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97 silverado Vortec 7.4 running like 7 cyl.


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harborguy
Novice

Jul 22, 2010, 8:02 PM

Post #1 of 6 (5549 views)
97 silverado Vortec 7.4 running like 7 cyl. Sign In

Engine 7.4L, fuel inj., seems like it's running with 7 cyl's. I've installed a new distributer, cap and rotor, new wires,
plugs, coil. Engine has 15500 miles on it. After all the above still runs rough. We made sure we put the distributor
in correctly by TDC #1 cyl. etc. (old dist. theeth were like razor blades, ). Engine did this same symptom with
old didtributor also. Compression pretty good 150# with two cyl. at 125#

No codes poping up, fire going to all 8 cylinders.......

Anybody have some ideas?


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jul 23, 2010, 1:10 AM

Post #2 of 6 (5544 views)
Re: 97 silverado Vortec 7.4 running like 7 cyl. Sign In

Are the two cylinders reading 125# next to each other? Did/does wet (squirt of oil) make a huge difference in compression on those two?

T



harborguy
Novice

Jul 23, 2010, 9:46 AM

Post #3 of 6 (5532 views)
Re: 97 silverado Vortec 7.4 running like 7 cyl. Sign In

The two cyl. Are 7 & 8 across from each other. What would the oil tell you? Bottom or top end? :.............thanks.

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Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jul 23, 2010, 10:17 AM

Post #4 of 6 (5527 views)
Re: 97 silverado Vortec 7.4 running like 7 cyl. Sign In

OK - When two together sharing the head gasket are both low you test for pressure loss between them but now if opposite that shouldn't be the case.

Oil (just a squirt) would seal rings if worn and dramatically improve pressure for the test. Just would mean rings, cylinder walls somehow were suspect.

Can you tell if this "7 cylinder" feeling is really picking on one cylinder or random by ..... one - check for a rythmic put at tailpipe vs random sometimes tells or cancel plugs. By that I mean take off the plug wire to suspect single cylinder and ground it. If no change it's that cylinder to further chase down why.

I plain don't know it the puter on this can tell you which cylinder(s) or random misfire with a code reading - some can,

T



harborguy
Novice

Jul 23, 2010, 10:38 AM

Post #5 of 6 (5520 views)
Re: 97 silverado Vortec 7.4 running like 7 cyl. Sign In

As we speak.....we are now having all 8 objectors checked. Will keep you updated......Al.

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Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jul 23, 2010, 5:16 PM

Post #6 of 6 (5514 views)
Re: 97 silverado Vortec 7.4 running like 7 cyl. Sign In

Gotta be honest with you harborguy - I haven't opened the hood of a "Vortec" 7.4 (454) but just checking out injection as I thought there was either a spider of fuel delivery injection instead of individual injectors to test like with a noid light?

There are a couple types used apparently for fuel delivery.

I do know this - the fuel pumps really want exact proper pressure or run like crap or hard to start if just a couple pounds too low. That feels more like starving for fuel and on the Vortec 5.7 one ran fine, started hard but not always and once felt like smooth loss of power. Finally caught it showing 54 psi when spec was 60-66. Started with starter fluid and the higher voltage brough pressure up to 58 - still under and wasn't thrilled with that.

At any rate there were some choices that I was unaware of to check a single injector and canceling a cylinder seems like the easier way if what I think it may be like. What I mean is if each cylinder in this has it's own injector you could have one that's weak or not working at all.

A lot matters if this is always the same cylinder causing the problem or random. IMO your compression probably isn't the up front problem even though by the book it's off on two.

When you replaced plugs - they should have all looked similar if random misfiring and one or perhaps two would look different if just those - color would indicate unburned fuel or too much. Then chase down why for just those. Either issue would set off a check engine light fairly easily with a code for info info.

There's three I deal with - all the Vortec 5.7s. They have plenty of power IMO and can imagine this is like nuke power when all is right,

T







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