Help! 944 Idling Problem
Discussion
Good advice and quite likely the cause - engine temp sensor or airflow metering unit temp sensor - also air leaks on inlet pipes/manifolds etc, etc but I am sorry to put a spanner in the works but as well as all the possible faults that could cause this - the 944S can also develop this fault as the inside of the engine carbons up, the rings wear, the cam timing wanders out (due to wear on belts, cam belt sprocket teeth (like the gear teeth shape that the belt runs on), cam chain stretch etc and the resulting deterioration of design breathing parameters results in the mixture not being right or soaked into the carbon. If this is the case then setting the cam timing a little earlier than stated in the book spec to produce better bottom end breathing at the expense of a little top end power - makes the engine much more torquey and more like an 8 valve to drive and in view of the age of the car - probably a more sensible way to look after the engine anyway. Unfortunately as the belts and chains stretch they all move the inlet cam timing the other way (later) and therefore away from good low rev breathing.
Baz
Baz
hartech said:
Good advice and quite likely the cause - engine temp sensor or airflow metering unit temp sensor - also air leaks on inlet pipes/manifolds etc, etc but I am sorry to put a spanner in the works but as well as all the possible faults that could cause this - the 944S can also develop this fault as the inside of the engine carbons up, the rings wear, the cam timing wanders out (due to wear on belts, cam belt sprocket teeth (like the gear teeth shape that the belt runs on), cam chain stretch etc and the resulting deterioration of design breathing parameters results in the mixture not being right or soaked into the carbon. If this is the case then setting the cam timing a little earlier than stated in the book spec to produce better bottom end breathing at the expense of a little top end power - makes the engine much more torquey and more like an 8 valve to drive and in view of the age of the car - probably a more sensible way to look after the engine anyway. Unfortunately as the belts and chains stretch they all move the inlet cam timing the other way (later) and therefore away from good low rev breathing.
Baz
Baz, Baz
Very interesting. Surely this will apply to all models, not just the S? (Certainly to the S2 as well.)
Oli.
When several manufacturers first introduced 4 valve heads, most mated them with inlet systems that were too big. This resulted in more bhp at peak revs but less torque at low revs (Golf and Peugeot Gti's etc). Many owners then found the old 2 valve head versions quicker than the new 4 valve versions despite the extra bhp claimed because overall the rev range they could use within the limits of the gear ratios gave a similar overall performance. Probably driven like a racing car to max revs the 4 valve head cars would be marginally quicker but most people do not drive like that all the time and start going quicker from lower revs or change up a little earlier - benefitting the improved bottom end of the 2 valve headed cars.
I don't want to get into arguments about torque versus bhp ever again but the upshot was that eventually manufacturers found ways to make 4 valve heads work well at low revs as well.
The 944S was typical and the consequence of having too large an inlet system is comparatively low gas speed at low revs and poor breathing efficiency. They solved this by using the same head on the S2 with an extra 20% capacity which improved inflow at low revs and produced much more torque. The same solution could have been achieved by reducing the sizes in the inlet system - but the extra capacity also made the overall performance better as well.
It is this low air speed and general iefficiency at low revs that results in problems setting up the idling mixtures because any slight variations in wear or timing etc mess up the original parameters beyond it running well. The S2 with more "suck" still works well even as things age and run out of spec.
More modern engines have gone to vary the inlet timing and lift to create good breathing over a wider rev range.
So in conclusion the S will always be relatively unstable at low revs compared to an S2 and more of a problem to set up as it ages. Making the inlet timing earlier simply improves breathing at the lower end but is compensated for by reduced efficiency at the top end. But if the probllem is at the bottom end it is a solution and you would hardly notice the loss at the top end anyway.
As stated it is important that the throttle switch works when the throttle is closed - it is only an ON/OFF switch and you can test it with a contiuity tester. This enables the idle speed controller to work and try and keep the revs right. They can also stick or wear out but can be cleaned with an unltra somic cleaner and may work again. The ECU uses 2 resistors to move the idle speed controller one way or the other and one of these can fail but can be replaced.
Good luck.
Baz
I don't want to get into arguments about torque versus bhp ever again but the upshot was that eventually manufacturers found ways to make 4 valve heads work well at low revs as well.
The 944S was typical and the consequence of having too large an inlet system is comparatively low gas speed at low revs and poor breathing efficiency. They solved this by using the same head on the S2 with an extra 20% capacity which improved inflow at low revs and produced much more torque. The same solution could have been achieved by reducing the sizes in the inlet system - but the extra capacity also made the overall performance better as well.
It is this low air speed and general iefficiency at low revs that results in problems setting up the idling mixtures because any slight variations in wear or timing etc mess up the original parameters beyond it running well. The S2 with more "suck" still works well even as things age and run out of spec.
More modern engines have gone to vary the inlet timing and lift to create good breathing over a wider rev range.
So in conclusion the S will always be relatively unstable at low revs compared to an S2 and more of a problem to set up as it ages. Making the inlet timing earlier simply improves breathing at the lower end but is compensated for by reduced efficiency at the top end. But if the probllem is at the bottom end it is a solution and you would hardly notice the loss at the top end anyway.
As stated it is important that the throttle switch works when the throttle is closed - it is only an ON/OFF switch and you can test it with a contiuity tester. This enables the idle speed controller to work and try and keep the revs right. They can also stick or wear out but can be cleaned with an unltra somic cleaner and may work again. The ECU uses 2 resistors to move the idle speed controller one way or the other and one of these can fail but can be replaced.
Good luck.
Baz
hartech said:
When several manufacturers first introduced 4 valve heads, most mated them with inlet systems that were too big. This resulted in more bhp at peak revs but less torque at low revs (Golf and Peugeot Gti's etc).
It is this low air speed and general iefficiency at low revs that results in problems setting up the idling mixtures because any slight variations in wear or timing etc mess up the original parameters beyond it running well. The S2 with more "suck" still works well even as things age and run out of spec.
More modern engines have gone to vary the inlet timing and lift to create good breathing over a wider rev range.
So in conclusion the S will always be relatively unstable at low revs compared to an S2 and more of a problem to set up as it ages. Making the inlet timing earlier simply improves breathing at the lower end but is compensated for by reduced efficiency at the top end. But if the probllem is at the bottom end it is a solution and you would hardly notice the loss at the top end anyway.
Good luck.
Baz
It is this low air speed and general iefficiency at low revs that results in problems setting up the idling mixtures because any slight variations in wear or timing etc mess up the original parameters beyond it running well. The S2 with more "suck" still works well even as things age and run out of spec.
More modern engines have gone to vary the inlet timing and lift to create good breathing over a wider rev range.
So in conclusion the S will always be relatively unstable at low revs compared to an S2 and more of a problem to set up as it ages. Making the inlet timing earlier simply improves breathing at the lower end but is compensated for by reduced efficiency at the top end. But if the probllem is at the bottom end it is a solution and you would hardly notice the loss at the top end anyway.
Good luck.
Baz
I looked this up recently and in comparison to other engines (specifically those tuned for special applications like MB 190 cosworth or racing) the valve sizes in the S would be ideal for an engine tuned up to make much more power. It really should have been making more like the peak power of the S2.
As for the high end breathing well it is likely that any pulse tuning in the inlet manifold is designed to give maximum volumetric efficiency (VE) at the point where most torque is made or rather is required. This tends to mean that it strangles the engine up at 6000 rpm or more. I wouldn't be at all surprised if swinging the cam looses maybe 5 bhp peak but gains 5 hp everywhere else, which in turn will make the car much faster.
I have some graphs of the result in changing the cam timing and will look them up anf try and post them after the holiday - but basically the valve and inlet size is too large to get enough air flow speed at low revs and the timing allows too much to spill out again as the inertia is so low.
You would have had to reduce valve and casting sizes to sort it out whereas effectively that is what they did by leaving it the same and increasing capacity - otherwise if they had just mirrored the engine @ 20% increased capacity thet would have had to increase valve and inlet sizes proportionally.
The earlier timing does make for a better drive but it is only a compromise.
Baz
You would have had to reduce valve and casting sizes to sort it out whereas effectively that is what they did by leaving it the same and increasing capacity - otherwise if they had just mirrored the engine @ 20% increased capacity thet would have had to increase valve and inlet sizes proportionally.
The earlier timing does make for a better drive but it is only a compromise.
Baz
If the cam sprockets are worn & you need new camshafts. Jon Milledge engineering have produced a new profile that works much better than the standard cams. The kit is cheaper than oem. Will still cost 1K to do it all but if the car is good enough would be alot of fun. OF course the rest of the mechanics need to be in good order to take the extra horsepower which may not be the case. Strangly this is the most tunable model though it is the least liked!
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