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Discussion Starter #1
I have had a couple of M5’s (first a ’91 and now a ’00) over the past 15 years and in the past I can usually find the answer to my problems from one of the M5 boards. However, despite several hours of searching and reading, I’m still not sure where to start looking for the source of my problem. Hopefully, another member will have a suggestion.

Currently, I have a 2000 M5 with 81K miles that needed a new flex disc. Since the car would be going into the shop, I checked the Peake codes and found a 69 (“Engine coolant temperature, Plausibility") which usually indicates a problem with the thermostat or sensor. Since this code could have been there for months, I cleared the code and drove it for a couple of days before checking the codes again before taking it to the shop. There was no code (but I’m not saying that there isn't a problem).

On Thursday I took the car to the shop that has worked on my M5’s for the past twelve years. They replaced the flex disc and took it out for a test drive. The flex disc was fine, but the car would not idle smoothly – it was constantly hunting between 400 and 900 RPM.

Their initial thought was that there might be a problem with the O2 sensors as a result of removing the exhaust. After checking the connections and wires, they checked the Peake codes. (They were using more sophisticated test equipment than the Peake reader!)

There were numerous errors (including the 69) and they felt there might be a problem with the DME. I didn’t think we should go down that path until we had eliminated several other possibilities including a vacuum leak. (I’m not a mechanic or an engineer, but I’ve seen vacuum leaks cause hunting at idle and a couple of threads have confirmed this as a possible cause.)

Over the past two days, the car has run well between 1000 and 5000 RPM. (I am a little reluctant to run it at a higher RPM without a backup car!) Usually, the idle hunts, but sometimes it is worse than other times and occasionally it is fine. I check the Peake codes frequently and sometimes get: the 69; 90 and 91 (Fuel control); and misfires on 6 of 8 cylinders. The misfires seem to occur when the engine is cold.

So, we finally get to my questions. It could simply be a coincidence that the problem appeared at the shop after the flex disc was replaced. However, this seems unlikely and I would like to start with the assumption that there is a correlation. What could have been done when removing the exhaust system that might cause the idle problem? Should we focus on the O2 sensor system (including connections and wiring)? Is there any way a vacuum leak could have been introduced from the undercarriage? Could the thermostat/sensor cause this problem? Where would you suggest that we start?

I would appreciate any suggestions that members can offer.

Thanks,

Wayne
 

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Just a long shot but I believe it is possible to put the rear O2 sensor plugs on the front O2 sensor and vice versa. worth checking.
 

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It's definitely possible they introduced a problem underneath when detaching/reattaching the pre/post-cat O2 sensors. Cracked bungs, misconnected wires, the sensors being in the wrong place or being now in need of replacement, etc. In the engine bay, if they touched the plenum or air intakes, they could have pinched a gasket or squish-tested-and-cracked a hose, etc. And rather than preach the perils of ignoring a 69 code, I'd suggest searching the FAQ for "secret menu" and watching your "KTMP" to confirm whether you need a tstat.
 

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The only vacuum source the is even close, is the one going to the fuel pressure regulator, mounted on the front
of the fuel filter. It is highly unlikely that could have been disturbed, since it's covered by the left side under-tray
panel.

You can't mix the O2 sensor leads due the leads for the post-cat sensor are much longer. If the senors were
swapped, the pre-cat sensor that is now in the rear, would not reach the connection box.

The only potential problem I can imagine, if the only work done was removing/installing the drive shaft, is the
mis-routing of the wiring harness to the O2 sensors. They are clipped onto the aft edge of transmission mount
frame (for the driver-side) and then both sides should be routed carefully behind the heat-shields, reappearing
just forward of the O2 sensor connector boxes. If they are not routed behind the heat-shields, they can come
in contact with the exhaust. You would certainly think if there is problem here, a fault would be recorded on
the DME. A cracked senor bung or sensor damage due to mishandling would seem more likely.

It's a puzzler! Be sure to report back when the culprit is found.

Regards,
Alan
 

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Discussion Starter #5
Thanks for the quick initial replies.

I was in the shop shortly after they took the test drive. Apparently, their first step was to ensure that the connections between the left and right O2 sensors were correctly routed; and they said that they checked the pin connectors as well. Of course, there could still be a short or open at a connector or somewhere in the wiring. Before I pursue this path, I would like to verify that the O2 sensors (or lack there of) could conceivably produce my symptoms at idle without creating other symptoms?

When I entered the shop, the hood was up, but the mechanic said that he had not raised the hood prior to the idle problem appeared. And since you don't need to raise the hood to replace a flex disc, I am inclined to believe him. Having said that, I suppose that it is possible that there was an initial problem (before I was on the scene) and the current problem was created while tracking down the initial cause. Rather remote, but then none of this makes much sense.

During my research of the code 69 prior to my trip to the shop, I did see a thread that suggested checking the secrete menu (which I had forgotten about) and I tried to check it on the way to the shop. First, I forgot to set it up immediately after starting the car and then a couple of bad pixils made it hard to read! I'll try to check it tomorrow. BTW, assuming an ambient temperature of 60F and street driving, how long should it take to reach operating temperature (80C?).

Thanks for the suggestions. Please keep thinking!

Wayne
 

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Discussion Starter #9
OK, here is an update on my problem(s). I began by repairing the obvious problem (Peake code 69). With the directions and suggestions of several posts, I was able to successfully replace my thermostat and flush the cooling system. I give full credit for a "one shot" installation without leaks to all those who have gone before me and posted their advice!

During the installation, I discovered that one of gaskets in the air intake tube connecting to the plenum was pinched. I purchased and installed the only two the deal had in stock and the idle returned to normal. However, I was/am still getting a code 90 (Fuel Control, Cyl 1-4) and misfire code on bank 1 cylinders. I switched the left and right pre-cat O2 sensors and the MAF's, but the codes remain the same.

A couple of posts have suggested that leaks on the intake typically affect both banks; and when the error code is for one bank, it is typically the result of a leak on the exhaust side. I have cleaned and checked the exhaust and O2 sensor bung, but don't see a leak. Therefore, I have ordered oil separator hoses and gaskets, vacuum lines, and a couple of fittings. Once they arrive I'll pull the plenum and install the parts. Even if this doesn't correct the problem, I'll consider this to be preventive maintenance.

Again, I would like to thank everyone who responded to my original post and those who previously posted problems and their solutions!
 

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OK, here is an update on my problem(s). I began by repairing the obvious problem (Peake code 69). With the directions and suggestions of several posts, I was able to successfully replace my thermostat and flush the cooling system. I give full credit for a "one shot" installation without leaks to all those who have gone before me and posted their advice!

During the installation, I discovered that one of gaskets in the air intake tube connecting to the plenum was pinched. I purchased and installed the only two the deal had in stock and the idle returned to normal. However, I was/am still getting a code 90 (Fuel Control, Cyl 1-4) and misfire code on bank 1 cylinders. I switched the left and right pre-cat O2 sensors and the MAF's, but the codes remain the same.

A couple of posts have suggested that leaks on the intake typically affect both banks; and when the error code is for one bank, it is typically the result of a leak on the exhaust side. I have cleaned and checked the exhaust and O2 sensor bung, but don't see a leak. Therefore, I have ordered oil separator hoses and gaskets, vacuum lines, and a couple of fittings. Once they arrive I'll pull the plenum and install the parts. Even if this doesn't correct the problem, I'll consider this to be preventive maintenance.

Again, I would like to thank everyone who responded to my original post and those who previously posted problems and their solutions!
Because you had a problem with the gasket, the trim may have been off or attempting to adjust. Now that you fixed the problem, Give it a reset, and a few miles to see if it comes back. The mixture may have started to go off far enough that when you fixed the problem, the change cause an error code again.
Also, I would check the exhaust to make sure there are no leakes. I have gotten a code 90 from a small exhaust leak before. They did have to remove it to get to the flex disk, and you may now have a small exhaust leak if you keep getting the error code after you have reset it and let the adaptation value come back up to speed.
 

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Do you have the VW air flow meters or the BMW ones?
 

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Discussion Starter #12
I installed BMW MAF's and pre-CAT O2 sensors 23,000 miles ago. However, a WOT test this weekend generated a number (114) which is considerably below the targeted 140. I'll retest them after replacing all the caskets in the intake tubes and replacing vacuum hoses and fittings under the plenum.

This evening I swapped the left and right post-Cat O2 sensors. The Peake 90 code quickly reappeared after a short drive. The problem appears to be isolated to bank 1 and a reexamination of the exhaust system while it is on a lift may be necessary.
 
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