Wobbly idle

[BicycleBoy]

I've been using a box of general purpose nitrile O-rings for years:
https://www.ebay.co.uk/itm/134114158713
..not on K-jet injectors though.

If OEM seals are failing then I'd look at the state of the alloy bores in the head, lest something be ripping or distorting the o rings.
Otherwise I'd reassemble with healthy lashings of red rubber grease to make sure.

[Othen]

Originally Posted by BicycleBoy

I've been using a box of general purpose nitrile O-rings for years:
https://www.ebay.co.uk/itm/134114158713
..not on K-jet injectors though.

If OEM seals are failing then I'd look at the state of the alloy bores in the head, lest something be ripping or distorting the o rings.
Otherwise I'd reassemble with healthy lashings of red rubber grease to make sure.

I'd tend to agree with BB here Juular, if it was me I'd find some slightly larger nitrile O rings for the leaky injectors.

[Juular]

I have a pretty good O-ring kit here which has a lot of in-between sizes. I've tried a few on the injector holders but still haven't been able to get it to seal properly. It is quite odd.

The seats in the head are clean and smooth, I've been over them with a scotchbrite pad to take off all the soot and residue.

I haven't been able to get to the dealer to pick up new seals, but I am thinking of RTVing them in this evening, if nothing else to see if them being fully sealed fixes my idle problem or if I have bigger issues. I can always go back and deal with the seal problem properly after that.

[Juular]

An update to this.

I have replaced the injector seals with a full brand new genuine set from the local Volvo dealer. Since it was only £15 for the full set it was cheaper than getting them online.

This sealed the injectors perfectly and there's no vacuum leaks there now. Clearly the aftermarket ones are total junk and possibly the 'genuine' set I received off ebay were fakes - it wouldn't be the first time.

I also replaced the intake manifold gasket as I had a spare.

I now can't find any vacuum leaks, but it's still running like a bag of junk, in fact possibly worse than it was before.

So to recap:

Fuel system - Passes all tests: Pump flow, system pressure, control pressure, accumulator pressure hold, injector flow, injector spray pattern. I'm 100% sure it's not fuel.

Ignition system - Everything except the distributor body and hall sensor has been replaced with known working items, of which I have since tested every single part with a meter just to make sure. I'm also thinking that since the engine is smooth as anything at higher revs and has loads of power, it's not ignition.

Air - New throttle body gasket, intake manifold gasket. Air sock checked for tightness. Injector seals done, Vac lines to servo, distributor and PCV checked or replaced.

Valve clearances - I just checked them and a few are a bit off. Not massively though, and I doubt this would cause a random misfire that only occurs at idle?

It's got to still be a vac leak I think?

[BicycleBoy]

100% sure on fuel?

Leaky cold start injector means rich running and crappy idle.

A sticky airflow plate, or one out of centre could also cause rich running...?

¯\_(ツ)_/¯

[Juular]

Originally Posted by BicycleBoy

100% sure on fuel?

Leaky cold start injector means rich running and crappy idle.

A sticky airflow plate, or one out of centre could also cause rich running...?

¯\_(ツ)_/¯

Yeah, the cold start injector has been tested by running the system with it pointing into a jar, and it's not leaking. I don't actually have it plugged in as I find it starts better without it the majority of the time. It only really kicks in in proper cold weather.

The airflow plate is free moving, as is the plunger inside the metering head barrel. The lumpiness / misfire disappears if you give it some revs.

I'm pretty sure it's not a mixture issue, as adjusting the CO screw raises or lowers the idle but has no effect on the lumpiness / misfire.

I rechecked the valve clearances last night. My other half did them last week but I think she was getting the feeler stuck on the buckets rather than the actual shims. Turns out they're all bang on, so it's not a compression issue.

I'm wondering about testing the hall sensor on the distributor with an oscilloscope.

[SalvadorP]

I have a similar issue on my 940. Idle swings between 800 and 900, looking like a rythmic misfire.

I have done this:
Compression test (good results)
Replaced plugs
Replaced leads
Sound checked injectors, they are firing properly.
Cleaned Distributor
Cleaned and tested idle valve
Cleaned and tested MAF
Tested for leak with carb cleaner, couldn't find any
Smoke tested for leak, couldn't find any
Replaced all vaccum hoses
Tested for vaccum leak with a leak gauge (there was no vaccum leak neither before or after replacing hoses)

I'm waiting on parts to refurbish the injectors. If Injectors are good and spray partern is fine... Well, then I guess I'm finally using that one grenade my pops brought back from the colonial war.

jk, if it's not injectors I have to assume it's MAF, O2 sensor or less likely idle valve. I would rule out the distributor because the injectors are firing steadily.
Hope this helps. I've been chasing that bad idle for 2 years now...