Tracing a misfire

graeme · Post by **graeme** » Mon Jul 09, 2007 11:00 am

Can anyone advise on how to diagnose a misfire? The S160 is slightly lumpier than usual at idle, and under load it's misfiring really, really bad. I *think* it's not just one cylinder either (it's really very very bad), which makes me want to discount a single plug or injector, but I'll listen to any advice given on diagnosing this issue. Never had to trace a misfire before...

I'm tempted to
1) Drive to garage
2) Get out credit card
3) Eat beans on toast for 6 months

but any other option is obviously better.

Surely nothing to do with this after all this time..? it's had a couple of tanks of optslop through it since then.

Rich H · Post by **Rich H** » Mon Jul 09, 2007 11:06 am

How many coils does a S160 have?

I would start there and work backwards....

Post by **ironside** » Mon Jul 09, 2007 11:12 am

Is there a distributor cap on the 160?

Skoosh in there with some WD40 and have a look at the state of it and the rotor arm while you're in there.

graeme · Post by **graeme** » Mon Jul 09, 2007 12:13 pm

OK, dizzy first then, if there is one, which I think there is. Might be that as we have had a spot or two of rain recently. I'll pop the plugs out for a while if we get a dry spell too and make sure all is dry round there and then skoosh around them and the leads.

As for ignition coil, pretty sure it only has one. Not sure which type though... Never poked around there before so I'll get the service manual out tonight.

Thanks so far. Anything else I can look for? (One thing at a time of course or I won't know what the fix was...)

mac · Post by **mac** » Mon Jul 09, 2007 12:15 pm

Or work cheaply and then forwards with -

New Rotor arm & dizzy cap (if fitted)
New Spark plugs
borrowed set of known plug leads (buy if fixes)
Coil pack(s) again borrow known set first if poss.

After that your looking at why the ECU is sending out a misfire or why the engine is misfiring - which isn't fun

Mac

graeme · Post by **graeme** » Mon Jul 09, 2007 12:28 pm

Cheers Mac.

Graeme

Post by **ironside** » Mon Jul 09, 2007 12:54 pm

If you have a distributor cap it'll just be the standard single coil (at the other end of the ignition lead that comes out of the middle of the distributor cap, underneath the inlet manifold.)

If changing it, it's easiest to access from underneath. If you're just replacing the ignition leads or just checking they're (HT and LT) attached properly you can do this from the top.

If you suspect your coil is iffy I have a spare new one here I can mail to you, I doubt that's your problem though.

graeme · Post by **graeme** » Mon Jul 09, 2007 1:47 pm

Cheers dude. Just had a peek at the car (in the dark work garage). It has a distributor cap, and following the middle lead, the coil seems to be located just under the TB/plenum (i can feel it but not see it, so could be wrong) and should be easily accessible by removing the TB if it comes to that.

Thanks for the offer. I'll let you know if I get that far.

G

robin · Post by **robin** » Mon Jul 09, 2007 1:47 pm

You may have gotten water into the plug bores - so if when you pull the HT leads off the plugs there's water or rust you should consider (a) cleaning it all off and (b) using some silicone sealant on the plug cover to keep water out in future.

Cheers,
Robin

mac · Post by **mac** » Mon Jul 09, 2007 2:05 pm

graeme wrote:OK, dizzy first then, if there is one, which I think there is. Might be that as we have had a spot or two of rain recently.

And the award for understatment of the year goes to ........................ Graeme Fraser

graeme · Post by **graeme** » Mon Jul 09, 2007 2:39 pm

robin wrote:You may have gotten water into the plug bores - so if when you pull the HT leads off the plugs there's water or rust you should consider (a) cleaning it all off and (b) using some silicone sealant on the plug cover to keep water out in future.

Cheers,
Robin

Ta Robin. Good idea.

Thanks,
Graeme

Chopperver1 · Post by **Chopperver1** » Mon Jul 09, 2007 3:58 pm

If it is a problem with a cracked distributor cap or leakage on the HT circuit you may actually see it when you run the engine in the dark. It will look like the Northern Lights in there.

Don't do what I've done in the past and touch an HT lead to find out if they are OK. (don't ask me how I would know) 20,000v on the outside gives you wee tingle

Rich H · Post by **Rich H** » Mon Jul 09, 2007 4:06 pm

And if its faulty you get a nice big belt...

robin · Post by **robin** » Mon Jul 09, 2007 7:35 pm

The coil is indeed mounted down under the intake manifold - access is a pain, but it won't be the coil

Check all the service items before worrying:

Distributor cap, rotor arm, HT leads, spark plugs. 10:1 it's one of these.

Kind of hard to test the spark plugs properly, so if you run out of options, replace the plugs. But inspection of the plugs (black/wet = bad) can be helpful in further diagnosis if needs be.

Cheers,
Robin

graeme · Post by **graeme** » Sat Jul 14, 2007 9:45 am

No sign of a cause, but a clean and skoosh of plug-wells, lead-ends and dizzy seems to have cured the misfire, so I guess it was just moisture.

Ta all.