Problem with 5cylinder 1

[Mike1002]

It concerns an Audi 2.3l 20v 5 cylinder. With pin coils and Bosch injectors. I have connected everything to the ECU and checked the wiring harness. All setting correct in the computer. Now the engine starts, only it runs on 4 cylinders. Cylinder 5 does not participate but the sparkplug does get wet. I made a list of what I checked.

What checked/checked/replaced

Wiring harness measured
New sparkplugs mounted
Compression measured
Fuel pressure measured 3/4bar
Mapping checked
Injector flow test done
Crankshaft signal test done
Checked firing order
Sparks from sparkplugs tested
Checked inlet valves to see if they open
Inlet gasket reassembled and additional liquid gasket used in between
Temperature sensor connected
Oil pressure sensor connected
Engine timing checked
Ignition time controlled with timing light

All sparkplugs spark when upside down. But when the sparkplug is mounted in the head, the spark is not strong enough or there is no spark at all.

 

[TugboatEng]

Inlet gasket reassembled and additional liquid gasket used in between

Don't do this!

Does cylinder 5 have compression?

This is not a forum for diagnostics.

 

[Mike1002]

Yes there is 8,5 bar compression with throttle
Closed

 

[TugboatEng]

Why would you take a compression reading with the throttle closed?

 

[Mike1002]

With throttle open the same bar at all cilinders around 8,5/9,5 bar

 

[TugboatEng]

Have you tried moving the plug and coil pair to other cylinders? Does the misfire follow the pair?

 

[Mike1002]

Yes I have tried it but the problem is than on the same cylinder as before so everytime on cilinder 5!

 

[TugboatEng]

I would suspect a faulty ECU, then.

 

[3DDave]

That or the wire to the coil pack is broken or a swage has failed in a connector. I suggest watching some South Main Auto Repair LLC videos.

https://www.youtube.com/c/SouthMainAutoRepairAvoca/videos

has terrific diagnostic walk-throughs.

 

[Mike1002]

Yes it’s a custom ecu but the strange thing is that all the coils spark on the top of the head and in the good firing order! So I don’t know if there is something wrong with ignition or fuelinjection or something with the cilinder. But before make this new setup for the engine(wiring, coils, intake and new ecu) the engine was running good on 5 cilinders. So I think the cilinders must be good ?

 

[LittleInch]

Fuel injection sequencing?

What type is it?

Is it firing the injectors in the right order?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[3DDave]

I'm beginning to think that last cylinder is a dummy one. It has compression, spark, fuel, air, timing? Should fire.

Move the injector to another or pull the fuel supply completely and try running the engine with starter fluid into the intake to see if it runs on all cylinders. Maybe the last one is so flooded that it cannot fire.

 

[Mike1002]

The injectors all connected to the same pins so they not sequencing. So all injectors spray at the same time before the intakevalve

I use bosch injectors number 0380155831

I already change the injectors to a working cilinder and no different. I think there is something wrong with the ignition or with the cilinder!

 

[3DDave]

as before ... pull the fuel supply completely and try running the engine with starter fluid into the intake to see if it runs on all cylinders. Maybe the last one is so flooded that it cannot fire.

I see from another system designed by Bosch that there are usually individual transistors to control the injectors. One failure mode for transistors is to short circuit so the injector would remain on continuously. You can check for power at the injector connector. Though they may operate in tandem, the transistors are individual for minimizing power handling requirements and to allow the engine to operate if one fails. Since it is port fuel injection they are simply reproducing the function of a carburetor to supply a fuel-air mixture in the intake at the intake valve.

 

[Mike1002]

So I have tried to let the car running with starting fluid. It really difficult to let it run and start at the same time. But the sparkplug on cilinder 5 is black

so I think there is a spark but there is also fuel, the plug is wet while running with fuelinjectors. Maybe to much fuel??

 

[TugboatEng]

Under the pressure of compression, the resistance of the air in the plug gap is much higher. Observing a plug spark out in the open does little to represent what goes on in the combustion chamber. Under pressure, it's possible the arc is running on the surface of the insulator.

 

[BrianPetersen]

Fouled sparkplug. Clean/replace.

If it fouls immediately again, you need to find out what's causing either the excess of fuel or the shortage of air in that cylinder.

An intake valve that doesn't seat/seal properly will allow some of the intake charge back out into the intake runner where it then gets mixed with even more fuel on the next intake stroke thus causing that cylinder to go rich. A little leak will make it a little rich. A big leak will make it obnoxiously rich.

Do a cylinder leak-down test.

 

[TugboatEng]

It's really not wise to clean fouled plugs, The fouling burns a carbon track on the insulator that's difficult to clean off. This will lead you on a wild goose chase if you're trying to diagnose problems.

Also know that a weak ignition coil can also cause fouled plugs. I chased this problem on a dirt bike once.

 

[mc5w]

Back in 9th grade Power Technology class the spark tester for lawnmower engines had a 1/4 inch or so spark gap to determine if there was enough voltage to fire a smaller gap at high pressures. If the spark cannot jump a 1/4 inch gap at atmospheric pressure it will not work with the spark plug installed at the higher pressure inside the engine.

Ir also looks like you have 1 of those fancy dancy spark plugs that are essentially a gimmick. Spark plugs that have a single spark gap work just as well and it is a lot easier to set the gap correctly.

 

[3DDave]

He started with new plugs and the plug is now fouled, that suggests there was enough spark to cause combustion, but too much fuel to completely burn. Since the main change was to the wiring/electrical system, I still think the fuel injection control to that cylinder is damaged or the injector is leaking in a coincidental failure.

Since I've not heard of follow-up to check to see that the injector is getting the right pulse it's an open hole in the diagnosis.

Also open to the idea the intake valve isn't sealing but the claim is that the engine was fine before replacing things that do not affect the valves. If the valve was bent due to mistiming at some point in the assembly process it is unlikely it would seal enough to produce the measured compression.