86 928s ignition problem

[Sertan]

Hİ
I have problem with ignition sytstem.My car have dual distrübitör and Lh jetronic for fuel,EZF ecu for ignition.My problem is wrong ignition timing.I think EZF ecu control to ignition timing,I sent to ECU for test and they sad there is no problem(acm elektronik).Also I find used ecu,I tried it.Same result.Ignition timing is always changing.Sometimes engine run advance sometimes retarted.What can be problem?
86 928s 4.7 16 valve euro car 5 speed

[MarkRobinson]

Check your vacuum: 17-18" at idle (800-ish rpm). Your timing is controlled via rpm & load: load is determined by the engine's vacuum: there is a MAP sensor inside your EZF module, and a black vacuum line & 180d elbow at the top of this computer. Follow this line through your firewall, up to the main vacuum tree, & see if it's unplugged. You might want to check all your vacuum connections, as most tee off of one point, so if any have a leak, all will "leak" from that point.

[porken]

Check that the idle switch is working/connected. If it has a bad connection, or the throttle doesn't close all the time, the idle advance will be 20°, instead of the normal 10° (cold engine 13°).

[Sertan]

I checked vacuum line and throttle switch.There is no problem.Have you got any different idea?Thanks

[John Speake]

Hi
If you mean the ingtion timing is varying randomly, then the problem is that the EZ-F ECU is not synchronising correctly to the flywheel sensor signal.

Does this random timing happen at high rpm as well as low ?

The problem could be due to a faulty flywheel speed sensor, if (for example) the sensor negative lead connected to pin 19 is at ground potential instead of "floating" etc.

If you have a 'scope, check flywheel signal are present on pins 7 and pin 19 of the EZF ECU.

This is not an uncommon fault with EZF ECUs, so check that the tests were fully carried out, e.g. actually checked on a car, as well as a bench test.

[Sertan]

If I rev the engine until cut off(red line) ,then my problem is starting.After this occured car don't start.As if there is backfiring.If I cahange the entrance of distrubitor then car begin to start.But in this position main spark cables stay reversed.If I rev the engine again till 6300 rpm, cut the fuel than no start symptom comes again.If I put the main spark cables in the original position starts well.Finally,If I don't rev the engine high rpm instantly,I can drive the car a few days.But than,slowly slowly it starts to run with erratic idle.I can drive the car like that maximum one week.After it doesn't work again.If I change the ignition cables again (from ignition distrubitor,in the middle)car starts to work. 86 928s 4.7 16 valve 5 speed

[John Speake]

I think it is an EZF ECU problem. As the fault is somewhat errratic, then this may have mislead the tester of the ECU.

See if you can borrow another EZF

You ought also to check the flywheel sensor waveform, as shown in the workshop manual, and also check that the -ve side of the flywheel sensor is not shorting, or a low resistance to ground (chassis)

[Sertan]

I tried with another EZF same result,also I sent EZF to ACM elektronic for test.They sad EZF and LH ecu are ok.I also tested flywhell sensor residance.900 ohm.Have you got different idea?Thanks for quick reply.

[John Speake]

Hmm, OK. Have you checked the relative position of the two distributor rotors is correct ?

Also, check the condition of the belt, it's worth changing it as preventitive maintainance.

Disconnect the flywheel sensor at the plug on top of the engine, and measure resistance to engine block from each of the connections. Should be infinite.

900 Ohms total winding resistance is OK.

[Sertan]

Hi John
There is 3 pin on sensor cable plug.First one infinite,second one (in the middle)171 ohms,3. is info ohm.(no contact).Also I check again distribitor position.They stay correct position.

[Sertan]

Should the timing mark ring gear moving(on flywheel)?One months ago I burned clutchs than I changed pressure plate and clutchs.

[Sertan]

I find the problem.Timing mark ring gear moving itself.(on flywheel ).It was not firm. it is always changing slowly slowly.

[MarkRobinson]

wow. 6 years ago I had that problem: I used vice-grips to secure the flywheel while I did a t-belt change (removed crank bolt when clutch was removed), & spun the ring. have someone tack-weld it once you find it's happy place.

My friend has a '86 flywheel sitting around in his basement: if you want, I can put you in-touch with him so he can guide you to zero-ing it in before you apply a tack-weld.

Mark

[John Speake]

Glad you found the problem !!

[MarkRobinson]

I logged at least 100hours back 6 years ago diag. my problem with my old black car...sucked big time: had to remove my exhaust/long-tube headers multiple times trying to get it right (which involved unbolting the engine/jacking it up to remove the headers & removing the clutch of course

[Sertan]

Thanks for all answers.I fixed the car.It is running very well.