Constant speed - 2000-3000rpm Misfire


A while back I posted a question about what the potential problem could be with a misfire that occurs ONLY at constant speed while in the 2000-3000rpm rev range.

Since then, my car was serviced at which point I told my mechanic and hoped he would find the problem. This was not the case, and it has now been into a Lotus dealer to be hooked into the diag computer to see if there are any error codes - the result - no errors whatsoever!

The answer from the dealer, and my mechanic for that matter, is that this is a trait of the car (the engine only comes on song at 5000) and that I need to live with it. I don’t know whether I am going nuts and becoming too sensitive to the car, but I know my car never did this before I had the recall work done, at which point my original Lotus 190 ECU was replaced for a new one.

So, am I over-sensitive? Are other S1 190 drivers experiencing a misfire at constant speed in the said rev range?

Please help?


PS: The car on throttle acceleration does not miss a beat at all, regardless of revs. This is purely when one is casually driving down the road in 3rd gear doing 2700 revs when the car is retarding / pulling back and certainly not smooth!

When you say misfire could it be back-firing. Even then it’d be odd at a constant speed.

Not surprised on the ECU read. An AA man I know said he’d never been able to get any K series to report anything.

Have you changed the leads? I don’t have a problem with them, but last Thursday two seperate people (at different locations) looked in my engine bay and said “Oh, you’re still on the standard leads.” Lots of people have switched to [I forget right now, somebody will say in the mo].

Good luck!


ps. And no, you’re not being over sensitive. If you don’t understand it, you don’t know what else it could cause.

Pre Emerald mine did that alot, was very annoying. I think it might have been the brown temp sender that was telling the ECU the engine was cold even when it wasn’t and messing up the mixture at the light throttle point.

Drove me crazy, but having the emerald now means i take people around the block as slowly as possible to show them all that nastiness has stopped.

First check is the Throttle Position Sensor
Then the Brown temp sensor
Then the greean air temp sensor

Propably is the TPS needing to be replaced, but it is not a trait of these cars it is a duff sensor somewhere, causing it to be using an over rich or over advanced map at the time it sounds terrible.

No, this problem is a misfire, not a “backfire”. I enjoy the odd pop and bang. The misfire feels like a hiccup in the rhythm of the engine and you can feel the car pull back for that nano-second as there is a loss of power.

I still have the standard leads on the car. I think you are talking about changing to magnecor?

Thanks Jason,

Then why the hell won’t mechanics and official dealers tell me this stuff or at least examine the sensors. Is it hard to detect a duff sensor?

How should I move this forward…Jason, y’know I’m no ace under the hood myself!

A duff sensor should show up as such on the dealer’s ECU monitor thingy!

It’s probably down to poor mapping/download at the time of the ECU recall. I’d get them to exchange the ECU (assuming to don’t want to cough up to switch to Emerald).

I still have the standard leads on the car. I think you are talking about changing to magnecor?

yes - that was the brand of leads a few people changed to in the early days. IMHO it was a bit of marketing hype about the benefits of these leads together with a range of non-specific ignition issues that lea=d to the change. I’m still on standard leads and would not swap them as there has never been a problem. |I also think the replacement ones are KV85 which are slightly oversize.

Leads CAN cause this. Something to do with electrical distortion caused by a certain pattern of events (eg. certain rev range vibriation, pulses etc). Possible that even moving/seperating the leads will help.

A mate with a 365 GT4 ferrari had precisely this problem and the only eventual cure was a set of leads direct from Ferrari that appeared exactly the same as others (but were far more expensive ). He had tried several other visually similar sets along the way. Go figure!

I can also say it can easily be the TPS, sometimes even just resetting it (need the Lotus data tool tho, the 5 pedal press is just for MEMS ECU’s).

Or if it’s fu’bar’d then it’ll over read. Plus the VHPD is known for eating these items

Or better still, bite the bullet and get an emerald, can save so many headaches!!!


Does the car idle as per usual, reason why i ask is the delay you are experiencing every now and then when putting your foot down to accelarate could mean that the idle control valve is open to far.

My first guess though is the TPS, sometimes they are ok when cold or at least not at usual operating temperature, hence not showing upon the scan.

Perhaps try cruising along on open section of road, take your foot completely off the throttle and then slowly start to depress it, if it reacts immediately with the slightest press then it is registering, if nothing happens until you have a fair amount of pressure on the throttle then it is more than likely the TPS.

Have to say that i doubt it is plug wires ot that it is running to lean, The dealers have pre programmed maps that are loaded on to ECU so the chances of it being wrong in that load site are minimal, could just be that for some reason the lambda is leaning it up to much because it is reading incorrectly.

So many variables and possibilities, The best thing to do is to narrow them down 1 by 1. Start with the TPS, you will propably need one eventually so it won’t hurt to change it now anyway. Thereafter borrow someones plug leads, i have my doubts that that is the problem though, then get dealer to re flash ECU.

If none of the above works avoid using less than 50% throttle by for example leaving earlier or later for work in the mornings when roads will be quieter and you will be able to keep your foot flat most of the time. This will also save on mapping time in the future as you will only have to map 1 load site, wide open throttle

Seriously though let us know your progress as you are troubleshooting perhaps i could be of more or less help, propably less but i will voice my opinion anyway .

After I replaced my coil pack earlier in the year I had a misfire return at 3500-4000rpm car would stutter but pick up again, replaced the spark plugs and it hasn’t happened since.

Thanks all for the contributions.

Jason, as far as I can recall, there is no hesitation on throttle when I depress the pedal. The engine is only being sh1tty when my foot is stationery and the speed is constant.

The severity of the condition does tend to change, but not necessarily consistently. I had been driving for an hour when suddenly it stopped doing it for the rest of the afternoon?!

…that was a couple of months ago now, and the car seems to do it every time now…some times worse than others.

I am in India at the moment so will have to wait till late Nov to give you all further feedback. Please, any more thoughts on potential reasons are MOST welcome.

BTW, Delhi is no place for an Exige. Take my word for it…


India, cool

Anyway, I’d say 100% the TPS, you hold a constant throttle, it jumps up and down and all over the place sending different amounts of fuel in all the time


I’m back from Delhi and want to sort this now.

The question is: What is involved in swapping out a throttle position sensor? Is it difficult or is this something I could do?


I understand it is simple. It’s at the top of the engine and I believe you could pick one up from a scrapy.

But that’s the best advice I’m afriad I can offer.