Recently I changed my plugs due to a cold start misfire, (about a month ago) which has so far been cured , no longer any hesitation with a cold engine. However NOW I have a misfire which is more difficult to understand. The engine starts up and drives away from cold just as it should. After about 10 mins,when the engine has been up to temp for at least 5 mins & by then usually cruising on the highway; about 2/3 of the way to work. Suddenly a light misfire or stumbling appears. It seems to be worse at light or part throttle & is not noticable if I sharply accelerate & go into open loop fuel condition. I have only just got a misfire code P0302, which is a different code from the cold start misfire from before I changed the plugs (P0306), which has not re-occured. When the misfire starts it continues to run rough at idle speeds & the only condition where it smooths out is under rapid (for an Aero) acceleration.

I hooked up my scantool & all the sensors seems to be reading as normal, ECT, ACT, O2 sensor voltage swing, & fuel trims seem to be normal. The IAC is working & controlling idle speed at around 900 roughly since it's not very smooth under the misfire condition. TPS measures good. Symptoms dont seem to jive with EGR valve, but maybe it could be that?

I'm kind of at a loss at the moment, If I had a bad #2 plug... it should misfire when cold & also with damp conditions ? Same with the plug wire, if it was a coil pack, should be 2 cylinders misfiring right? Could it be an injector & why only when engine is warm/ hot?

MPG's seems to be normal.

So far only the one code. One thing I noticed yesterday, I shut the engine while the stumbling/ misfire occurred, when I turned the key on I listened for the fuel pump, could not hear it properly since there was a groaning sound..... I presume until the system pressurised & the pump switched off (about 2 seconds), could this be a sign of a failing fuel pump? I have yet to test for fuel pressure or injector pulse width since my computer (laptop ) is under self demolition! and I cant run the analyser at the moment.

Any suggestions more than welcome.
Regards, Aeroman

 

Had the same thing happen when summer heat kicked in. I found that the coil pack was slowly going bad on my 94 aero. The pack has 3 coils,1 went bad. Gota go

hope it helps

 

what does the spark plug electrodes look like on the coded cyl?

i'd go with the injector also. may be the injector wire connector was disturbed when plugs were changed. they are not the tightest connection and are susceptible to corrosion and coolant spray.

exchange 2 injectors and see if prob. tracks to new hole. pain to pull on the Aero because of lack of room

some intermittents that appear to be hot run associated are actually physical movement caused

can also be a cracked plug insulator, crack opens up with warm up. seen that one. far worse with resistor plugs and their internal gap that the spark has to jump across.

we've got an old section of Eisenhower concrete freeway with the tar expansion joints and the concrete pads that keep tilting and moving. best place in the world to test for physical movement intermittents.
have to use extra false teeth glue when i drive it.

 

Well tonight I took out the #2 plug & it looks like new. The plug wire did not look good though, some corrosion like crud on the electrical clip in the boot, so I've changed out the 3 wires on that bank, they still say 1996 on them, thought I'd swapped them out at the last plug change... guess not! There are 5 retainer clips on those short wires ! took quite some time to re-route the wires & install them. Also noticed the plug was on the loose side probably a bit too loose. Could have had a slight bit of debris fall onto the threads as I installed it maybe. I don't like to crank very hard on plugs.... 10 Lbft is not very much torque & not much room to get a torque wrench on it.

Took her out for a 30 km spin & so far so good, I'll report back in a week or so with any other developments.

 

Been out a dozen or so times & no sign of the Misfire. CEL has gone out by itself. Engine feels great.

As far as I can make out, loosened plug causing eratic ignition when hot..... or possibly bad plug wire. I'm thinking the loose plug since if wire was bad engine would have been harder to start. Also I don't see any signs of insulation break down or spark tracking on/through the wire.

Either way problem has gone,

Regards, Aeroman