misfire on #5 code….help

[tom]

got engine code ( misfire on #5 ) ….did compression test on #5 and #7 and got about 125 reading on both cylinders… plugs,wires,cap and rotor are couple months old.
Replaced spider fuel injector assembly, replaced #5 fouled plug, cleared code, ran engine…. same code returned (misfire on #5 ) immediately… I’m stummped on what to do next…
1998 GMC Sierra k1500
5.0 305 vortex 2wd (301,100 miles)

Topic

[Its me]

What were your readings on other cylinders? 125 seems a bit low…

Karl

[tom]

only tested those two

[Its me]

Ok, test them all…. FWIW compression tests should be done on a hot/warm engine at WOT…. What has to be determined no matter how you did it, is that compression is not off by more than 20% between cylinders. ..

125 seems pretty low.. so test all cylinders in the same conditions… If over 20% off from other cylinders then you have a mechanical condition to deal with. ..

Post your results and we will give you input…

Also 5 and 7 are next to each other. .. It could be a blown head gasket. ..

Good luck. .

Karl

[Bill]

I might be wrong but I believe the vacuum supply for the power brake booster is adjacent to #5 & 7 runners on the intake manifold. If the booster is leaking internally it could be causing a vacuum leak and the misfire you describe.

Try pinching off the hose to the booster to see if anything changes. Be sure there are no vacuum leaks anywhere else by spraying water around the intake manifold. If there is a leak the sound of the engine will change.

[tom]

i’ll take a look and test it out
thanks

[angelo paloumbis]

are 5 & 7 plug wires crossed on the cap? common mistake on a small block chevy engine especially if it is a standard round cap. firing order is 1 8 4 3 6 5 7 2 with cyl 1 on the front drivers side and distributor turns clockwise as your facing the engine #1 plug wire should be on the front of the dist. almost offset to your right(drivers side)

[Author]

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