Sudden drop of MPG

[mcblfc88]

Hi guys, been a while...

My mk5 GTI over the last few tanks of fuel has dropped from circa 28mpg combined to a consistent 22.

Same fuel being used, the car was serviced less than 2k miles ago.

I’ve done the PCV test, oil filler cap came off but the engine went crazy when I removed it, suggesting to me the PCV is fine?

Checked tyre pressures, all good, DV was changed less than 5k miles ago so would assume that’s fine.

Any suggestions? Just seems strange.,, the type of driving being done hasn’t changed either!

[FJB]

When my mpg dropped it was the no 2 injector that was bad. At idle VCDS was showing a slight missfire on cylinder 2. I moved the coils and plugs around but the miss stayed on cylinder 2. I did a compression test and had the same results for all cylinders. I tried some injector cleaner, but it made little difference. I replaced injector 2 and the missfire went and mpg returned to normal.

[fakie1977]

Sticking calipers another possible cause.


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[mjmallia]

Carbon build up also reduces efficiency and then gets to a point where it will get mild misfires before they get worse.

A failing MAF and/or lambda sensor can mess with fuel and air mixtures too.

[mcblfc88]

Would most of these issues not throw up EMLs?

I think the sticking caliper could be the one! That comment reminded me that it was mentioned one of the rear calipers was a bit sticky when we got the car 2 years ago, then a few months ago a noise appeared that got worse when you lightly touched the brakes but went if you pressed the pedal hard, it’s all coming together now and I’d imagine that is the issue... will check it out today

[bobby_fodge]

I had a rear sticking caliper. It was red hot when I came to a stop.

[Havelin150]

I had very bad mpg when my thermostat was sticking open. Effectively running rich all the time trying to get to temperature. Does your engine warm up and stick at 90 on the dash? If it drops on motorways then this couple be your issue.

[mjmallia]

Would most of these issues not throw up EMLs?

I think the sticking caliper could be the one! That comment reminded me that it was mentioned one of the rear calipers was a bit sticky when we got the car 2 years ago, then a few months ago a noise appeared that got worse when you lightly touched the brakes but went if you pressed the pedal hard, it’s all coming together now and I’d imagine that is the issue... will check it out today

Never had any on mine when I plugged it into VCDS

[mcblfc88]

So sticking caliper looks to be the issue.

Went for a short drive yesterday in the car, came back and checked the disc temps with my IR temp gun... both fronts and drivers side rear were around 28 degrees, passenger rear was 60...

Is it worth rebuilding the caliper or just get a new one? I see there are plenty of aftermarket calipers available remarkably cheaply

[shail]

Could it also be a sticking handbrake, rather than caliper? 

[mjmallia]

Rears are a common problem and would just change it out.

[bobby_fodge]

So sticking caliper looks to be the issue.

Went for a short drive yesterday in the car, came back and checked the disc temps with my IR temp gun... both fronts and drivers side rear were around 28 degrees, passenger rear was 60...

Is it worth rebuilding the caliper or just get a new one? I see there are plenty of aftermarket calipers available remarkably cheaply

I just stuck a new one on. You can rebuild it with Big Redd parts. Depends if you can handle the car being off the road whilst you rebuild the caliper.

[mcblfc88]

I’ll just fit a new caliper, not worth the hassle of rebuilding this one

[pudding]

Get new calipers, not worth p1ssing about rebuilding with shagged ones in my experience.  It's not really a 'rebuild', rather a band aid attempting to drag them out for a bit longer.

Get a pair, because we all know it, once you've done one side, the other side will go soon after.

It's a w@nky design of caliper and is an 'every 5 year' job it seems.  For regularly used cars that see winter road salt at least.

If you're on a budget, get a pair of aftermarket unpainted TRW units and just sprinkle on some paint yourself.  OEM calipers are eye watering, but they are just the same TRWs painted red for you.

Pay attention to the carriers.  Grease up the sliding pins but if they are rusty and dry, it's too late, the damage has been done, replace the carriers as well.