Spark Plug Change - Now Misfiring

[ard]

I searched (before tackling the sparkplug DIY), but didn't find this problem addressed. However, if I overlooked a thread, feel free to redirect me.

 

PROBLEM:

----------

* prior to my changing sparkplugs truck ran just fine - smooth idle and no codes

* changed the sparkplugs (since PO last changed them about 3 years / 30K miles ago according to service records and I didn't know which ones were used)

* now I have very rough idle and when I put it in Drive it sounds like it wants to stall

* "Service Engine Soon" light on the dash is now illuminated

* no backfires though

 

WHAT I CHECKED:

-----------------

* checked (twice) to make sure I didn't have any plug wires crossed. I didn't replace the wires since I think they were replaced 30K miles ago and didn't seem to show any external signs of wear / cracking

* all vacuum lines connected correctly

 

I used a sparkplug boot puller, and made sure that I gripped the boot only and not the wire. I twisted before pulling and was able to get all 8 wires off without any drama.

 

I used antiseize on the plug threads and dielectric grease inside the boots. I checked the gap of the plugs before I installed them. When I got the misfire I actually went back and put the OLD sparkplugs back in (they were actually in very good shape), just on the (very) off chance that I had the wrong sparkplugs, but the misfire persists

 

Does this sound like I broke a wire internally? I plan on pulling the wires and measuring their resistance next, but I would be grateful for any insights.

 

 

VEHICLE is a 1997 GMC Suburban with the 7.4L motor.

 

TIA

 

/alex

[[email protected]]

You probably either didn't gap a spark plug right or you damaged a wire in the process of changing the plugs. The wires are very fragile.

[dewfpo]

Did you disconnect any other components (vacuum hoses, electrical connectors, etc....) to get ant any of the spark plugs? If so, re-check all those connections.

 

DEWFPO

[ard]

Thank you for the prompt replies - great forum.

 

You probably either didn't gap a spark plug right or you damaged a wire in the process of changing the plugs. The wires are very fragile.

 

I did verify that the plugs were gapped to 0.060", which is what the specs call for. I am therefore leaning toward a bad wire. It looks AWEFULLY tight at the distributor. Can a person of average dexterity get new wires on there or does "stuff" have to be removed? If so, what "stuff"?

 

 

Did you disconnect any other components (vacuum hoses, electrical connectors, etc....) to get ant any of the spark plugs? If so, re-check all those connections.

 

DEWFPO

 

I did accidentally pull a very small line off of a small cylindrical device on the right side of the engine bay when trying to get to the #6 and #8 spark plugs but I did plug it back in.

 

I checked the vacuum diagram in the engine and made sure that all vacuum hoses are connected. As far as electrical connectors are concerned, I am quite sure I didn't unclip any, as they are all secured between the male and female parts with locking clips.

 

Keep the suggestions coming.

 

TIA

[[email protected]]

Spark plug wires are easy to replace, just make sure you use the right length wires for each cylinder. And do ONE at a time and find the one that's the same lenght and use it where the old one came off.

[Black02Silverado]

It sounds like you damaged a plug wire. It could be possible that you got a bad plug but not likely. I would check all the wires as suggested above.

[pm26]

There is a possibility that you cracked one or more porcelain insulators on the new plugs when tightening them. This happened to me years ago on an 83 Pontiac Bonneville. A crack in the porcelain will cause the plug to misfire. Be very careful when tightening new plugs and always use special spark plug sockets lined with rubber.

[sic lsx]

Also if up have the wrong plugs to cold it can miss fire. Have up hooked a scanner up to seen what cylinder or cylinders are missing?

[dlewis1111]

You said you put the old ones back in and the problem was still there. You have eliminated the plugs as the problem It is either a wire or something else that you have disturbed. Put on some new wires.

[jblakeney]

Spark plug wires are easy to replace, just make sure you use the right length wires for each cylinder. And do ONE at a time and find the one that's the same lenght and use it where the old one came off.

 

Come on man.... I love it when people pull them all off then take the cap and rotor out and chunk it on the ground..... I like it even better when they do it in the front door of MY NAPA store......

 

And if the guy with the 1987 Jeep Wrangler with the 350 Chevy is out there... You know the guy that did this the LAST TIME in my parking lot.... He even pulled the distributor out and dropped a re-man distributor in toooo. Not to mention it was 5:00 on a Friday afternoon..... Yes, I stayed and got the chevy-jeep back in time and got the plug wires going to the correct cylinders........ I did take my time to explain to him next time do this at his house or take it to a shop....... I doubt it worked thou. About 6:30 I finally retrieved the lock down bolt for the distributor (he dropped it ) and tightened up the clamp. Cranked it up and he drove off..... And my weekend officially started.

 

Oh yeah, my money is on a bad plug wire or broke cap....

 

Jbo

[[email protected]]

Spark plug wires are easy to replace, just make sure you use the right length wires for each cylinder. And do ONE at a time and find the one that's the same lenght and use it where the old one came off.

 

Come on man.... I love it when people pull them all off then take the cap and rotor out and chunk it on the ground..... I like it even better when they do it in the front door of MY NAPA store......

 

And if the guy with the 1987 Jeep Wrangler with the 350 Chevy is out there... You know the guy that did this the LAST TIME in my parking lot.... He even pulled the distributor out and dropped a re-man distributor in toooo. Not to mention it was 5:00 on a Friday afternoon..... Yes, I stayed and got the chevy-jeep back in time and got the plug wires going to the correct cylinders........ I did take my time to explain to him next time do this at his house or take it to a shop....... I doubt it worked thou. About 6:30 I finally retrieved the lock down bolt for the distributor (he dropped it ) and tightened up the clamp. Cranked it up and he drove off..... And my weekend officially started.

 

Oh yeah, my money is on a bad plug wire or broke cap....

 

Jbo

 

 

Yeah, I know, some people just don't think about how an engine actually runs until it's too late and they don't know what to do. If I could get apid to put plug wires on a 350 all day, I definitely be doing it in my off time.

[sic lsx]

why i love lsx motors! i would love to see u cross them lmao

[dlewis1111]

funny but true story about plug wires: Until I was 25ish, the only V8 engines that I had ever really worked on were Oldsmobiles. Their firing order is the same as a chevy, but the distributor turns the opposite direction. I helped a buddy replace the 350 in his 88 pickup and it took us forever to figure out that i routed the plug wires perfectly for the olds motor. ran perfect after the snafu was sorted out.

[jblakeney]

dlewis1111, my first car.... was a 1979 Hurst/Olds...... Which the only reason the previous owner (which was the original owner) sold the car was the guy at the local chevy dealership didn't know how to adjust the timing on a Olds 350..... He created a overheating problem. Work out great, after adjusting the timing and a carb rebuild all was back to normal and it still runs great... I still have it and since have added a couple other Hurst to my collection....

 

The lady that owned the car from 1979 till May 5 2000..... Actually owned 2, 1979's at one point. Her son ended up wanting to drive her's so she bought him one to drive. She did not want him driving her car......

 

Cut me and I bleed Oldsmobile.

 

Jbo

 

Picture 1 1986 R10 - 1983 Oldsmobile 98 Regency - 1979 Hurst/Olds

Picture 2 1979 Hurst/Olds

[Silverado4x4]

There is a possibility that you cracked one or more porcelain insulators on the new plugs when tightening them. This happened to me years ago on an 83 Pontiac Bonneville. A crack in the porcelain will cause the plug to misfire. Be very careful when tightening new plugs and always use special spark plug sockets lined with rubber.

 

The same thing happened to me, they crack easy.