slate blue

Guys my second 928 a late 84 year, model 85 with a 4.7 liter engine running the LH 2.2 has a rough idle.

I put the timing light on it and found that it is jumping all over the place. From about 5 degrees to 20 degrees. I disconected the vacuum line and observed no difference in its behaviour. It's late in the evening here so I wont do anything till morning but has anybody had this symptom? I have previously changed the computers and the MAF, in regard to the MAFs they are new on both my cars. So should I be looking at the induction sensor with suspicsion? If you can shed light on this puzzle great.

slate blue

O.K just had the Impulse flywheel sensor changed for a brand new one, at first the car runs nicely, admitadly the timing is at 5 degrees where the book says 10 degrees plus or minus 3. The thing with this is that my other car also runs the same timing. Go figure! So it runs well and then after a minute it starts to get the wobbles. The timing starts to go all over the place. I will have the timing computer changed over tomorrow, if that doesn't work I will change the temp 11 sensor.

Any suggestions?

Greg

Vlocity

Is the timing just bouncing around because the idle is not stable? Can you increase the idle to a stable point and then get the timing numbers?

Ken

Lizard928

how much of a idle leap do you see?

slate blue

I have to say I don't know, the idle is stable at first but then when you notice the idle rough the timing is everywhere. What comes first the chicken or the egg? As far as the timing jumping around what you have to remember is that the engine speed is quite low so why would the timing go to 20 degrees advanced? That is with the vacuum line disconected. It does it also with the line connected.

I am now off to change the computer, not that I'm thinking that will help.

Greg

PorKen

I think maybe the idle speed is too low. Have you tried increasing it at the TB? The EZF will increase the advance, briefly, if the rpms drop too low.

If the idle switch is not working or disconnected, the idle will be 20°, instead of the normal 10° (both +3° engine cold). The idle will stumble and drop out, and it will be hard starting with that much advance at idle.

Side note: with 32V LH2.2, if you jumper B->C on the test port, when setting the idle, the advance is ~6°. (B is the full load switch input to the LH, C is ground.)

Mrmerlin

are the spark plug wires in good condition and did you use the right plug? Is the dist cap new? did you replace the TPS while the manifold was off? are the wires that go to the TPS in good condition?

slate blue

By Mr MerlinThe spark wires have been checked and swapped over with my other car, no difference. I have checked the TPS with my multimeter and found it to be working, in fact I have had the car hooked up to a diagnostic tool and it found no faults. I would add at that stage I didn't know if the timing was jumping all over the place.

WallyP

I'm with Ken - check the idle switch on the throttle.

slate blue

Wally P and Ken, you're right on the money. Changed the switch and the problems have gone away. So much for the diagnostic tools, I can't explain why that missed it twice, sent me on a wild goose chase, well at least all the hoses got changed because of it. Learnt something too, that is the best positive spin I can put on it, it has been a right royal pain.

Thanks guys!

Greg