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Astra F misfire from low revs when accelerating

1998 geg Astra F Arctic I 1.6i petrol (92 - 97 model).

for quite some time now this has been misfiring from low revs when asked to accelerate. There is no misfire when cold so that suggests that the problem might be fuel supply, or possibly a vacuum leak somewhere into the inlet manifold.

Joining a main road with a uphill gradiant is a problem, once 2nd gear is engaged and the revs drop down it hops like a kangeroo. Opening throttle wide when warm at tickover makes it cough violently. OK if revs are kept above 1250-1500 when going round corners.

Braking from 60/70 down to 40 and then trying to accelerate in a high gear makes it stutter too. Plugs are GM twin head and look perfect even though they have not been changed for 40K miles. Fuel filter has also not been replaced during the same period. Do these get clogged up ?.

I cannot hear any air leaks.

The tubing to the MAP sensor and the activated charcoal tank vent valve is ok (afaik). I have checked and cleaned the contacts on the MAP sensor (because this connector is not gold-plated but uses 5 volt logic, i.e 5V plus Gnd IN and 0 to 5V variable OUT depending on absolute vacuum levels).

Bolts holding EGR valve were only finger tight (??), so EGR was removed and checked for crud - all seems ok.

Engine management light has never come on, apart from briefly when EGR connector was removed, then went out after connector was replaced.

? any ideas

Andrew

Reply to
Andrew
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Leads, plugs, coil pack Most likely a lead. Might be tracking across the coil pack mind, have a good look for grey hairs- or sparking in the dark.

Reply to
Tim

On 04/04/2015 15:39, Tim

I appreciate that, and having just got to 80K miles it's on the shortlist, but it doesn't happen when the engine is cold !.

Flooring the throttle during the first few minutes after starting from cold gives the response that you would expect. And pulling out of a side road from cold doesn't show the low revs hestitation.

Only when the engine has reached normal operating temperature does the problem appear. And nothing has ever made the engine management light come on. My neighbour (who had a SAAB business and is a retired MOT tester) doesn't think it is electrical for this reason.

All four (GM) plugs 'look' fine, a nice brownish colour, but I'll try some news ones for starters.

Reply to
Andrew

Cold engines run richer than a hot engine, richer is easier to get lit with a weak spark, when the engine is hot the much weaker mixture of a hot mode rn engine is much harder to light (hence massively increased spark voltages over old engines, and the need for better plugs, typically multi earth plu gs (what you should have) and platinum/iridium plugs) I have known the mul ti prong plugs fail in 12,000 miles (three pot corsa), let alone 40k

Reply to
mrcheerful.nm

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