CPI pressure problems difficult to diagnose

Posted by jdemaris on April 29, 2005 at 05:07:26 from (209.23.30.175):

In Reply to: O/T: Pesky GM 4.3 V-6 Vortec posted by Maine Fordson on April 28, 2005 at 15:21:17:

You didn't mention if it has VIN code "W" or "Z" engine. If it is a "W", I can almost guarantee you the CPI "tarantula" module is causing the problem. The "W" code does not have throttle-body injection, but instead uses a central injector hooked to 6 output lines and check-valves and it is serviced/replaced as an assembly. It is prone to the sort of problems you've mentioned once it gets some mileage on it. It is very difficult to diagnose with pressure gauges. The problem is, the electric fuel pump, unlike that used in TBI, is a high-pressure unit and runs around 55 - 65 PSI, constantly. The check valves in the CPI pod are set to open at 45 PSI. So, in theory, if the electric fuel pump pressure falls below 45 PSI, the engine will not start. The reality is, I have worked on many that have fuel pumps putting out 55 PSI and they still would not run. There is a very small margin of pressure area where the engine will start and run. To compound the issue, if your pressure gauge is just a few pounds off, it makes things even more confusing.
You say your CPI is almost new - and so is your fuel pump, so I guess you do have a "W" code engine. Check fuel pressure at the schrader valve at the back of the engine. I'm wondering what exactly was installed for the new CPI and fuel pump. Were they AC Delco or an equivalent? I like to know the part numbers. Some of the AC Delco replacement parts are absolute crap. What ticks me off is that AC Delco knows it, but seems to be unloading a lot of bad parts anyway. Was your CPI installed new, or was it a "rebuilt" unit? Also, the fuel-pulse dampener, hooked in line with the electric fuel pump can cause a lot of trouble - you're much better eliminating it when installing pump. Many of them leak down. It too, can cause the problem.
By the way, if you don't have the proper hookup for the fuel pressure Shrader valve, and air conditioning manifold line will fit, if it's the old style for R12. You also might read some tech. articles on the CPI problems, e.g. at: http://www.asashop.org/autoinc/feb2000/techtotech.htm

Follow Ups:

• One more thing . . . jdemaris 05:10:34 04/29/05 (2)
  • Re: One more thing . . . JHT 07:00:37 04/29/05 (1)
    • Re: One more thing . . . jdemaris 07:19:14 04/29/05 (0)

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