What Causes This Then

[stevef 0]

My VR6 has developed a mis-fire.

 

It is worst between 1000 + 2000 RPM on light or trailing throttle but can still be noticed under hard acceleration as a noticable surge at about 4000 RPM then another surge at about 5000.

 

I did a back to back drive with Stu's VR and the car doesn't seem to lack performance in comparison.

 

I have replaced the spark plugs with no change. I have also run the engine in TOTAL darkness and can see no arcing from the coil pack or leads.

 

Any help much appreciated.

[Kevin Bacon 5]

When you say misfire, do you mean like it's running on 5 or 4 cylinders? Or just feels like a couple of cylinders are only getting about a 1/4 strength spark?

 

You don't always get fireworks under the hood at idle, try revving the engine quite hard whilst spraying a fine mist of water all over the engine.

 

If still nothing, then suspect the MAF sensor.

 

EDIT: Try unplugging the MAF connector when the engine is up to temperature and running. If it splutters and dies, the MAF is OK and the ECU was using it to meter fuelling. If the engine continues idling as normal, the ECU has already bypassed the MAF and is using the Throttle position sensor.

 

K

[stevef 0]

Thanks for that Kev.

 

I have rev'd the engine in the dark but not with a water mist - I will try that.

 

Regards the MAF - I read previous posts prior to this so did as you suggest with the MAF.

 

The engine carried on as normal on dis-connecting the MAF in that the idle didn't change. On re-connecting the MAF the engine gives a slight shudder. I tried this several times with the same result. I reset the ECU and tried again. The engine started first time after ECU reset and idled correcetly but dis/re-connecting the MAF still had no effect.

 

I then drve the car with the MAF dis-connected. The engine still misfired but the idle was all over the place and coming on/off the throttle caused the car to lurch. The fuel consumption on the MFA went to 23MPG when it normally sits around 28MPG.

 

I reset the ECU and re-connected the MAF and idle returned to normal and lurching stopped. MPG back to 28.

 

The misfire remained in both states. Feels like running on 4 or 5 cylinders. As I say, the power seems to come in waves at 3000,4000 and 5000 RPM.

 

The MAF is a film type not hot wire - how can it be tested off the car?

 

Cheers

[Kevin Bacon 5]

Your MAF sounds like it's OK then, but may have an intermittant fault. The film type are generally more reliable than the wire type.

 

Have you checked the 02 sensor? When the car is at idle and up to temperature, get a decent multimeter with a 10Mohm input resistance (Maplin sell them for about £20) and poke the red probe into the Lambda's white wire and the black probe onto the car body somewhere. The voltage should constantly fluctuate up and down from 0.1 to 0.8V. If it stays at 0.45V, the probe is dead.

 

Do you have access to VAG-COM as it could be one of a myriad of sensors?!

 

K

[Robbo149 0]

Hi Steve, this sounds like exactly the same problem I've been having on my VR. I thought I'd tracked it to a knackered MAF sensor, as disconnecting it made little or no difference, as you describe. But I got hold of a couple of s/hand MAFs & tried them (resetting ECU with each one) and these were no different. I then took it for a diagnostic test & this showed a Hall Sensor (camshaft position sensor) fault. I've just got a new one (£32) & will be fitting tomorrow so I'll let you know if this cures the prob.

 

Cheers,

Robbo

[stevef 0]

Hi Robbo + Kev

 

I have forked out and had my ECU read.

 

The garage used a Sun tool to do this so not 100% that codes will be correct but here goes.

 

17978 ECU Blocked

 

01259 Fuel pump Relay intermittently SC or OC to ground

 

00524 Knock Sensor 1-G61 SC or OC to ground

 

00561 Mixture Adjustment Adaption Range (ADD) intermittently below minimum level

 

Thats a fair old list for you to ponder

 

It looks like a chicken and egg situation to me in that the problems are related to one cause. The fuel pump error could be down to me trying to start the car forgetting to disarm the immobiliser.

 

Where is the Knock Sensor?

 

Any help much appreciated!!!!

[Kevin Bacon 5]

Cor that is one unhappy VR6!

 

There are two knock sensors, one at the back of the block and one on the front. Wiring connectors for both are on the associated engine mounts.

 

I think the mixture adapation error could point to the MAF or the Pump relay as the ECU is 'adapting' to use other sensors for fuelling. It's reached the limit of adapation and can't meet the engine's requirements.

 

Maybe the surging is due to the pump relay intermittantly shorting....but I wasn't aware the ECU even monitored that......very strange!

 

K

[stevef 0]

Can you get to the knock sensors from above or do you need to go from underneath?

 

Do you need to remove other items to gain access?

 

I am going to need a wiring diagram to investigate the relay error. Does anyboby have one?

 

Where is the fuel pump? I need to look at the wiring at that end too!!

 

Any help much appreciated!

[Kevin Bacon 5]

Can you get to the knock sensors from above or do you need to go from underneath?

 

Do you need to remove other items to gain access?

 

I am going to need a wiring diagram to investigate the relay error. Does anyboby have one?

 

Where is the fuel pump? I need to look at the wiring at that end too!!

 

Any help much appreciated!

 

At the front - Follow the wires from the engine mount to the block. The white plug is the crank sensor and goes to the right of the block, so you need the other other wire! Just follow it until it meets up at the block. Same with the rear mount, two connectors, one is the Lambda probe and the other is the Knock sensor. Access is easier from underneath looking up. Nothing needs removing AFAIK.

 

Fuel pump is in the petrol tank! I wouldn't go in there unless you're sure it's knackered, which I don't think it is tbh.

 

2CC is your man for fuel associated wiring as he spent weeks tracing a fault and has a good understanding of it all.

 

You could try replacing the fuel pump relay if you haven't already. They do have a reputation for packing up .

 

K

[dr_mat 0]

Mixture adjustment adaption range could indicate the MAF (i.e. the ECU doesn't know how much air is going in) or the lambda probe (the ECU doesn't know how rich the resulting mixture is) I'd guess. I would think either could result in poor fuel mixture calculations. The lambda probe is a cheaper fix!!

 

But thinking about this some more, it could mean neither of these are faulty. All that's reporting is that it's adapted fuelling ratio is below what it considers to be a "normal" engine, i.e. it's putting in less fuel per measured unit of air than it considers to be correct. Could this even be dodgy injectors? The ECU asks an injector for a short burst, but the injector is stuck open so floods that cylinder, the ECU picks up the extra fuel/air at the lambda probe and adjusts it's injector timing shorter to try and compensate?

 

Pure speculation, but...

 

Dunno what the hell ECU blocked means!!

[stevef 0]

Started my investigations today.

 

Tha output on the white wire to the Lambda sensor read 0.1 to 0.8VDC and constantly changed. I think that is OK acording to Kev.

 

I removed the knock senors and sensor 1 ( the one on the back of the engine) is just a blob of melted plastic with a broken piezo crystal disc sticking out of the side.

 

I have been quoted £75 for this part from the stealer and it will be with me on Tuesday.

 

Have looked at the fuel pump relay and cleaned it. I drove with my finger on the relay and was not able to detect it dropping in/out but without a wiring diagram cann't check the wiring. I will get a replacement when I collect the knock sensor.

 

I will let you know how it goes with the new sensor.

 

Cheers.

[DaveStorm 0]

Nice to hear your problems nearly sorted, tis a tidy VR you got there. 8) :)

[Robbo149 0]

Sorry to hear about the list of faults Steve, at least with a C is going to be worth fixing! BTW, fitted my new cam position sensor today and (fingers crossed) seems to have cured my poorly VR. I've only been out for a couple of short runs but seems much smoother. Will get out for a longer blast see if there seems to be any noticeable hike in power.

 

Cheers,

Robbo

[stevef 0]

Nice to hear that the cam position sensor has cured your fault.

 

At least I found one of the reported errors on my VR. Its difficult to continue investigating once you have found a problem as I have no real way of knowing if all of the errors are interrelated.

 

I may have another look at the fuel pump relay issue once I have a replacement. I can doctor the old relay to fir test leads so I can drive with my meter connected.

 

I will report back once the knock sensor is fitted.

 

Cheers

[stevef 0]

Ah well!!!

 

I have replaced the faulty knock sensor and fitted a new fuel pump relay.

 

Unfortunately the problem is not cured. The engine is a little smoother but still misfires at around 1500-2000 rpm on light throttle. The problem is that this co-incides with 30-40mph in 3rd,4th and 5th gears so appears omnipresent.

 

When you open the throttle the engine pulls cleanly- maybe a slight misfire but I may be paranoid about it!

 

Next is to take the imobiliser out of the fuel circuit to ensure the pump runs OK.

 

Any other ideas!!!

 

I will keep you informed.

[Kevin Bacon 5]

Ah well!!!

 

I have replaced the faulty knock sensor and fitted a new fuel pump relay.

 

Unfortunately the problem is not cured. The engine is a little smoother but still misfires at around 1500-2000 rpm on light throttle. The problem is that this co-incides with 30-40mph in 3rd,4th and 5th gears so appears omnipresent.

 

When you open the throttle the engine pulls cleanly- maybe a slight misfire but I may be paranoid about it!

 

Next is to take the imobiliser out of the fuel circuit to ensure the pump runs OK.

 

Any other ideas!!!

 

I will keep you informed.

 

Ah....I think I know what you're talking about now. Mine has the same hesitancy (constantly accelerating/deaccelerating) on part throttle at those exact speeds. It is possible to get it in 4th gear at exactly 2000rpm and it'll buck (engine rock) slightly....

 

That isn't an electrical misfire as such, but usually a MAF/TPS related fuelling issue on part throttle. My old BMWs used to do exactly the same thing and I never found the cause.

 

Same again with the VR6....I've replaced pretty much every sensor on the car and it still does it!

 

You may have to learn to live with it as VAG-COM reports diddly squat.

 

K

 

P.S Let me know if the removing the immobiliser helps as that's the only thing I haven't looked into, but then it's a Clifford and not an OE one.

[stevef 0]

May not be related in any way but have just noticed on my wiring diagram that post '94 cars supply power to the film type MAF via the fuel pump relay.

 

The earlier cars with the hot wire MAF supply it via the engine control relay.

 

It could mean that the MAF is causing the relay error I am getting?

 

I will bypass my imobiliser when it stops raining!!!