Slight stutter / hesitation at low revs - BMW M5 Forum and M6 Forums
E34 M5 Discussion 1988-1995 Sedan and Touring

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post #1 of 18 Old 12th February 2019, 11:12 PM Thread Starter
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Slight stutter / hesitation at low revs

Hi,

I have noticed that there can be a slight stutter/hesitation in higher gears at low revs (1500 - 2500rpm). It is most apparent when going up hills in 2nd /3rd / 4th gear when the engine/oil is cold. If the car revs a little higher through the gears it doesn't falter at all. It also doesn't seem to happen when the engine oil has come up to temp on the gauge.

I have been doing a bit of research and am lead to believe that the ignition system is under a fair bit of load in these sorts of circumstances, is that true?

The spark plugs were changed roughly 3 years/4,000 miles ago, so I would imagine they shouldn't be knackered yet? What else would be the first port of call to check?

You will have to bear with me as I am a bit of noob when it comes to working on engines, but anything I can swap/clean myself without too much mechanical knowledge I'm happy to try.

I should add that the car drives pretty faultless apart from this, and no warning lights have been showing.

Thanks in advance,
Luke

Last edited by lukai; 12th February 2019 at 11:56 PM.
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post #2 of 18 Old 17th February 2019, 08:01 PM
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Quote:
Originally Posted by lukai View Post
Hi,

I have noticed that there can be a slight stutter/hesitation in higher gears at low revs (1500 - 2500rpm). It is most apparent when going up hills in 2nd /3rd / 4th gear when the engine/oil is cold. If the car revs a little higher through the gears it doesn't falter at all. It also doesn't seem to happen when the engine oil has come up to temp on the gauge.

I have been doing a bit of research and am lead to believe that the ignition system is under a fair bit of load in these sorts of circumstances, is that true?

The spark plugs were changed roughly 3 years/4,000 miles ago, so I would imagine they shouldn't be knackered yet? What else would be the first port of call to check?

You will have to bear with me as I am a bit of noob when it comes to working on engines, but anything I can swap/clean myself without too much mechanical knowledge I'm happy to try.

I should add that the car drives pretty faultless apart from this, and no warning lights have been showing.

Thanks in advance,
Luke
Hi Luke,

Noob or not, these cars are far easier to diagnose (and fix) yourself than the new cars! I’m happy to see you diving into this, as specialists qualified to fix these cars are dwindling in number and we are seeing more and more shops end up letting owners down.

Friend and fellow user Nanajoth is actually chasing a similar fault with his car. I don’t want to overwhelm you with possible faults, but I would continue checking on the ignition system - which on the B36, is very low-tech. And thankfully still easy and cheap to service.

The spark plugs are only the tip of the iceberg for this system: Do you know when the cap/rotor/wires etc were last replaced?

-Brent www.angryasssolutions.com
'91 M5 BK06091 256k miles
'84 Land Cruiser
'10 Mini Justa (hers)

Last edited by Bleedsblue; 17th February 2019 at 08:02 PM.
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post #3 of 18 Old 17th February 2019, 11:18 PM
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for low rev issues its also worth doing the 2 min check for a vacuum leak..... spray some carb clearner around the intake areas when the car is idling..... if the revs change you have a leak in that location. in particular check the throttle body gaskets - where the throttle bodies meet the head.

in mt experience dizzy cap and rotors are particularly weak parts on the s14 - i assume its the same for the s38 3.6
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post #4 of 18 Old 18th February 2019, 05:39 PM Thread Starter
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Hi Luke,

Noob or not, these cars are far easier to diagnose (and fix) yourself than the new cars! Iím happy to see you diving into this, as specialists qualified to fix these cars are dwindling in number and we are seeing more and more shops end up letting owners down.

Friend and fellow user Nanajoth is actually chasing a similar fault with his car. I donít want to overwhelm you with possible faults, but I would continue checking on the ignition system - which on the B36, is very low-tech. And thankfully still easy and cheap to service.

The spark plugs are only the tip of the iceberg for this system: Do you know when the cap/rotor/wires etc were last replaced?
Nice one thanks, I've only owned the car for about a year and I can't see anything in the history mentioning the cap/rotor/wires. Would you say that is a good place to start?

Thanks,
Luke
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post #5 of 18 Old 18th February 2019, 05:41 PM Thread Starter
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Originally Posted by MattH View Post
for low rev issues its also worth doing the 2 min check for a vacuum leak..... spray some carb clearner around the intake areas when the car is idling..... if the revs change you have a leak in that location. in particular check the throttle body gaskets - where the throttle bodies meet the head.

in mt experience dizzy cap and rotors are particularly weak parts on the s14 - i assume its the same for the s38 3.6
Nice one, I'll give that a go.
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post #6 of 18 Old 18th February 2019, 05:41 PM
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Does your issue happen only after the car sits for long periods of time? Like Brent said, I am dealing with something similar. I suspect it is moisture in my distributor cap or a coolant temp sensor.

My symptoms (let me know if they are similar)

Lower RPM than typical 700-800 (I usually idle around 1K)
Hesitation starting under load
When it warms up it acts completely normal
Problem will not show itself when the car is driven daily, only after sitting for a week or two.

I have not had the time to really dig in to it, but I do not believe my problem is vacuum related as I have already done all the lines recently and the symptoms do not match my previous experience with vacuum leaks. If your lines have never been done, it is very possible that is your issue or it will be down the road. I would start with the simple stuff on yours, make sure that your plug wires are all seated correctly. If you don't know the health of the cap and rotor, pull them off and inspect them. I believe moisture might be my issue since my car sits outside and showed evidence of it when I replaced the cap and rotor a few years ago. I am planning on replacing the coolant temp sensor on mine since I have never done it and it can cause fueling issues.

BK05864
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post #7 of 18 Old 20th February 2019, 07:40 PM
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My $.02...so many variables can be eliminated on these older cars by "baselining" the ignition, fuel and vacuum systems. If one is even slightly mechanically inclined...replace the fuel lines from tank to rail; cap, rotor and plugs; all rubber leading up to the intake plenum. Replace the Coolant Temp sensor that controls the fuel mixture in part. These items are not expensive and will eliminate 90% or more of the common issues related to the older cars (e-xx from 1982 to 1995 and beyond). Fuel air spark...its not throwing away money but rather baselining the vehicle to ensure components are operable in good working order. Cheap insurance. Fuel leaks or rotting lines can cause similar issues. Vacuum leaks as well. Etc etc.

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1986 535 B35, stahl headers, LTWFW, Supersprint
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post #8 of 18 Old 20th February 2019, 10:48 PM Thread Starter
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Originally Posted by Nanajoth View Post
Does your issue happen only after the car sits for long periods of time? Like Brent said, I am dealing with something similar. I suspect it is moisture in my distributor cap or a coolant temp sensor.

My symptoms (let me know if they are similar)

Lower RPM than typical 700-800 (I usually idle around 1K)
Hesitation starting under load
When it warms up it acts completely normal
Problem will not show itself when the car is driven daily, only after sitting for a week or two.

I have not had the time to really dig in to it, but I do not believe my problem is vacuum related as I have already done all the lines recently and the symptoms do not match my previous experience with vacuum leaks. If your lines have never been done, it is very possible that is your issue or it will be down the road. I would start with the simple stuff on yours, make sure that your plug wires are all seated correctly. If you don't know the health of the cap and rotor, pull them off and inspect them. I believe moisture might be my issue since my car sits outside and showed evidence of it when I replaced the cap and rotor a few years ago. I am planning on replacing the coolant temp sensor on mine since I have never done it and it can cause fueling issues.
Hi, thanks for the reply. It seems we share some symptoms but not all:

Idle is still around 1K
The hesitation does appear in the lower rev ranges when under load yes.
My car also acts completely normal without any hesitation once it has warmed up properly.
Annoyingly, my issue is present regardless of being driven daily or when left sat for a while.

Thanks for the advice, I have ordered a new dizzy cap and rotor so will replace them at the weekend while making sure all the plug wires are seated properly. If I get chance I'll check for vac leaks as well.

The coolant gauge seems to act as expected by slowly making its way to dead centre and then stays there constantly. Would there still be cause for concern with the coolant temp sensor, and if so are they fairly easy to replace?

Thanks,
Luke
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post #9 of 18 Old 20th February 2019, 10:56 PM Thread Starter
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Originally Posted by SCBMW View Post
My $.02...so many variables can be eliminated on these older cars by "baselining" the ignition, fuel and vacuum systems. If one is even slightly mechanically inclined...replace the fuel lines from tank to rail; cap, rotor and plugs; all rubber leading up to the intake plenum. Replace the Coolant Temp sensor that controls the fuel mixture in part. These items are not expensive and will eliminate 90% or more of the common issues related to the older cars (e-xx from 1982 to 1995 and beyond). Fuel air spark...its not throwing away money but rather baselining the vehicle to ensure components are operable in good working order. Cheap insurance. Fuel leaks or rotting lines can cause similar issues. Vacuum leaks as well. Etc etc.
Thanks, great advice.

As I have just mentioned above I plan to tackle the cap & rotor first as they seem cheap and fairly straight forward. Ignition leads seem to be fairly costly so I'll leave them be for the time being.

Is the temp sensor easy to get to, and does it require any extra bleeding of the system etc?

I take it changing the hoses to the intake plenum would be fairly straight forward too, so long as they are all fairly easy to access?

Last edited by lukai; 20th February 2019 at 11:02 PM.
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post #10 of 18 Old 21st February 2019, 02:23 AM
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On the Coolant Temp sensor..it measures the coolant temp but controls fuel...not intuitive I know but....in my opinion should be replaced every time the plugs are changed. They are cheap (BMW brand is $55 or so) and when they go bad cause rich mix and other issues. They can also cause high idle if completely disconnected/shot and the ECU defaults. The M5 has several sensors but one controls the mix. By hoses I meant vacuum hoses. Also check the bypass hose that the IASV (commonly called the idle control valve). It allows air to enter the intake when throttle plate is closed. If stuck or sticking or bad it will cause idle issues - high idle or pulsing idle.These can be removed and cleaned with brake cleaner. The bypass hose connected to this and then to the intake can get hard and brittle and crack (if never replaced) and also lead to higher idle. Vacuum leaks will also cause stumbling under load or higher idle...as unmetered air is entering the intake. I believe the CTS is located on or near the Tstat housing. IIRC its light blue in color 17mm and screws directly into the housing. One simple way to check if its reading...with engine running disconnect the plug from the sensor...the idle should increase substantially to 1k+. It can also be tested for ohms with a meter. Change when coolant is cool! Hope this helps. Also if you buy a cap and rotor, buy Bremi vs Bosch. A lot of the Bosch stuff is now made "offshore" whereas Bremi is still I believe a German product. I have seen premature failures >5k miles on Bosch rotors. Again..my $.02. I would check your plug wires for connectivity but in all honesty they seldom go bad unless the car has been abused or there are 300k miles plus and even then they are probably not the culprit on an e34 S38. Hope this helps.

2003 M5 Titanium / Imola, bone stock
1986 535 B35, stahl headers, LTWFW, Supersprint
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