1979 Other BMW Models BMW 633csi Cold Running Issue

Tiny
BMW633
  • MEMBER
  • 1979 BMW
  • 6 CYL
  • 2WD
  • MANUAL
  • 216,000 MILES
I have a 1979 BMW 633csi that stumbles and smokes (dark gray) when cold. It idles around 900 rpm and sounds like it is missing. The problem is worst at part throttle, when the engine bucks and shakes the car significantly, but when accelerating it runs relatively smooth. When warming up, the car hits a point when it suddenly runs smooth and the idle jumps up to around 1800 rpm, from which it gradually decreases to about 1200 rpm. Sometimes it will start running smoothly like this, but revert momentarily to running poorly when I give it part throttle. I figure the high idle speed once it's running smooth is probably a separate issue from the cold running problem. I've replaced the mass airflow meter (with a used unit), the plugs, and distributer cap. I tested the fuel pressure, which remained within the 33-38 psi specifications, but started at the high end of the range and decreased suddenly by about 5 psi when the car got warm. The residual fuel pressure dropped very gradually (about a psi per minute). I've also put one batch of injector cleaner through it, but I don't know where to go from here. It's been getting horrible gas mileage (in the 10 mpg range) and I'd love to get this sorted out.
Thanks!
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Thursday, February 19th, 2009 AT 9:56 PM

32 Replies

Tiny
DR. HAGERTY
  • MEMBER
Try replacing the coolant temperature sender.
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Thursday, February 19th, 2009 AT 10:07 PM
Tiny
BMW633
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Thanks for the quick reply!
Isn't there a way to test the coolant temperature sender by looking at resistance?
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Friday, February 20th, 2009 AT 12:27 AM
Tiny
DR. HAGERTY
  • MEMBER
It is a negative coefficient type sensor and I dont know the parameters. It will also need to be accurate at the "brain" not just the correct resistance at the sensor, inspect the wiring and the connectors.
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Friday, February 20th, 2009 AT 12:34 AM
Tiny
BMW633
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Ok, thanks!
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Friday, February 20th, 2009 AT 12:45 AM
Tiny
DR. HAGERTY
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I think a "hot" resistance of about 80 ohms was normal for that sensor, try substituting a carbon film resistor in its place.
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Friday, February 20th, 2009 AT 12:49 AM
Tiny
BMW633
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I was actually looking at this sensor earlier today, and it seems like if I was looking at the right one, there was only one electrical connection on it. Is that right? I'm looking at a sensor going into the coolant housing/passage at the front of the engine, close to the radiator. There was a second wire very close by, but nothing to connect it to.
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Friday, February 20th, 2009 AT 1:00 AM
Tiny
DR. HAGERTY
  • MEMBER
It will be a sender with a single lead that goes to the L-Jetronic brain. I will call my sources in San Diego today to see if I can get more info to you.
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Friday, February 20th, 2009 AT 11:23 AM
Tiny
BMW633
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Ok, that would be great!
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Sunday, February 22nd, 2009 AT 3:48 PM
Tiny
DR. HAGERTY
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Sorry I haven't got back sooner. My source was not available and I will try again tomorrow, stay tuned.
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Sunday, February 22nd, 2009 AT 7:49 PM
Tiny
BMW633
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Ok, I replaced the coolant temp. Sender, and there doesn't seem to be a difference.
What should I go after next?
Also, the "reactor" and "egr" test panel lights are on. Could either of these cause the cold running issue?
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Sunday, March 1st, 2009 AT 12:25 AM
Tiny
DR. HAGERTY
  • MEMBER
The reactor and EGR lights can be reset from under the car. Follow the speedo cable to the box with two "buttons" on it. Push to reset. The cold start issue could also be the cold start system is inop, does the Thermal time switch have power to it? Has the cold start valve been tested?
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Tuesday, March 3rd, 2009 AT 12:44 PM
Tiny
BMW633
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Ok, I'll reset the lights, but could the EGR system have an effect on the cold running problem. I've heard EGR valves aren't supposed to open when cold.
I haven't tested the voltage on the Thermal time switch or the cold start injector itself. However, I disconnected the fuel line to the cold start injector when I turned the car off one day and the cold running issue remained next time I started it up (fuel line still disconnected from cold start valve).
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Tuesday, March 3rd, 2009 AT 8:49 PM
Tiny
DR. HAGERTY
  • MEMBER
EGR does not usually show up as a cold start issue. Cold start requires the cold start valve has fuel pressure and a signal to work correctly.
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Wednesday, March 4th, 2009 AT 5:11 AM
Tiny
BMW633
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I know that it has fuel pressure, but I don't know for a fact that it has a signal. However, the car has no problem starting up when cold, it just runs very rich when it does (seems like lack of cold start signal would cause it to run too lean at first). Plus it runs poorly for much longer than the maximum 30 seconds or so that the cold start injector would be doing a job.
Seems unlikely to me that it's the cold start injector, I'm just not sure what it could be.
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Wednesday, March 4th, 2009 AT 9:10 PM
Tiny
DR. HAGERTY
  • MEMBER
Well you have a 30 year old car with worn parts and a cold running issue, short of replacement of all the possible components including harnesses, there is one thing to keep in mind, how much is this car worth to me? I am not advocating crushing the car, but if there is an emotional attachment here, you may need to do some soul searching. I have worked on older BMW cars and the first thing I tell the people is that this can become very expensive very quickly. If the air flow meter is bad, if the harness has an internal issue, if they have to spend several dollars more than the street value of the car, then they need to know that so that an informed decision can be made and once that decision is made, then a plan can be formed, do we have a plan? Is it complete with every contingency thought out? If not then I can try and help you find one. Lets start with the basics like a baseline of the condition of the power train and engine management systems.
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Thursday, March 5th, 2009 AT 12:47 PM
Tiny
BMW633
  • MEMBER
That makes sense, the car isn't worth particularly much and I don't want to just sink money into it. I plan to sell it at the end of this summer. I bought it as a project for $400 (without an interior) and I've been transferring the interior from a parts car into it. Naturally, nothing on these cars are in perfect shape, so it's no gem. The car drives and shifts well, but it has a slight steering pull, the cold running issue, and some little things that need attention - tires, oxidized paint on the hood, etc.
However, it's a good looking car with a lot of replaced parts, including tuned suspension, cold air intake, strut brace, and so on.
Basically, I'm reluctant to spend a bunch of money to replace all the worn parts, I'm just looking for an basic idea of what I should be checking into next on this problem. I'm generally more willing to spend time on it than money. I appreciate all your help.
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Thursday, March 5th, 2009 AT 8:59 PM
Tiny
DR. HAGERTY
  • MEMBER
Thank you for sticking with this project. Lets see if we can do some good. Valve adjustment, spark plug gap, cap and rotor, wires, can all be contributing factors, as can throttle plate adjustment and warm up regulator, plus all the plumbing, how does this stuff appear?
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Friday, March 6th, 2009 AT 1:44 AM
Tiny
BMW633
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I just adjusted the valves a couple days ago, including tightening a loose (but still connected) banjo bolt. The valves made it run a little smoother, but the cold running problem persisted. I also replaced the spark plugs, cap, and rotor. I haven't replaced the wires, but they look like they've been replaced relatively recently. It seems that the warm up regulator is probably fine. It gradually brings the idle down to 950 rpm when warm (I adjusted the idle control screw since I originally posted this query). I've looked at the throttle plate and it seems to close completely. In addition, I set the timing a few days ago.
As for plumbing, there were a few problem areas that I've worked on, including a disconnected vacuum line going to the fuel pressure regulator (fixed now), and the leads to the electric valve control on the vacuum advance are broken off (I'll replace this soon from my parts car). I've experimented bypassing this valve and I couldn't notice any difference. Otherwise, I've taken a look and I can't find any gaping issues with the plumbing. Vacuum lines seem relatively new and crack free.
As a side project, I'm working on removing the exhaust reactor and replacing it with a non-reactor manifold from a 630csi.
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Friday, March 6th, 2009 AT 9:05 PM
Tiny
DR. HAGERTY
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Those thermal reactors are trouble, save them for the future. The spray pattern from the injectors can be an issue or just the fact that it is a 30 year old car with antiquated injection can be an issue.
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Friday, March 6th, 2009 AT 11:06 PM
Tiny
BMW633
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Well, I've sent through 2 doses of techron injector cleaner in hopes of improving the spray pattern or unclogging injectors, but it didn't seem to make much of a difference. I could take each injector out and look at the spray pattern, but that would be many hours of work because they are so inaccessible, and it seems unlikely that they are bad considering the cleaner didn't make a difference. What do you think?
I have done a fuel pressure leak down test and determined that the injectors are not leaking.
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Saturday, March 7th, 2009 AT 8:43 PM

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