View Full Version : 1985 280i - stalls when warm

04-11-2014, 04:59 PM
Trying to help a friend. He is a good mechanic and experienced with Bosch CIS. He is finding that the signal to the frequency valve goes away when the car warms up. One can hear the engine tone change and then the engine slowly dies. MANY other hands have been in this car before my friend. He found vacuum leaks, disconnected wires, each time thinking "Oh Boy, that's it" and yet the problem persists. It has a Bosch ECU # 0280800050. Can anyone shed some light on this ? Thank you for your time,

04-11-2014, 09:31 PM
The loss of signal to the frequency valve has been confirmed,yes? What is happening to the engine once it's warm;does it die,sputter,etc? I don't know what troubleshooting steps you and your friend have gone through yet,and I'd like to have some details about how the engine is running. If the fuel distributor/metering plate adjustments are good,I would first check the O2 sensor and then the coolant temp sensor(the one mounted by the engine,not the radiator fan temp switch). Easiest way to do this is simply disconnect the O2 sensor wire--the ECU should see this as an O2 failure and revert to 50% duty cycle(if my memory is correct),but the engine will continue to run,and run well. Same with the coolant temp sensor--if the O2 sensor checks out,disconnect the wiring harness plug from the coolant temp sensor--same as with the O2 sensor,the engine should continue to run,cold or warm. Let me know what you find,ok?

04-12-2014, 07:38 AM
Thanks for the reply. I am going by my memory, but here goes. The engine runs well when cold, when it warms up it dies, slowly petering out. The single wire O2 sensor has been replaced. When my friend first got the car, it had been in a lot of hands and he had to straighten the throttle plate and readjust the mixture screw which, of course, had been messed with by someone(s) with NO idea what they were doing. My friend has mentioned that the coolant temp switch seems to act as a ground until it warms up and then turns on, perhaps he is looking at the wrong temp switch. Thank you. I will suggest to him to try, in turn, disconnecting the O2 sensor, and then the coolant temp sensor. He is quite experienced working on CIS in German cars and Volvos, but this has him stumped so far. He is suspecting the ECU. Are these a known point of failure ? The # is 0280800050. This is a low mileage car, I believe around 20K, I was wondering if there are fuel pump, or fuel system problem spots, for instance the fuel pickup screen or ...? Thank you once again, more as I know more.

04-12-2014, 09:58 AM
My friend has tried disconnecting the O2 sensor but still has the same problem, but he did say that the engine will only die if you let it idle. If you rev the motor it will stay running. I hadn't been told this before. He has disconnected the coolant sensor at the thermostat housing, still no joy. It turns out that the owner had the A/C removed and there are Hi and LO pressure switches that are not exposed to pressure. He is also aware that there is an engine run sensor, but believes that if this was at fault, it wouldn't run at all. I hope that knowledge is power and this is some help, Thanks Again,

Ps If you go to YouTube and enter the search term "Antiqueynot", there are two videos, one about a friend's '56 Nomad for which I merely did the video for YT, and another about an American Bantam I brought back to life after a long rest.

04-12-2014, 10:13 AM
Wow. My experience with CIS is like your friend's--30-40 year-old German cars. I'd like to know what he's done so far,because there are quite a few things to check out once you start down this road,and it's hard to know where to start when I don't know what he's already checked. It could indeed be the ECU,but good luck finding another to test/swap on this side of the Atlantic. How long does it run before it begins to shut down? Can he keep it running with the throttle pedal,or does it just starve and die no matter what?Has he checked that there's sufficient fuel pressure to open the 6 main injectors? He knows to check the warm-up regulator,too. The engine can start and run on the cold-start injector,but if the main injectors don't open,the engine will peter out and die once the cold-start injector drops offline. I'm sure he knows the fuel pump relay trick,too--jumping two terminals at the fuse box so that the fuel pump runs continuously. The relay can sometimes cut out when hot--bridging the terminals is an easy way to check the relay is good or not. Don't drive the car with the relay jumped,just use it for testing. I have no idea what the TVR fusebox looks like,but there is another relay,sometimes called the control relay--this can directly affect the ECU. On cars I've worked on,there's always a non-essential relay that is identical to the control relay--like the horn relay.So if the horn(or whatever non-essential circuit you find shares the same type of relay) works,swap that relay with the control relay and see if that's the problem. It may not be labelled "control relay" in the TVR,so you may have to use your powers of deduction to find it. If your friend wants a sounding board,he can write me directly and I'll give him my phone number--I already sent you my email address via PM. Oh yeah,there's probably also a wire running from the coil negative terminal to the CIS control relay--if that is corroded or has a break,the ECU won't send a signal to the freq valve--something else to consider. Check any grounds for the ECU. Check continuity of the wiring from the ECU to the freq valve--always check the simple stuff first,right? The freq valve not getting a signal is the one identified problem so far,so I'd make ****ed sure everything in those interconnected circuits is operating properly first,before looking elsewhere. I'll have to find my old CIS shop manual and take a good look at those circuits again;it's been a while,and so far I'm going off memory,so there's probably a few things I haven't thought of. Good luck to you both;CIS is simple,but I know it can still be really frustrating;if I can help at all,I'd be happy to,because I've been where you guys are,and it can drive you nuts!

04-14-2014, 10:39 AM
I had the same problem with my 280iFHC about ten years ago. Converted to a carburetor and I've been much happier since.........

04-14-2014, 10:25 PM
Which manifold did you use--something for a Ranger,Bronco,or Capri? I have a 4-bbl Offy intake for a '74 Capri,but it won't fit the later 2.8 heads. By the way,after a few emails,Tony and I kind of got a handle on the symptoms they're having with the TVR. His friend knows what he's doing with CIS;unfortunately the last person who had his hands on it did not. He's having to correct some serious damage to the airflow sensor(it was BENT somehow),idle mixture control screw,etc--sounds like a real mess. Last I heard he was going back to square one,checking fuel pressures,which is smart. I believe if he can get the airflow sensor to operate properly and idle mixture set correctly,he'll have a good chance of solving this problem.