Flat spot with air con on

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Chrispy
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Flat spot with air con on

Post by Chrispy »

Engine query this time for you folks!
On my TD Xantia, whenever I have the air con on, at about 2000rpm, there is a rather noticable flat spot, then a sudden surge of power after it. This goes away with the air con switched off. Now, I have adjusted the idle control lever on the back of the fuel pump to permanently be on the hot setting at 850rpm as the idle control mechanism (a vacuum pump thingy, a relay and a lever) doesn't work. Are the 2 related? Does the timing need to be advanced all the time with the air con on to handle the extra load? Is this a normal symptom I am seeing?
When I first got the car the idle was set on the high position permanently as the cable was slack due to the idle control thingy not working, and the air con didn't have any effect on things then.
Cheers,
Chris.
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Kowalski
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Post by Kowalski »

The idle cable on mine seems to get pulled or released at all sorts of different times, i.e. hot / cold idle adjustment is just one of the things it does. For example, when the engine is cold, revving it gently the engine speed goes up, the comes down to hot idle before the cold idle takes it back to the hot idle setting. Conversely when its hot you can get the opposite to happen, i.e. hot idle.
The cable is also used to raise the idle for the air con, but if you've got it stuck in position there should be no reason for turning the air con on to have any effect on your engine, unless it talks to the ECU and the ECU adjusts the timing.
I can't see any reason for the air con system in itself to be varying the load on the engine to cause a flat spot at a particular speed.
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Post by NiSk »

I think you will find that most cars with AC suffer from this effect, especially turbo diesels with restricted power outputs. I know that my old XM TD12 with the 2.1 litre turbodiesel engine certainly became slow off the mark when it was hot and the AC was turned on. I had to turn off the AC if I was going to do any spritely overtaking, otherwise it seemed like ages before the engine "came on turbo". This engine has relatively little power/torque below 2000 rpm and the added load of the AC (which can steal up to 10-14 hp) really causes the turbo to take along time to wind up. And yes, it feels just like flat spot!
//NiSk
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Post by CitroJim »

Absolutely Nisk, Go along with that totally. All the 1.9 TDs I've known are dead below 2000 and then off they go... Peak torque occurs at 2200 rpm if what my tacho tells me when towing is correct! This is the reverse of a Normally Aspirated 1.9 which runs out of breath at 3500 rpm but pulls like a tank from very low rpm. At least the TD keeps on going until the speed limiter....
My old Pug 405 refused to start if the aircon was accidentially left on when stopping the engine..
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Post by Chrispy »

Really? 10-14hp???? Bloody hell..... It's almost like someones stuffed a rag up the exhaust pipe which is suddenly removed at varying points in the rev range. It's not the end of the world but it can feel annoying at times, almost like the engine's hesitating every son often, especially if you're on and off the throttle like in town.
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Post by CitroJim »

When I towed with the 405 I never used the aircon for the reason it consumed so much power and arguably, that's just when it's most needed. My Xantia is a non-aircon model and I'm actually quite glad. no access problems in the engine bay, no complication and no power loss. I find the combination of 4 open windows and a wide open sunroof works well for me. On the 405 I really really missed a sunroof giving air in the summer and a nice ight, airy cabin in the winter. I may create a few enemies by saying this but in the UK an aircon is a bit like a rag-top: Great on the two days in a year when it's nice enough to really enjoy it but a pain on the other 363 days of the year...
Anyone know how much power the hydraulic pump consumes when it's working?
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Post by RichG »

Chris,
I have had my car from new and it has alwys had a hesitation at about 2000 rpm with the air-con on. It hasn't got any better or worse so i just live with it.
Richard
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Post by alan s »

On the series one CX with air/con they had a "step up" that increased the revs when the air/con was switched on. With EFi cars I understand it's done via the ECU and I always get the impression that it throws the settings a bit out of whack as it's a flat spot like a dodgy accelerator pump on the carby cars rather than a power drop.
Air/con in Cits is possibly the most efficient I've ever come across and added to this, it has very little effect on performance overall apart from this flat spot that's being spoken about, that is if it's operating as it should.
My son has recently fitted air/con to his BX 16V and already had a MoTeC programmable EMS in it, so he set it so that when the accelerator pedal is above 80% the compressor cuts out and when backed off to around 4000RPM cuts back in again. That way he gets the best of both Worlds; air conditioned comfort and no loss of performance when overtaking. I have heard of similar systems being used on other cars in a more primitive manner by fitting micro switches to the accelerator pedal.
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Post by jeremy »

Does the cutting in and out at high speed affect the aircon clutch arrangement adversely?
I've never really thought about it but do remember someone telling me that he always switched it on at idle to lessen the load on the clutch. From what I remember from my Jaguar days there was nothing you could do about it - if you were running the aircon it clutched in and out whenever it decided it was the right time. (And yes - I did have a clutch disintegrate on the Jag!)
jeremy
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Post by alan s »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by jeremy</i>

Does the cutting in and out at high speed affect the aircon clutch arrangement adversely?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Not really. Car air/con systems are designed to operate under extreme conditions and realistically, the compressor should be doing more cycling at high revs and speeds than low due to the greater volume being pumped at the higher revs which is circulating the gas faster within the system and increasing efficiency and the greater volume of cooling air passing over the condensor, so it should be cutting in and out more often anyway.
I once was speaking to a guy who had done this arrangement on a GS which would have been operating at extreme loads and he had no problems so I can't see it happening on a modern system.
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Post by oscarloco »

Same problem on my car. But in the right conditions, say leaving a red light on low revs and the compressor kicking in at just the right time makes my engine stall. Hate when that happens.
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