I have had a problem with my ’84 733 (automatic) it has not run properly for well over a year.
I have posted on every board I can find and e-mailed everyone I know. I have tried and tested a long list of possible causes and remedies and have zero results.
Here is a brief summary of what happened:
I have owned the car for many years. It has approximately 129 k miles and the engine has always run well. I was driving in town. The engine was warmed up and the car was driving and idling fine.
I stopped at a stop light. Just after the light the lane merges with another lane.
I accelerated hard to merge, probably in 2nd gear. The next light changed so I lifted off immediately. Sudden acceleration followed by sudden deceleration.
As I stopped at the light I noticed the idle extremely rough, the engine almost stalled. I managed to limp home. The symptoms are very rough idle, running way too rich, and will foul the plugs at idle.
The engine will start and it is possible to speed up the engine with no load. It sounds OK at about 2000 – 2200 RPM, smooth and spark plugs are clean. But I can’t speed up or slow down without it running very rough. Misfires and sometimes spits back through the intake.
1. Is it possible that the timing chain has jumped a tooth and my valve timing is incorrect?
Compression test is OK on all 6 cylinders.
2. Could the symptoms above be caused by incorrect valve timing?
3. Has anyone had this happen to a big 6?
I have owned several with a lot more miles and driven a lot harder than this one and never had a problem like this.
4. How can I check the valve timing?
I am most appreciative of all the help and advice from the forums but please note I have tried, tested and diagnosed a long list of possible causes for this problem and published most of the information on this and other forums but so far I have had zero results. What I am trying to do is establish if valve timing is the cause of my problem.
Thanks, John aka Kenneth
1984 733 129k
1991 735 IL 190K
Highly doubt you skipped a tooth on the timing chain, pretty hard to do on a stop light to stoplight sprint. I'd check some basics first, unplug CTS on t-stat housing, running the same? Check for resistance accross the CTS terminals. Pull valve cover, (while there, check banjo bolts on sprayer bar) look torwards the front, any broken plastic pieces in the valve train (front?), look down into the timing chain area with a light, tensioner/guide intact? Dark Brown or off white?
A non op CTS will make the car run JUST as you're describing, don't panic yet.
I'm thinking it's not a jumped chain either. I've seen M30s with a cracked chain guide and loose oil pump drive that were still in time.
If it's running rich, I'm with kojo...I'd suspect the CTS immediately. Test it for resistance or a broken resistor inside the boot. With the 2000-2500 RPM smoothness, I'd also want to test the AFM & harness. Less likely but possible causes are the fuel pressure regulator, dead O2 sensor, bad distributor or coil wire, stuck cold start injector, something like that.
Thanks Kojo & DtD, the CTS is the middle one of the three and controls the cold start valve correct?
I have tested the switch and also swapped it out for another. Makes no difference, hot or cold or either switch.
The cold start valve is disconnected. High pressure fuel line to the valve is also disconnected and attached to my fuel pressure gauge so there's no way it can be the cold start valve.
Fuel pressure is normal so it's not the regulator. Return is clear.
Does the CTS also control fuel delivery to the other injectors? I don't think so.
All plugs equally black and wet at idle so probably not a stuck injector?
O2 sensor doesn't make any difference to a cold engine or at idle?
AFM was replaced years ago by me but doesn't have many miles. Appears to be as new and tests perfect.
If it was distributor or HT wires, why does it run OK at some RPMs?
Wiring and connections on a 33 year old car though is anyone's guess, maybe between the AFM and ECU or something as it sure seems like the fuel delivery is only right at 2000 to 2500 RPM and no load.
It also seems strange that this happened in a split second. Running great then running terrible.
But consensus is that it's not the valve timing also, I think the compression test confirms that.
Appreciate the advice, please let me know if you think I'm off track on any of this or if you have any other ideas. Thanks, John aka Kenneth
How's your distributor cap and rotor look inside?
CTS is coolant temp, and is not associated with the cold start valve (which is controlled by the thermo time switch).
The CTS is the #1 thing to affect mixture, followed by the AFM and O2 (but of course only after warm up).
If the FPR has tested good UNDER LOAD- idle measurements are useless-then it has to be the CTS or AFM.
You will have to test for continuity of the wires from the CTS to the ECU plug, and I Think the possibility that a resistor installed between the CTS and ECU burnt out or other wiring condition failed is high.
--Jay3->Sic gorgiamus allos subjectatos nunc
84 €735i manual, 88 528e/i
Motronic 1 and 1.3 wiring
Transmission & Diff gearing sheet
This was part of a smooth idle campaign by BMW and I don't know if it was for this early an M30. The dealer installed a resistor in line with the CTS wiring, usually under the boot at the connector to fool the ECU into thinking the car was a bit cooler than it was and thus providing a bit more fuel outside of the oxygen sensor area, specifically idle for this use. If the solder joint has broken or otherwise the connection is bad, the car gets ∞ for the CTS and causes high fuel, just like the CTS were disconnected. Great cold start, idle creeps up to 1200 and stays about there and the car will not warm start, too much fuel.
I've definitely seen a resistor on B32 cars, just not sure if it was part of the dealer campaign or an earlier mechanics work.
--Jay3->Sic gorgiamus allos subjectatos nunc
84 €735i manual, 88 528e/i
Motronic 1 and 1.3 wiring
Transmission & Diff gearing sheet
Did you ever inspect your crank sensors for visible cracks?
One of the top connections was and still is in sad shape, the plastic gets old and crumbles but the connections still contact. Possible cause of the problem of course although I think the usual symptom of a bad sensor or connection is that it won't run at all? I have another pair of connections in good condition to try when the weather improves, unfortunately the car is outside.
Also, responding to DeskTopDave and Jay535is:
I got my sensors confused. CTS was replaced with a new one from BavAuto. All the original sensors checked out per Bentley.
I have an "off the wall" question though: If the resistance of the sensor varies with temperature, there should be a difference when the car is running between an open circuit with the connector disconnected and a closed circuit with the connector jumped??? will I damage the ECU or anything if I try this??
Also, with an old car those thin brown wires could have a break somewhere. I am going to have to go through all the continuity checks in Bentley.
Finally, comment regarding the dealer added resistor, I have seen this before but I don't think it made any noticeable difference to driveability, now if the soldered connection failed that could cause a problem like this?
Any and all help is appreciated, sorry I don't check the board and respond more often, John aka Kenneth
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