New exhaust affecting idle speed?..

Ok weird one but I've just stuck a stainless system & fanny moulds on the old girl (lse) and I assure you I touched nowt else and despite knowing she's due an engine rebuild that I've not finished yet the car always ran and idled fine but now it's doing the start up rev malarkey and the idle is not settling below 1100 to the point it's annoying while driving as it's still pushing the car along on a closed throttle?..
I used my lambdas from my old leaky original system which are only a couple of years old any ideas?
I was gong to just check base idle anyhow but seems strange that the idle has been altered

Without doing some investigating I suspect the answer won't be obvious, that said changing the exhaust may have altered the way the engine breathes by changing back pressure and depending on your cam it is not impossible that the new exhaust system has changed the way the engine idles and if the ECU is not able to compensate with the stepper then it might result in a higher idle.

All I can offer is to check your ignition timing, clean your stepper, or at least make sure it is working then with ignition timing correct, set your base idle to 500rpm and see how it goes.

TheEFI ECU should have compensated as it did on my engine after fitting a cat-back system, on starting up the rpm went straight up 2000 and stayed there for a minute and then dropped down to the 700s as normal, if it hadn't I would have checked my engines base idle.
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Thanks guys, took me a while to find the time to fiddle with the old girl.
I'm not entirely sure what fixed it but it's running sweetly again now.. I checked the timing which was a couple of degrees out, guess that tired old timing chain is getting longer.. Checked base idle roughly and fitted a good 2nd hand stepper but didn't seem to cure it.
Went out to it again today, timing still fine, checked base idle properly with a tacho which was a bit high at 650 so sorted that, pulled out stepper, chucked o ring away and seated it on some silicone instead and voila! Revs to about 1200 upon start and very quickly settles to 650-800 depending on its mood..
I can only guess the stepper gasket was letting by and the high base idle wasn't helping. Glad about that as the new exhaust has definitely perked up performance and response, probably mostly due to the old original manifolds and down pipes leaking like a sieve for ages..
Thanks guys

Thanks for letting the Forum know the outcome, I'm clad u had a good result as u deserve after the money u must have spent on a full system.

it was nice to have an easy result this time.. To be fair in the decade I've had the car it's rarely run badly and other than one coil, lots of plugs and a pair of lambdas it's needed nowt in the running dept. it's usually rubbish bushes or transmission bits that fail!
The full system was a lucky find on a totally rotten rangie which was sadly off to the crusher..