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Thread: Weird idle on cold start

  1. #1
    Join Date
    Sep 2018
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    TN
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    1987 BMW 635CSi

    Weird idle on cold start

    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?

  2. #2
    Join Date
    Oct 2015
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    Illinois
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    740 735 635 535 318is
    What is the battery voltage when it is idling without the charger on it?

    Sent from my SM-G900V using Tapatalk

  3. #3
    Join Date
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    +1 on the battery voltage...Bosch electronics are picky about the charging system (IMHO they're not nearly as robust as American & Asian electronics). It's important for these cars to have a nice stable 13-14 volts at idle. A good battery should hold a little over 12VDC with the car off at 80 deg F ambient temp. Battery voltages will vary depending on ambient temp, of course. If the voltage is a lot lower, like 11.5VDC or lower, the ECU/DME gets into trouble supplying a good reference voltage and you start to get all sorts of interesting problems.

    Vac leaks are another big problem, seems you're pretty aware of that one. On the M30B34 that ICV hose is pretty weak. I've also found a few fun ones on the fuel injector o-rings, interior temp sensor line, and the cold-start injector.

    I'd also check the air, coolant and "thermotime" temp sensors. If they're reporting the wrong temp during the warm-up cycle, it'll run the motor lean and you'd get a very low idle or even occasional stalling. The CLT is one of the small sensors on the coolant neck, along with the larger thermotime and the coolant gauge temp sender. The IAT sensor is integrated into the Air Flow Meter. There are four pins on the AFM, two are the flap, two are the IAT.

    Here's a really helpful troubleshooting article on the early Motronics. It's a LONG read, with a few unrelated sections, but IMHO well worth it. I'll give him a free plug too, the silicon hoses he sells are good quality. I print one out and leave it in the glove box, along with spare relays, bulbs, relay jumpers and the usual junk. I added a few notes to his guide on my blog here.
    Save the manuals!
    '08 128i 6MT, '86 635CSi 5MT (B32, Motronic 1.3, WBO2, G265, 18# FW, 3.46 torsen)
    Past projects:
    '96 318iS, sold 4/23 '90 535iM, RIP 1/23 '90 535iA RIP 6/22 '91 318i, sold 7/19 '97 M3 sedan, sold 11/18 '85 735i, RIP 2/18 '92 325iC, sold 7/16 '91 318iS, sold 6/16 '84 745i, sold 10/14 '82 633CSi, traded 9/12 '90 325i, RIP '87 325 sold '89 525i, traded '87 325iS, RIP

  4. #4
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    1987 BMW 635CSi
    Quote Originally Posted by DesktopDave View Post
    +1 on the battery voltage...Bosch electronics are picky about the charging system (IMHO they're not nearly as robust as American & Asian electronics). It's important for these cars to have a nice stable 13-14 volts at idle. A good battery should hold a little over 12VDC with the car off at 80 deg F ambient temp. Battery voltages will vary depending on ambient temp, of course. If the voltage is a lot lower, like 11.5VDC or lower, the ECU/DME gets into trouble supplying a good reference voltage and you start to get all sorts of interesting problems.

    Vac leaks are another big problem, seems you're pretty aware of that one. On the M30B34 that ICV hose is pretty weak. I've also found a few fun ones on the fuel injector o-rings, interior temp sensor line, and the cold-start injector.

    I'd also check the air, coolant and "thermotime" temp sensors. If they're reporting the wrong temp during the warm-up cycle, it'll run the motor lean and you'd get a very low idle or even occasional stalling. The CLT is one of the small sensors on the coolant neck, along with the larger thermotime and the coolant gauge temp sender. The IAT sensor is integrated into the Air Flow Meter. There are four pins on the AFM, two are the flap, two are the IAT.

    Here's a really helpful troubleshooting article on the early Motronics. It's a LONG read, with a few unrelated sections, but IMHO well worth it. I'll give him a free plug too, the silicon hoses he sells are good quality. I print one out and leave it in the glove box, along with spare relays, bulbs, relay jumpers and the usual junk. I added a few notes to his guide on my blog here.
    Thanks for the links. Figured I'd post an update:

    Checked the battery and it holds 12.6V with the car off and 13.3-13.6V at (rough) idle with no accessories on. Cleaned the ICV and inspected its hose, which appears to be relatively new as the rubber was still soft and had no cracks. After cleaning the ICV the car still takes a few minutes of regular driving to return to it's appropriate idle but does seem to breath much better. Will move forward with checking the other sensors.

  5. #5
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    Sorry for the double post here, forgot to add that I 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.

  6. #6
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    That sounds OK to me, maybe just a wee bit too low. I don't think that'd be enough to cause a low idle. Regardless, I do the same test you've done...turn on the headlights & HVAC fan for load, then test. Across the battery terminals I'm usually looking for 12.5VDC with the car off, better than 13.5VDC at idle, maybe 14.5VDC at 2K RPM. It should never get higher than 16.5VDC at any speed. Those cheap little cigarette lighter voltmeters are surprisingly good troubleshooting tools BTW. I use them frequently on my vehicles.

    The voltage regulator is easily replaced instead of the whole alternator; I've done that a bunch of times just to test them out. You know the drill: disconnect the battery...disconnect the alternator wires...remove the two screws from the white plastic plug on the rear of the alternator. It'll slide right out, brushes & all. Check the brushes for wear. Keep in mind that the battery and alternator act as a team, damage to one of them may cause the other to fail.

    IIRC another (rare) possibility is that some starters or ignition switches can fail and the starter bendix won't retract. Did you test the thermo sensors?
    Last edited by DesktopDave; 09-09-2018 at 06:40 PM.
    Save the manuals!
    '08 128i 6MT, '86 635CSi 5MT (B32, Motronic 1.3, WBO2, G265, 18# FW, 3.46 torsen)
    Past projects:
    '96 318iS, sold 4/23 '90 535iM, RIP 1/23 '90 535iA RIP 6/22 '91 318i, sold 7/19 '97 M3 sedan, sold 11/18 '85 735i, RIP 2/18 '92 325iC, sold 7/16 '91 318iS, sold 6/16 '84 745i, sold 10/14 '82 633CSi, traded 9/12 '90 325i, RIP '87 325 sold '89 525i, traded '87 325iS, RIP

  7. #7
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    Welp, replaced the voltage regulator, coolant temp sensor, and fuel pressure regulator. Car seemed to like those changes but can't say definitively as I'm currently dealing with a new problem: Car will crank but won't start. So until that gets fixed I can't give an update...

  8. #8
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    Bad crank sensors caused the no start issue. Those were replaced and car starts...

    However, I've replaced the intake boot and reinspected vacuum hoses for leaks and found none. Cleaned the AFM and checked resistance between all terminals and all were in spec. Coolant temp sensor was replaced as stated above. Thermo time switch shows appropriate resistances of 0 ohms and 35 ohms between W/ground and G/ground respectively. Car continues to idle rough during warm up unfortunately.

  9. #9
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    Sounds like you have had multiple issues with the new 6er. What is the condition of the engine? A few things I recommend with any M30 is a compression check, the correct plugs, and valve clearance check / oil spray bar check. Have you had a look under the valve cover? The last 3 M30's I've had all had issues with the spray bar having loose banjo bolts, and all needing valve lash adjustments. Lash, compression, and not having the right plugs will all make for a poorly running M30.

    Sent from my SM-G900V using Tapatalk

  10. #10
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    Quote Originally Posted by Dschart View Post
    Sounds like you have had multiple issues with the new 6er. What is the condition of the engine? A few things I recommend with any M30 is a compression check, the correct plugs, and valve clearance check / oil spray bar check. Have you had a look under the valve cover? The last 3 M30's I've had all had issues with the spray bar having loose banjo bolts, and all needing valve lash adjustments. Lash, compression, and not having the right plugs will all make for a poorly running M30.

    Sent from my SM-G900V using Tapatalk
    Swapped out plugs today for bosch silvers and did a compression test while I was there. Compression is superbly consistent: 145, 143, 145, 144, 148, 148. Old plugs had a nice even brown color as well. Motor appears remarkably healthy. Valve adjustment will be done once my new gasket gets here. I believe I've narrowed this down to to either the AFM or the thermo time switch since those are the only things I haven't replaced at this point...

    Checked the TPS, it's in spec. Clicks immediately once off idle.

  11. #11
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    Replaced the ICV hose today just as a last ditch effort. No change. In fact, symptoms are progressing. Car is starting to idle rough intermittently when warmed up now.

  12. #12
    Join Date
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    I just re-read your first post, had a similar problem a while back. One of my fuel injection harnesses wasn't powered. The car ran surprisingly well on three cylinders, but it had a lumpy idle...sounded an awful lot like a cammed V8. The solution to that problem was a few overhauled injectors. Your plugs don't seem to have any mixture problems though.

    Cold solder joints in the DME can occasionally happen, and usually present symptoms only when warmed up. Have you checked the DME transisors? There are two injector drivers on most 80s BMWs, as well as an ignition driver and (I'm pretty sure) an ICV driver too.
    Save the manuals!
    '08 128i 6MT, '86 635CSi 5MT (B32, Motronic 1.3, WBO2, G265, 18# FW, 3.46 torsen)
    Past projects:
    '96 318iS, sold 4/23 '90 535iM, RIP 1/23 '90 535iA RIP 6/22 '91 318i, sold 7/19 '97 M3 sedan, sold 11/18 '85 735i, RIP 2/18 '92 325iC, sold 7/16 '91 318iS, sold 6/16 '84 745i, sold 10/14 '82 633CSi, traded 9/12 '90 325i, RIP '87 325 sold '89 525i, traded '87 325iS, RIP

  13. #13
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    Pretty sure I have this solved and it was so simple... The throttle body butterfly was completely closed at idle. A little adjustment of the linkage and it's back to idling consistently at 750. I'm assuming there was probably an air leak I fixed along the way that contributed to the madness. Will do more driving to verify all is well once the weather here clears up.

  14. #14
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    I'm walking this path right now.
    I testing the thermo time switch, I get 68 ohms on terminal G and 136 ohms on terminal W.

    Hot or cold.

  15. #15
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    Update: This morning, I have 36 ohms on terminal G and .2 ohms on terminal W.
    So it looks like the TTS is not the problem.

  16. #16
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    If poor idle when cold and fine when warm, consider looking at the cold start valve. Also take a peek in the gas tank and pull the auxillary pump and make sure the screen is clean and the tank has no debris, you may be pulling some stuff up into the screen at start up.
    Rob E3

  17. #17
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    Quote Originally Posted by imo6en View Post
    Pretty sure I have this solved and it was so simple... The throttle body butterfly was completely closed at idle. A little adjustment of the linkage and it's back to idling consistently at 750. I'm assuming there was probably an air leak I fixed along the way that contributed to the madness. Will do more driving to verify all is well once the weather here clears up.

    Check if your TPS clicks when the throttle is released fully, if not then adjust it. If it doesnt click when released it stays in the afm load mode instead of switching to the idle mode which turns the idle valve on. Does it randomly die when coming to a stop (if manual)?

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