Renault Scenic occasional misfire

[Space cat]

We have a Renault Scenic 1600 RT (petrol), about ten years old and 52,000 miles on the clock. In the last week or so it's suffered from a very occasional misfire.

It's most likely to happen when pulling away up a hill or out of a junction and always at the same time. You change to second gear and, just as the clutch comes up and the throttle goes down, there is a momentary loss of power. I'm not an engine expert but it certainly feels like a misfire to me.

It never happens when idling or driving down the open road and seems to be limited to that one time when using quite a lot of throttle at low(ish) revs. I'm also inclined to think that it's more likely to happen after the car has just rounded a bend.

One bit of information might be relevant. Nine days ago we filled up at an unfamiliar garage - which I won't name in case it isn't relevant. I think the misfire started after that though it's hard to be sure when it occurs so infrequently. Is it possible that poor petrol - or the wrong kind of petrol - could be the cause or is it mere coincidence and I should be looking for something else?

Any advice will be greatly appreciated.

 

[Space cat]

Update:

The problem just became persistent. Almost without exception, the engine does not respond correctly to the throttle. As before, it's worse at lowish revs when it feels as if the thing just isn't firing on all cylinders. It can be provoked while stationary by revving at just above idle speed and then the exhaust gas comes out in bursts instead of the usual steady flow. On a test drive through city traffic, there was just one instant in second gear when the engine responded with normal power but it only lasted a couple of seconds.

I checked the air filter (mostly because it was easy to check) and it wasn't bad. I was hoping to find the thing clogged with dead leaves but no such luck! Then I remembered an incident with a Mini some thirty years ago. There was virtually no power and I limped home in first gear on full choke. The cause: a lump of fibrous material jammed in the fuel pipe!

Suspecting a similar problem, I tried to get the petrol pump out to check its filter. "Tap the locking ring around with a screwdriver" it suggests in the Haynes manual. No ****ing chance, so I had to abandon that one. The fuel filter under the car is a sealed unit so there was no point taking that out.

Next, I remembered the obvious and tried to look at a plug. For those who don't know this car, each plug has its own little HT coil perched on top, fixed down with a single torx screw - and guess what; I couldn't budge the things. But I don't really see how a plug problem could go from occasional misfire to persistent fault in a single day.

Unless anybody has some suggestions, I'm afraid it'll be off to the garage tomorrow.

 

[wotan]

Coil packs, or individual coils can show intermitant fault, then fail completely, so don't discount this as a possibility, you may possibly have water in the fuel, this would cause stuttering too.

Please let us know the cure, when it is found.


Wotan

 

[jetex]

It's almost certain to be one of your ignition coils. Remove the plastic cover from the top of the engine (if it's got one), to expose the ignition coils. You do this by lifting up the front of the cover, then pull it towards you. It's quite tight as it's on rubber grommets. You will then see the 4 ignition coils with the leads going to them. The coils will normally have grey tops unless they've been changed before, Press down on the clip on the plug connections, and pull off the plugs. Note: don't pull the wires as you may break them. Remove the bolts securing each coil, then refit the coil plugs. Start the engine then lift up the first coil as if you were removing it. You should hear the engine note change, and a cracking sound as the coil is shorting to the cylinder head. Repeat this for all the coils until you find the one that makes no difference to the running of the engine. That is the one you change. Renault number the coils from the fan belt end of the engine 4-3-2-1. Once you remove the faulty coil, it should have a code date on it e.g : 252-2 meaning that this coil was made on the 252th day of 2002. Change all coils with a code date below 330-3. They are usually marked Renault/Sagem.

 

[jetex]

Sorry, just re-read your post, so ignore first part of mine ! The Torx type bolts were fitted to the early Renaults, they were later swapped to conventional bolts with a 10mm spanner sized head. It's important that you use the correct size Torx bit (T25 ?), otherwise you'll never get them out. Put some copper grease on the bolt threads when you refit them. Coils are around £45 each. It's a good idea to change the spark plugs as well while you're in there, as these can lead to premature coil failure.

 

[Space cat]

Well, I got the car into the garage first thing this morning and it looks like jetex is spot-on. I demonstrated the ropey engine idle to the mechanic and he was straight in doing something out of sight underneath the front of the engine. In less than a minute he pointed to cylinder number one and said "That one's not firing. It'll probably be the coil. It's a common fault on these cars"

Intrigued, I asked him if he'd been squeezing fuel pipes to see if any made no difference but it was easier than that. "Injectors" he said and unplugged one again to show me. If that trick is in the Haynes manual, I never saw it!

Thanks for the advice. I have two torx tools: a screwdriver that didn't give enough torque and an angled key that I couldn't get in. I was never going to get those coils out myself. If I could - and if I'd known about the injector trick - I would have swapped coils around to see if the fault moved. Too late for that now.

It's a good idea to change the spark plugs as well while you're in there

As luck would have it, I thought of that one. Knowing that the garage would be half way towards removing a plug anyway - and I would have to get a special tool - I asked them to change the lot while they were at it.

 

[Space cat]

It's fixed - and the root cause was scarcely credible but true. I know it because I've got the old bits.

The nearside washer pipe had sprung a small leak which sent a jet of water onto the engine every time we used the washer. From there, it ran down into the recess containing the number one plug and coil. (For those who don't know this engine, the plug recesses are very deep.) This had obviously been happening for some time because the plug body was heavily rusted, and so it was a race between leaking pipe and engine heat which the pipe eventually won!

So I ended up paying for a new coil, the new plugs I asked for and two feet of new washer pipe. £105 in all. It could have been worse.

 

[jetex]

Well done