MC2 runs lean and misfires

[little_e]

Early this summer my 1990 Audi 100 TQ Sports started to misfire when I first started it in the morning. I had to rev it at about 2500-3000 rpm for like 10 seconds before it fired on all cylinders. At the same time it started to misfire occasionally when the turbo was boosting.
The latter problem got worse to a point where the car is now undriveable. It even misfires before the turbo kicks in.
The spark plugs were checked, and the car runs very lean; they're white as snow. I've also checked all kinds of hoses,
I can't find any leaks anywhere. I've checked voltage at the fuel pump, that's 13,64V. No errors in the fault memory.
It boosts 0,85 bar of turboboost.

All sensors, hoses, injectors, gaskets/seals, ignition parts are new, new spark plugs (Bosch WR6DP),
new Bosch 040 fuel pump.

I've checked pressures in the fuel system, and all pressures are within spesifications.

Before the engine started to misfire I hadn't touched anything. What could be wrong? :/

[BIGSTEV]

First in German:

Servus!

Wenn ich das richtig verstanden habe hast du ein weisses zündkerzenbild. Also schonmal schlecht. Fehlzündungen beim start und mittlerweile auch die ganze zeit.

Hast schonmal gecheckt ob öl im wasser ist? bzw. anderst herum??? Sind alle zündkerzen weiss? Evtl mengenteiler verstellt? Wenn nur eine kerze weiss ist könnte dort die kopfdichtung doch kaputt sein!?


Now in English:

Hello,

Have you checked your cooling system? Is there oil in? Or ist coolingfluid in the oil of the engine? Are all spark plugs white or only one? If all plugs are white, so your flow divider have to be adjusted!? Check your duty cycle with a multi-meter. If only one plug is white, probably your zylinder head gasket is damaged.

I hope that you understand my bad english.

best regards

BIGSTEV

[little_e]

No oil in the coolant. Coolant is red and clear. Nor is there any coolant/water in the oil. And yes, all the spark plugs are the same shade of white.
The reason I don't want to adjust the Co% (mixture) is because the spark plugs where initially the correct colour. The car just started to run lean all by itself
I didn't adjust anything, change anything...

[BIGSTEV]

1. have you checked your "michelin"-hose from downside? My own Audi had a shrewd michelin-hose. so i had nearly the same problems. the hose teared at full turboboost because of it´s old age.

2. Only check your CO-mixure at the 2 pins with your multimeter.

3. (but not often) maybe the lever from the air sensor plate to the pin of your flow divider doesn´t moves easily up and down!?

best regards

[little_e]

1. The Michelin-hose is a new silicone-hose, so that's ruled out

2. I've never really understood how to check the Co%-mixture with a multimeter using the two pins. However, I turned the mixture screw half a turn clockwise to enrichen the mixture, and this
didn't make the car more drivable at all. To turn the mixture screw half a turn is really a great deal and if everything is in fully working order, this should make the mixture get really rich.

3. What you're thinking of is that that the little control plunger inside the fuel metering device might stick, thus making it hard for the lever attached to the air metering plate to go up and down?

What about the frequency valve? Could that be buggered?

[ben.seifert]

Hi!

Have you also checked your Crankcase ventilation hoses? Charge-air-cooler leaking? What happens, if you pull out your oil-measure-pin? does it drop with the engine-speed for a short time? When you open the oil-refill, does it stop immediately?

The measuring of the mixture-regulation is not that complicated, but you need a multimeter with duty-cycle function [%]. (In german: "Tastverhältnis").
Actually this is just the activation of the N7 valve for Lambda regulating.

There is a little white 2-contact-plug hanging somewhere near the Idle-regulation valve. The duty-cycle can be measured between these 2 contacts. if you connect + and - wrong, you will get the realtive "off time", not the "on-time", i.e. 60% instead of 40% or 20% instead of 80%. If your fuel-cutoff for highspeed idling (Schubabschaltung, don't know the correct english word now) is active, you have a constant 40%. so bring it up to 3000 rpm, release the throttle, and you schould measure around 40%, if 60%, change +and -.

Best regards, Ben

[little_e]

I can't find anything wrong with my crankcase ventilation hoses.
If I pull the oil level stick out the engine speed drops for a very short time. If I open the oil refill lid the engine stops immediately.

I'm trying to get hold of a multimeter with duty-cycle function, but no shop in the vincinity seems to have one, so I've put that on hold
for the moment.

I've also tried to run the car without the oxygen sensor (lambda sensor) connected, but the car runs just the same. No difference.
I also ohmed out the temperature sensor for the ECU (it's fairly new), and got 89 ohms. That's within spesifications.

If the intercooler was defective shouldn't the engine be running rich, and shouldn't I also have problems with drops in turbo pressure?

[little_e]

For the record, I measured fuel delivery from the pump today; about 850-900 ml / 30 seconds. Should be more than enough.

[serdar187]

hi,

make a pressure test, to have certainty. It looks like a leakage.
Have you read the error memory?



Best regards,
Serdar

[little_e]

Yes. I've read fault codes (error memory), no faults stored.
I will do a pressure test and post the results.

[little_e]

I've now done a pressure test. I pressurised the system from the 90 degree hose that goes onto the turbo. The only place I can hear air coming out is from where the little hose attaches to this big 90 degree hose, and this is actually the suction-side of the turbo, so normally it's not 1 bar of pressure here.

What to do?

[little_e]

I've now also inspected all hoses in the engine compartment. Mind you, all hoses are new, even the big rubber boot over the fuel distributor (gummihutze auf mengenteiler #034 133 357 P). I've also replaced all hose clamps. I really can't find anything wrong.

Are you guys here familiar with the Gunson's Colortune? It's a see-through spark plug for adjusting the fuel mixture. You adjust mixture by checking the the combustion color. By using this tool I found out that at idle (it's to be used at idle only) the car get's the correct mixture.

So what I do know that somehow it's getting the correct mixture at idle, but that's about it. I can step on the accellerator to about 25% before it starts to misfire.

Can this be a failure to either the ECU or let's say the fuel distributor itself? I'm getting a bit annoyed.

[little_e]

So, after much back and forth I finally replaced the tuning chip with the original chip inside the ECU, since it's sitting in a socket.
The engine now runs like a dream.

What's the fault likely to be? The tuning chip gone down the drain or the ECU? The first I thought of was the tuning chip, but could it be a failing ECU, and it just got a jump start when I took it out of the car and put it back in several minutes later?