Suggestions Please - Rough Idle and Misfire under load

Yes, there is a definite fuel smell at idle.

I get the hesitation/misfire under heavy acceleration right across the rev range - dropping back on the throttle and moving more smoothly then resolves the issue. Given the relative low cost, I've also ordered a crank position sensor just to rule this out.

In the early days of my problem, I replaced nearly every sensor - made no noticeable difference and then with Pscan I plucked up courage and altered the idle screw and finally, my problem was solved.

I doubt it is the CPS - it generally either works or does not (ie you cannot start the car).

I do suggest replacing the TPS - it is relatively cheap, easy to replace and can be part of the problem if its butterfly degrees open reading is erratic/wrong. If you replace the TPS, remember to do a TPS reset.

I did think that, but a new TPS was included in the throttle body so I can't see this being the issue?

More adjustment on the idle screw this evening, and it’s still no better unfortunately. To add insult to injury, I wanted to top up the washer fluid (and check the battery connections just in case) only to find that the bonnet release isn’t working.

Frustrated isn’t the word!


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Try this...
See https://landroverforums.com/forum/discovery-i-39/base-idle-speed-setting-96-disco-95426/. Follow these instructions (but get the engine oil to 85 degrees + - ignore instruction 1.) which takes the reset back to basics. i.e. It is ensuring that only air through the butterfly valve is getting through (use clean rags to blank off the hose from plenum to IACV - there is obviously a special Land Rover hose for this back-to-basics reset [instruction 4]). Reconnect everything, restart the engine and then slowly adjust the idle valve in small steps, leaving 2 mins between each turn until you are at 26 steps/steady idle at 870 rpm or so.

Still no improvement

Tried the suggestion on the Land Rover forum, and apart from the obvious change in process the result is the same. The best way I can describe the idle issue is that it never seems to settle, including the stepper position. Took the car for a long-ish drive and the misfiring under load is still there.

The plugs and leads were only replaced at the end of last year and look to be good quality and ok in condition. Short of replacing these again, I'm at a loss as to what I can try next. Unfortunately, the MOT is due this week and I suspect that it'd fail on emissions if tested as is.

I don't approve of guesswork against reasoned diagnostic procedures but on the basis of your answers, ignition system supposedly ok, miss under load and rough idle, I would proceed as follows: remove each plug in turn . Is one very much cleaner than the rest? Yes. Suspect HG.
No, is one darker? Yes. Suspect leaky injector.
Are they all about the same colour? Yes. Consider the possibility of a faulty fuel pressure regulator but first rule out a bent or burnt valve with a compression test.
An afterthought. Just check the seating of your throttle body. If the gasket got pinched or a screw worked loose around the back, you could be sucking in too much air and the lambda sensors will be trying to compensate by enriching the mixture. For that reason, check all the air pipes to the manifold as well for cracks or splits.

Thanks minimax.

The plugs looked fairly consistent when I checked them last week, so it sounds like a compression test is the next course of action. I’ve already checked the inlet piping and can’t see anything obvious.

Despite having fairly amateur mechanical knowledge, like you I prefer to stick with tried and tested diagnostics but this car has me stumped! I haven’t checked/replaced the lambda or cat yet but I suppose issues there would reveal themselves in the MOT result.

I bought the car with an inheritance as a bit of a gift to myself, so more than anything I think I’m just disappointed that my seemingly well looked after example is proving so troublesome! ☹️


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To be fair to the car, it isn't troublesome, it has one fault that resists diagnosis. It is only an engine, though and a good diagnostic analyser run at a decent garage might be worthwhile, if only to rule out the things that aren't wrong. There are no oil or water leaks and the level in the header tank stays constant. The engine does not overheat so it is fundamentally sound. It is just a matter of diagnosis. The repair is likely to be easy to do, once you know what is wrong.

When I had a similar problem to you, there were NO fault codes, and most sensors, including the IACV and the TPS, were replaced including the leads - but no difference. It took several weeks for me to get the right idle screw adjustment to get the car in equilibrium.

In the end, to ensure that the butterfly valve was shut and that I was adjusting from 0 degrees, I adjusted the idle crew so that it was clear the valve was shut. I then worked from there adjusting the screw in small increments until I hit 26 steps.

I assume you have replaced the faulty coolant sensor (the one feeding the ECU). Replacing the IACV is an option on the basis you have cleaned it but not operating right (not the cheapest of sensors) and perhaps if you still have the old TPS put that back in and see if this makes the difference.

I fully understand the frustration that you are going through. I spent a fortune on sensors and I ended up messing around with the idle screw and going on short test rides for several weeks before I got it right.

I learned to follow the data/evidence but the lack of any fault data being during this period made the whole episode even more frustrating. The problem is that the fault data is based on whether the ECU is getting data from sensor X and but the ECU is not smart to check whether the data is accurate.

Nah. I'm not buying this. Fiddling with the tb is getting you nowhere and the fault appeared suddenly. Check the fuel filter and the pressure regulator.

I’ve got someone coming to look at the car in the next couple of days with the view to completing a compression test to rule out anything there.

I did manage to get the stepper to settle around 26, but there’s still a hint of a miss at idle which makes itself know under sudden load. I’ll try to capture this on video tomorrow if I get the chance.

The fuel filter was replaced shortly after the issue started, but are there any checks I can do on the fuel regulator before looking at a replacement?


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