The UK quattro Network
The UK's mobile Audi 5-cylinder mechanic - 07833 654800, 24x7
Specialist in turbos, quattros, electrics and non-starts

This is a handheld-friendly site.

The Audi ur-quattro MB Engine

The five-cylinder inline turbo-charged MB engine fitted to the European models of the Audi ur-quattro in the 1988 and 1989 model years was stunning for its time and one of the best designs Audi ever produced. A significant number of MB-engined cars are used as daily drivers and many have reached the 200,000-mile stage. As of February 2006, my own engine has run 208,000 miles and an identical engine in my Type 44 has 309,000 on the clock. Not bad for 200bhp turbo engines that spend their lives working hard.

I have produced this page to supplement Audi's service information with data gained as a result of experience - maintaining several MB engines on a frequent basis, seeing a great many more occasionally, and stripping and rebuilding a specific high-mileage example (engine MB001293).

Some words of caution to the would-be amateur mechanic: The engine bay of an ur-quattro was not originally designed even for a turbo engine, much less an engine of the MB's complexity. It is perhaps the world's most cramped and worst organised engine bay, with hours of disassembly required to access some components. It's necessary to remove the front right headlight, for instance, to change a bulb or the air filter - the exhaust manifold nuts cannot even be inspected unless the whole of the fuel injection system and the inlet manifold are removed. There are no Haynes or Bentley manuals available, and original Audi documentation is now long out of print, error-prone and occasionally only available in German.

Audi MB engine - Theory of Operation:

The MB engine uses well-proven hardware. The I5 (inline 5) cylinder block design was around ten years old when the MB engine was first shipped - the hydraulic lifters were a novelty in the ur-quattro but had been used over three years before (in essentially the same cylinder head) to power cars as prosaic as the VW Passat (USA - Quantum). The hardware, fuel supply and ancillaries are almost identical to the MC engine used in the US Audi 5000. There essentially three modes of operation, not including OFF.

• Idle:
  This mode is selected when the throttle is shut - the centre (common - pin 18) and upper (pin 2) contacts of the throttle body switch (Audi 034 133 093F, Bosch 0 280 120 308) are closed. The purpose is to keep the engine at a stable idle as close as possible to 800 rpm with the vehicle stationary and good battery voltage. Control of the engine is effectively handed over to the Idle Stabilisation Controller (auxiliary fuse panel - double-width device with a VDO sticker) and the Idle Stabilisation Valve - a bright cylindrical object with two hoses attached, mounted transversely on the rear of the inlet manifold, which acts as a computer-controlled throttle. Idle speed is deliberately raised if the throttle is closed with the vehicle still in motion - this is to provide extra power for the steering and braking assists - and if the battery voltage falls below a certain point.
• Cruising:
  This is the normal mode of operation, selected when neither of the idle (see above) or the full load (see below) switches is engaged. The purpose is to obtain maximum fuel economy at the stochiometric mean - about 14.7 parts of air to one part of fuel. The electronic control unit (a Hitachi MAC-12D ECU) varies engine timing and the fuel frequency valve duty cycle according to inlet manifold pressure, air temperature and rpm. It also checks for knocking and can switch to alternative ignition maps for 98 or 95 RON fuel. Note that - unlike the MC from which it is derived - the MB has no oxygen sensor and thus runs 'open loop' at all times.
• Full load:
  Full load (or 'WOT' - wide open throttle) is selected when the centre (common - pin 18) and lower (pin 1) contacts of the throttle body switch (see above) are closed. It is somewhat of a misnomer, since the mode is active from about 2/3rds throttle onwards. The purpose is to obtain maximum power from the engine, temporarily discarding fuel economy considerations to run at an air:fuel ratio closer to 12.4:1. The engine management unit continues to manage engine timing and fuel frequency valve duty cycle, but adds active boost management via the wastegate frequency valve. The ECU has the ability to protect the engine by reducing boost, retarding the ignition and ultimately cutting out the fuel pump. Because of this capability, unmodified MB engines very rarely suffer major damage.

Diagnosis:

Not starting:

The MB's ECU performs a number of checks during the start sequence. Amongst these are using the flywheel pegs to make sure the engine is rotating the right way, checking the Hall sensor to distinguish the compression/power TDC from the exhaust/induction TDC, checking the coil's resistance, etc. If the ECU is not satisfied by these checks, it will store fault codes and refuse to start the engine.

Poor running:

NOTE: Some of the following tests place considerable strain on the engine. One of the potential causes of poor running is physical damage to the turbine, and stress-testing can cause further damage or even cause shrapnel to be sucked into the inlet manifold. Even though the probability of this is very small, it is recommended that the turbo-to-intercooler hose be pulled off the car and checked for metal fragments before proceeding.

Perform the output tests and pull the fault codes after a reasonably long run - at least ten miles. Address any issues found in the order that they are presented, and repeat the test run between fixes. Once the ECU is storing 4444 or no codes at all attach a boost gauge, a wastegate frequency valve light and a manual WOT override switch (if available) and repeat the test run. The boost test is performed from 35 mph in third gear without applying the brakes. If possible, the engine should be run right up to the fuel pump cut-out at redline. The boost gauge should show an initial over-boost spike (which is not measured) followed by boost pressures up to 0.69 bar relative - the wastegate frequency valve should flicker during acceleration. If this does not occur and the manual override switch is available, repeat the test run closing the WOT switch manually during acceleration. Pull the fault codes after each run.

Tuning

A great many people seem to want more out of their engine than it currently delivers. There are three things to be considered before embarking on a program of modifications:

• In the vast majority of cases presented to me, the dissatisfaction is the direct result of a sick engine. The MAC-12D engine control unit (as explained above) responds to out-of-specification conditions such as bad sensors by deliberately degrading performance - depending on the severity of the error, the driver may remain completely unaware that the ECU is doing this. Because the MAC-12D's fault memory is volatile (it is erased when the ignition is switched off) and only severe errors are shown to the driver via the "Check Engine" light, many owners drive for years unaware that a simple and cheap sensor replacement would give them 50% more performance.
• There are no free lunches. The MB is designed to deliver 200bhp - it can easily be made to deliver about 280bhp without hardware changes, but the downside is increased wear and a shorter engine life. The equation is somewhat biased with the MB ur-quattro, because of the cramped layout of the engine bay. Cylinder head removal, for instance, is an arduous eight-hour job - major engine repairs are hideously expensive in removal and refitting labour costs alone.
• It is not easy to increase the performance of an MB engine properly. This is a direct consequence of its ancestry. It was originally designed for closed-loop operation in an emission-controlled environment and the modifications to allow the engine to operate without oxygen sensor feedback are fairly crude. The basic issue is full load enrichment - the MAC-12D code contains pre-programmed assumptions about the volumetric efficiency of the engine and needs to be changed in a sophisticated way if the engine geometry is changed by, e.g., a different cam, throttle body, air filter, gasflowing, etc. It is easy to demonstrate (using a VAG 1348/1 service tool, a good digital multimeter and a special test lead) that useful enrichment input from the air mass sensor ceases at about 4500rpm. In direct contrast with its WR predecessor, the MB engine has no boost-derived mechanical enrichment facility - above 4500rpm enrichment is achieved by the ECU varying the duty cycle on the fuel frequency valve.

One of the major problems is the heritage of the WR engine. The earlier WR is a completely mechanical design - 'chipping' is extremely easy because the chipper doesn't need to worry about ECU control of either boost or full load enrichment. A stronger wastegate spring and a voltage divider on the ECU's pressure sensor cost only pennies and can be sold at huge profits. This technique works on the WR engine because full load enrichment is entirely mechanical, and many chippers use it on the MB engine - but the engine will receive little or no full throttle enrichment above about 4500rpm.

What this means is that the engine will run very lean (and hence hot) right at the point where it produces most power. Since cylinders 5 and 4 have a tendency to leanness in the MB in any case, this greatly increases the chance of melting a hole in one of the rear pistons.

There's another problem - controlling boost. Unlike the WR, boost control in the MB uses a servo system - the ECU is able to switch either boost or vacuum to the upper wastegate chamber to control the level of boost. The boost response of an MB engine is not linear - when full throttle is first engaged, boost is deliberately driven high to minimise turbo lag. Later, it is modified and can be significantly reduced by the ECU if it feels it sees a problem. These controls are buried deep within the actual code executed within the ECU - they are a mystery to most chippers who are not Audi specialists.

So the chippers modify the ECU in a very cheap and easy way - by putting a voltage divider circuit costing only pennies on the output of the ECU's boost sensor. This 'fools' the ECU into generating higher boost pressures until it sees the voltage from the sensor corresponding to the boost it is trying to generate. The engine, however, is seeing much higher boost.

Fooling the ECU is not a very good idea. For one thing, it may defeat the ECU's safety cut-off of the fuel pump if something like a hose failure (manifold to wastegate centre chamber, for instance) causes a sudden and massive overboost. For the second, the ECU derives ignition timing in part from boost levels (this is the only function of the WR's ECU). And for the third, it causes incorrect values to be calculated for driving the fuel frequency valve.

The first problem could be countered by adding a simple boost limiter to cut the fuel pump supply. The third can be biased to some extent using the CO adjustment in the metering head, although this causes the air mass sensor cone to be biased to the wrong point. There is no solution for the second problem other than reprogramming the timing tables in the ECU - and no UK chipper is currently doing this.

THIS PAGE IS STILL WORK IN PROGRESS

Kerbside Motors Home Page and Contact Information