Question re. constant miss....Non-valk.

[John Schmidt]

My son-in-law has a 6-cyl. work van(Chevy Express 2500) with about 200k on the clock. A few weeks back it developed what appeared to be a random miss and plugging in the analyzer didn't pin it down, just stated the obvious...random missfire. He took it to his local "go to" mechanic who eventually replaced the original plug wires, plugs, everything associated with the distributor, and it seemed to correct the problem. A couple days ago it started doing it again with the same readout. Last night I asked about total miles and that's when he told me 200k, I have no experience with how the distributor functions on it(an 02) so I'm wondering if it might have a problem in the drive of the rotor. Back when you could work on motors without an engineering degree and $50k worth of test equipment, an old motor would sometimes wear the rotor shaft enought to cause a random missfire. Looking for advice on where to look, could it be a collapsed lifter or broken valve spring? The exhaust smells very rich, so maybe low compression in one cylinder on an old motor might be causing trouble. Any thoughts or ideas?

[bentwrench]

A good scan tool will show the misfire counter. That will help you pin down the offending cylinder. not all mifires are ignition related, if I'm not mistaken that engine has the vortec injection system with its many faults.

[old2soon]

            At 200K maybe a valve somewhere. Or as suggested a cylinder or 2 or more over fueling? Air cleaner clean? Fuel filter Clean? Run a can of B-12 chemtool thru it? RIDE SAFE.

[da prez]

Crank position sensor.  OEM only. After market usually fails


         da prez

[Skinhead]

If I remember correctly, (no guarantees on that) we're talking about a 4.3L V6 with either SCPI (Sequential Central Port Injection) or CPI (Central Port Injection).  The difference in the systems is the injectors and their locations.  On the SCPI the injectors are located on the end of a tube in the intake port of each cylinder.  On the CPI the injectors are located in the CPI body and there is a poppet valve at the end of the tube in the intake port.  The SCPI system is more robust to leakage problems than the CPI system.  Any foreign material or deposits on the poppet valve of a CPI will cause a leak or excess fuel being delivered to that cylinder, which may result in a misfire.  If you have a fuel pressure gage you can check for this by monitoring the fuel pressure after shutting the engine off.  On a normal system there will be a sudden drop in pressure when you shut the engine off to about 40 psi (I think) and the pressure should hold there for 5-10 minutes at least.  if the pressure continues to drop you have a leaking Poppet(s). 

This can sometimes be corrected by pinching off the fuel return line, which increases the fuel pressure and causes the poppet to open further than normal and may clear the debris.  You can also try adding Techron fuel injector cleaner.  If that doesn't work, injector/poppet needs to be replaced.  This only applies if your misfire is fuel related and not spark.  Also it's been several years, since I've work on any of these systems so some of the details are a bit sketchy.  A google search may confirm some of this.