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Weird hesitation/lumpy acceleration at low RPM


cb350

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Hi all,

 

My '03 JDM 350Z has suddenly developed a strange issue, immediately after I took it in for an MOT.

 

Under light acceleration, from about 1500 - 2500rpm in any gear, I can feel an occasional hesitation which appears to be getting worse. At first it seemed to do it just when cold, but after a week now it's doing it more consistently even when hot.

 

I had a misfire last year which turned out to be a coil pack, so I replaced the spark plugs and coil packs. This problem almost feels like a similar misfire, but it's just not quite the same, and it's not causing the engine warning light to come on. At idle it seems fine, and so does under hard acceleration.

 

One thing that's worrying me is my car has always used a lot of oil. I chuck in nearly a litre every 1000 miles and I have done for years. I've never seen any leak out under the engine (I've always done oil changes myself, so I've had a good look before now), I've never seen any blue smoke, and it's never got any worse, but these past few weeks I think the oil consumption is up a bit. I won't know for sure until I've done a few more miles.

 

So, I'm worried that maybe low compression is causing something like a misfire.. and that, maybe my engine is getting somewhat worn out and somebody with a lead foot during the MOT test may have helped put a nail or two in the coffin while getting ready to run the emissions test? I usually hang around during the test but I couldn't this time due to work.

 

Does that sound feasible, and does anyone have any other less expensive ideas on what might be the problem?

 

Cheers,

 

Chris.

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Definitely worse driving it today - and it seemed to turn into a full-on misfire at idle while waiting at a set of traffic lights. Pulling away, the misfire suddenly disappeared while accelerating. Again, the engine warning light didn't come on. Are there any known issues that sound like this?

 

Chris.

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So, the misfire is even worse this morning. I know from when I had the coil pack go down that I would expect to see the engine management light come on, and the VDC and SLIP warnings at this point, if it was an ignition problem.

 

Does anyone know, could a faulty injector cause a misfire *without* illuminating the EML? Just trying to think of things I can try.

 

Chris.

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Just to update this - the misfire got really bad, still no engine management light or error codes.

 

Out of desperation I poured a bottle of Redex in the tank, and the misfire has magically gone away!

 

My thoughts are either:

 

1. Blocked injector, fixed with Redex.

 

2. Redex was superfluous to proceedings and I had a dodgy tank of petrol?

 

It still seems a bit lacking power, but I don't know if that's my imagination or not. Probably a good idea to replace the injectors?

 

Chris.

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On 13/06/2019 at 21:06, Kieran O'Quick said:

Good that it seems to have gone away. Hopefully it was just bad fuel or a piece of crud that's gone. Redex can actually clear stuff in my experience. Fingers crossed it'll pick up now.

 

Sadly, it ran OK for a day (although I did think it still felt a bit down on power), and then the misfire was back, coupled with what almost certainly sounds like really bad pinking under acceleration between 2000-3000rpm! A day later, it sounds really awful. Still no error codes, no engine warning light. A total mystery.

 

I'm going to change the injectors and the knock sensor, and will take it from there.

 

12 hours ago, TT350 said:

I had this problem. It was one of the coils.

Did you get an engine warning light when the coil went down? I had two coils fail last year - both gave me warnings and I replaced all the spark plugs and coils at the time. It ran perfectly fine afterwards. I have wondered if maybe one of the new coils has failed, but surely I'd expect a P0350 error code?

 

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It can be really slow to throw a precise code sometimes. But it usually at least gives the 0300 'random misfire' code. Have you checked the connectors on the coils? I've had some work loose on the pattern coils. Just a couple of weeks ago actually. 

Coils can start to fail repeatedly if the seals on the rocker cover are on the way out. Zmanalex sells a replacement kit. 

I suppose it could be injector problems. The only other time my car ran like you describe though was when a Cat  was breaking up and clogging the exhaust up. 

Good luck. 

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9 hours ago, cb350 said:

 

Sadly, it ran OK for a day (although I did think it still felt a bit down on power), and then the misfire was back, coupled with what almost certainly sounds like really bad pinking under acceleration between 2000-3000rpm! A day later, it sounds really awful. Still no error codes, no engine warning light. A total mystery.

 

I'm going to change the injectors and the knock sensor, and will take it from there.

 

Did you get an engine warning light when the coil went down? I had two coils fail last year - both gave me warnings and I replaced all the spark plugs and coils at the time. It ran perfectly fine afterwards. I have wondered if maybe one of the new coils has failed, but surely I'd expect a P0350 error code?

 

No, it never gave a light or an error code.

 

It was really annoying. 

 

Can you hear it spluttering when it hesitates? 

 

It'll sound different, obviously, if one or more cylinders isn't firing.

 

What I did was, just bought a full set of coils from a breakers. Changed them. Sorted it out.

 

Coils, in my experience, can just go.

 

 

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On 15/06/2019 at 17:33, Kieran O'Quick said:

It can be really slow to throw a precise code sometimes. But it usually at least gives the 0300 'random misfire' code. Have you checked the connectors on the coils? I've had some work loose on the pattern coils. Just a couple of weeks ago actually. 

Coils can start to fail repeatedly if the seals on the rocker cover are on the way out. Zmanalex sells a replacement kit. 

I suppose it could be injector problems. The only other time my car ran like you describe though was when a Cat  was breaking up and clogging the exhaust up. 

Good luck. 

Thanks to everyone for the suggestions and help. I was about to dive in and remove everything to get to the injectors, when I thought I should probably take one more look at the connectors to the coil packs that I could see. I had looked at them previously, but there's one which I was barely just able to get my hand on, and as I pulled on the wires, the connector came off the coil pack!

 

I plugged it back in and drove it today - no misfire. The pinking was there initially, but as I drove, it started to die down. After about 30 minutes, it seemed to have gone completely.

 

My only guess as to what was happening is that for some reason, the ECU wasn't recognising a randomly disconnected coil pack as a fault. Believing that the car was running fine, it would have possibly figured the car was running rich (or something like that), and then would have been compensating by adjusting the fuel map - eventually leading to it pinking like crazy(?).

 

I'll see how it goes over the next day or two - I'm hoping I've got this figured out now.

 

Thanks all,

 

Chris.

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5 hours ago, Kieran O'Quick said:

I'd be getting a small cable tie or something around that little bleeder....  Or wedge it on somehow? 

 

Great news if that's the fix. Fingers crossed.

 

Thanks, yes I'm keeping my fingers crossed! It's still pinking a bit, but nowhere near as badly as it was, so I'm hoping the fuel map just needs to get back in tune. Will need to keep an eye on that coil pack plug.

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