Compression?

[nautiduck]

Need some help. My faithful steed (99 k2500 w/5.7 vortec) has been giving me a stumble every now and again. Once and awhile when I lift off the gas my truck stumbles like all 8 cylinders misfired, and twice now while sitting in traffic it starts running really rough until I push the throttle down just a little for about 30 seconds. Recently I had to replace the distributor because the disc that the rotor bolts to literally sheared off in the middle of BF WV so that maybe an issue. Not really knowing how to check the mechanical advance on the distributor, I figured well she 240k miles on her lets check the compression. Now maybe I did this wrong because it seems a little high but all the cylinders came out with in 4-5 lbs of each other but it was reading between 180 and 185psi this with a warm block and all plugs out.. Now I know the vortec heads have a smaller compression chamber than the old heads but I dont think it amounts to that much pressure difference does it? Im pretty sure my old 350 with the old .202 heads were like 125 - 130 psi. Does anybody know what the compression should be? I just hooked up my compression tester to my compressor and it matched the gauge on it at 120lbs so I think its fairly accurate. Maybe I held the starter on too long doing the test? Its been so long since Ive done a comp test I dont remember how long to hold it on, just a bump or let it spin for a while. I heard three distinct compression hits before I let off the key I did test one cylinder longer but the compression didnt change. If anybody can help Id appreciate it. Thanks Nauti

[Big Fish Billy]

Sounds like you did it right, three hits, 180 is about right , my 5.2 Dodge Ram is 175 all around. Two things that come to mind immediately are the coil and spark plug wires. Believe it or not they can cause all sorts of issues as they get bad in various ways. As an example a buddy of mine was complaining because his transmission on one 454 boat engine was making all kinds of banging sounds, and he was going to replace it. I asked, "how long since you changed your spark plug wires?" Never was the response. Put a new set on and noisy tranny went away. For the price of a coil and some wires, it's worth a try, if not you have spares. Report back if it works out for you...

Billy

[nautiduck]

The plugs and wires are less than a year old I think April, not ruling them out just throwing up a timeline. The distributor, cap, and rotor(again) were replaced in the last week of June. I checked the cap last week and the mechanic (friend of mine)who set the distributor must have twisted the screw in the back a bit much cause it was loose and it wouldnt tighten down. I replaced it with a screw one size larger and it is clamped down tight, hoping that was the cause but its still doing it. Ill double check the wires tomorrow maybe one rubbed through or cracked and is shorting out.

Maybe 180psi is right then it just seemed high to me. If thats the case then Im happy 5 psi difference at 10 years old and almost 250,000 hard towing working miles and the only thing fixed engine wise was the intake gaskets at 190,000 miles and a freak broken distributor. Now if I can just figure out the stutter.

[34SILVERTON]

check youn ignition module and coil. you said the dist was new did it come with the module?is your check engine light on?

[Big Fish Billy]

When my truck was doing what Nautys was, it was the coil. I bought a new MSD coil and put it in, and it also was bad. Bought a new coil at Autozone and it was fine, there is a test for checking coils but I forget how to do it, I'll try to figure it out. As for plug wires, if they cross at the cap or along the line, as they do without the plastic seperators, on damp days the spark finds other places to go other than the plug. Once a wire finds a convenient place to short out, due to the resistance of going to the plug, the wire begins to deteriorate. As I said I'd change out the coil first ($35), and then the wires ($40). After that, then you can go after the more expensive stuff.