5 cyl Petrol T4 'missing' in action

[robs wedge]

I bought an '02 T4 2.5 petrol auto several years back. I had a bottle shop, and I loved it (despite the auto)

So much so I wanted another. I just bought a '96 T4 2.5 petrol MANUAL with 227,000 k's on the clock. It had a miss and hesitation which the dealer assured me would be 'fixed' before I picked it up. It wasn't and there is a bit of a dispute. I have replaced the usual suspects (plugs, leads, rotor, cap, O2 sensor, vacuum leaks) but I think the map sensor in he computer is mis-reading ...

Any clues?
Rob

[gregozedobe]

I've a vague idea there might have been a thread about a similar problem a while back, but it may have been more severe symptoms. Could have ended up being a faulty ECU, but my memory is pretty hazy on this one. Might be worth you while doing a bit of searching and looking at old threads ?

Electrics (incl coils), fuel pumps and filters are always good places to look for these sorts of problems. IIRC if it runs better with the MAF sensor disconnected that is a good indicator of a fault.

Let us know of your progress, it is always good to add to the collective knowledge.

[robs wedge]

I will sift through the old posts.

Do these computers have a 'limp home' mode

It starts and runs fine cold. It starts to play up as it warms up, worst at operating temp.
Symptoms:

It will cut out at idle.
Big hesitation at application of throttle.
loss of power
doesnt rev out freely
misses and hesitates through rev range.
misses and hesitates at constant speed, not much fun around town!

So far:
Repaired boot air filter to inlet manifold (cracked, new one soon)
Plugs, Leads, Rotor, Cap, O2 sensor.
checked for vacuum leaks.
I have checked the tube to the map sensor inside the computer. It did have a bit of crud in there, it did not appear to be oily, more like mucous ...

Is there anything else I should look at?

Cheers! Rob

[gregozedobe]

Sorry to be so generic in my advice, but I have near zero practical experience with petrol VWs, but anyway, my $0.02 worth is:

No idea about a limp home mode on these, but if they do have one I would expect it to be more a revs and power restriction, and running reasonably smoothly otherwise, so probably something else is the problem, not just a limp mode.

The symptoms you describe point to a lack of petrol (running too lean). That could be either lack of fuel being supplied (problems with pumps, lines, blocked filters etc), or issues with the electrics controlling how much fuel is allowed in.

From what you said it sounds like the "cold running" side of things is working properly, which makes it much more likely to be an electrics/control issue rather than a simple mechanical lack of fuel being delivered (it would be even worse when the engine is cold if it wasn't able to deliver enough fuel).

If the old plugs were very light in colour from warm running (ie engine not cold) that would support this diagnosis. That's about as much as I can say, given I don't know much about this specific model.

Hope you fix it without too much trauma.

I think in one of the old threads they swapped the ECU to locate the problem.

[Silver Caravelle]

I would also suggest checking the coil, as a faulty coil can break down when reaching normal engine temperature. Also doubly check for major vaccum leaks on the inlet side.

Also Engine temperature sensor (green or black in colour?) that provides one of the inputs to the engine computer have been known to go bad and may effect warm running, but I wouldn't have thought to an extreme extent.

One thing that is over looked is a blocked CAT, which shows up more when exhaust temperature is hot....leading to effectively a partially blocked exhaust therefore reduced power and engine missing.

Bad fuel/water in the tank? Fuel pump on the way out?

Or worse a burnt exhaust valve?

Actual distributor fault in the circuit?

Things like a faulty MAP sensor, minor vaccum leaks shouldn't affect the engine running to the extent you describe.

Regards,

Scott

[robs wedge]

#30 ECU relay

I checked the ECU relay, it had a bad joint on the 'make' contact side. Soldered it up and it runs alot better. I had already drained the tank, and installed a new fuel filter. It does feel now like exhaust restriction and/or fuel starvation. A new ECU relay is on the shopping list, I might include a temp sensor. I will check the fuel pump as well. I understand the fuel injectors dont play up often and cope well with high mileage but they are on my check list ... Thanks for help so far! Rob

[gregozedobe]

If the fuel supply is OK but there is an ignition problem the spark plugs will have a brown to black "sooty" appearance from the unburnt petrol - in bad cases they will even be wet and smell of petrol.

[robs wedge]

eureka!!!

Well, I had time to have a bit of a think about this problem. Miss almost gone, more of a hesitation now, really. Power OK at start-up ... then power would drop off as it warmed up. I was thinking along the lines of say exhaust restriction (cat falling apart), not enough fuel, or timing ... I picked timing to start. So I advanced the timing a couple of degrees. WOW different vehicle!! I understand mechanically advancing the dizzy increases the base-line timing setting, the ECU takes it from there. Bloody hell! what a difference a couple of degrees makes! A much MUCH happier vehicle, and a much happier owner. I hope my posts help someone else out there in T4 land. Cheers! Rob

[Silver Caravelle]

Well, I had time to have a bit of a think about this problem. Miss almost gone, more of a hesitation now, really. Power OK at start-up ... then power would drop off as it warmed up. I was thinking along the lines of say exhaust restriction (cat falling apart), not enough fuel, or timing ... I picked timing to start. So I advanced the timing a couple of degrees. WOW different vehicle!! I understand mechanically advancing the dizzy increases the base-line timing setting, the ECU takes it from there. Bloody hell! what a difference a couple of degrees makes! A much MUCH happier vehicle, and a much happier owner. I hope my posts help someone else out there in T4 land. Cheers! Rob

Rob,

Did that totally solve the problem? To resort to altering the base timing setting would suggest either it has been tampered with before, or the timing belt might be a tooth out of alignment which has exceed the limits of the ECU to compensate. Could it be that altering the base setting is masking the problem?

Cheers, Scott