Runs like crap after plug change

[silky28]

THey all,

03 'hoe 5.3 flex...

Decided to do some routine maintenance on the beast - plugs, wires, air filter - and afterwards the engine is running like crap, most notably at idle. On the highway the truck seems to have power and is smooth...at idle the engine runs slightly lumpy and, listening to the exhaust, it is obvious that there is a misfire.

I hooked up my scanner and go the reliable p0300 code but that does not tell me much. I cannot see it being the intake manifold gasket because I've changed that and because I tested it by spraying carb cleaner around the engine with no change; it doesn't seem like there is a vacuum leak at all. The live data on the scanner shows a defective O2 sensor on B1S2 (pegged at about 835mv). The other sensors seem fine though B2S2 seems a bit sluggish. The fuel trims are slightly negative ~ -3.5 on average. Would a -3.5 stft really cause a noticeable issue?

The engine seemed to run fine with the old plugs, single Platinum NGK plugs that were in it. I changed them because they were old and the center electrode was essentially gone on all of them. I changed the plug wires because some of them ripped when I removed them.

Parts used:

Air filter: regular fram filter

Plug wires: NGK

Plugs: Bosch 8104 Double Platinum

I see that many recommend only using the AC Delco plugs so that may be the issue. Other than that, I did not check the gaps on the plugs when I installed them. I assumed they were at the right .40 as the box says and threw them in. I guess I should check that. Next I will replace the fuel filter becuase I am quite sure it has not been replaced in 60,000 miles.

 

[afpj]

Which wires Did you get? I seem to recall a difference between the wires depending on whether you have the round or the squarish coils. Also how many miles on your rig? Did you clean the MAF? Can you see if misfire data on your scanner? As in live misfire counts?

 

[avalonandl]

Pull the BOSCH plugs out and put in NGK , AC or ND.....Bosch plugs work like SZHIT in the LS engines.....

 

[silky28]

The bus has about 200,000 miles on it. I did not clean the MAS and I got the right plug wires for the coils. I have the round coils that part number end in like 2018.

scanner does not show misfires unfortunately. I am going to try and isolate the misfiring xylinder(s) with an infrared heat gun. Ie look for cylinders that are below the temp of the others.

it’s obviously not a serious misfire because There is plenary of power on the road...

 

[silky28]

Pull the BOSCH plugs out and put in NGK , AC or ND.....Bosch plugs work like SZHIT in the LS engines.....

can you recommend some cheap copper plugs I can put in temporarily just so I can make sure that is in fact the problem before I buy more plugs?

 

[avalonandl]

No these engines require Platinum plugs.

On the 2004-6 GTO forum we had numerous members try the Bosch plugs with same issues. Put the old plugs back in.....

 

[Danny3737]

I’d put in the AC Delco’s and call it a day

 

[PNWYukon]

The PCM is detecting a crankshaft rotation speed variation indicating a misfire sufficient to cause emission levels to exceed mandated standards.

Bad gaps, janky plugs.

Does your scan tool tell you which one is misfiring?

 

[Fless]

At night, start it up and open the hood. Watch for the light show.

 

[silky28]

No these engines require Platinum plugs.

On the 2004-6 GTO forum we had numerous members try the Bosch plugs with same issues. Put the old plugs back in.....

I was under the understanding that the only reason they used platinum plugs was for the extra lifespan and that coppers work just fine but need replacing much more frequently.

 

[silky28]

Ok so it turns out it was not the plugs but the wires...more specifically the connection between the wires and the plugs. Long story short the connections on 2 plugs were loose. It was hard to notice...wasn’t arcing or anything...but the wires seemed slightly loose. Basically I just reshaped the connectors a bit and after that everything snapped together. Truck seems to run perfectly now.

...other than the down stream o2 sensors which are both pegged ~800mv and the short term fuel trims which bounce between 0 and -4% but mostly between 0 and -.7%

I guess next thing is to swap the o2 sensors.

 

[Danny3737]

It’s always good to put some dielectric grease on inside of the spark plug boots so they slide on easily

 

[avalonandl]

Ok so it turns out it was not the plugs but the wires...more specifically the connection between the wires and the plugs. Long story short the connections on 2 plugs were loose. It was hard to notice...wasn’t arcing or anything...but the wires seemed slightly loose. Basically I just reshaped the connectors a bit and after that everything snapped together. Truck seems to run perfectly now.

...other than the down stream o2 sensors which are both pegged ~800mv and the short term fuel trims which bounce between 0 and -4% but mostly between 0 and -.7%

I guess next thing is to swap the o2 sensors.

OK so that would cause an issue....

 

[avalonandl]

https://www.gm-trucks.com/forums/topic/119068-prob-with-bosch-spark-plugs-in-a-60l/

 

[brichter]

S2s are post-cat, and not really indicative of driveability issues. For those issues, look at the S1 readings.

 

[Rocket Man]

It can be hard to get the wires to snap onto the plugs and even harder to know when they do. Dilectric grease like mentioned helps but twisting and pushing at the same time also helps. Glad you got it figured out. I always put the wires on the plugs, then onto the coils so I can twist them onto the plugs.

 

[LARRY M VANDIVIER]

ive had this problem for some time my self if gm knows of this i guess vortec has same issues no only ls gm need to do a recall way to many
problems in this area .

 

[Rocket Man]

ive had this problem for some time my self if gm knows of this i guess vortec has same issues no only ls gm need to do a recall way to many
problems in this area .

Exactly what issue are you referring to which would warrant recalling 15-20 year old vehicles? The OP discovered he didn’t have the spark plug wires snapped all the way onto the plugs which was the reason for misfires.

 

[Sweetie101]

THey all,

03 'hoe 5.3 flex...

Decided to do some routine maintenance on the beast - plugs, wires, air filter - and afterwards the engine is running like crap, most notably at idle. On the highway the truck seems to have power and is smooth...at idle the engine runs slightly lumpy and, listening to the exhaust, it is obvious that there is a misfire.

I hooked up my scanner and go the reliable p0300 code but that does not tell me much. I cannot see it being the intake manifold gasket because I've changed that and because I tested it by spraying carb cleaner around the engine with no change; it doesn't seem like there is a vacuum leak at all. The live data on the scanner shows a defective O2 sensor on B1S2 (pegged at about 835mv). The other sensors seem fine though B2S2 seems a bit sluggish. The fuel trims are slightly negative ~ -3.5 on average. Would a -3.5 stft really cause a noticeable issue?

The engine seemed to run fine with the old plugs, single Platinum NGK plugs that were in it. I changed them because they were old and the center electrode was essentially gone on all of them. I changed the plug wires because some of them ripped when I removed them.

Parts used:

Air filter: regular fram filter

Plug wires: NGK

Plugs: Bosch 8104 Double Platinum

I see that many recommend only using the AC Delco plugs so that may be the issue. Other than that, I did not check the gaps on the plugs when I installed them. I assumed they were at the right .40 as the box says and threw them in. I guess I should check that. Next I will replace the fuel filter becuase I am quite sure it has not been replaced in 60,000 miles.

 

[Sweetie101]

ive had this problem for some time my self if gm knows of this i guess vortec has same issues no only ls gm need to do a recall way to many
problems in this area .