Classic LSE cutting out at idle...

Hi folks,

Car is a '93 LSE hard dash with a 4.2 V8 Petrol.

Problem started off with the car starting from cold with no issues, but would hunt and cut out after a few seconds. On the second attempt, the car would start and run perfectly all day, with no further cutting out even after being parked up for a while. I'd say the idle was a little rough even when warm, however sits at 750rpm on the rev counter. Up to speed though, and it ran smoothly with no misfires.

Within the last two weeks, the problem has got progressively worse, with it taking a few starts each morning to get it to hold an idle so I can drive. Now today, it simply cuts out as soon as I start it so it's now undriveable. Coming back from the last drive, I let it rev out in second gear, and I'd say it felt down on power, like something was holding it back. Not massively, but enough to be noticeable.

No warning lights on the dash. All ignition components replaced with genuine items and the coil is a new Bosch item. Ignition amplifier was replaced at the same time as the rest of the other components. Alternator and battery are new as well.

Any ideas?

Time to look at fueling.

Thanks West Slope. Fuel pump was new a year ago, but is a Britpart item.

Forgot to mention when I got the car back from the garage a few weeks ago, the exclamation mark was lit up on the dash. The hand book states this is an EFI issue. I reset the ECU by disconnecting the battery and it hasn't reappeared, even with the current issues.

You're going to have to systematically start looking at it. A simple start would be just giving a listen to your pump's activity. Is it making noise? Second, replacing the fuel filter would be an easy, cheap test. A clogged filter can cause such issues. You can pull a spark plug after cranking the engine for a bit to see if it is wet or dry. That will tell you if fuel is getting to the cylinders.

The engine fires up ok and will rev no problem, and will drive as long as I don't let the revs drop down (not easy!), but if I let the revs drop to idle it'll cut out. So there's definitely fuel getting through. I'll change the fuel filter tomorrow and see if that helps, but if I was getting fuel starvation due to a blocked filter I'd expect an idle but no revs, whereas I've got revs but no idle?

VogueLSE4.2 said:

The engine fires up ok and will rev no problem, and will drive as long as I don't let the revs drop down (not easy!), but if I let the revs drop to idle it'll cut out. So there's definitely fuel getting through. I'll change the fuel filter tomorrow and see if that helps, but if I was getting fuel starvation due to a blocked filter I'd expect an idle but no revs, whereas I've got revs but no idle?

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What do the connections to the coil look like? The wires can get brittle and cause issues Also try a clean-up of the earth underneath of the coil housing.

Have you checked/cleaned the air idle valve?

VogueLSE4.2 said:

Hi folks,

Car is a '93 LSE hard dash with a 4.2 V8 Petrol.

Problem started off with the car starting from cold with no issues, but would hunt and cut out after a few seconds. On the second attempt, the car would start and run perfectly all day, with no further cutting out even after being parked up for a while. I'd say the idle was a little rough even when warm, however sits at 750rpm on the rev counter. Up to speed though, and it ran smoothly with no misfires.

Within the last two weeks, the problem has got progressively worse, with it taking a few starts each morning to get it to hold an idle so I can drive. Now today, it simply cuts out as soon as I start it so it's now undriveable. Coming back from the last drive, I let it rev out in second gear, and I'd say it felt down on power, like something was holding it back. Not massively, but enough to be noticeable.

No warning lights on the dash. All ignition components replaced with genuine items and the coil is a new Bosch item. Ignition amplifier was replaced at the same time as the rest of the other components. Alternator and battery are new as well.

Any ideas?

Click to expand...

Sounds like a fairly easy one - check the base voltage for the throttle potentiometer, if it is too low the engine won't idle.
You only need a fraction of movement on the throttle pot to be able to change it, the later type potentiometers do not have a slotted hole for adjustment but if you carefully enlarge the existing holes with a needle file you can get enough movement on the mounting screws to be able to correct the base voltage. IIRC I am sure I have posted on this a while ago so do a search.

Idle Air Control Valve sticking...??

Coolant Temp Sensor?

classic kev said:

Sounds like a fairly easy one - check the base voltage for the throttle potentiometer, if it is too low the engine won't idle.
You only need a fraction of movement on the throttle pot to be able to change it, the later type potentiometers do not have a slotted hole for adjustment but if you carefully enlarge the existing holes with a needle file you can get enough movement on the mounting screws to be able to correct the base voltage. IIRC I am sure I have posted on this a while ago so do a search.

Click to expand...

Thanks, I'll look into this.

TVRConvert said:

Coolant Temp Sensor?

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Where would I find this about?

Saint.V8 said:

Idle Air Control Valve sticking...??

Click to expand...

Is this the round thing that screws into the plenum? Do I just remove it and spray it with carb cleaner?

West Slope Rover said:

What do the connections to the coil look like? The wires can get brittle and cause issues Also try a clean-up of the earth underneath of the coil housing.

Click to expand...

These all look ok and were checked when the new Bosch coil was fitted. Definitely getting fuel and at the right time so not a blocked filter or fuelling issue either.

VogueLSE4.2 said:

Where would I find this about?

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lr call them stepper motors ,located at the rear of plenum you need to check it moves and pipe work and openings are not blocked, it takes air from before butterfly , you should hear it move when you turn ignition on

If you go in to the V8 engines section and do a search for idle problems, throttle potentiometer, stepper motor etc you will find lots of information on how to check, clean and set up the engine for a hopefully smooth and even idle speed. From your description, I doubt it is a coolant temp sensor problem. Throttle pot or possibly an intake air leak are more likely. Sticky stepper maybe, but doesn't sound like it. If you remove the stepper, you will almost certainly damage the gasket, you can use RTV silicon in place of the gasket but make sure there is no air leak around the stepper motor when you have put it back or you will have idle speed problems.

If the throttle pot base voltage is too low, the ECU won't read it and as a result it won't turn on the idle stabilisation which in turn means without using the accelerator, it won't idle. It is also probable that if the throttle pot is not able to reach full scale voltage, the ecu is not seeing the 4.8v - 5.0v at WOT so it doesn't give the max injector pulse which might explain why you said the engine felt like it was holding back and down on power.

You can check the throttle pot with a multimeter, better still, get yourself an "ecumate" which is a useful piece of kit for reading your ecu, it will show you what the throttle pot is doing and also if the ecu is switching idle stabilisation on and off, it also allows you to move the stepper motor. You should also be able to see what the O2 sensors are doing, your car will be running cats so this is something that you need to remember as the fuelling will be influenced by the O2 readings once it is up to temperature because the ecu will then ignore the CO trim from the air flow meter and adjust fuelling based on O2 readings. On a cat equipped V8 the CO trim voltage is set at something like 1.8v (I'd need to check) whereas a non-cat car the CO trim is 1.0v or less.

If you are reasonably happy that the engine is fundamentally good, then I would strongly suggest you need to think about removing the plenum and ram housing, clean it all up and also clean the throttle disc (butterfly) and set the clearances. Doing this will also give you an opportunity to to check for split, perished or missing hoses that might be causing inlet air leaks. It only takes a couple of hours to remove, clean, check and set everything up if you remove the plenum and ram housing. You will also need to set the base idle as part of this process but it isn't difficult. I can understand if you are nervous about taking off what looks and feels like a major part of your engine but seriously, it isn't that bad, I remember the first time I did it and afterwards thinking "that was actually quite easy".

So, some food for thought there, but do a search in the V8 section and let us know how you get on.

Right...decided to remove and clean the stepper motor/air idle control valve, and it felt loose as I removed it. Cleaned it, and as I gently screwed it back in the threaded part sheared off. So...I'm assuming it had split and was leaking, hence my idle issue.

Unfortunately the threaded part is now stuck inside the plenum with none of it protruding, so at best I might get it out with a hammer and a small chisel.

Glad to have found what should be the cause of the problem.

You can unbolt the AIV spigot from the back of the plenum, 3 M6 (I think) cap bolts, and you should be able to dig the remans out of the threads.
Good luck

But more head scratching required...

Replaced the stepper motor today with a new one and it made absolutely no difference...car starts and revs but cuts out once I let off the throttle.

Next step is the throttle potentiometer. I've located this but haven't taken it off yet. It doesn't appear to have any adjustment, will I find a screw to turn when I take it off? How do I go about adjusting it?

If it's the potentiometer, why do they become faulty?

Thanks for all the help so far folks.

VogueLSE4.2 said:

But more head scratching required...

Replaced the stepper motor today with a new one and it made absolutely no difference...car starts and revs but cuts out once I let off the throttle.

Next step is the throttle potentiometer. I've located this but haven't taken it off yet. It doesn't appear to have any adjustment, will I find a screw to turn when I take it off? How do I go about adjusting it?

If it's the potentiometer, why do they become faulty?

Thanks for all the help so far folks.

Click to expand...

early engines pot had slotted holes later dont