DDK - Die Deutschen Klassiker

I've been fairly thorough by my standards in trying to fix this - does anyone have further suggestions where to go next?

SYMPTOMS / SIGNS:
Carrera 3.2 Emissions fail, CO reading is circa 5.5% should be around 1% I think. MOT limit is 3.5% or 4.5%. The car sometimes idles fine and other times fluctuates up and down wildly.

FURTHER INVESTIGATION AND FIXES:
-Car runs and drives beautifully apart from rough idle. Fuel economy good. Burning a little oil.
-Car apparently has oxygen sensor deleted
-I thought inlet manifold gasket was leaking, so the manifolds have all been off and the gaskets replaced. I'm confident it's all sealed nicely now, but the problem remains.
-No apparent vacuum leaks
-Cylinder head temp sensor apparently working fine: 250 ohms on hot engine, climbs as engine cools
-Air temperature sensor apprently working fine - 1,400 ohms on a hot day
-AFM potentiometer within acceptable range - 250mv closed, 4v wide open
-Idle valve apparently working: I removed it and plugged up its vacuum hoses, leaving the wires connected. Raised and lowered engine idle using the throttle bypass screw and the valve could be seen to open and close as it should with rising and falling revs. Also, it responds correctly to 12V on each of its positive terminals.
-HOWEVER: If I just disconnect the cable to the idle valve, the engine stops hunting. It idles perfectly. Reconnect it and the engine hunts wildly.

Next I moved on to the idle mixture screw on the AFM. As I began to turn it I realised it was so much unscrewed it was no longer in the threaded part of its tube and it was falling out. Presumably somebody has been in before me and tried to lean the mix for the same reason as me, and gone too far. The manual talks about some special tool to turn this but on mine it just takes a standard allen key. Is this because I have a simplified air filter system?

I have been reading about others with similar problems and threads point to: a) take apart the AFM. I took it off but can't see where to start disassembling it, b) leaking FPR - replace if leaking and c) bent fuel return line. Any further suggestions? How does the AFM come apart? Can I test and/or service the FPR? Where does the fuel return line run? There are a lot of fuel lines!

Also, this is the idle mixture screw and its home is just a hollow tube with nothing else in it - is that right?
https://photos.app.goo.gl/QnzVpahHBcF8i8EJ9

So after a fairly rigorous work through the possible causes I have found the fuel quality adjustment switch under the passenger seat had bee nset to position 6 instead of the stock position 0. This increases fuel delivery by 6% as well as retarding the timing. Having returned it to 0 and re-set the idle speed, the idling seems to be much more stable. I'll follow up with MOT test result and some calcs on how 6% mix should increase the CO emissions to confirm whether this is responsible for the high emissions.

I should also say thanks to Mike Bainbridge for the DME Test Plan without which this exercise would have been impossible.

Nice work.

The only question now is why someone changed it. The risk is that it was to cure lean running and that putting it back melts the engine when it’s hot under load...

If so I guess you should be able to hear pinking and see the temp rise before its terminal.

Sam wrote:Nice work.

The only question now is why someone changed it. The risk is that it was to cure lean running and that putting it back melts the engine when it’s hot under load...

If so I guess you should be able to hear pinking and see the temp rise before its terminal.

Thanks. I'll keep an eye out. The idle mixture screw was so far out that it was out of its thread and had fallen an inch - so perhaps it was in response to a lean idle. Also, a vacuum hose was off the plenum which I guess might have caused a lean running condition which was being compensated for. Alternatively, perhaps somebody was trying to eek out some extra performance on the track. Finally, Mike Bainbridge told me some people twizzle the cog inside the AFM to get some extra horsepower. Perhaps that has been done here and it was compensating for that. I doubt it was the previous owner though because somebody has been in trying to fix this before me and failed - new idle control valve, new cylinder head heat sensor, I can see where somebody's nicked into the insulation on one of the AFM leads to get a multimeter on there before me too.

Yeah. When these cars were cheap enough to make into race cars or track day hacks you’d just run them as rich as possible for safety as much as performance.

Sam wrote:Yeah. When these cars were cheap enough to make into race cars or track day hacks you’d just run them as rich as possible for safety as much as performance.

Hey, I just saw you have a 70% built house just like me! You're not at Cambridge Uni are you?

No, I’m 42.

I guess most students here are probably wealthy enough to build a house, but it took me a bit longer to save up!

Sam wrote:No, I’m 42.

I guess most students here are probably wealthy enough to build a house, but it took me a bit longer to save up!

But he looks 52, saving up that money took its toll…


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42 my arse

I think you’ll find I’ve shaved off my beard AND dyed my hair this week so I pretty much look like I did when I was a student.

It’s a kind of Keifer Sutherland in Lost Boys x Riz Amhed in The Sound of Metal vibe. Only even moar sexeh.

Love DDK, technical info and humour in the same post

Just updating this old post... I ran the car with a bit of a high idle and that fixed the rough idle problem. There has also been an MOT since this and although the emissions were a little lower, they were still high. I had to deploy a bacon butty. But later investigations ruled out a lot of things and I found the car has no O2 sensor. Replacing that stabilised the idle considerably, so we'll see at the next MOT how it gets on. I read that without O2 sensor, Motronic enriches the mixture slightly.

Where is the 02 sensor on a 3.2? Mine has definitely never had one, its an 88 also, is yours a US car?

Mine usually needs a bit of MOT trickery to get the emission correct. More so these days as it's getting on a bit.

As you look into the engine bay there's a 3-pin round connector on the left below the fuel filter. Mine had a plug in it with a loop of wire. The plug comes out and the electrical connector goes in. Below that there's a round hole hole in the engine tinware which the wire passes through. The sensor wire has an integral grommet. Below that there's probably a socket on your exhaust where the O2 sensor end goes in. The genuine Bosch one is about £70 https://www.ebay.co.uk/itm/334918664887 versus a lot more for the Porsche one - probably the same part.

P.S. Mine is a UK car.

My 73T has a 1986 3.2 in it, sis headers etc and wired by Paul Broke at Bob Watson place in 1994.
That round plug is there in the hybrid loom but no sensors etc. On the engine or connected or bypassed.
Has run perfectly since installation.