ok here is one for you,

91 EB, start it in the morning and it behaves how it should, idles properly drives properly, drive it 1/2hr to town and slowdown, pull up at lights and it starts to idle realy rough, like it is not firing on a cylander, take off from lights, all is well again, pull up at next set of lights and it starts the rough idle again, give it a boot full from the lights and by the next set it is not doing it as bad, drive like a granny to the next set and it is doing it bad.

I bought a new rotor button as I have never changed one since I owned it and it was starting to look a bit tacky, put it in - no change, pulled the ISC off cleaned it out made sure it sealed, put it back on - no chance, pulled the ECU out to make sure the pins were ok and they were, reset the ECU did a self test all seems ok, but still no change.

It has not lost any power, with it sounding like a lumpy V8 at the lights give it a bootful and it loves it sits u back in your seat doesn't miss a beat yet when you stop it is not happy.

What do you think I should try next?

It has 10mm leads on it and if the leads were at fault I would of thought it would be unhappy under load. The plugs are new as well about 5000k ago and are motorcraft platinums

Jeff

sounds like a lead or dizzy cap problem to me. how old are the leads? and cap?

Dissy cap got replaced with the plugs

ah ok, leads? also temp sender might be faulty causing mixture problems??

Should be a simple answer, take it to a mechanic with a scope, should be able to nail the problem pretty quick.

James

Could possibly be a vacuum leak too, not ignition. Check your hoses and manifold too.

I am not taking it to a mechanic, no one I would trust here, and some of the stories I have heard, anyway I dont think I have ever taken any of my cars to mecahnics, I always find the problem and fix it,

I might try the leads first, they are 10mm Top Guns and I know what the opinion of top guns are on this site. and they were on it when I bought it.

Yeh had a bit of a scount around, couldn't find any even checked the PCV valve, seems ok too

I tested some brand new top gun leads resistance the other day out of curiosity. They were about double that of some 30,000km old Bosch leads. and wouldnt fit right on the dizzy. (Weren't for a Falcon though)

Sounds like a vacuum leak to me too, but it could be a number of other things too, running the fault codes would be my first step,

A faulty or dirty MAF sensor can cause very lean running and bad idle, if you run a reusable air filter like a K&N this is quite common. It could also be dirty or blocked injectors, might be worth running some injector cleaner through the petrol.

I pulled the MAF out, It looked clean but I gave it a touch up anyway, and it is running a K&N

Yeah even if it looks clean, the K&N oil can coat the sensor wires which throws the readings off and makes the engine run lean. Lean conditions at idle can often cause a rough idle, especially on older engines.

Best way to clean it is to remove the electronics and spray the sensor wires with carby cleaner, also spray the body and air passages with cleaner too.

Yeah vac leak worth a look, may be that something is 'opening up' as the engine heats up and stuff begins to expand, may explain why it takes a bit to come on.

Hook a gauge up if possible, or do a physical inspection of the hosese/manifold.

Would think it'd have to be a fairly bad leak to cause that kind of idle.

Not sure that it is a vacume problem, it has good vacume, as in it is not loosing it anywhere, I pulled a line off while it is running and it is sucking hard. I am leaning towards an injector, it realy feels and sounds like it is not firing on a cylander and it is progressivly getting worse, as in when you take off it is taking longer now to return to normal.

earlier when you opened the throttle it would instantly be better again, but now it has a bit of a cough and splutter and then goes back to normal.

Ok done a bit more investigation, poured some injector crap into the old girl and it didn't make any difference, wasn't sure it would I only had the injectors cleaned about 10k ago, got a new set of leads for it, goes like a charm again

thanks for all your help

Jeff

Jeff,

similar problem here...EB XR8 1991....exactly issue as you describe. I have pulled the injectors down and had them cleaned, replaced the HEGO sensors, new dizzy cap, rotor, plugs, leads, reset ECU, cleaned the MAF sensor and had the thing down to specialist tuners. They replaced the temp sensor switch because the test gear said it was faulty and checked all vacum lines for leaks by spraying carby cleaner around and looking for bubbles etc.. No problems found tho. He said that on the dyno the thing would lean out beautifully under load, then when she returns to idle after a while the mixture seems to get a bit too rich...or something like that!

She has done nearly 270,000kms and still pulls hard...doesn't leak a drop of oil...but has this intermittent rough idle going on.

My last option that I will try on the weekend is something the tuner pointed out. He said on test drive, that he felt the car was running too cold. Its true that my temp gauge hardly ever moves off of the blue marker, except on hot days. He reckons that the ECU is very temp sensitive and if the thermostat is no working proberly it can effect it in terms of mixtures.

I am gonna replace my thermo and clean out the throttle body while I am at it and see what happens. I may even look the the fuel pump as well...tho I am not sure whether a faulty pump would cause something like this. After that I will have to try looking at mechanical stuff...compression test etc.. But the power and perfromance of the engine just doesn't indicate engine probs.

Good luck with your quest too, Jeff. If you have any sucess in finding a solution please PM me or post an update and I will do the same.