Thread: k i got a cylinder 4 miss and im running out of options

I can hear mine misfire at idle but my scans come up clean.

Originally Posted by Bio248

these cars misfire at idle. if its not causing any problems and all the ignition goodies are fine then who cares. my car always misfired at idle.

But that doesn't make it right.

OK... I'm new here. I just got a '99 Firechicken 3.8 efi with the same #4 idle miss that occasionally throws a code. Like another poster, mine's also really bad at start up and seems to smooth out some as the engine gets closer to operating temp. Even then, if you creep the throttle forward from a full stop the engine stumbles and then surges into passing gear, kinda like a carbureted engine with a sticking float. But heavy throttle from a full stop gives smooth acceleration.

I'm just getting started with the elimination process (wires, injector, ign pak, fuel pressure, etc.) and am interested to see how the OP's problem plays out. I'll definitely post back if I find a cure for mine. ~FH

yeah i hear those too... but i dont care because it runs fine.

Originally Posted by DaytonaGTP

i just kinda gave up on mine. it doesnt throw a code anymore. its almost not noticable and sometimes it doesnt happen at all. the only thing i can think of that i did was do something to the rear head when i had straight pipes. but yeah in case you are all wondering i never fixed it i just let it go... doesnt really bother me. 230k miles so i figured its a normal thing now

Mine is high mileage, too. 180k. As I said earlier, it seems to clear up to an acceptable level as the operating temp rises, which got me thinking about it being a "cold" plug and maybe I would bump the heat range on #4 up a little. Hard to figure an engine that operates "normally" close to 210F could have a cold cylinder though.

BUT... I have also recently noticed the miss comes back if I shut down and re-start...like on a trip to the store. That kinda tells me the problem might reside in the closed loop "warm up" cycle of the computer. So now I'm wondering if at some point these high mileage cars might need to be re-flashed. Anybody tried that? ~FH

three and four are the worst cylinders for adding heat.

they're already prone to popping.

Originally Posted by 01gpgt01

I wouldnt go mix matching heat range on plugs. These cars are notorius when moded at having mis and surging during closed loop operation. Some people have had luck with tuning.

No mods on this engine... that I know of. Just came out of a plug/wire/oil change. Just for giggles & grins I'll have the 'puter reflashed and see what happens.

As widespread as this #4 misfire seems to be on the 3.8 it's surprising no tech bulletins or fixes have been found by now. I do a lot of multiple stop daily driving and if it weren't for the closed loop miss on restarts I guess I could live with this particular gremlin. But just knowing the engine is out of balance for a few minutes and remains an unresolved problem bugs the crap out of me. ~FH

Originally Posted by matt5112

I can hear mine misfire at idle but my scans come up clean.

OK... update to my last post...
First, I mistakenly said the miss was most noticeable in "closed loop" (warm up) mode. Apparently the warm up mode is open loop. And once I got on the same page as my mechanic he said "COOLANT TEMP SENSOR"|". He said the computer won't throw a CTS code until the temp reading is zero. So if the sensor is sending erroneous temp to the computer it will mess with the fuel flow and make the engine run rich and rough. Why the problem seems to manifest itself with a miss and code in #4 is a mystery.

I replaced the CTS ($16) and the 3.8 is now running smooth as a baby's butt and NO SEL! The CTS is directly under the top radiator hose on the intake. You need to loosen the serpentine belt to get to it with a 3/4" deep socket.

I don't know if this will cure ALL the #4 misfire problems, but it sure sent mine packing in a hurry. ~FH

My coolant temp sensor is right under the thermostat in the LIM...

Nowhere near the belts on a transverse 3800... I seem to recall you mentioning you have a F body...

Though I have noticed that my PCM thinks the ECT is 177 at cruise when the T-stat is supposed to be a 180. Newish too.

OK...It's been almost a month since I changed the Coolant Temp Sensor and... the miss in #4 came back in about a week, complete with P0304 code. Back to the shop and my mechanic finally decided to scope the cylinder through the plug hole. Something is stuck to the exhaust valve. He said it looks like an errant piece of manifold gasket that slipped past him during the gasket replacement. His best hypothesis about the coming and going of the miss is the gasket chunk gets softer as it heats and allows the valve to close a little more.

Anyway... the heads are now scheduled to come off to get a full inspection and cleaning... no charge... since he feels it's his fault he missed it.

And just a sidebar about this 181k 3.8 engine... even with the #4 misfire I got 27 mpg on a trip this weekend. And that was just on back roads. Not the Interstate. Other than the plastic intake, GM sure got this one right. ~FH

Originally Posted by bluegtp91

Sticky valve...run some seafoam through that b1tch.

For realz.

There's a difference between a sticky valve and something stuck TO the valve. 2 cans of Seafoam didn't faze it. Heads will be coming off. ~FH