General Silver tube on exhaust manifold purpose

[tobywood13]

Hi all, recently after some slightly disappointing fuel economy figures I did some research on ways to improve it. I came across the warm air pipe that sits on the top of the exhaust manifold and goes into the bottom of the airbox, some people suggested having this disconnected would worsen fuel economy.

I've had two Pandas, and on both this pipe was always disconnected, standing shiny and proud at the front of the engine bay without me ever knowing what it was for. I have now connected it to the air box, and immediately I noticed the car's performance seemed to worsen slightly when stomping on the throttle at low revs, as if it were bogging down slightly. It felt a bit like too much fuel with not enough oxygen, a slight delay before revs picked up and the acceleration kicked in. Or maybe I'm imagining it.

I have a 1993 999cc single point injection car, so I suppose it would be vulnerable to fuel icing in these winter months and I should probably leave it connected. My question is, does connecting this tube actually improve economy? In my head, having warmer air entering the intake would be a bad thing, as it would be less dense and therefore less oxygenated. This would cause a slightly rich mixture, worse power, and worse economy. Is this correct?

Any thoughts on whether the silver pipe does make a difference on performance/economy? Is it something you're supposed to connect in the winter but disconnect in the summer?

 

[tobywood13]

General update. Completed my first tank of fuel since re-attaching the silver pipe. Mostly B-road driving, no motorways. Averaged between 40 and 60mph. My MPG is now down to 37MPG, compared to 43-45MPG average before.

Not sure if it is just the pipe or if another problem has developed, but I will leave the pipe reattached for one more tank of fuel, then detach and see if there is a noticeable improvement. Perhaps the pipe is intended only for cold-weather driving to prevent icing? It seems to be a very significant drop to have been caused just by some hotter than usual air.

 

[Didge3]

The air box should have a flap inside it (I believe) which only opens to the warm air intake if the temperature is cold enough, I think in the UK climate it isn't really necessary, back on my old 750 the pipe had a split in it so most heat was probably being lost anyway.
It shouldn't really have much of an effect on the fuel economy, if it were stuck open you might see slightly less power as you're drawing hot, less oxygen dense air all the time but the effects on fuel economy are very low

 

[tobywood13]

The air box should have a flap inside it (I believe) which only opens to the warm air intake if the temperature is cold enough, I think in the UK climate it isn't really necessary, back on my old 750 the pipe had a split in it so most heat was probably being lost anyway.
It shouldn't really have much of an effect on the fuel economy, if it were stuck open you might see slightly less power as you're drawing hot, less oxygen dense air all the time but the effects on fuel economy are very low

Interesting. I’ll check to see if the flap is stuck open, otherwise I’ll probably just disconnect it until I suffer icing problems (unlikely).

 

[tobywood13]

It shouldn't really have much of an effect on the fuel economy, if it were stuck open you might see slightly less power as you're drawing hot, less oxygen dense air all the time but the effects on fuel economy are very low

Took the air box off, the flap is indeed stuck in the open (cold) position. I soaked it in hot water for ages, and it wouldn’t budge. Interestingly when manually opened, it would close when cold water was dribbled on the thermostatic component, but not the other way round.

I’ll try some WD40 and wiggling it around, but this would certainly explain one reason why my MPG is down. Is there a good method to free the flap off so it operates properly?

 

[Didge3]

Warmer intake temperatures typically increase MPG, this is due to the higher intake temperature having less air density meaning less fuel is injected to keep the air:fuel ratio correct, this in turn has the side affect of reducing engine power so you need more throttle to maintain the same speed which then equals lower pumping losses as you've got the throttle open further.
However with all this in mind the measureable amount would be quite negliable, certainly not 6+MPG

How recent in this drop in MPG? I always find my MPG falls in the winter, especially on short trips as the engine spends longer running rich until it is up to temperature

 

[RalphM]

Icing is more a carburettor thing it usually happens when the air is moist and just
above freezing within a mile or two of start up, engines run best on cold denser air
you get more oxygen and explanation, if you cant get the flap working correctly
I would leave the pipe off.

 

[tobywood13]

How recent in this drop in MPG? I always find my MPG falls in the winter, especially on short trips as the engine spends longer running rich until it is up to temperature

I’ve only had the car a month so haven’t seen any change, just from the research I’ve done it seems to be lower than it should be. When I connected the pipe it did get noticeably worse though, which makes sense as the flap is stuck. I’ll disconnect the pipe and have a go at freeing off the flap.

 

[Didge3]

Could be low for all manner of reasons, poor spark or air restriction would be a couple of easy ones. Have the spark plugs, air & oil filters been changed recently?

 

[bugsymike]

As has been mentioned many things can affect fuel economy.
As a small example in the 1970s we had a customer who bought a brand new Mazda and complained bitterly to us about it's fuel economy, we thoroughly checked and got excellent results, finally after many months we got the area rep to engage with the owner and on road test was getting good figures with the owner driving. Next he was asked what his typical driving was.
It turned out he worked as a car park attendant, would drive from his home around a mile to work on choke (pre fuel injection) park until lunchtime, drive home the same way, back to work an hour later and the same process in the evening! He was convinced to fill his car with fuel, do a long drive , the refill and check his economy, that was the end of his complaint. so driving conditions have a large influence.
As a youngster I had a V6 Ford Zephyr that I used to drive like a total di**head and averaged 12 mpg around town, on a fast run to London I more than doubled that figure. Mind you then petrol was 35 pence per gallon (yes 4.5 litres) not the stupid price we pay these days per litre.
Generally the reason for the "hot spot" pipe from exhaust warming the inlet air is to aid response from cold in cooler ambient temperatures.
Most people in my experience do not notice if that function is working or not, even after I find it faulty and correct it when servicing the vehicle.
The performance increase on a cold /damp morning as mentioned due to dense (oxygen rich) air was more apparent in the old days of carbs, before ECU controlled fuel injection connected to multiple sensors etc.

 

[tobywood13]

It’s on fresh oil, fresh plugs, new ignition leads. Commute to work is 30 miles each way, so plenty of time to warm up and (in theory) switch to its most efficient running conditions. I’ve disconnected the pipe now, will see if it makes a difference at my next fill up.

 

[bugsymike]

It’s on fresh oil, fresh plugs, new ignition leads. Commute to work is 30 miles each way, so plenty of time to warm up and (in theory) switch to its most efficient running conditions. I’ve disconnected the pipe now, will see if it makes a difference at my next fill up.

All sounds good, temp gauge in middle (90 degrees) cruising at 2000 rpm in top gear, not stuck in traffic.
No brake drag, soft tyres, tracking out wearing tyres and causing drag?
Even lights, heating, aircon all have an effect. Sorry air con?
I see on your second post you mention B roads, are you now enjoying more "spirited driving" now you are reliving the pleasures of driving nippy small cars?
Also 60 mph in older cars was fairly high up the rev band compared with modern 6 speed models doing that at under 2000rpm.

 

[tobywood13]

All sounds good, temp gauge in middle (90 degrees) cruising at 2000 rpm in top gear, not stuck in traffic.
No brake drag, soft tyres, tracking out wearing tyres and causing drag?
Even lights, heating, aircon all have an effect. Sorry air con?
I see on your second post you mention B roads, are you now enjoying more "spirited driving" now you are reliving the pleasures of driving nippy small cars?
Also 60 mph in older cars was fairly high up the rev band compared with modern 6 speed models doing that at under 2000rpm.

I do a mix of A and B roads mostly, not much town driving/traffic. Tyres are ok and properly inflated, tracking does need doing but it’s not terrible. Don’t think the brakes are dragging, but I’ll see if any feel especially warm after a drive.

Temp gauge at 60mph is around 75 degrees, is that a bit low? Perhaps the thermostat is stuck open.

 

[bugsymike]

I do a mix of A and B roads mostly, not much town driving/traffic. Tyres are ok and properly inflated, tracking does need doing but it’s not terrible. Don’t think the brakes are dragging, but I’ll see if any feel especially warm after a drive.

Temp gauge at 60mph is around 75 degrees, is that a bit low? Perhaps the thermostat is stuck open.

I personally like to see around 90 degrees and in the centre of the gauge after the first mile or so if possible, so by then in Winter the car should feel warm and demisting well.
Cars run more efficiently and quieter when in the middle of temp gauge, it also reduces condensation in the engine oil.
In the old days the common thermostats ran at 82 degrees Centigrade and I found a 88 degree one a good improvement, mind you the thermostats were made by Smiths industries and were interchangeable between manufacturers unlike today.
One of my daughters had an issue with DPF light warning and apart from using the additive in the fuel and giving it a good run which fixed , I noticed her temp gauge ran around a third so quite cool especially as her run to work was short. I replaced her thermostat ( a right sod of a job behind the alternator on a 1.6 VW Golf diesel) however it has much improved things.
Others on Forum will tell you what temp your thermostat should open, but it sounds pretty low to me, so may well be stuck open.
I found this in one of my old manuals for the early Pandas, it does seem to indicate yours is running cool.

 

[tobywood13]

I’ll add a new thermostat to the list. Probably over the Christmas break I’ll do a thermo replacement and flush the entire system (again). Couple of drippy hoses need replacing too so it’s a good excuse to go over everything at once.