Weird idle on cold start

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imo6en
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Weird idle on cold start

Post by imo6en »

Hello all! I'm very recently new to the 6er world, having just bought my first one last week. It's a 1987 635csi and so far it's been a great car except for the following:

Whenever the car is started cold it will idle fine for the first few seconds and then the idle will drop to around 500rpm (sometimes just under) and the engine will start to idle very rough, like a v8 with a large cam. However, after driving for about 5-10 minutes the car idles fine (right around 750). Revving seems to be smooth regardless of the idle problem. Car will idle fine while warm. Interestingly, I had a dead battery the other day and had to use the engine start function on my battery charger to start the car. With the charger still connected and in start mode the engine idled fine, but as soon as I removed the cables it started to idle rough again.

I've inspected the vacuum lines and don't see any obvious source of leak, however, I've already ordered new lines and a new intake boot with plans to replace them all anyway. I've pinched the fuel supply to the CSV without resolution of the symptoms. I don't believe either of these are my source though, since the problem eventually goes away after warming up and a vacuum leak or leaky CSV doesn't really change with engine temp. My other thought was the AFM but I plan to go with a Miller MAF in the future anyway.

I've searched on here for causes of a rough idle before posting this but haven't quite found a problem like mine. Any thoughts?
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Brucey
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Re: Weird idle on cold start

Post by Brucey »

you may be experiencing a simple lack of volts somewhere; if the alternator doesn't start charging straight away (which it might not after a start at low engine revs for all kinds of reasons) then the engine will be trying to run on 12V or less rather than 13.8 volts.

A dodgy connection somewhere may go bad at the lower voltage, or the coil output may drop slightly and this may be causing a misfire somehow.

The low idle speed may be caused by the ICV jamming; (it may need cleaning anyway but ) it won't so easily unjam itself if there are not enough volts.

So I'd suggest that you

a) check for volts after a start; is the alternator actually charging? Does the charge warning light work?
b) check and if necessary clean the ICV
c) check (by substitution) the operation of the main and fuel pump relays.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
imo6en
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Re: Weird idle on cold start

Post by imo6en »

Brucey wrote: Wed Sep 05, 2018 11:52 am you may be experiencing a simple lack of volts somewhere; if the alternator doesn't start charging straight away (which it might not after a start at low engine revs for all kinds of reasons) then the engine will be trying to run on 12V or less rather than 13.8 volts.

A dodgy connection somewhere may go bad at the lower voltage, or the coil output may drop slightly and this may be causing a misfire somehow.

The low idle speed may be caused by the ICV jamming; (it may need cleaning anyway but ) it won't so easily unjam itself if there are not enough volts.

So I'd suggest that you

a) check for volts after a start; is the alternator actually charging? Does the charge warning light work?
b) check and if necessary clean the ICV
c) check (by substitution) the operation of the main and fuel pump relays.

cheers
Checked the battery: 12.6V with the car off and during rough idle it's getting between 13.3-13.6. Also checked the ICV and cleaned it. Inspected it's hose and this appears to be relatively new as the rubber is still soft and has no cracks. After cleaning it still idles rough until the car has warmed up but does appear to breath better afterwards and throughout the rev range.
imo6en
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Re: Weird idle on cold start

Post by imo6en »

Replaced all the vacuum hoses except the ICV (which I inspected for leaks) and the intake boot itself. There was a small crack in the breather hose off the intake boot but otherwise all were in relatively good shape given their age. Idling the car after the vacuum hose maintenance the car continues to idle rough at around 500. During this rough condition I turned on the headlights and the symptoms worsened. Battery voltage with just the headlights on and the car running was around 12.8V. I'm thinking I may have a bad alternator.
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Brucey
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Re: Weird idle on cold start

Post by Brucey »

at 500 rpm a good alternator will struggle to run the headlights.

It occurs to me that you may well have the usual problem with the throttle body; that is, the linkage is worn and the throttle plate is too far closed at idle. This will cause the ICV to open fully when the engine is cold, and it may not modulate smoothly and hold the idle constant, like it should.

Note also that in a car with an oxygen sensor fitted, it only runs 'closed loop' when the sensor is warmed up. Before that (i.e. when the engine is cold) the engine runs 'open loop' and all kinds of things (mostly air leaks and bad fuel delivery i.e. not in proportion to the airflow/temperature) will cause rough running.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
imo6en
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Re: Weird idle on cold start

Post by imo6en »

I realize I haven't updated this in a long time so let's review what has been checked/replaced:

Checked:
ICV- In spec, cleaned several times.
Thermo-time switch: In spec, 0 ohms and 35 ohms between W/ground and G/ground respectively
TPS- Clicks right off idle
Cap and Rotor- Newer and in good condition
AFM- Cleaned and resistance measurements appear to be in spec (was hard to get a solid reading with only 2 hands)
Compression Test (while I had the plugs out)- 145, 143, 145, 144, 148,148

Replaced:
Voltage regulator- No change in idle, but battery seems to charge much better now and less drop in RPM with accessories
FPR- No change in idle
CTS- No change in idle
ALL vacuum hoses including intake boot, ICV hose, and interior climate sensor- No change in idle
Ignition coil- No change in idle
Spark plugs- Bosch silvers, gapped at 30. No change in idle
Crank position/reference sensors

Still idles rough during cold start/warm up. Appears to be progressing too, as it's starting to intermittently run rough after it's at operating temp.
plip1953
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Re: Weird idle on cold start

Post by plip1953 »

Could it be something like a faulty/failed water temp sensor, so the ecu doesn't know to run for a while in cold start mode?
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imo6en
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Re: Weird idle on cold start

Post by imo6en »

plip1953 wrote: Sun Oct 21, 2018 12:07 pm Could it be something like a faulty/failed water temp sensor, so the ecu doesn't know to run for a while in cold start mode?
I've replaced the coolant temp sensor. I checked the voltage from the harness side and I'm getting 4.75V which I'm assuming is in spec?
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Brucey
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Re: Weird idle on cold start

Post by Brucey »

imo6en wrote: Fri Oct 19, 2018 9:42 pm I realize I haven't updated this in a long time so let's review what has been checked/replaced:

Checked:
ICV- In spec, cleaned several times.
Thermo-time switch: In spec, 0 ohms and 35 ohms between W/ground and G/ground respectively
TPS- Clicks right off idle
Cap and Rotor- Newer and in good condition
AFM- Cleaned and resistance measurements appear to be in spec (was hard to get a solid reading with only 2 hands)
Compression Test (while I had the plugs out)- 145, 143, 145, 144, 148,148

Replaced:
Voltage regulator- No change in idle, but battery seems to charge much better now and less drop in RPM with accessories
FPR- No change in idle
CTS- No change in idle
ALL vacuum hoses including intake boot, ICV hose, and interior climate sensor- No change in idle
Ignition coil- No change in idle
Spark plugs- Bosch silvers, gapped at 30. No change in idle
Crank position/reference sensors

Still idles rough during cold start/warm up. Appears to be progressing too, as it's starting to intermittently run rough after it's at operating temp.
you have spent a fortune in both time and money but you don't mention that you have done the very simple (and free) check to make sure that the throttle body is set correctly.

I repeat; it is not sufficient to ensure that the TPS is 'working', it is vital that the TB is open the correct amount too. The linkage wears and the idle setting needs checking at fairly regular intervals. There is a procedure telling you how in the workshop manual.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
imo6en
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Re: Weird idle on cold start

Post by imo6en »

Brucey wrote: Mon Oct 22, 2018 8:34 pm you have spent a fortune in both time and money but you don't mention that you have done the very simple (and free) check to make sure that the throttle body is set correctly.

I repeat; it is not sufficient to ensure that the TPS is 'working', it is vital that the TB is open the correct amount too. The linkage wears and the idle setting needs checking at fairly regular intervals. There is a procedure telling you how in the workshop manual.

cheers
I have inspected the throttle plate and it appears to be pretty well closed. Will try to track down a feeler gauge small enough for this spec. If my google fu is correct, this should be .002". As in 2 THOUSANDTHS of an inch. Correct?

My smallest measurement is .005", not quite small enough.
imo6en
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Re: Weird idle on cold start

Post by imo6en »

Sorry for double post but I spaced the throttle blade to .002" and the car idles magnificently smooth...at 1200rpm. And that's with adjusting the TPS as far as it would manage. Initially set the TPS to click off idle, as it did before, but that set the idle at 1750ish rpm. Ohms continued to read 0 between pins 2 and 18 at closed throttle regardless of TPS position. Not sure what that's about so I just set the TPS based on RPM at idle.
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Brucey
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Re: Weird idle on cold start

Post by Brucey »

IIRC the TB gap ought to be 0.5mm, i.e. 0.020".

If it doesn't idle slowly enough with this TB gap it usually means the ICV isn't working/ is jammed open and/or there is a massive air leak.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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