I have a 2000 Blazer LS 4x4, there is a large amount of power loss when I drive it now, I've replaced the FI spider, fuel pump and filter, distributor, shift and TCC solenoids in the tranny. When getting love stream data from the OBD2, I noticed the down stream sensor has voltage but not registering fuel trim. Is my cat clogged or is the sensor bad? I thought of straight piping and getting rid of the cat and adding a sensor spacer for the O2 sensor. FYI fue; pump is getting 50 to 55 PSI running and key on engine off but leaks down rapidly, maybe a check valve?

 

You have brought up more than one issue so let’s break it down:

If you don’t hold 55 psi after key on engine off then you need to find out why. The first step is to divide the problem in half by repeating the test at the fuel filter outlet. That way you know if you need to work in the tank or the intake plenum. Either way this needs to be fixed. The most likely causes are the wiring/power/ground, fuel pump, regulator or leaking injector. Unfortunately new does not rule out failed.

The downstream O2 sensor. I don’t know what you mean by not registering fuel trim. That sensor should show a relatively steady voltage somewhere between 0.5 - 0.7v. This sensor is not the primary control of fuel delivery, that is performed by the upstream sensors. Fuel trims are calculated by the engine computer by comparing the actual fuel delivery requested by the upstream sensors with the theoretical delivery implied by the other relevant sensors so looking at a graph of any of the sensors tells you nothing about the fuel trim calculation. Speaking of fuel trims what are your four values at idle and again at 2500 rpm?


George

 

Okay I'm holding 50 to 60 PSI for fuel, and the cat is not clogged. Now I cannot rev the engine beyond 2500 rpm. I'm going to list the parts I've replaced
TPS
CKP with relearn performed
distributor
plugs and wires
fuel pump and filter
injection spider and pressure regulator
IAC
MAF
I'm going to replace the O2 sensors and MAP next weekend. If this doesn't work should I replaced the PCM?
FYI here is the info you wanted

 

Do you have any codes?

The spec is to hold 55 psi after 10 min. What brand of fuel pump did you use? Have you measured leak down at the fuel filter outlet? This would tell us if the problem is in the tank or in the intake manifold.

We also need the fuel trims at idle but your 2218 rpm numbers look very good. An engine that cannot rev above 2500 rpms but shows near perfect fuel trims at that rpm is not likely to be having major fuel delivery or air intake problems. So this is either likely:

• An ignition problem
• A sensor problem
• An exhaust restriction
• Mechanical problems

How do you know that you have no exhaust restriction?

Has there been a recent compression test?

Can you graph the precat O2 sensors like this at idle and 2500 rpm?:


What are all your sensor values at

• Engine cold, ignition on, engine off
• Idle
• 2500 rpm

Can you measure Cam sensor retard?


No. I would not replace the engine computer at this point and it is better to find the problem through testing instead of using the parts cannon because of cost, time and in some cases replacing perfectly good AC Delco components with potentially inferior after market parts causing more problems.


George

 

Do you have any codes? No
The spec is to hold 55 psi after 10 min. What brand of fuel pump did you use? Bought the pump off eBay
Have you measured leak down at the fuel filter outlet? No, I measured the pressure at the fuel line outlet by the distributor. If I have 55 to 6- PSI there why would there be an issue before the filter?
How do you know that you have no exhaust restriction? I removed it, straight piped and added an adapter to the downstream O2 sensor. Keep in mind the rev issues was happening before the delete, hence the reason for deleting it.
Has there been a recent compression test? Yes on all the cylinders, 160 to 170 PSI
I have a complete Autel report but it's in PDF format of 2218 RPM and 625 RPM

 

I was reacting to your initial rapid leak down comment on fuel pressure. If you are holding 55 psi on leak down now then scratch the fuel filter question. That said, we have a long history here of fuel pump problems when the replacement is not AC Delco or Delphi. Have you watched your pressure when it runs like crap?

You definitely don’t have a restricted cat then. Lol. I don’t know much about the whole cat delete/O2 sensor adaptor thing but since it did not generate a code and it ran lousy before let’s assume that’s not your problem.

I asked about the O2 sensor graphing because it’s hard to see in tabular data if they are switching properly. Without that I can’t help you asses if they need to be replaced or not.

Do you have the fuel trims at idle?

I asked about your sensor values at three points to rule out a sensor related cause for this. Without this I can’t help you with your MAP sensor or any other one for that matter.

I don’t know if that scanner can measure cam sensor retard but that’s one of the first things we like to get in spec on problem engines but you would probably have misfire codes.

I would still suspect ignition if you fuel pressures are good when the trucks runs bad, your idle fuel trims look good and all the sensor data looks good.


George