6 cylinder misfire?

Hi I have a misfire in the 6 cylinder on my 98 Chevy c1500 5.7l 350 v8 2wd. I changed the spark plugs and wires, cap and rotor, and changed the spider injector i put the spider injector in from another engine I had. After doing all this the code still comes up. The truck runs fine and exept it idiles slight rough not bad at all and now it says I have a vacuum leak and while I’m driving you can feal the loss of power. I tryed all the ways to do it like on YouTube. Can somebody plz tell me what is wrong with my truck? Thanks

Have you checked the compression? I am thinking either a intake leak or burnt valve.

Year, make, model really helps when trying to help someone over the internet.


The compression is low in the 6 cylinder

@Deerslayer For your information, no fault code specifically states that you have a vacuum leak

For the time being, I’ll assume the fault code said “lean condition” or something to that effect. Either P0171 or P0174, correct?

What exactly is the compression on #6 . . . what is the compression on the other cylinders?

Out of curiosity, this donor spider, are you absolutely sure it’s fine? Are you sure the #6 poppet isn’t partially plugged? It’s extremely common for one or more of them to partially plugged. A fuel pressure gauge and an OEM level scan tool with bidirectional controls can quickly determine if you’ve got a fuel problem

If one of the poppets is plugged, running the engine on fuel cleaner hooked up directly to the fuel rail often clears them up. it’s not the same as dropping weak cleaner in the fuel tank. The stuff that gets hooked up to the rail is much more potent

Anyways, have you determined if the cause of compression is top end or bottom end? That would be one of the early steps in my diagnosis

Back in my days with GM this was a common situation. The truck would have one cylinder run lean from an injector starting to fail, sometimes it would go undetected until an exhaust valve burnt. The low compression on #6 leads me to believe this is what has happened.

"98 Chevy c1500 5.7l 350 v8 2wd."
That’s from the original post, tester.

As regards the problem, I’ve nothing to add to what’s been posted. Except that the low compression on #6 leads me to suspect a valvetrain problem. Probably have to pop the head to get to the bottom of this one. A listing of the various compression readings might change my mind and lead me to suspect a “bottom end” cause, but without that I’m going to be an optimist and say “valve”.

With compression down on a cylinder you can throw parts at it until hxxx freezes over and make no headway. A slightly rough idle is to be considered expected and normal when a cylinder is dropping.

It’s also common for an engine with a weak cylinder to idle slightly rough and smooth out off-idle when the RPMs start climbing.

It would be interesting to know the compression numbers on all cylinders. It could be just a tired engine whose time has come.

I will let you all know what the compression is on all cylinders soon and I changed the spark plugs and the 6 cylinder was the only 1 that had corrosion on it and in it. I put the new spark plugs in yesterday and checked the codes after running the truck around yesterday and today and here are the codes that are coming up p0174 p0306 p0420 po507
Co216. The power came back to the truck and does not feal like its loosening power anymore while I’m driving.


P0174 is lean bank 2 . . . possibly because of the excess unburned oxygen from #6 misfire

P0420 is catalytic converter low efficiency . . . bank 1, I believe

p0507 idle speed higher than expected . . . possibly dirty throttle body

Do you live in an area where emissions testing is required?

Yes emisions is are required here and I cleaned the mass airflow sender and throttle body