1993 VR6 excessive fuel consumption challenge

[ismith 0]

I've had the car 10+ years. It used to do 30+mpg, now 25mpg over same journeys. Performance off.

Lambda adaptation values look bad on VAG-COM. Lambda probe switching ok at idle.

Lambda probe, MAF, ECU, temperature senders, distributor, injectors all been changed.

Fuel pressure measured and ok.

 

Any ideas?

[Kevin Bacon 5]

MAF

 

You may have replaced it already, but that doesn't necessarily mean you put a better one on, especially if it was s/hand.

 

The lambda adaptation is usually a good clue to that.

 

Either that or your hall sender in the dizzy has gone and it's running in batched injection mode, which will blunt both the performance and economy.

[ismith 0]

Thanks for your suggestions.

The MAF on this vehicle is the wire, not film type, so generally give few problems. I actually replaced it with a new one from the dealer.

The hall sender is brand new new too, as the last one fell apart.

Any other clues? This is beyond the dealer's ability.

[Supercharged 2]

Rear brakes binding slightly??

 

Changed tyres recently??

[ismith 0]

All brakes are fine. They have needed maintenance in the past year. Tyres unchanged. We're talking about a lot of fuel here and the lambda does have a film of black soot, so the car is definitely deciding to over-fuel. Performance is significantly off.

[Kevin Bacon 5]

The wire MAF is actually the more troublesome of the two types.

 

Have you had it scoped at a Bosch agent?

[floppyman 0]

So whats your lambda adaptation values at on vag-com. Maybe make shure the plug for the lambda probe is nice and clean and theres a ground wire from the same plug that connects to the chassis. Had a problem with that one before

[Conker 0]

Some random suggestions:

 

Cam sensor maybe? Throttle position sensor? Is the coolant getting hot enough - maybe the thermostat is stuck open? Broken wires to coolant sensors? Rusty ECU plug? Have you started using cheapo 95 ron fuel?

 

If all else fails, give it a wash and polish, that'll sort it! :)

[andy 0]

I'd definately check she's getting hot enough, I've had horrendous consumption before from cars that are running cold or a duff temp sensor that tells the ecu the engine is cold. It would be running permanently rich if thats the case. (sooty Lambda)

[Kevin Bacon 5]

Aye, the infamous 'blue temp sender'. The warm up map switches off at 70 deg water temp and hands over to Lambda control, which is + / - 15%. 15% also happens to be the reduction in mpg, so if the lambda adaptation is maxxed out, (control limit surpassed error in VCDS), there you go.

 

Good call guys.... but as per the OP, Mr Smith is savvy with this kind of stuff and has already checked the BTS.

 

A defective MAF is also capable of misreading to the tune of 15 or more %, which is again outside of the lambda's control range.

[Wendy 0]

Have you put on weight ? !!!

 

Is it a case of harder right foot ? !!!

 

Sorry - I will get back to work now !!!!

 

 

Where are the smilies ?

[ismith 0]

Brilliant list of suggestion, folks. Thanks!

Cam sensor was replaced, even though it was not faulty.

TPS dig go a couple of years back. VAG-COM shows correct operation.

Car gets nice and hot. (Been through a couple of temp senders in her life.) VAG-COM diags show correct temperature too.

VAG-COM logger shows the lambda probe switching at idle and when driving. (The lambda on the VR6 is different from the other models in that it has four-wires and does not rely on a local ground.)

VAG-COM adaptation at idle varies from 0.4-0.8, depending on when I've checked it. It should be pretty close to 1.00 from what I can rememeber.

Last year I had the valve guides replaced, as a valve was sticking, but that didn't fix the fuel consumption. It made a huge difference to the oil consumption!

 

The ECU was dodgy. The fuel-pump relay started clicking rapidly just before I started. A scope showed the ECU driving it 200ms on, 1ms off. Sounds like a reset watchdog going off in the software. Ebay got me a good replacement one. Still didn't impact the mpg.

 

I just wonder if the old ECU could have damaged the MAF. I have two of them, but there's a chance they both could be damaged I guess. I'll see if I can get them tested.

 

Otherwise I can only thing of an obscure air leak. I did find one before when a garage didn't replace the injectors properly and damamaged their seals. That still didn't make a significant difference.

[Supercharged 2]

Be good to try another MAF I think and go back through basic settings after an ECU adaption reset. I think it's good in a way that performance is down as it should make it earlier to test when you think you've fixed the issue

 

The other thing that damages MAF's is aftermarket imobilisers that cut the power to it when it does it's cleaning cyle after the engine is shut off.

 

I assume the actual MPG your getting corresponds to what you see on the MFA?

 

Not sure where RW1 is but he may have some good input on this thread... Maybe also worth a trip to somewhere like Stealth or DG Autotech and get them to have a look and maybe map it afterwards, I know there has been a lot of praise for the DG map on standardish cars.

[ismith 0]

Well I got a new MAF today, reset the ECU, and no difference :-(

 

I remember once when I had the throttle body off to clean it and the ISV, the car was back to normal for the drive in to work and then horrible from then on, but I tried that again for luck and no joy. Any bright ideas?

[Kevin Bacon 5]

Where did you get the MAF from? I had a couple of DOAs before finding one that worked. GSF MAFs are bad for that.

 

How many miles has it done?

 

Chains been replaced yet?

 

Checked the intermediate shaft sprocket timing is at 0 or 180 degrees?

 

Your right foot getting heavier?

[ismith 0]

190k miles. New chains and valve guides. That didn't make huge difference, but is smoother at some revs. I suspect thererfore that the garage got the sprocket on the right way!

Were your MAF wire or film types? I've tried three (6-pin) replacements in total: two scrap and one from VW dealer, to no effect.

Performance off - it's like a 2 litre. I get 23mpg on a 10 mile commute which is 60% country lanes and 30% urban. When she was working properly, I'd get 30mpg.

[Supercharged 2]

Compression ok?

[ismith 0]

Compressions spot on. When the head was off, the bore wear was bearly discernable. The bore expert said he couldn't charge me for anything!

I just wonder if there's a hard-to-spot air leak. I need some flammable liquid to spray around the intake.

[boost monkey 0]

No-one has mentioned injectors.

 

Have you checked those?

[ismith 0]

Injectors have been out, overhauled by an agent and back in. Not them sadly. I even tried a second set just to be sure.

[floppyman 0]

So you still havent found the problem.... I would like ro know what it was in the end, I average 25mpg and this was after changing relay box and ecu. I can only get around 30 mpg in the motorway. Keep us updated

[C488ADO 0]

i got 39.5 today on a 30 mile run ok ok ok it was @ 70 mph!......

[lukeyy 10]

my rado vr does 30mpg on a daily run poodling on the motorway at about 65/70mph. is this normal? i had problems with the car stalling sometimes when coming upto junctions. as a temp fix i unplugged the throttle position sensor, this has cured the stalling issue, however would this hinder my mpg? the car idles at around 800rpm which i thought is fairly normal.

Ive rfan a load of redex through the car and cleaned throttle body and breather pipes. car seems to have good performance and is still very quick. any ideas to how i could improve my mpg? as ive seen people saying they've been getting 35+mpg on the forum.

car also has milltek exhaust system and k&n induction kit. would these make any difference?

 

luke

[fla 9]

i too get about 30-32 mpg max on the motorway, so i really dont know how some of the astronomical mpg figures are reached. Mines only done 70k too so less isseus with perhaps it being worn out. I do tend to drive like an old fogie at times too, especially due to the ridiculous fuel price rises but cant better 32mpg. I dont think its anything to worry about. OBD2 however, does seem to massively improve efficiency and stops the dreaded stalling.

[lukeyy 10]

i think ive cured the stalling problem. i reconnected it the throttle position sensor. then cleaned up the three plugs on the front of the bottom block. yellow, blue and black. im guessing the blue one is ecu temp sensor? these looked rather dirty. cleaned em and the car ran beautifully. took it for a thrash up the road with hard stops at junctions and it didnt stall once!! then reset the mfa and drove home normally and saw 28mpg!! pretty happy with that. gonna take it for a longer run tomorow and see what it puts out then.