Misfire under load

coil pack (assuming the leads and plugs were genuine and the correct plug gaps )

"Mrcheerful" wrote in news:DfaZl.42408$ snipped-for-privacy@text.news.virginmedia.com:

All parts are the correct Motorcraft ones. Plugs gapped to 1mm instead of

Thanks Mrcheerful.

in that case it is (almost) guaranteed to be the coil pack, nothing else springs to mind.

What brand of leads did you use?

Otherwise Coil pack.

My Mondeo is doing something similar, also with newish Motorcraft leads and plugs. Could be the coil pack, but ISTR reading about some part of the EGR system (attached to the bulkhead) which caused a msifire at about

Anybody know what I'm talking about?

Those symptoms are usually that of failed leads. Clean and check the coil pack first though, then oik the leads off the plugs, and look for grey dots or tracking lines on the surface of the rubber boots to be sure.

Tim.

The most common cause of misfire under load but not at cruise is too weak a spark for the plug gap. It afflicts heavily modified turbo engines which can actually snuff the spark out completely at high boost unless the plug gap is reduced. As you've already gapped the plugs down then coil pack would seem to be the most likely culprit. You can test by reducing the plug gap even further and see if things improve. Back in the day engines managed quite happily on plug gaps of about 0.75mm so that's clearly plenty of spark to fire an average mixture. Modern high output coils and high voltage sparks from big plug gaps make a further miniscule difference to mixture combustion but primarily it just loads up everything else that can provide a current route to earth and shags the plug leads and other parts of the system. I'm a great believer in using only as a big a plug gap as you need to fire the fresh charge. Your HT leads will last a hell of a sight longer anyway.

"Mrcheerful" wrote in news:XEaZl.42442$OO7.42355 @text.news.virginmedia.com:

I have changed the leads and coil pack. No difference. Chief suspect is now the TPS, as it hangs slightly when taking throttle off and revs sit a bit high at approx 900-950.

Tunku wrote in news: snipped-for-privacy@pipex.net:

Well, that has improved things a bit. I've just swapped out the TPS. I am thinking that I may also take out and clean the Idle Air Control Valve. Anyone know where this is on a '98 Zetec 1.8 silvertop ?

In message , Tunku writes

Its to the rear of the block on the inlet duct near the throttle body. It will have an electrical connector going to it. Its tucked up under the inlet manifold and quite tricky to get at. On the Silvertop, you will really need to replace the gasket which IIRC is a paper gasket and generally not re-usable.

Paul Giverin wrote in news:rwBBlwCn7nOKFw0u@10.0.0.3:

Cheers for that. I've had a look and the way to get it seems to be from below. Looks bloody awkward.

"Dave Baker" wrote in news:h1ba3c$bfd$ snipped-for-privacy@news.datemas.de:

I'm inclined to go along with that, I'm looking up the gaps I used to use on my old Volvo for a comparison. What is the idea of such a large gap anyway? Bigger spark for a bigger burn? I don't see the connection, petrol will burn anyway.

"Mrcheerful" wrote in news:XEaZl.42442$OO7.42355 @text.news.virginmedia.com:

As you have probably read, it is not the coilpack. I had my money firmly on that. It is a lot better after changing the TPS. I have put on the old coilpack just to make sure. I have read that I should take off the ISCV, (idle speed control valve for those doing a search) and clean it at the very least before buying a new one. Can I clean this in situ through the hole in the side of the throttle body by spraying carb cleaner down the hole? I am assuming that this hole upstream of the butterfly valve connects to the ISCV and reconnects downstream of the butterfly valve. If I squirt copious amounts of carb cleaner in there, leave it for awhile, then start the engine, all the fluid, crap, etc. will be sucked through and out the exhaust?

Actually fuel/air mixtures under high pressure don't ignite as readily as you might think. There is still a certain amount of cycle to cycle variability in torque output per cylinder depending on how well the mixture ignited and how quickly the flame front spread to the rest of the combustion chamber. Mixture turbulence has as much to do with this as the initial spark. A bigger spark reduces this variance somewhat and also helps ignite very rich and very lean mixtures. Unfortunately cat equipped engines can't run very lean mixtures because the ecu controls everything to stoichiometric so huge plug gaps are even more redundant. They made more sense when lean burn engines were being developed.

If you put a Zetec engine on a dyno I doubt you'd see much difference between the power output with a 0.75mm plug gap and with 1mm or bigger. Maybe a fraction of 1 percent. You certainly wouldn't feel anything on the road. The small gap would probably make your HT leads last the life of the car rather than 40k miles though which is when they seem to die on these engines. I'm just surprised that fewer people have realised that it's the huge plug gap which kills these leads rather than there being any defect in the lead design.

that is the method most people use. a faulty iscv will not cause a misfire, only a problematic idle. you can just disconnect it, the car should still run ok but may not idle, same with the tps. changing the tps may have improve matters simply by richening things a little (which gives the sparks an easier time)

So, I am wondering what the reason is behind Ford specifying this huge plug gap of 1.3mm on alot of their current Engines (including the 1990's Zetecs..) ??

Tim .

"Tim.." wrote in news: snipped-for-privacy@bt.com:

They have shares ? :)

Paul Giverin wrote in news:rwBBlwCn7nOKFw0u@

I took the bull by the horns and decided to take the IACV off. My Zetec appears to have a different set up, as the IACV was doable from the top. Anyway the first thing I noticed was the bolts were finger tight, the second thing I noticed was the alloy of the facing was all oxidised. I thing there has been an air leak here. I tidied everything up washed it all in carb cleaner and put it back on with a gasket and tighened the bolts. It has had a short test run and seems to be much better. Would the air leak on the facing of the IACV cause misfiring etc.?

an air leak means a weaker mixture which is more difficult to ignite