Mondial8 running rich & fluctuating tach

I can think of a few things that could cause this, but one (which is easy to test) would be the operation of the cold-start injector. It should only be allowing fuel to flow during cold starts, but either an electrical fault (e.g. thermo-time switch) or a mechanical fault (injector stuck open) could be an issue. I have to dash off and I can't describe the procedure right now, but there should be info on testing this on F-chat or perhaps a German car forum.

I would steer clear of any adjustments and just test components in isolation at this point. A set of pressure gauges for Bosch CIS is extremely helpful, and pretty much required, for most fuel system work. Also get a copy of Probst's book on Bosch fuel injection... will pay for itself the first time you use it.

Not sure about the tach fluctuation on revving. Is the problem constrained to the tach indication, or can you sense the actual RPM fluctuating? If its the former, it may not be related to the over-rich situation... if the latter, then maybe you're getting periodic partial or no combustion on some cylinder(s) due to the rich mixture during acceleration.

The backfire on shutdown is consistent with unburnt fuel from an over-rich mixture passing into the exhaust where it is subsequently ignited.

Also, this could be ignition related. Might be good to check for consistent spark on each plug, etc.

 

Just for completeness: Are there other symptoms of rich running? How does the engine run and drive?

More thoughts:

Losing ign. on one bank would allow loads of fuel into the exhaust system.
Does it have an air pump... excess air into exhaust? Exhaust system tight?
Exhaust valve clearance or (gasp) seat erosion/damage?

I would try to confirm rich running with an exhaust gas analyser, notwithstanding any other obvious symptoms.

 

Let me know what you find out about the Tach fluctuations. I have the exact same problem with my QV. Disconnected the CSI injector as well, some improvement. Did the test for the vacuum valve under the coolant tank and my valve did not close at all. Went to full warm-up and still pulling strong vacuum. Taped both ends up and it made a world of difference in the idle, and strong rich smell at idle seemed to go away, and no POP when turning car off.
Can any one tell me what effect this vacuum valve might have on the running of the car when it will not close? To rich perhaps being it is constantly drawing vacuum/air from the intake pipe?

Thanks!
Mark

 

Well, at least you know it is spark and not fuel.

Try swapping ignition modules to see if the problem switches sides. If it does, you have the answer.

I would think it unlikely the wires on one side are all bad at the same time.
Regards,
Jerry

 

There is a wire from each coil to each digiplex, check the continuity. That wire fires the coil for each bank.

 

I assume that the rear bank was still firing when switching the digiplex units. If so, that's checked.

There is a red and yellow wire going into in each coil. Check if either of the two (cannot remember which one) has 12volt on it with the ignition on (don't start). If the affected side has not it means it doesn't get voltage. Make sure the black connectors on top are not wet or oxidated.

If this still turns out to be ok, swap spark wires from front to rear. Measuring wires only means that they have a connection, but still could have a severe crack. I guess I would begin with the one from the coil to the distributor, since all four are not firing.

 

If you swapped the digiplex and had no change then one constant is the trigger wire from the digiplex to the coil, check the continuity as that wire fires the coil. The other constant is the voltage supply wire to the digiplex. Check the input voltage to both digiplex plugs.

 

One other constant is the wires from the flywheel sensor to the digiplex. Might want to check the continuity there also. The two ends are at the digiplex connector and the connector by the sensor. Not sure what reading across the two wires you should get but the two sensors should read the same. The wires themselves from the connector to the digiplex should be solid wire on all four wires.

My thoughts are if the coil does not fire and you swapped all other items then some connection for the one digiplex unit is bad somewhere.

 

Ok that's a different symptom. I was thinking you had no spark. If the spark is the same on both banks, check that 1st by testing a spark plug on each side while cranking with the fuel shut off. I shut the fuel off by removing the two relays (I have 2v 308i). The tach is driven off of only one digiplex. Not sure if it is the same as the bank you having troubles on.

 

There are three trigger sensors around the ring of the flywheel. One for each bank, and one for the tach. They sensors are small coils which are triggered by the passing of a node on the flywheel, and determine the firing times, and the tach pulse seen on the instrument.

I would venture that you have a problem with one or more of those ignition or tach signals. They fail by heat opening the windings on the coil. They are swappable if you want to move them around to test.

The actual ignition wires should have little/no resistance. The plug extenders should be about 1k ohm as ballast. If you measure from the inside of the cap, to the end of the plug extender, you'll get about 1.1k ohms. If you get more than 1.2k ohm, investigate the wire or the extender.

 

Just to confirm.
Are you saying that the faulty bank has no spark at idle but kicks in at higher RPM?