JustinTime

Hello, hoping someone out there has a cure for my concern.
A3 2002 1.8T Automatic Cold Start Misfire Under Load fault.
Starts fine, idling up for cold/choke type conditions; but as soon as the transmissions drive is engaged (engine loaded) the engine will kick and buck as if it has a coil or sparkplug misfire, so bad it’s not driveable.
Put it back into Park the miss will clear; the cold start idle is around 1200rpm and will eventually drop to around 800rpm after about a minute maybe two (like waiting for a kettle to boil, seems longer than it probably is). Drive can now be engaged and the vehicle will perform perfectly all day, no matter what the engine or transmission load is. Allow it to sit long enough to go totally cold and the “Cold Start Misfire Under Load” returns.
Had the car 9 months, fault always been there but getting worse (longer wait) now we are in winter (NZ).
Have new plugs, air filter, oil and filter and coolant. All still in good condition.
No Diagnostic Trouble Codes (DTC’s) recorded – I have access to an OTC Diagnostic Tool, unfortunately I am unable to get live data readings as we think the units software is old or car too old for accessibility.
The temperature gauge appears to be working fine.
The temperature sender unit is a 4 wire type. Dead cold the reading across one pair of terminals is 1000 ohms and 2400ohms across the other pair, which quickly halves (500 and 990ohms) after about a minute (tested on vehicle).
The fault is definitely temperature related, so I’m wondering if one of the pair of resistance readings is giving the Engine Control Unit (ECU) false info resulting in incorrect fuelling while cold.
Does anyone have resistance range specifications for this type of temp sender?
Does anyone have a circuit diagram for the engine electronics and wiring?
Any help greatly appreciated.

Prospeeder

Sounds like a big air leak causing a lean misfire and bucking. Like when they have all the crankcase breather parts fall apart. See the rubber hose on the valve cover thats like a Y shape, look at the pipes attached, see the one on the back going down to under the intake? follow it, make sure its not broken, theres a rubber T it goes to that rips, theres the PCV valve in the T that goes bad, theres also finally a largish plastic tube that does a 90 degree down into the block on the last part of the T break.

if one parts bad, replace them all, you wont be able to remove any of it without destroying them so.

JustinTime

Sorry about late reply, spent last couple of weekends following your lead into vacuum leaks.
Looks like I may have purchased problem off previous owner as the section of piping you refer to has already been replaced; nice flexible rubber and plastics with shiny new hose clamps.
Working on vacuum leak theory, I checked all other potential sources of unmetered air and found hairline cracks at a one-way valve (?) coupling (see attached pic’s), in the line between the top right-hand end of the inlet manifold and the brake booster. I wrapped the section up in a self-bonding rubber tape; and for the next 2 days the problem appeared to be solved. Alas this was short lived as we have had a couple of extra cold mornings and the fault is back with a vengeance!
Any other idea’s would be greatly appreciated.

Prospeeder

your on the right track. Check that the Diverter valve holds vacuum, they rupture and are rather large air leaks, soo it in the picture with the vacuum line attached pushed into the rubber pipe from the air filter. You may also want to check your tape job, it gets super super hot under the hood and most rubberized and plastic tapes will just expand and melt away. Iv used black silicone and just sealed up the cracks.

You can tell its a major lean condition thats compensated for as soon as the engine starts reading from the oxygen sensors. Mass air flow sensor is extremely important for cold start running before oxygen sensor control is being used. I would try a different one if possible, or replace it. If you can see of live data from the engine, MAF is typically 2.5-3.2 grams per second on a warm idle on 1.8T cars. Depends on altitude and what not. Good luck from here

JustinTime

Thanks.
Did a vacuum and pressure check on the valve you described, also rechecked the brake booster line valve, all checked out ok.
It then dawned on me, when the problem disappeared for a few days, I had been working around the Mass Air Flow sensor. I removed the unit from the intake tube for inspection only to find the unit is a solid state type device, i.e. no moving parts; also no visible hot wire and nothing scratched, damaged or broken. I refitted the MAF sensor and used an electrical contact cleaner on the sensor wiring connector. The fault almost completely rectified, at least a 90 percent improvement, and it has stayed like that over the last 7 days since cleaning the connector.

I’m working on a couple of theories:
1. I may have a bad terminal contact at the MAF sensor connector.
2. I may have a bad Mass Air Flow sensor and by reducing some electrical resistance at the connector may have brought the signal from the sensor into a just manageable range.

I would like to get hold of an Engine Management Wiring Diagram and a Range Chart for the Mass Air Flow Sensor.
Can you point me in the right direction to obtain these?

I am currently looking for second hand Mass Air Flow sensor here locally.

Regards
Justin.

Prospeeder

Bentley Manual for the wiring diagrams. As far as testing the maf, its pretty difficult. So much depends on it, Temp, altitude, engine condition, and air filter brand/condition all change how much air physically enters the engine and is metered. I usually just rule out everything i can, air leaks, low fuel pressure, check maf readings at idle, see if they make sense, and if everything points at the maf, i replace it.

JustinTime

Thanks for your help. Really appreciate the input.
Cheers
Justin.

JustinTime

I've been a bit lax in reporting this problem solved (several years late) but here is the cure: Vehicle was still giving me occasional Cold Start Misfire, more evident the colder the day. Then one morning I started the car, it idled, but there was no throttle response, engine would not rev. The code that was logged stated Throttle Valve Control Unit fault; sorry I can't recall the code. Fitted new Throttle Valve Control Unit - problem gone. 3 years 30,000 kilometers on and we are still running strong.

Regards
Justin.