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Discussion Starter · #1 ·
My 3.8 has always had a slow idle compared to the 3.6 I had previously. It idled just under 1000 RPM, but the 3.8 runs slightly below the 750 RPM mark, and hits 750 with AC on. Occasionally it dips to about 600, but settles a little higher when blipping the trottle.

Since having read about weak batteries causing slow idle, I had a look at it and changed the non-stock battery which had a too low Ah-rating to a Porsche (!) battery (simply because my cousin had got it from someone, and gave it to me) which was about 80-85 Ah and performed well with a battery load tester, but the idle was exactly the same as before. Is it still likely to be an electrical problem (alternator/bad contact etc) causing this slow idle? I have at least never seen the battery charge warning light come on...

Could this idle speed be bad for the engine in any way, or maybe it doesn't matter? The engine never shows any tendency to stall when just idling, but the 3.6 was nicer to launch from standstill since the engine is quite strong already around 1000 RPM. I think the idle is pretty smooth to be an M5, just a little "weak" since running slow.

Well, maybe I'm just having to much spare looking for troubles ouich , but it's always nice to know the beast is ok. :)
 

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Mr. J said:
My 3.8 has always had a slow idle compared to the 3.6 I had previously. It idled just under 1000 RPM, but the 3.8 runs slightly below the 750 RPM mark, and hits 750 with AC on. Occasionally it dips to about 600, but settles a little higher when blipping the trottle.

Since having read about weak batteries causing slow idle, I had a look at it and changed the non-stock battery which had a too low Ah-rating to a Porsche (!) battery (simply because my cousin had got it from someone, and gave it to me) which was about 80-85 Ah and performed well with a battery load tester, but the idle was exactly the same as before. Is it still likely to be an electrical problem (alternator/bad contact etc) causing this slow idle? I have at least never seen the battery charge warning light come on...

Could this idle speed be bad for the engine in any way, or maybe it doesn't matter? The engine never shows any tendency to stall when just idling, but the 3.6 was nicer to launch from standstill since the engine is quite strong already around 1000 RPM. I think the idle is pretty smooth to be an M5, just a little "weak" since running slow.

Well, maybe I'm just having to much spare looking for troubles ouich , but it's always nice to know the beast is ok. :)

Hi!

I've had my E34 M5 3.8 for nearly 3 years now. The idle has always been around 750 rpm which I think is normal for this motor.

When I changed the battery for another BMW battery it is rated at 95 Ah so yours should be OK.

It really sounds like the idle is normal.

:cheers:

ralph (vadas1)
 

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Idle speed shoul be around 950RPM to secure enough oil pressure for proper lubrication. The M88/3 (E28 M5) idles at 850RPM, but following Stevie's advice, I set it to 950RPM. However, the S38 uses a different hardware setup for the idle control. On the S38, an idle control valve (ICV) contols the volume of air that bypasses the throttle bodies (TB) when these are closed (i.e idle). The idle speed is proportional to the opening of the ICV. Unlike the M88/3, there is no adjustement screw for the idle-speed.

There can be two faults. 1: A not functional TPS switch (black box on the front of the TB for cyl #1 and #2) or 2: A contaminated ICV.

The TPS switch should click when you slightly operate the throttle lever from the idle-stop! If it doesn't, the DME doesn't know that the TB's are fully closed and keeps the ICV closed.

If the TPS switch works, chances are that the ICV is dirty. Use some carburetor-cleaner to clean the ICV. However, to gain access you'll have to remove the intake plenum, which is a 15-30min job.
 

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raymondw said:
Idle speed shoul be around 950RPM to secure enough oil pressure for proper lubrication. The M88/3 (E28 M5) idles at 850RPM, but following Stevie's advice, I set it to 950RPM. However, the S38 uses a different hardware setup for the idle control. On the S38, an idle control valve (ICV) contols the volume of air that bypasses the throttle bodies (TB) when these are closed (i.e idle). The idle speed is proportional to the opening of the ICV. Unlike the M88/3, there is no adjustement screw for the idle-speed.

There can be two faults. 1: A not functional TPS switch (black box on the front of the TB for cyl #1 and #2) or 2: A contaminated ICV.

The TPS switch should click when you slightly operate the throttle lever from the idle-stop! If it doesn't, the DME doesn't know that the TB's are fully closed and keeps the ICV closed.

If the TPS switch works, chances are that the ICV is dirty. Use some carburetor-cleaner to clean the ICV. However, to gain access you'll have to remove the intake plenum, which is a 15-30min job.
Thanks for the info Raymond! If I recall correctly, this black switch made a more distinct click on the 3.6 I had, than on this engine. Does it control the idle only or is it giving information about throttle position with open throttle bodies too? I experience no problems when running the car with open throttle. How will the engine react if the switch is disconnected - ie. can I do that to test whether the TPS switch is faulty?

Patrik
 

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Mr. J said:
Thanks for the info Raymond! If I recall correctly, this black switch made a more distinct click on the 3.6 I had, than on this engine. Does it control the idle only or is it giving information about throttle position with open throttle bodies too? I experience no problems when running the car with open throttle. How will the engine react if the switch is disconnected - ie. can I do that to test whether the TPS switch is faulty?

Patrik

Patrik,

The TPS switch functions as a sensor ONLY. With other words, it doesn't control anything, it only tells the DME which of the following three conditions apply:

1: idle
2: Partial throttle
3: Full load

At idle: The TB's are fully closed and the DME operates the ICV to set the idle speed.

During partial throttle: The DME closes the ICV (when the TB's open). This is a delicate setting as the switch over point determines the throttle response, hence the TB's synchronisation need to be checked and if necessary adjusted once in every few years. In this condition, the DME uses the MAF and the O2 sensors to operate as economical and clean as possible.

At full throttle The DME denies the information from the MAF and O2 sensor. It just injects as much as fuel as possible to ensure a rich operating condition. This protects the engine from capital damage (self destruct mode :1: ).

To check the TPS. Do not start the engine. Operate the bowden-cable lever manually towards full throttle. You should hear a slight click directly after the lever is being pushed. That is all. Repeat this several times. If the results are consistant,teh TPS should be OK. If the results are not consistant, the TPS switch is eithers set incorrectly or the idle-stop isn't set correctly. In that case, you are better of with a synchronization of the TB's as that includes the TPS switch settings. However, this is not as easy as it sounds. I have little trouble describing it, but the procedure itself is rather tricky if you carry this out for the first time. Don't ask me how I know.
 

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Raymond,

If I remember correctly, the TPS on the 3.8 doesn't click, it's a variable resistor. If you measure the voltage on the pins of the TPS while opening the throttle, you'll see a wide range from fully closed to WOT.

There isn't a microswitch in it like the TPS on the 3.6.

In my case, I had a similar problem of very poor/low idle because the throttle bodies were very much out-of balance. After synchronising them, the idle was 850-900rpm and cold starting was very much better.

Cheers,
Andy
 

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Thanks Andy,

The B38 operates with Motronic 3.3 whereas the B36 uses Motronic 1.2. The main difference is that 3.3 allows eight cylinders to be conneced on one DME, but some of the auxiliaries as you pointed out are different.

But the principle stays the same. If the TPS (throttle position sensor) doesn't work correctly, one gets the same results. But a full synchronization of the TBS's is better indeed. Allows one to check the TPS and clean the ICV as well. :D
 

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raymondw said:
Thanks Andy,

The B38 operates with Motronic 3.3 whereas the B36 uses Motronic 1.2. The main difference is that 3.3 allows eight cylinders to be conneced on one DME, but some of the auxiliaries as you pointed out are different.

But the principle stays the same. If the TPS (throttle position sensor) doesn't work correctly, one gets the same results. But a full synchronization of the TBS's is better indeed. Allows one to check the TPS and clean the ICV as well. :D

Interesting point about 950 rpm idle to keep oil pressure up. Does it apply to 3.8 as well? At 750 rpm mine is a little rough which one expects but the pick up is very clean and the power appears excellent especially in third gear.

On a quick aside, I am going to Amsterdam shortly, what is the weather like? Any particularly interesting places to see?

:cheers:

ralph (vadas1)
 

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vadas1 said:
Interesting point about 950 rpm idle to keep oil pressure up. Does it apply to 3.8 as well? At 750 rpm mine is a little rough which one expects but the pick up is very clean and the power appears excellent especially in third gear.

On a quick aside, I am going to Amsterdam shortly, what is the weather like? Any particularly interesting places to see?

:cheers:

ralph (vadas1)
----------------

I hear the coffe shops have an excellent selection this time of year... :7:
 

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Discussion Starter · #10 ·
AndyE said:
In my case, I had a similar problem of very poor/low idle because the throttle bodies were very much out-of balance. After synchronising them, the idle was 850-900rpm and cold starting was very much better.

Cheers,
Andy
I will talk to my mechanic about the idle when he's back from vacation. I think the idle smoothness is pretty good, judging from the 3.6 I had, and from an M5 touring which was slightly rougher (and I know that car is being very well maintained). So maybe it's problems with my ICV rather than the throttle bodies then. (My engine runs almost as smooth as my father's M50 520i when my AC kicks in. :) ) Andy, did your unbalanced throttle bodies affect the smoothness much when at cruising speeds too, or just the idle?
Of course I do not want too low oil pressure, so I will have this checked.
 

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vadas1 said:
On a quick aside, I am going to Amsterdam shortly, what is the weather like? Any particularly interesting places to see
Whether zzzzz zzzzz zzzzz zzzzz Not that good, mostly cloudy and rainy :crying:

Sight-seeing points: I don't live in Amsterdam, but close to the German border instead where the Oberhausen straight (A31) is much more interesting :1:
 

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On a quick aside, I am going to Amsterdam shortly, what is the weather like? Any particularly interesting places to see?

:cheers:

ralph (vadas1)[/QUOTE]

Ralph, there's plenty of Window shopping to be done! :blink: cherrsagai cherrsagai :cool: ,
when you exit Centraal Station the vvv tourist info is to the left. The road more or less straight ahead (Damrak) will take you to Dam Square, going down this road on the l/h/s as the buildings start there is a street to the left, go down there you will see the Grasshopper on l/h/s where you can get a beer, a smoke or a meal. If you continue on that street you will soon notice some interesting window's. Alternatively you can head to Dam Square and continue south west-ish to the museum district which is near Rembrant Square/plein ?, plenty of bars, restaurants etc, no dodgy geezer's trying to sell you class A drugs - stay well clear, there's plenty of them in the red light district, ignoring them is the best policy. You can visit a couple of Breweries (Amstel and Heineken I think) as well. Wear comfy shoes if you are walking as some places are 1-2 km from the station, depends where your staying.
Have fun.:eek:
Cheers,
Phil.
 

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My beast idles at ~900 rpm, I think it's normal hmmm
 
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