Chasing a slightly lumpy idle...still

[Laird Scooby]

Originally Posted by GreenBrick

Yeah the donor (Wira), with good compression, was a recently MOTd car that I bought for the engine/drive train and interior, after I got the blown one (Jumbuck) home.

The blown engine had done 45K and the donor had done 35K
I will see what changing the o2 sensor does next.
Just hope my Jeep (blown HG also) still has a good o2 sensor (it just passed an MOT so fingers crossed).

Both Proton cars are 14yrs old, but low mileage.

Be careful undoing the donor Lambda sensor, they have a tendency to get stuck because of the heat and just the shock of the threads releasing can cause them to fail, either instantly or very soon after - it may be more cost effective to simply renew the sensor and have done.

Likewise with the Jeep - what's the mileage on the Jeep by the way?

[AllHailKingVolvo]

I had a bit of a revelation over the weekend, which I think may provide some clues to sorting the last bit of unnevenness out...

Yesterday daytime it was hot again down here in Dorset. The first properly hot day in a couple of weeks. I had a wedding gig in the afternoon and drove there at the peak of the day’s heat. I noticed that a little bit of the hesitation under power had returned, especially when pushing the engine hard (ie up hills, fully laden with PA system etc).

However, by 11pm when I left, the night was significantly colder (bordering on chilly) and all hesitation was gone-the car was absolutely perfect, smooth and grunty.

So, I deduced that the lower the ambient temperature, the better the car runs. I know this isn’t uncommon (having raced piston port two stroke karts for many years, i know that the winter always brings more power) but it would seem that the unevenness issue is due to something on the air intake side of things...it was greatly improved by cleaning the throttle body, but when i changed the injectors I noticed that there was a lot of black gunk in the intake manifold...I’m now thinking that I need to whip off the manifold and give it a thorough de-coking, and clean the intake ports on the side of the block.

Am I barking up the right tree here, do you think? I figure if there’s a lot of gunk and gak in there, it’s going to disturb the flow of air and compromise atomisation etc.

My other suspicion is the outside chance of the MAF reacting badly to hot air and giving wonky readings. I’m pretty sure the temp sensor is fine as the gauge reacts as expected, and disconnecting it has no effect on the running of the engine.

[Laird Scooby]

That's a major clue Phil!

Have a look at this pretty picture :



Look at items #11 and #13 - they fit onto a T-shaped valve, the "leg" of which takes air into the air cleaner.

On mine, the heated side is smalller than the cold side, on a previous 740, they were the same size and some clever idiot had fitted it the wrong way round so when i hoofed it, i was getting hot air coming in and bad driveability when cold because it was pulling cold air - turned it round the right way and problem solved!

However, that doesn't solve your problem which i suspect is actually a very common problem. The thermostatic valves inside the T-piece fail, only allowing hot air into the air cleaner from the exhaust. It could be that yours is round the wrong way as well or perhaps just seized in position. It would account for the ingress of black gunk into the inlet tracts though as it would be drawing hot air over the bottom of the engine where oil and dust are prevalent, although blocked breathers will also put oil into the inlet manifold.

I'd try removing the T-piece or at least the hot air trunking from it and see if things improve. If so, you'll know you're in the right area.

[KBB]

If you have got a high miler then excess carbon build up on the intake and in particular on the back of the inlet valves can cause poor idle conditions. Not so much that air flow is disrupted but that carbon is very porous and absorbs fuel (that's why evap canisters have carbon in them). This was a well known issue with early Vectras, the fuel from the injector would get absorbed by the carboned up inlet valves creating a lean condition at idle, they were often incorrectly diagnosed leading to the unnecessary changing of MAFs, coils plugs and other sensors, a TSB was issued by Vauxhall. The new Mini was another sufferer of this problem which led BMW to designe an on-board inlet valve cleaning process using adapted sandblasting with walnut shells. Other OEMs are now adopting this cleaning process, there is a lot of industry talk of considering this process and off engine injector cleaning as a service requirement at 50k mileage as this goes quite some way in improving economy and emissions on older vehicles.

[Laird Scooby]

Originally Posted by KBB

If you have got a high miler then excess carbon build up on the intake and in particular on the back of the inlet valves can cause poor idle conditions. Not so much that air flow is disrupted but that carbon is very porous and absorbs fuel (that's why evap canisters have carbon in them). This was a well known issue with early Vectras, the fuel from the injector would get absorbed by the carboned up inlet valves creating a lean condition at idle, they were often incorrectly diagnosed leading to the unnecessary changing of MAFs, coils plugs and other sensors, a TSB was issued by Vauxhall. The new Mini was another sufferer of this problem which led BMW to designe an on-board inlet valve cleaning process using adapted sandblasting with walnut shells. Other OEMs are now adopting this cleaning process, there is a lot of industry talk of considering this process and off engine injector cleaning as a service requirement at 50k mileage as this goes quite some way in improving economy and emissions on older vehicles.

Very interesting post that - i wonder how relevant it is to the "good ol' redblock" though as i've had 3 high-milers (>200k miles) all of which had the MoT guy looking for the cat as they ran so cleanly and even my current B280E had him looking for the cat on the first MoT - for the second i had carried out a lot of remedial work and set things back as Mr Volvo intended and the CO was bang on 1% as per the spec for the car.

Also the newer cars you mention all run a bit warmer than most of the older stuff because they run lean, i wonder how much that has to do with the carbon build-up on the back of the valve heads?

[AllHailKingVolvo]

Really interesting stuff, thanks gents!!

I pulled off the hot air trunking, and lo and behold, the car is indeed hoovering air in from the hot air part of the T-piece when running at full operating temp...I left the trunking disconnected and took it for a run, and it definitely seems smoother. Next up I’ll have the intake manifold off for a thorough clean and a new gasket...one step closer!

[AllHailKingVolvo]

Looking online, it seems a new thermostatically controlled T-piece is rather hard to come by. I wonder whether Removing the airbox entirely and replacing it with a K&N style cone filter would be the way to go to tidy it up?

[GreenBrick]

Originally Posted by Laird Scooby

Be careful undoing the donor Lambda sensor, they have a tendency to get stuck because of the heat and just the shock of the threads releasing can cause them to fail, either instantly or very soon after - it may be more cost effective to simply renew the sensor and have done.

Likewise with the Jeep - what's the mileage on the Jeep by the way?

The donor's o2 came out easily, so I am optimistic about this one!

The Jeep has done 130K, don't how much of that was on gas, but the conversion looks a good few years old.

[AndyV7o]

I wouldnt go sports filter, yeah k&n are very good, but any higher flowing filter lets more particles through, which in turn can cause more wear and residue regarding valves/inlet/bores/combustion chamber etc. Not a problem for motorsport and highly tuned cars as they need frequent rebuilds anyway.
Maybe try and make your own temp valve in some fashion, or recreate days of yore by having some kind of winter/summer diversion etc??

[Laird Scooby]

Originally Posted by AllHailKingVolvo

Looking online, it seems a new thermostatically controlled T-piece is rather hard to come by. I wonder whether Removing the airbox entirely and replacing it with a K&N style cone filter would be the way to go to tidy it up?

What Andy said Phil! ^^^^^

It would be nice to have a K&N cone air filter but after a lot of thought about my own air filter housing, i came to the conclusion Volvo designed it that way to restrict the flow at high revs/load deliberately.

Like Andy suggests, it should be easy enough to create your own diverter, even if you just drill a hole in the flap and thread a bit of strimmer line through it so you can pull it remotely and then lassoo the top of the shock absorber with the other end or some other convenient point.

Originally Posted by GreenBrick

The donor's o2 came out easily, so I am optimistic about this one!

The Jeep has done 130K, don't how much of that was on gas, but the conversion looks a good few years old.

It's 30k overdue a new Lambda sensor by the Jeep service schedule then! In the USA, the have an "Emissions Timer" and when the car gets to 10 years old or 100k miles, it puts a fault on the system to say the Lambda sensor needs renewing.