missfiring @ 3500rpm upwards

[lifebegins@400]

Folks - forgive me for a well worn topic but I have searched previous threads and can't quite find the info I am looking for.

car symptoms as follows (Chim 400):

- missfire starting at 3500rpm - random and clearly more than just one cylinder - almost the same sensation you get if you hit rev-limiter
- only occurs under load, not when vehicle stationary - you can't 'drive through it' to higher revs either
- doesn't matter what gear or throttle position you are in
- doesn't seem related to specific engine temperature
- seems to be have been accompanied by a slight worsening in the 1500rpm - 5th gear shunt which some owners experience but difficult to seperate this symptom out from its normal behaviour
- otherwise starts on the button and runs very smoothly - seems to have full power up to the point of missfire.

Car was serviced earlier this week incl new plugs and clean bill of health on a diagnostic check - car ran perfectly for one day and then the above symptoms started! I am going to take a peek at the dist cap / rotor arm for cracks and deterioration tomorrow - HT leads also - my gut feeling though is that the coil is giving up.

Anyone got an opinion? - all comments gratefully received (and if it can still stagger up the M40 I hope to see you all at ROAR!!)

thanks,
Andy

[GreenV8S]

I would say (1) probably weak spark and (2) take it straight back to the people who just 'serviced' it to check over because that timing seems rather suspicious, maybe they have accidentally left a plug lead loose or something like that.

[humpy]

Chap with similar symptoms in a different car, turned out to be water ingress into the fuel - via a dodgy fuel cap.

Hope you get it sorted

[Timja]

Check the condition of the leads. I had a similar sounding this and the leads had perished and the spark was arcing accross the the exhaust manifold leaving a tell tale white mark.

[TVR-Boy]

Hi there. I have had my Chim 400 for 2 weeks now and when I bought it, I had the exact same simptons. It got so bad the day after I bought it that on the way down to Andover to visit my dad, the car stuttered, popped and eventually gave up the ghost right on a hill. I couldn't get it started again, but loads of people kept on stopping to help, think they just liked to look at the car. After a few minutes, it started and off I went. It did it again on the way home.

Here's what I did next. I replaced all the spark plugs with brand new correct ones, the old ones were knackered. I ripped out all the crap HT leads which were performace ones and fitted propper standards leads, (note: I did one lead at a time so as not to mess up the firing order, this will become relevant in a moment). I then ordered and fitted a new dizzy cap, rotary arm, coil and replaced the auxillary drive belt. Checked all fluid levels including the obvious such as water, oil, power steering, clutch, brakes etc. I cleaned all the electrical contacts, stepper motor, cranks case breather, and finally I disconnected the ECU, cleaned the contacts, reset it (by removing it) reconnected it, checked all the fuses & relays as well as the battery terminal connections. Lastly, I have order a fuel filter, fuel pump & 100amp fuse, all yet to be fitted.

The up shot is that the car still ran like a bag of nuts and bolts. I wasn't that bothered as I really enjoy tinkering with cars and as I already own and run 15 cars I don't rely on this one at all.

So, I thought I would check through everything starting with the obvious - firing order. Like I said above, I had already replaced the HT leads, but did it one by one, and so the firing order was the same as when I bought the car the day before. I numbered all the leads 1-8 on the dizzy cap in a clockwise direction, removed them all and using a diagram I found on the net, I reconnected them one by one, and to my surprise they went like this: 1-2-3-4-5-7-6-8 so numbers 6 & 7 were around the wrong way, and so I fired her up and bingo! She now runs like a dream, perfect, hasn't missed a beat since that day!

So I would suggest checking those leads.

HTH

[lifebegins@400]

many thanks everyone for your advice and comments. I got to ROAR OK nonetheless and the guys on the rolling road have narrowed it down to a fuel issue - air/fuel ratio suddenly increases at the point of misfire to approx 16:1 (should be 13 or 14:1 apparantly), in other words it's running very lean. Cars going in to Crazy Horses HQ next week in Borehamwood for a more specific diagnosis but it's going to be fuel pressure / blocked injector / duff output from engine sensor or the like - for completeness I'll report back on the final outcome, might help someone else with similar symptoms in future.

Cheers,
Andy

[blitzracing]

That could be as simple as a blocked fuel filter if you have run the fuel low.

Mark