What governs my diesel fuel mixure?

[martincloud9]

People on this site tell me my car doesn't have an o2 sensor. Ok, that is probably true.

What then checks the exhaust gases to make sure that the right mixture is being created?

How does it know when to change the mixture? and when it can use the EGR valve?

I have been reading loads about cars and they all say that the exhaust gases get checked for fuel mixture. So I am lost on how my car does it.

Oh, it a Cupra tdi 150 PD ARL.


Thanks

 

[Cupra Ross]

People on this site tell me my car doesn't have an o2 sensor. Ok, that is probably true.

What then checks the exhaust gases to make sure that the right mixture is being created?

How does it know when to change the mixture? and when it can use the EGR valve?

I have been reading loads about cars and they all say that the exhaust gases get checked for fuel mixture. So I am lost on how my car does it.

Oh, it a Cupra tdi 150 PD ARL.


Thanks

Diesels take a full charge of air on each induction stroke (hence no throttle butterfly) and your ECU will allow a predetermined amount of fuel to be injected for a given throttle position and engine speed. AFAIK there is no reference to exhaust gas composition. The EGR valve automatically opens at low to medium throttle to keep combustion temperature down. Its also supposed to help fuel economy but the evidence for this particular virtue is questionable to say the least. EGR valves on diesel passenger cars are a waste of metal and cause more problems than they solve in my opinion.

 

[martincloud9]

...AFAIK there is no reference to exhaust gas composition...

Interesting. So what tells the injector to inject that amount?

The reason I am asking is my car's MPG has gone down. I have done loads on it and replaced loads of bits too (turbo, MAF, injector seals, air filter, fuel filter plus seals). I am now down 10MPG from what I was getting last year. After cleaning the thing goes like the bats out of hell again, but fuel economy is well out of the window.

So I guess, what is telling my injectors to add too much fuel, and how can I stop it doing it?
I have a VAG-COM and am kinda new to it. I know how to log and check data, but from there on I am scratching my head. Can I check if the turbo pipes are leaking with it?

Thanks

 

[Cupra Ross]

I think you are getting confused as to the differences of petrol and diesel engine fuel ignition and metering. Your car is only overfuelling if you have plenty of black smoke from the exhaust. Otherwise it will be fuelling just fine. Cold weather amongst other things increases fuel consumption. I'd look elsewhere for the cause of your drop in economy, it is almost certainly not a fueling issue.

 

[martincloud9]

Maybe I need to change my approach.
My car has decreased in fuel economy. It has gone from av. 50MPG to av. 40MPG. When I put my foot down there is sometimes a cloud of smoke that follows. Not lots, but you can see it if you are driving behind. There appears to be too much fuel being used somewhere. My driving style hasn't changed, and if anything, most of the engine is new new or cleaned like new.

If it is not a fueling issue Ross, what else would decrease my fuel in this manner?

One odd thing is this: If I try and start the car too early (before the dash lights have done there 'thing') the car is a pig to drive, even if it is a 40 mile journey. Poor throttle response, turbo overboost, rough running.
When I try after the dash lights have settled it is a great car to drive again.
Would this be a symptom that might be related?

Thanks

 

[Cupra Ross]

Have you tried an ECU reset by disconnecting the battery for 30 mins or so?

A puff of smoke is entirely normal from a diesel engine. Check that your air filter is clean and the MAF is in good order. I'd be suspecting something MAF related at this stage, a new one is not necessarily a good one.

Your rough running, poor economy and poor throttle response could very well all be down to a dodgy MAF sensor. 10mpg down is not entirely out of the question in winter vs summer.

 

[Robb1990]

You had it on a code reader? fuel temp or coolant temp sensor could be sending your MPG out. Im getting a coolant temp sender fault some times, and i feel my consumption could be a little better. You will probably know if its gone compleatly though, its a 4 pin type and feeds your dash guage too, so that will read wrong ect and it will probably over fuel consistantly. Id consider getting your codes read though, especially if you can get a code reader cheap from the bay or the like, they always come in useful, even if its just a mates VAG car.

 

[martincloud9]

...You had it on a code reader?..

...I have a VAG-COM...

And I have replaced and checked the coolant temperature sensor

Have you tried an ECU reset by disconnecting the battery for 30 mins or so?

Check that your air filter is clean and the MAF is in good order....

.... 10mpg down is not entirely out of the question in winter vs summer.

I am now down 10MPG from what I was getting last year (i.e. March 2008).Also when I changed the turbo I disconnected the battery overnight.
I've checked the MAF and Turbo on VAG and all is rather great. The peak turbo boost is higher up the rev range than on the rosstech site. Peaking at 2600 rpm, unlike the 2000 rpm. Not sure if that is a good thing?

If it is not a fueling issue, I'm not sure what else I can check or replace. I've almost done everything.

 

[Cupra Ross]

I'm still not convinced this isn't MAF related mate. Have you tried driving the car with the MAF disconnected?

I'm assuming its a Garrett VNT turbo that you have fitted? If so, have you checked that the vacuum actuated VNT mechanism is working correctly?

 

[martincloud9]

I'm still not convinced this isn't MAF related mate. Have you tried driving the car with the MAF disconnected?

I'm assuming its a Garrett VNT turbo that you have fitted? If so, have you checked that the vacuum actuated VNT mechanism is working correctly?

I've tried driving it with the MAF unplugged. Awful driving experience, almost rather dangerous. On VAG the MAF reads an almost constant 1100 on the test. From what it says, I have a goodun.

I put a reconditioned Garret VNT turbo, I checked it all worked properly. I even gave a suck on the vacuum pipe and checked the N75 valve. All worked swimmingly.

If it is a new MAF, and it tests ok on VAG....

How does the injector know how much fuel is going in to it? If the injector is wrongly assuming the amount going in, it could push too much in.

 

[Robb1990]

possibly because you have fitted all the new parts, things are cleaner allowing a cleaner burn which will in turn allow more fueling and more power. How are you measuring your MPG? per full tanks? not going from a laptop/dash read-out?

 

[Robb1990]

How does the injector know how much fuel is going in to it? If the injector is wrongly assuming the amount going in, it could push too much in.

On your PD engine it is controlled VIA the ECU kind of like a petrol injector but obviously more pressure in involved.

On a Non-PD they are controlled by the injector pump overcomming spring tension sinside the injector, thats my basic undersanding.

Edit, should of said, on your PD, you have a common rail, which is under injector pump pressure. If either your pump or injectors were to **** you would know wbout it. Suppose you could re-time your pump, but again, if it was marginally out, your would probably experience either flat-spots, or bad idle.

 

[martincloud9]

... How are you measuring your MPG? per full tanks? not going from a laptop/dash read-out?

no, not from the dash. It has changed since I bought it. One thing about your car, you do notice when things are not quite right. Like the amount of time between refueling is decreasing.

How easy is it to remap your ECU? I am a software programmer and have been for 40 years.

 

[Robb1990]

no, not from the dash. It has changed since I bought it. One thing about your car, you do notice when things are not quite right. Like the amount of time between refueling is decreasing.

I did notice considerably lower MPG when i used the shell garage/pump near me, i used them because they were still selling at 97.9 and tesco where i usually go was still at 99.9. But on the other hand, when ive used the shell a few miles away, my diesel seemed to last longer. I dont usually keep track of exatly how much/how many miles i do a week, i just put £30 in a week. Ive since gone back to using the tesco pumps around the corner from me, and my MPG has returned, or rather, stayed consitant.

I still need to do my coolant temp sensor, and i hope that will send my MPG up a little.

 

[Robb1990]

How easy is it to remap your ECU? I am a software programmer and have been for 40 years.

You can buy kits from ebay to link upto your ECU and flash it with a new map, but firther than that im not sure how easy/hard it is to obtain or create our own map.

 

[martincloud9]

This input is great, and I do appreciate it, but I feel no closer to solving my dilemma. I know for a fact that the A/F ratio is not quite correct. You can smell it if you idle it. You can see it if you stop at a junction and floor it. If there is nothing else electrical I can replace or check, and from what Ross says it has nothing to do with fueling. What else is there?

 

[Cupra Ross]

Fuel measurement on the PDs is a function of the ECU assuming a given fuel pressure from the injector pumps and adjusting the injector open duration to supply what it sees as the correct volume of fuel for a calculated air mass, engine speed and accelerator position.

Don't get disheartened, I think we are narrowing this down. What is the mileage of the car? Have you ever cleaned or blocked off the EGR assembly?

 

[martincloud9]

Yeah, cleaned the EGR last week, and the manifold too. Was full of s**t. Oil from the breather pipe was mixing with the soot and really smegged up. Took ages to clean. All works and seals good now. Was considering getting a bypass kit.

The car has now done 130000 miles.

Could anything possibly have gone wrong in the injectors?

Disheartened...? Broke is what I'm getting.

 

[Cupra Ross]

130k miles is pushing on for the PD engines. Your injectors will not be in peak condition at that age. The biggest mechanical problem on high mileage PDs is camshaft lobe wear. The lobes that fire the piezo injectors wear down over time causing fueling problems.

I'd remove one of the injectors and send it to a diesel specialist, this will give a definitive verdict. If the injector is ok, its possible that the camshaft lobes are worn, if not they will recondition your injectors and the difference will be very noticeable.

 

[martincloud9]

I guess that is the next step. I have had the head replaced because the cam wore through a hydraulic tappet. So new cam in it about 20000 miles ago.

How much is it to get an injector cleaned? and where would I go? I live near Stansted Airport.

I took the car to a Bosch diesel injection specialist in Hoddesdon a few weeks back, They replaced the injector seals, but when I asked if they checked the actual injector unit they said 'no'. Apparently they didn't have the equipment there to do it. Specialists, my arse!