thanks for the info.

Does the ECU soley rely on the coolant temp sensor, mad, and airtemp then for open loop operation? What other sensors are heavily leaned on in open loop that differs from closed? I'd change my coolant temp but the dash reading is fine.

[Speed Monkey]

Dude, that’s a whole lot of questions, but there are many answers. You made me geek-out 🙄 and look through my PDFs; here is the info you seek. I hope it helps you.

[eacmen]

Dude, that’s a whole lot of questions, but there are many answers. You made me geek-out 🙄 and look through my PDFs; here is the info you seek. I hope it helps you.

What PDF is that from? link?


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[Speed Monkey]

My ancient copy was taken from old M3F.

In any case, I Googled it for you 😁; here it is:

http://www.unofficialbmw.com/images/m3engineelectronic1.pdf

[Icecream]

My ancient copy was taken from old M3F.

In any case, I Googled it for you 😁; here it is:

http://www.unofficialbmw.com/images/...lectronic1.pdf

I have been looking all over for definitive information on the ICV, it’s function and how much it really affects power response at low rpm. This answered it. Up to 15% load (which sounds significant), the icv is in control. Thanks for posting!

[High.miles.big.smiles]

Smoke tested the car, no luck at finding a leak. Convinced it's a sensor at this point.. all sensors look fine on inpa but cannot see maf readings. It doesnt seem to make a difference when it's unplugged so I'm not sure. Have a fourth injector code and misfires but I think those are derivatives of a bigger issue

[eacmen]

You can't see MAF readings? That sounds like a problem. But not likely to affect cold start.

Have you checked ignition coils?


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[bigjae46]

I was going to say probably an issue with your idle air or the SAP on the pass side.

Idle air is a more likely cause. The valve gets filled up with crud. I’d pull it out and check to see if the valve moves freely.


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[High.miles.big.smiles]

Replaced maf with a used until. No changes. I'm beginning to think it's a cam position sensor. In going to clean the crank pos sensor and order a cheapo Rock Auto cam sensor to troubleshoot, then invest in a BMW unit if it fixes it. I hate throwing parts at the car but at this point I don't have any more leads.

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[Speed Monkey]

OP, perhaps there might be some good info in these videos that can shed some light on your problem... Even though Nathan’s sled is not an E46M, much of the info crosses over to the M.

[High.miles.big.smiles]

Still chasing this down. To me, it all adds up to a vacuum leak but I can't find anything with starter fluid or a smoke test.

Getting an EGT code and injector 4 code. But I believe that injector code is a symptom not a root cause.

Maf has been replaced.

Car runs brilliant once the o2s warm up, and if I clear the light, it won't come back on until the next day's cold start

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[JeremyJames]

Time for the peanut gallery 😁 to chime in - cars in general (gasoline ones that is...) when they are stone cold, they run in an open loop, meaning without the use of O2 sensors to govern the air-fuel mixture. The ECU relies on a temperature sensor to default to closed or open loop. Old 911s use a sensor in the block, as with the family of VW/Audi/Porsche products that use early Bosch Jetronic CIS systems, which include K, KE, etc. Our modern M’s use the water temp sensor, or this below:

Take this as information rather than a diagnosis or an instruction/suggestion to spend $50 bucks on a sensor. Like others stated, do some testing to determine your faulty part(s), then proceed to buy. Good luck squashing your gremlin.

PS, Every bit as important as the sensor is, it is only good enough based on the connector’s or wiring’s condition. On a car that’s now 15 to 20 years old, the connector might have oxidized a bit which degrades the signal, or if the wiring is moving around, this may lead to slightly damaged conductors that lead to an intermittent connection. Moisture, being that this sensor is very near a water/coolant source, that wicks and works its way under the insulative casing will have a corrosive effect and degrade the conductor’s ability to carry a signal.

Monday fun-day where I chime in again, from ECS’s site, temp sensors can be had for the cost of an In N Out double-double combo meal Including animal fries w/extra grilled onions. That seems fairly inexpensive. The original image got deleted somehow, so I am just re-posting the same info.

Let’s talk about cold start some more as I think it might help the discussion. First, some definition of terms: Open Loop engine operation means the engine is running without O2 sensors engaged, hence “open loop”; fueling is predetermined by a set parameter by the ECU. Closed Loop means the engine is warm and the ECU now engages the O2 sensors to govern the AF mixture.

Cold Start - When the engine is stone cold, the ECU knows this from the temp sensor, because it is cold. When one turns the key, a cold start (open loop) program is used, which separate and not related to warm engine operation (closed loop). This cold start consists of injecting more fuel in the bore, because when it’s cold, the very finely atomized fuel droplets are attracted to the cold cylinder wall where some fuel droplets condense while others remain suspended just long enough to touch-off from the spark plug, leaving sufficient fuel for cold start combustion. Next, more fuel requires more air, which means the ISV opens up resulting in increased idle. Engine timing as part of the cold start is retarded to compensate for the condition of cold start - remember this, as this is key.

With that background, let’s run through a scenario of insufficient fuel at cold start. Like that stated, very finely atomized fuel will condense on a cold cylinder wall, hence the term “washing of the bore,” which means condensed fuel washes off the thin oil film on the bore, which leads to ring wear that everyone is familiar with. Anyways, insufficient fueling when it’s cold will lead to a hard start because much of that fuel is condensing on the bore wall, and a rich mixture is mandatory.

Next for this discussion, let’s say the injectors are still providing insufficient fueling for cold start - at some point when the engine is cranked continuously and long enough, enough fuel will be available for cold start but it’s a rough idle. What causes the rough idle, a combo of a lean mixture and retarded engine timing. Next, you’re late to work as usual so it’s damn the torpedoes and full speed ahead. You hit the gas, the mixture is lean, engine timing is advanced on an engine with retarded timing and bingo, the engine stumbles hard, and the car bucks like your girlfriend when... - well, you know.

Fueling is key to a cold start. Without enough fuel, some of you may like the feeling of your M3 bucking like your girlfriend, but it’s bad for the sled. Tackle your fueling problem, the hard cold starting, stumbling in cold operation and a bucking ride’m cowboy M3 will likely be resolved.

Best troubleshooting advice I have seen. I learned so much, more as a bi-catch researching something completely off of this topic. None the less I enjoyed the read. Thank you for the education!

[y2k_o__o]

Still chasing this down. To me, it all adds up to a vacuum leak but I can't find anything with starter fluid or a smoke test.

Getting an EGT code and injector 4 code. But I believe that injector code is a symptom not a root cause.

Maf has been replaced.

Car runs brilliant once the o2s warm up, and if I clear the light, it won't come back on until the next day's cold start

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sorry to bring this back, I have the exact same symptomes as yours and I am still chasing what it is

[ac427]

sorry to bring this back, I have the exact same symptomes as yours and I am still chasing what it is

Did you get to the bottom of the issue?

[y2k_o__o]

Did you get to the bottom of the issue?

I think I did, but I haven't driven long enough to said fully resolved as currently the interior is gutted for refreshing and cleaning

I smoke tested it, turn out the intake elbow is causing a very very tiny leak (high positive LTFT +14%, but not great enough to trigger CEL). I thought I clamped it at the right location, but no... I ended up rotating the clamp and wiggle the elbow a little and tighten it. It seems to bring the LTFT down a bit which is a good sign. However, prior to re-tightening the intake eblow. I replaced pre-cat O2 sensors, fuel pumps (Hella) and injectors that fixed the CEL or P030x misfire at start up, even cold start, so far. I threw alot of money to catch up the maintenance as it was neglected by the previous owner.

Here's my journal of my E46 m3, I tackled all hectic oil leak and CEL one by one since I bought the car 3 years ago.
https://www.e46fanatics.com/threads/my-02-m3-steel-grey-oem-build-journal-finally-back-to-the-road-after-1-5-year-of-diy-fix.1300910/page-2?post_id=19241887&nested_view=1&sortby=oldest#pos t-19241887