Can an exhaust leak cause poor idle

Could an exhaust leak in the mid pipe
section, under the heat shield cause poor idle?

David …

A rough idle is one of the hardest things to diagnose because it can be caused by mechanical, electrical, fuel problems, or vacuum leaks.

But I can’t see how a leak in your exhaust can possible cause it. Especially since your leak is after the O2 sensor.

I would start my troubleshooting by doing a compression check. This will give you a good idea of the internal condition of the engine. Next check for vacuum leaks and then move on to plugs, wires, cap, and rotor. last but not least low fuel pressure (fuel pump), bad fuel pressure regulator or fuel injector problems.

Usually with a car 3 decades old it’s not just one thing but a lot of small problems.

Could be, if massive. Can also affect lambda reading due to higher flow rate.
How rough is your idle? Everyone perceives something different under this term…
Here’s the reference for AJ6:

Is it shaking badly always or sometimes? Any different when starting from cold? Idle stabilises when placed in Drive etc -?

Janusz …

The O2 (lambda) sensor in the '89 XJ40 looks at the percentage of oxygen particles in the air (exhaust) leaving the engine. Since there is only one and it is located before the catalytic converter It doesn’t care about anything past that point where it’s installed. Depending on that percentage of O2 information the ECU then trims the fuel to the injectors by changing the duration and timing of the fuel pulse to each injector. Of course this is in addition to the other sensor inputs, water temp, crank position, etc.

I disagree that scavenging the exhaust quicker will add more or less O2 to the exhaust mixture prior to the O2 sensor.

I should clarify, it is hunting at idle sometimes but not always. A/C on helps and sometimes it purrs all day. Done most of the normal things mas , set .002, new t stat sender ,plugs rotor cap, etc.

On my 1991 with the A\C on the idle is smooth but once I shut it off the idle gets rough like it’s missing. Weird. Haven’t had time to mess with it though still trying to find my coolant leak

Mine acts the same , hunting when ac is off.

David you might want to read this …

A few years back I solved a searching idle problem by removing and disassembling my IAC valve. I gave it a thorough cleaning (it was full of grease and carbon) and after reinstalling it the engine idle was greatly improved.


Maybe I’m aligned too much to the modern engine’s ECU…
Maybe I’m not.

The exhaust is tuned to the nominal pressure. This pressure defines the amount of gases in the gas. This pressure also protects your cylinder head. That’s why you should not cut/change it in the length of 60ish cm from the engine. So the amount of O2 will be different with different pressure of the exhaust gases…
Havung the leak in exhaust downpipe and ignoring is sounds like carving in poop.

Plenty of causes. To eliminate all you may have plenty of work. Start from the easiest one. What is your mileage?

  1. Primarily -Any vacuum leaks will cause that
  2. Start the engine with disconnected stepper motor. Improvement suggests it’s simply worn
  3. Alternator. Rear (small) bearing is the common cause. Remove the belt and start the engine
  4. AC compressor - ssme as per pt.2
  5. Spark plugs / leads /distributor
  6. MAF sensor (diconnect it and start in limp mode)
  7. Failing bearing on viscotic clutch - look for reddish dust in the area of input shaft
  8. Valve clearanances - can’t do much with those apart from checking (remove rocker cover)

All idle issues will occur without the engine load (hence better in Drive/with AC on)

Ill second the IAC valve-motor sticking, if its shaft is not super clean it will not be able to respond fast and smoothy. Cheap ones available at RockAuto but give shaft a good clean… I was able with a signal generator to extend the shaft and clean it but spray the heck out of it…and reinstall.

Hello Steve - Have you found the solution for the “idle hunting” problem on your 1991 - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 11/26/2020 0001hrs. EST USA.

No not yet, I usually drive with the A/C on so it doesn’t really bother me. What’s weird is I believe it only happens when I use A\C and turn off A/C but if I never turn the A/C on it idles fine. I’m actually trying to find a coolant smell when driving, that I think it might be a head gasket :cry: might try Barr’s before I go that route though and prolly take it to a professional to get their opinion

Hello Steve - wonder if you have a leak in the heater core, since you state that you can smell the coolant while driving - even though our cars are different, I am still curious as to the reason for the “idle hunting” - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 11/26/2020 1326hrs. EST USA.

Hello David - have you been able to find the reason for your “idle hunting” problem - I see that you have check many items - I also see that Steve has the same condition of the “idle hunting” after use of the AC - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 11/26/2020 1329hrs.

I had a problem with windows filming up, Barry leak fixed it

I am going to try setting the idle as per the above link on Saturday,I will let you know how it turns out.

That’s usually because your A/C drains are blocked

Steve …

There are a lot of places that you can have a coolant leak but I think the type of head gasket leak you’re talking about is the least likely. Of course an internal coolant leak from a cylinder into a water gallery is a real possibility (I just had that happen not too long ago) because that’s where all the pressure is but you wouldn’t smell anything.
head gasket

For a water gallery to leak to the outside without a cylinder involved is rare because the water isn’t under that much pressure. So don’t be in such a hurry to pour any crappy water sealer that can cause more problems than it solves.

The key is to do a compression check to make sure the head gasket isn’t leaking from cylinder to cylinder. This could be the source of your poor idle.

If you’re smelling antifreeze in the cabin with the windows rolled up I agree that the heater matrix or it’s fittings are a serious suspect.


totally agree - lots of places a small sepage can vaporize in the engine compartment and be drawn into the cabin, heater valve to cylinder head connection/hoses come to mind