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Discussion Starter #1
I have been searching for a hesitation under the 2000 rpm mark. The car seems to be running rich except for the first minute or so during cold start up. The replacement list goes as follows; NGK spark plugs, valve cover gasket (oil on tip of spark plugs), air filters, new tps (left bank), new O2 sensors (post cat), new thermostat, two new MAFS (VW and OEM), and 2 new oil seperators. New oil filler cap and vent hoses from valve cover to oil seperators are on order. Oil filler cap leaks oil (steady stream after 12 mile drive) and vent hoses are pretty much putty. I have been to three different dealers with no fix. They have pointed out parts that need replacing, but not the one that is making my car hesitate. It seems that the problem is lessened severely at cold start up, but gets worse as the engine warms up. If anyone has any more ideas please feel free to comment. Let me know if you need more info. No trip to the dyno yet. Too far away and my schedule doesn't permit.

Thanks,

Jeff
 

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I had a similar problem... mine was intermittent and I couldn't tell what was causing it. The only thing that I could somewhat attribute it to was that it seemed to happen more often when it was very hot. At first, this caused me to believe it was a heat soak issue. After further thought, I don't think heat soak would cause the car to hesitate, maybe feel lower on hp, but not cause the car to stumble.

I was perplexed until I recently replaced my vent hoses and I think it solved my problem. I noticed that the hoses became very soft when the car was warmed up. Perhaps when the car warms up and the hoses are like putty, they are so soft they collapse and don't allow any vapor flow when you punch it?
 

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Discussion Starter #3
I hope you are right. I have some new hoses on the way. Mine feel like putty/clay when they are cold. I will be relieved, but sick if that turns out to be the problem. Hundreds of dollars of diagnostic time/parts to be fixed with $20 hoses.

Thanks,
Jeff
 

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I'm NOT sure if my situation was related to yours. But, it could be.

On a different BMW, I had trouble starting it when it was warm. Everything was fine when cold. Even the BMW dealer I worked for couldn't find anything wrong.

You say it has a problem when it is warm.

On my car it turned out to be the pressure regulator (near the fuel pump ... could be anywhere on an M5, or maybe even a different approach to maintaining the pressure in the fuel system). It had a small internal leak and couldn't maintain pressure. It was supposed to hold pressure for at least a day. When the engine was cold, it knew it and built up pressure when you started. But, when the engine was warm it assumed it had enough pressure to start.

When I ordered the replacement part, I was asked by the parts manager (an old friend), are you sure you need a new pressure regulator? (I had destroyed my MityVac finding the problem.) "Yes," I said. So he ordered it. He said he had NEVER replaced that part before on a BMW!

It fixed the problem. I wasn't expensive and was an easy DIY job.
 

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Discussion Starter #6
The latest codes were bank 1 tps and misfire cyl. #7. Since then, I have replaced the tps, valve cover gaskets and ngk spark plugs. My car starts just fine when it is warm or hot out. It runs great at higher rpm's, but stumbles at low rpms. I still have black tail pipes and the smell of eggs when driven for a while then parked.
 

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I noticed that you changed your post-cat O2 sensors. They only test the efficiency of your cats and don't have an effect on the air/fuel ratio.

The pre-cat O2 sensors are the ones that control this. If your pre-cat O2 sensors were bad, I'd imagine it would throw a code though. The other thing I'd also change if you haven't already done so is the fuel filter, which is under the car on the drivers side I think. This is a relatively cheap part ($80 or so) and I was told by my Indie it should be done as part of the Inspection II. Not sure if that's true, but hesitation can be caused by a bad fuel filter.
 

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Discussion Starter #11
I only changed the O2 sensors that were after the cats. I was under the impression that the pre cat O2 sensors were for emissions and didn't affect the way the car ran. Did I change the wrong ones? The car is running rich..black tail pipes, blackned spark plugs,and egg smell. If I clean out my tail pipes, they will be black and sooty 25 miles later. I have not done fuel filter yet. The car is bone stock except for the Magnaflow tail pipes. I put the pipes on the car. The problem was there before and after exhaust installation.

Thanks,
Jeffcherrsagai
 

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Yes, you changed the wrong ones. The post-cats measure the efficiency of the cats. The pre-cats are the ones that are used to help adjust fuel trim. That could be why you're still running rich, especially after installing the new MAF's. After installation of the 2 new pre-cat O2 sensors be sure to reset your adaptations so your car will "learn" the new sensors. If you don't reset the adaptations, your car will eventually "learn" the new sensors, it'll just take longer.
 

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I only changed the O2 sensors that were after the cats. I was under the impression that the pre cat O2 sensors were for emissions and didn't affect the way the car ran. Did I change the wrong ones? The car is running rich..black tail pipes, blackned spark plugs,and egg smell. If I clean out my tail pipes, they will be black and sooty 25 miles later. I have not done fuel filter yet. The car is bone stock except for the Magnaflow tail pipes. I put the pipes on the car. The problem was there before and after exhaust installation.

Thanks,
Jeffcherrsagai
Absolutely wrong. All the postcats do is make sure the CATs are functioning. The DME ignores them for adjusting AFR (and this makes sense since, if the cat is working there will be NO variation in the postcat sensor output.)

Good catch gmak, I didn't see that in the OP.

Jeff- It shouldn't be the exhaust- if you did headers perhaps an air leak could trip a cat efficiency code.

Change out the Precat O2s - they are the ONLY ones that control air/fuel. As I understand it, the DME is looking for a 'zero crossing' from the sensor as the AF ratio slightly varies around the target- if the o2 sensors are dying, then the signal gets sloppy and imprecise... leading to bad AFRs.

Also, if you have been running real rich, change your oil at 3k intervals if it is on 5w30, 5k if it is 10W60. You'll be getting fuel shear.

Good luck!

A
 

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Discussion Starter #14 (Edited)

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Discussion Starter #15
Pre cat O2 sensors are installed and I still have the problem. I pulled out the plugs and they looked to be heading in the rich directions (50 mi. on new plugs). Cyl #7, which was the one with the misfire code, had a shiny wet spark plug tip. No new oil accumulation since the new valve cover gaskets. Any new ideas?
 

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Pre cat O2 sensors are installed and I still have the problem. I pulled out the plugs and they looked to be heading in the rich directions (50 mi. on new plugs). Cyl #7, which was the one with the misfire code, had a shiny wet spark plug tip. No new oil accumulation since the new valve cover gaskets. Any new ideas?
Did you give the car 50-100 miles for the ECU to readapt if the old O2's were off?
Regards,
Jerry
 

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Discussion Starter #17
I unplugged the battery and waited 30 min before hooking back up. I then drove around 100 miles and no change. I did notice the hesitation gets worse as the oil temp rises more than with the temp of the engine. I don't know if this helps. The warmer outside it is, the worse the car runs. Today it hit 85 degrees and the car felt like it was down 25hp or better. The mornings are warmer too. This has made the problem more prominent on cold starts. I am getting new vent hoses and gas cap tomorrow. I really hope this does something.

Jeff:M5thumbs:
 

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Have you looked into the engine coolant temperature (ect) sensor? BMW calls them double temperature switches. If the sensor is bad, it could tell your ECM that your engine is cold even though it is warmed up. In other words, it's telling your ECM that the car is cold and to run a rich A/F mixture. From what I understand, the sensor that operates the dash gauge is not the same sensor as the one that communicates with the ECM.

There are 2 sensors on a 540, so I am not sure if the M5 has the same configuration. One on the water pump and the other on the radiator. Cost about $20 and $40 according to realoem.com. Both have easy access.

I am not 100% sure about this, but I am looking into this for just the opposite reason. My car acts like it is warm at start-up, thus, giving me the chug-a-lugs in the morning from running too lean. <!-- / message --><!-- sig -->
 

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There are 3 coolant temp signals on an M5

One on the radiator outlet, I think this is mainly for contolling the auxilliary fan.

The other two are combined into one sensor housing on top of the thermostat housing. One to supply temp to the ECU, the other to supply temp to the gauge.

If you do the diagnostic function on the display, the coolant temp it shows is from the ECU, not the gauge. I would check it's temp, it should be in the 80-90C range when fully warmed up.

A good OBD2 reader should be able to tell you the O2 sensor adaptation %, along with the current A/F % modification. Ideally your adaptation (long term fuel trim or LTFT) should be as close to 0 as possible, usually within 1-2%

It is possible that you have bad cats, and as they heat up, they are causing high backpressure. The egg smell could be an indicator of this. New factory cats aren't cheap, and I'm not sure how you would actually confirm they are bad. Ususally it would be more noticable at high RPMs though, and it sounds like you're just having a low-RPM stumble

Maybe bad CPS?
 

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If you do the diagnostic function on the display, the coolant temp it shows is from the ECU, not the gauge. I would check it's temp, it should be in the 80-90C range when fully warmed up.

Maybe bad CPS?
I am curious to see what jcdavis's temperature readings are.

In my case I have hesitation when the engine is cold, but idles perfect hot or cold and run great otherwise. I checked my temp and I got 35c when cold and 85c when warm. 35c is about 95f. Does the temperature seem high for a cold engine? The temp outside was about 50f. Any help?
 
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