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Did you ever resolve this ?

I'm a fellow Kiwi (living in Tasmania) with a Smart Roadster that has the exact problem you described. No misfire at idle or at high revs - but as soon I try to hover around 2200-3000rpm it misfires on cylinder 2

I'm going nutty trying to work out why its doing it. I've got great compression and passes leak down test etc, tried coils, injectors, plugs, leads etc etc....
The only thing I've found that makes it 'go away' is unplugging the coolant temperature sensor - which I think in turn makes the ECU think the coolant is super cold and so makes the injectors inject more fuel. Makes me think the whole misfire thing is a fuel/ratio problem but for the life of me I can't work out how/why and why only on one cylinder. Its either getting too much air or too little fuel, but checked for leaks and tried swapping injector and neither have made a difference.

In desperation I've ordered a 2nd hand ECU from UK (which I'll recode) to see if its an ECU issue - there's just not much left to check!

Anyways, really hoping you had a solution that worked for you!
 

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I should add, I've also got a new coolant temperature sensor and fuel pressure regulator on the way. I'm fairly sure my current coolant sensor is fine (OBD shows what looks like realistic temp) but figured why not. Pressure regulator does make a buzz and seeing as I think its a fuel or air related issue its just another cheap component to rule out.
 

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Discussion Starter #25
I hope you get yours sorted. I never got mine fixed. One of mine's plugs on piston no 1 burnt away again and a piece of the bottom of the plug got stuck in the valve and damaged it. I got another second hand engine. I think I found the problem for piston 1 running lean - there was a hairline crack in the silencer close to piston number one, think this might be the reason why piston one was running lean.


Let us know if you get your's sorted.
 

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you may have already done this, but check compression on all 3 cylinders. You will probably find #2 is much lower than the others. You could try some Marvel Mystery Oil, or other such product to release stuck rings, or it might be something more complex
 

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Sadly no =/ thanks anyways. Almost unbelievably all three cylinders are on 180psi, test done with two different compression testers, both hot and cold, as I didn't really believe the results of the first tester. Leak down was just as good, sitting on around 22% for all three cylinders (well in the green area). While I was waiting for the leak down tester to arrive I was really pinning my problem on being a valve issue but not looking like it now with good leak down results.

After that I went about unplugging every sensor one by one and observing results - which is how I discovered unplugged the coolant temp sensor made the misfire 'go away'. So I'm feeling confident its a fuel/air issue now, just no idea why just that one cylinder. I've swapped injectors around to no avail. Wondering if there might be enough of a vacuum leak on just that cylinder somehow to make it happen, but only in the 2200-3k~ rpm range, high rpms has no issue, same with low rpm. Some things I've read suggest coolant temperature also affects timing so if its that causing the issue then perhaps a replacement ECM will clear it up.

Running out of options, Mercedes dealer here is no good as they've hardly ever seen a Smart so I'm left with replacing parts one by one until I find the culprit! Intake and exhaust manifolds might be next on the list
 

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You haven't given a location but a good foreign car shop with a STAR computer is probably a better investment than just throwing expensive parts at it. They can check out the car on the road and probably identify the issue. :)
 

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I'm in Hobart, Tasmania, Australia. I'll have a hunt around and see if I can find a STAR equipped shop. Being a remote place has its disadvantages sometimes though lol

I do have an Autel AL319 scanner, that's letting me read and reset faults as well as monitor live OBD data. Only fault it is picking up is the P0302 misfire when in the right rev range
 

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Did a fuel pressure test on the inlet line today. Came back at 48psi (3.3bar) at idle. Spec is 55psi (3.8bar) I think or 58psi (4.0bar) if you're running the Audi FPR. Maybe my FPR or pump aren't good.

Wondering if this could be it.... almost certain its a fuelling issue. New Audi 4bar FPR is on its way so will report back!

ps If either the new FPR or replacement ECM don't cure it I'll buy a Chinese STAR, already have the laptop so just need the multiplexer and cables, only slightly more expensive than getting the city Mercedes blokes to scan it apparently =/
 

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I have a 451 that used to do this. It turned out to be the seal between the throttle body and the intake manifold. It had developed a wrinkle in it. Twice. I used Honda Bond on it the last time I put it together, and that solved it. I don't know what the throttle body or seal look like on the 450, but I thought I would mention it. It was aggravating to say the least. My drive home was on a length of road with a 40 mph speed limit that seemed to bring out the misfire at that speed which was right around 2000 rpm. If I recall correctly.
 

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Well nothing good to report =/ Chinese STAR arrived and works well, but doesn't really give me any more info that I already had - a misfire on cylinder 2, but only at mid-rpm. No other codes present and all live data etc looks good.

Fuel pressure regulator did nothing, only thing that helps (makes it go away) is unplugging the water temp sensor (which tricks the pcm into giving it more fuel). Put new intake seals all round for good measure

At my wits end now. Wondering if it could be a PCM fault or an invisible vacuum leak in the plastic intake manifold around cylinder 2. I'd like to get a smoke test done for that and then maybe a good 'scope guy to observe pulse width on the injectors.
 
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