gearheads78

OK my new to me daily driver / tow rig is killing me. A couple days after I started driving it I randomly started getting a misfire. Over a few weeks it started getting worse and I watched it while driving on a scanner and its coming from #4. To get a base line I did a tune up with plugs, wires, cap , rotor and new 02 sensors. Fixed!! Drove like a dream for 3-4 days then started to get a occasional miss again Over the coarse of about 3 weeks its gone from occasional to 15000 misfires on a 20 miles commute home. It's still #4.

Start looking deeper. Misted water on the wires in the dark no arcing. Pulled #4 plug looks great but replaced anyway with no change. Replace wire with another new wire.. no change. Compression test on 2, 4, 6, both hot and cold all 135-138 psi. Injector balance test... all 8 with in 2psi.

It makes no sense to me! What the h##l am I missing? Anyone have any idea? I have an injector on order and hope to have it in over the weekend but if that does not fix it my next step might be a match and a can of gasoline.

65Delta

I have a '98 with the Vortec 350 and there's a TSB for tight valve guides that can result in a multiple misfire when the engine is pulling a long hill. I haven't looked at the 454 but perhaps it has the same problem. The fuel injection spider has some issues (there's an improved Delphi CSFI to MPI retrofit available), the plastic intake gaskets degrade over time, and the distributor body is made from a phenolic resin as opposed to AL or steel. Any (or all of these) may be creating the perfect storm if applicable.

I'll check the FSM to see if the 454 has the same components.

gearheads78

I would think that would show up in the compression test though? Also does it from just off idle to under light to medium load

Mr Shifty Sidney

I chased a bad ignition module in my 93 350 Chevy truck. Would miss fire and and eventually it started shutting the engine down. Had it tested and it tested fine. I suspected it at first but since it tested good I did not replace. I do not like just replacing parts trying to fix something. Eventually I replaced it and problem was solved. May be worth a cheap fix??? Good luck.

Don W

65Delta

The tight valve guides only cause the valves to stick when the engine is red hot & working hard so it likely wouldn't show up in a compression test. The "fix" is to pull the heads and increase guide ID 0.001-0.0015".

Based on the fact that it does it off idle and light load the tight guide issue isn't your problem.

Are your crank/cam sensors in sync? Should throw a code if you're outside the window but might be worth running the crank variation learn program on the scan tool.

gearheads78

Only misfires #4

Will check that. I have been googleing for that last hour and ran in to a thread on another forum from a guy with a similar issue. Turned out to be a wire in the injector harness rubbed through touching ground. Maybe there is hope!!

joe_padavano

I have the same engine in my 99 K3500. I had a similar misfire problem on #2. Brand new wires. The one of the new wires was bad. I'd also try a crank sensor learn cycle, since you have the scan tool.

nonhog

Not sure if it applies to this engine or situation. The rubber O-ring on the cranks sensors age and shrinks causing issues that are hard to find due to the sensor out of alignment. Cant see how that would show on #4 though?

cfhcar

check for intake leaks

gearheads78

For anyone following this I am 98% certain I have found the problem. The light blue wire with black stripe is the #4 injector wire. Yes that is copper you are seeing.

oldcutlass

Glad you found it.

CRUZN 66

Interesting, my 2002 with the 8.1 has recently started misfiring on number 5... I haven't checked it out as yet, but if it's not the injector, I may go right to the harness... Thanks for your follow up...

lemoldsnut

I had the same issue and it was a bad injector wire and then had it again and it was the wire to the TPS. Fixed and 40k later no issues. 262k total now

gearheads78

Aaaaaand fixed. $60.00 for a new harness. I replaced the whole thing since it was so hard and brittle

lemoldsnut

Where did you get the harness??

gearheads78

I ordered it from work (gm dealer)

lemoldsnut

Can you give us the part number? I would order one for mine.

gearheads78

I can't find the number now. I will try to find it but any gm dealer can look it up by your vin. It was in our local warehouse so its still readily ava.