Petrol engines intermittent 2.1

From VW T25(T3)-Tech
Jump to navigationJump to search


Beaker: Check that you have 12v on pin 30 in the distribution box left side front of engine as this supplies the fuel and ecu relays, then check the outputs from both the relays, if the ecu doesn't get it's ignition switched 12v it won't start.

Check spark at plugs

Check connections on temp2 sender and afm

Check connector to ECU (maybe the wind and damp got to it)

Check that air inlet not blocked somewhere (eg birds nest material in cyclone)

Check trigger signals to injectors

Check fuel pump running

Check connections hall sender and isv and idle control unit - try taking the idle out of the system too - a faulty control unit may affect the start up, the ecu is told it's starting by the ignition switch and throttle switch and adjusts accordingly

Classic 'Vanagon syndrome'

Vanagon syndrome can take a variety of forms, but was originally intended to cover cases of injected US origin MVs misfiring after a random mileage from start of journey right up to stopping completely. Symptons usually disappear after stopping for a while. But do not assume that the fix is always the std. vanagon syndrome fix which only works in some cases.

Std. Vanagon Fix was originally a VW replacement sub-harness for the AFM (see links below) which removed potentiometer voltage 'spikes'.

There was an identified cause covered in the links below: AFM potentiometer tracks, AFM trigger signal instability etc.

However, on both DJs and MVs electrics/electronics can become flaky after 15 ~20 years from new. Check and remake good engine and engine bay earths, main engine star-earth, connectors, and on MVs always think 'Lambda sensor' (O2 sensor failure) or its wiring. ECU or Ignition Control Unit intermittent. It has also been suggested that floating engine potentials of up to one Volt can be a problem when the alternator is charging at high amperage due to poor earthing, so a very heavy earth wire from alternator body to engine-bay firewall could resolve this.

vanagon syndrome

capacitors for above fix

Intermittent cut-outs, bad running or poor hot starts (non-vanagon syndrome)

Symptons like vanagon syndrome but not cured by the normal vanagon syndrome fixes...

On both DJs and MVs electrics/electronics can become flaky after 15 ~20 years from new. Check and remake good engine and engine bay earths, connectors, and on MVs always think 'Lambda sensor' (O2 sensor failure) or its wiring (IIRC - a bad sensor can be disconnected to trouble-shoot)

TPS (throttle position sensor) wear/adjustment - check using voltmeter and make/break position

Fuel pump, fuel pressure regulator or fuel filter (blocked)

But always first check for:

Dirty fuel tank, water in fuel, dirty fuel filter

Fuel pump itself


Decent plugs, decent connections to injectors, and to Ignition Hall unit (& hall sender in dizzie (which usually fails completely - heat)...

Fuel pressure, 28~30 psi

Ignition Amp, Main ECU (fail when warm or hot or vibration)

Blackdog: Bad multi-connectors behind AFM (corrosion) - Fuel pump supply connector General - fuel pump supply wiring, trace and check condition.

Throttle idle-cut-off position sender and cable adjustment (unless been verified OK recently)

Possibly Idle Stabiliser Unit (check function and cleanliness, can be bypassed (see Bentley) or IS Control Unit (check for signs of damp exposure)

ECU (substitute to test)

Ignition - Hall sender in distributor (usually completelty u/s), ignition control unit (substitute) Leads/plugs - usually if load or weather dependent, not truly intermittent - problem may come and go but generally gets progressively worse!

Syncrosimon: I would check the following (not starting intermittently, then stumbling - new AFM has not fixed)
  • Coil HT plug for signs of coil failure, could be cracked or with residue.
  • Check that the fuel pump relay is working. When you switch the engine off the temp in the engine bay rises as there is no airflow through the to cool it, and the increase in temp can cause intermittent failures.
  • Have you changed your temp 2 sensor for the ecu. Can cause hot start problems.
  • Also could be your fuel pressure regulator. If when hot it is failing and you have no residual pressure left in the system then starting will be harder. You can get a fuel pressure gauge on ebay for 16 quid. This will show your pump is working, your fuel regulator is working, and that you have residual pressure. If one of your injectors is weeping it will flood the engine and cause staring problems. (would not explain stumbling after start up(but a knackered regulator would)). Fuel press ~28>30 psi (HM)
  • If your van is an auto, could be the no start if not in park switch.
  • These links might be some help.



Aidan: If it starts and runs OK but won't drive then it must be to do with variable inputs

i.e. throttle body switch must be working, if it's not then ecu doesn't know difference between idle and open throttle

AFM signals - ecu needs to know how much air coming in and what temp it is - temp1 is in afm

Hall signal - ecu needs to know revs/position of rotor arm

Temp2 - what temp engine is.

...or a controller that's intermittant eg a relay (fuel pump relay) or a breaking down component that can't provide full delivery eg king lead, coil or fuel pump.

or ECU which is looking favourite from the heat cycle failure mode

AFM fix with modern hot-wire anemometer($695)

I bought the harness thing and that didn't do any good

Hesitation, Cough, Cough, Cough: The Vanagon Highway Hiccups

Ever drive down the road in your 86-91 VW Vanagon, all well, stars aligned, moon in Venus then all of the sudden: Hiccup! At first you think it was a gust of wind, or a maybe you just drove over a racoon. Then moments later: Hiccup! After that, maybe back to blissful wonderful moment with stars aligned again - or maybe a total loss of power and to the side of the road you go. You check the fuel level on the gauge, tyre pressure, stars and moon, get back on the road, and all is fine again! Then: Hiccup. Damn it all!

The dreaded Vanagon highway hiccup syndrome has to do with the Digifant fuel injection system found on 86-91 Vanagons (and some other VW vehicles of the same vintage). The culprit is often the air flow meter (AFM). The AFM is the metal box attached to the air filter housing, that has an electrical fuel injection harness plug attached to it. This device does just what its name implies, it meters the air flowing into the engine. There is a spring-loaded, swinging door inside this contraption that is displaced by the air rushing into the engine. Just about all electronic fuel injection systems from around 1974 into the early 90’s uses a device like this. This swinging door rotates around a pivot point, above which is located an electrical “wiper” that rubs on a conductive strip (potentiometer). The Digifant computer sends a 5 volt signal to the AFM, and depending on how much air is entering the engine, this potentiometer sends back a proportion based on the amount the door is displaced. It’s a pretty simple, fail-safe idea, right? Wrong…

What happens over time is that the electrical wiper and conductive strip begin to wear. When this happens the Digifant computer gets confused, and goes into shut-down mode. You are probably thinking, “OK, so what, everything wears out, as long as it lasts long enough and you can get a replacement when the time comes.” Herein lies the problem. New replacement air flow meters have been discontinued by Bosch. And, even with a new air flow meter that were almost $600 back when they were available, these symptoms would return as early as a year or two, or as little as twenty or thirty thousand miles. VW recognised the problem early on, and back in the late 80’s/early 90’s introduced a “filter” cable that would plug in between the air flow meter and fuel injection harness. This cable was another $275, and even with it and a fresh air flow meter (we are talking close to $900 now), it was STILL not a for-sure, forever fix. For the last three years or so GoWesty has offered a rebuilt air flow meter with the filter cable circuitry built-in for $295 exchange. It was also not a for-sure, forever fix, but at least it was more affordable.

So, we decided to go in a completely different direction. 

In the late 80’s/early 90’s most manufacturers got away entirely from the AFM concept, and instead switched to an Air Mass Sensor (AMS), commonly referred to as a “hot wire” system for reasons I will explain, read on. Instead of measuring the air entering the engine mechanically and converting the mechanical movement to an electronic signal, the AMS does it directly. Instead of a swinging door, a very fine wire of known resistance is placed across the intake duct of the engine. The fuel injection system’s computer sends a 12v signal to this wire, and the wire begins to warm up (thus the “hot wire” reference). As the wire warms up, its resistance changes, and the computer makes a note of that resistance. When the engine is started and air enters the engine, it rushes past the wire thus cooling it down, and making its resistance change. By calibrating the resistance of the wire for different air flows, this data can be used by the computer to determine air flow directly. This system has two main advantages: 1) There are no moving parts, and, 2) It offers much less resistance to the air flowing into the engine than the swinging door arrangement. The former translates to a much more predictable behavior and longer life, and the later to better performance and engine efficiency.

Rather than completely redesign the intake system of the Vanagon, and completely change the electrical harnesses and such, we took a much more simple approach. Starting with a normal AFM, we completely gut it, fit it with a AMS sensor, which is physically very small and easily fitted within the housing. We take the signal from the AMS to a small black box we mount behind the left tail light via a small wiring harness. On the way to the black box behind the left tail light, the harness comes in close proximity to the ignition coil. It is there we pick up 12V and engine speed (tachometer) signal from the coil. The little black box converts the signal generated by the AMS to a language the Vanagon’s Digifant computer understands: fluent AFMish. This translation is sent back to the converted AFM housing where the fuel injection harness is plugged in as usual. The signal that now looks just like the old AFM signal is delivered to the computer by the existing, factory harness. The whole thing takes about 10 minutes to install, problem solved once and for all!

Now the bad news: The system costs $695, and is sold exchange (add $250 deposit until you send in your old AFM). Exchange because we need your old AFM housing to gut and convert. And pricey, yes, but is the LAST TIME you will EVER have to mess with this part of your Vanagon again! We guarantee it, FOR LIFE. That’s right, these systems come with a lifetime guarantee. If it fails for any reason, we will replace it at no charge. Guaranteed. Really

Editors note: All presuming the AFM is the problem!

Bad starting/flooding/rough running until hot

Usually the T2 temp sender fails or its wiring from under or behind the thermostat. Can be intermittent and can stop the engine through flooding, usually when starting. Sometimes the ISU (idle stabiliser unit), or the IS control unit on the n/s bulkhead or behind the o/s tail-light (Syncros)can prevent running