Car Starts Good, Runs Good, Then Misfires After Warmup

[MrAl]

Hello there,

Any ideas what would cause this?

I changed the plugs yesterday and today i went out there and started it and the check engine light went out to my surprise.
However, after 15 minutes running it started to misfire a little and the CEL came back on! Nuts.
It is just a tiny misfire it happens about once every 10 seconds it is not continuous. You can here the engine run smooth then after maybe 10 seconds you here the engine almost cut out for a 1/2 second, then it's ok for 10 seconds, then again it makes a misfire like sound.

Dont know what would cause that.

[abscate]

Plugs cap rotor wires status?

Try reseating all the Plug wires, including coil wire.

[matthew1]

What Abscate said. Re-seat plug wires and coils.

[June]

Why did you change the plugs? Was this a preexisting condition?

I just had cap, rotor, wires, and plugs on my 740 turbo due to a damaged plug insulation. Then my car afterwards started a similar hiccup every few seconds at warm idle like yours. Ran fine cold. Hiccup caused the tachometer to fluctuate at idle. I ran a can of Redline fuel system cleaner in a 1/2 tank of gas. Within 50 miles the car began to idle smoother then it has since I bought it. Then I ran the next tank of gas down to 1/2 and repeated a second treatment. I filled it up with Shell premium today and the car zips up to 80 mph with ease now. So in my case I think it was a gummed up injector. I always thought fuel system cleaner was just snake oil... Apparently some can help. I ordered a new set of injectors arriving this Friday. So don't discount the fuel aspect of combustion. June

[MrAl]

Why did you change the plugs? Was this a preexisting condition?

I just had cap, rotor, wires, and plugs on my 740 turbo due to a damaged plug insulation. Then my car afterwards started a similar hiccup every few seconds at warm idle like yours. Ran fine cold. Hiccup caused the tachometer to fluctuate at idle. I ran a can of Redline fuel system cleaner in a 1/2 tank of gas. Within 50 miles the car began to idle smoother then it has since I bought it. Then I ran the next tank of gas down to 1/2 and repeated a second treatment. I filled it up with Shell premium today and the car zips up to 80 mph with ease now. So in my case I think it was a gummed up injector. I always thought fuel system cleaner was just snake oil... Apparently some can help. I ordered a new set of injectors arriving this Friday. So don't discount the fuel aspect of combustion. June

Hello June,

Well i had misfire on cylinder #3 so i had to try something. The plugs were not changed in 7 years and i dont know what was before that because that's when i bought the car.
As it turns out, lucky i did. I found one plug only hand tightened and another plug with the little nib worn (or burnt) nearly flat.
I checked the gaps carefully before installation and then got them in there. It was a battle though with the plug cover the bolts that held that on were almost frozen in place. Broke my T handle wrench getting one out.

[MrAl]

What Abscate said. Re-seat plug wires and coils.

Hi Matt,

Oh ok for sure then i'll try that first before replacing anything else. Thanks.
I didnt know they were that hard to seat.

[MrAl]

Plugs cap rotor wires status?

Try reseating all the Plug wires, including coil wire.

Ok i will try reseating the plug wires, i did not know this was a problem area where they may not seat right. It is strange to me too to see the plugs recessed so much.

I did not replace the dist cap or rotor yet. I think you have to take the battery out to do that, and that is so you can get the air box out i think, does that sound right to you or can i get away without taking the battery out? If i take the battery out the monitors all reset.

I also did not get a new coil but it would seem a coincidence that the misfire was on cylinder 3 and after replacing the plugs misfire on the same cylinder, so i would not suspect the coil, but i guess that could be part of it right?

Thanks much Steve.

[abscate]

Just try removing and reinstalling the number 3 plugwire first. At night, look for spark misfires between the plug wires and the head with the cover off, too.

If the plugs are that old it is a cinch the cap and rotor are old and crusty too, that will cause single cylinder misfires.

[454cid]

I don't think I had my 850's battery out. I don't recall if I removed my whole airbox, or just the lid. It's not hard to remove. The couple of times I've done it, I removed the lid first then the bottom. Good times to clean under and around it too.

[matthew1]

What Abscate said. Re-seat plug wires and coils.

Correction to my post…

Coils should be singular: coil. Sorry about the confusion, if any. I need a periodic reminder of the single year 98 that were still mostly mechanically an 850.