Diagnosing A Misfire?

[ATK 143 1 Cars]

Got a misfire that has suddenly appeared.

 

Only happens under load and the car stutters. Happens in all gears. 

 

Coil plugs ht leads ignition amp and relay, dizzy cap rotor arm coolant temp sensor etc all less than two years old.

 

idle is fine. Does it cold or warm.

 

rather than simply

changing components blindly, any tips on how to diagnose?

 

cheers all

 

[welshpug 1,666]

check the condition of the plugs, check compression figures.

[pugdamo 111 2 Cars]

I know you said they were replaced 2 years ago but leads breaking down can be a common cause of misfires under load. Also worth a check in and around the cap, rotor arm leads and plugs for water/moisture. 

[tartanbloke 35 1 Cars]

Check condition of Distributor as this may be on its last legs.

[ATK 143 1 Cars]

Hi all, appreciate all these things can cause misfires - but just to clarify, I was wondering if anyone had any tips on how to test any of these components, rather than replacing them blindly.

 

I don't have spares to swap in and out, and I don't want to start buying components for the sake of it particularly as most parts are only;y 2 years old at max.

 

Thought there may be some ways of testing to see what is causing it (aside from the compression test welshpug mentions above which is an option).

[pugdamo 111 2 Cars]

Visual checks for moisture is the easiest and cheapest thing, as for tests on the leads you can do resistance checks on the leads, I wouldn't know what the values were off the top of my head I'm afraid, If i couldn't get hold of any donor leads and couldn't get any resistance values check them all and see if any one value looks odd, you can expect the readings to change with lead length though. 

[jackherer 543]

If the leads are arcing you can sometimes see it if you look at the engine when it is running in the dark.

[jj007205 0]

I'd check the plugs first. Then leads as a start. Quick check with multimeter will let you know state. Longer leads will have slightly greater resistance.

[tartanbloke 35 1 Cars]

In order to quickly check the condition of the distributor, you will need to remove it but this will also give you a chance to check the condition of the other items at the same time (rotor arm, leads etc).

 

In short, if the distributor is on its last legs, it will freely spin round with little to no resistance, indicating that the springs have broken on the inside, which can cause misfire and rough running under load. Also, in extreme cases, the inside contacts the body, causing it to grind, pop off the cover and loose vacuum.

 

Some of these are really old now and after having mine swapped with a refurbished unit, made a massive difference on how it drives, idles and accelerates.

[ATK 143 1 Cars]

Well, looks like it might have been a false alarm.

 

After initially not turning over (engine/starter was trying to start but wouldn't catch), got her started, had a long blast (italian tune up!) and all appears to be well. Might have been moisture as she'd been sitting for a few weeks.