cylinders 5,7 running lean

[99 2dr]

Well I decided that Friday night was going to be the night that I installed my headers and shored up my exhaust leaks. Upon removal of my old manifolds and spark plugs I noticed that I have a problem. Cylinders 5 and 7 are way lean; all other cylinders are just fine. I have been having problems with detonation/ pinging for a while now and have been running premium fuel to help mitigate some of the problem. If I run premium fuel and a bottle of octane booster the problem almost stops completely.

I installed the updated injection spider about 80,000mi ago.

I hope to get the headers buttoned up on Tuesday and will be installing a new cap/rotor and spark plugs. I am currently running 8.5 mm MSD super conductor wires that are about a year old.

The ECM is not throwing any codes, but its obvious that there is a big problem when you look at the spark plugs.

My guesses are:
Intake manifold leak near 5-7 ?
Spark plug wires are touching and causing 5-7 to crossfire under load?

Has anyone else had this problem? I tried searching for lean conditions on cylinders 5,7 but everything gets mixed up with people who typed 5.7 liter.

 

[Phone Man]

I had a problem with MSD wires in general. Chech resistance values on each one. I had 2 that were really bad. I have since changed to Taylor 8.2 Thundervolts. They are good wires.

 

[SunlitComet]

have you looked at compression level for those two cylinders compared to others at all?
did not cross those injectors during install of spider did you? should not be physically possible tho i think. but also 5 and 7 intake runners are pretty far apart too.

 

[99 2dr]

I haven't checked the compression on 5,7 but I will this evening. I'm all but positive that the injectors are in the correct spots; the truck ran fine until 6 months ago. I have some factory wires that I was going to try on those cylinders as well.

 

[99 2dr]

Well I checked the compression last night and that appears to be good. The highest cylinders were 175 psi and the lowest was 163 psi. More importantly the two cylinders running lean were #5 @ 170 Psi and #7 @ 175 psi. This doesn't seem too bad for an engine with 283,000 mi. I'm hopefully going to get her all back together tonight.

 

[gpracer1]

Well you did say the injectors are 80k miles old, maybe they are partially clogged and need some techron, etc.

Maybe a vac leak?

 

[99 2dr]

I'm guessing that it's either a vacuum leak, clogged injectors, or spark plug wires cross firing. I did replace the cap and rotor last night; didn't notice any excessive wear in those areas.

 

[SunlitComet]

assuming that is not carbon build-up in the combustion chamber making the reading high that is pretty damn good for 283k. mine was at 150ish range last year at 233k.

 

[99 2dr]

Sunlit, do you have any other ideas as to what the cause of my pinging/ detonation might be?

 

[SunlitComet]

Carbon build up or restricted coolant passages by cylinder running to hot for the pinging. Or dirty injector if it is actually lean on those two. A shop with the right scanner can run a injector balance test to find an injector problem. Usually take less then an hour of labor.

---------- Post added at 05:17 PM ---------- Previous post was at 05:14 PM ----------

At 280k you are bound to have build up in the coolant passages on the head gaskets. Perhaps a chemical flush is in order but get a balance test done first.