96 Suburban runs bad after it warms up

[chris peter]

My 96 suburban runs bad after running 20 minutes from cold start. It idles fine but if you push on the gas at all it hesitates no power at all once warm. Turn it off and restart it and it runs good for 5 minutes then it's the same problem. The check engine light is on code PO151 Low Voltage on O2 sensor bank 2 sensor 1. I have tried a new fuel pump and filter. Fuel pressure is 60 psi and holds. Tested the back pressure looking for bad converters but they are OK. Replaced both O2 sensors checked all wiring and grounds, replaced MAP sensor and the power control module. The timing will not change from -8 degrees I am thinking of trying to replace the crank sensor or maybe the distributor would like to see if anyone else has had this problem. I found one poster here with a 99 suburban had the same problem but he never posted the repair that solved his problem. His subject was (my 99 suburban dies after 20 miles) what he describes is just like mine. HELP!!!

[maxwedge]

Welcome to AF, assuming an accurate gage, 60 is too low, 65 is required. See if the fp holds for a few minutes or drops off quickly as the Vortec injector units have been troublesome. Yes the dist gear can wear also on these trucks, usually will set a different code though. That could account for the timing. Check this link for the p0151 parameters. http://service.gm.com/gmspo/mode6/pd...t57R_LD_yE.pdf

[chris peter]

Thanks for the reply the pressure holds and 60psi is with in specs. I was thinking some type of sensor is failing because the timing is not changing. I think the timing is set by the computer and its getting bad information from some where.

[777stickman]

You cannot check timing on these motors with a light. It has to be done with a high end scan tool that can put the VCM in "Cam Retard or Cam Offset". This is the "base" timing mode. When in this mode timing needs to be 0 +/- 2 degs as read on the scan tool.

Other things to look for on the code you mentioned are exhaust leaks, vacumm leaks and fuel trim values as read on the scan tool.

With all the stuff you've replaced and no change in conditions, that code P0151 says that the motor is running too lean on that bank.

Also please tell us what motor you have.

[chris peter]

My truck is the 1500 with 4wd it has the 5.7L engine with 150,000 miles on it. I did use a scan tool to check the timing it bounces -8 -9 mainly holds at -8. when you give it any gas it does not change. It does not appear to have any vacuum leak. Something is telling the computer to dump fuel at 28% rich.

[chris peter]

Update on repairs, I changed distributors and now the timing is changing but the truck still runs bad I was wondering if the computer needs to be relearned does anyone know if that is true. I was considering changing the crank sensor or the throttle positioning sensor next. But wanted to know about this relearning first.

[777stickman]

If all you did was replace the dist then no the VCM does not need to relearn anything.

Tell us how you timed the replacement dist (in detail)..

Your reference to 60 psi fuel press being within limits is correct. However that is the bottom of the scale limit and if your fuel press guage is off by a couple of pounds then it really could be 58 psi. When "maxwedge" says it needs 65 psi you should listen to the advice.

[chris peter]

The timing was done at the shop so it was done properly so I can't give you an answer. But the timing now changes when you start cold and change RPM. I also had the MAP sensor tested did not replace it but that was 1.2V at idle and 4.7V at wide open throttle the voltage at ignition on engine off was 4.79V I believe thats ok. Both banks fuel trims are now runnng 15% rich. Oh the ambent temp sensor that measures start up air temp say -41 degrees at first then seems to adjust from there. He did change one other temp sensor and that seemed to make a little difference but it still has the same problem. The temp sensor he change was not water temp but something else it was not working at all. If you block the return fuel line the pressure goes to 80psi. I hate to change fuel pumps again at this point but I am considering it. Also not ruling out bad CATs maybe just not totally plugged.

[chris peter]

Repair update tried running the truck with both O2 sensors disconnected and the truck runs much better. My question is now is this a bad fuel pump or pressure regulator. Its seems to be a fuel delivery problem.

[discnik]

Try a fuel presure test at idle then disconnect the vacuum line from the fuel pressure regulator you should see a 8-10 p.s.i. increase in pressure, if not I would say the regulator is bad.

[chris peter]

Well it now looks like the Auto Zone fuel pump maybe my problem it is a Airtex brand I installed it when I needed to replace the sending unit and all my troubles started 1 week later. Has anyone had trouble with brand? I should have left the old pump in there.

[j cAT]

Originally Posted by chris peter

Well it now looks like the Auto Zone fuel pump maybe my problem it is a Airtex brand I installed it when I needed to replace the sending unit and all my troubles started 1 week later. Has anyone had trouble with brand? I should have left the old pump in there.

this is not a good manufacturer...use the OEM type...Delphi

[chris peter]

Well here is what solved my problem I replaced the fuel pump with a Delco but also replaced the pulsator as well. The pulsator can fail causing a fuel delivery problem. The pulsator is that odd looking part that connects between the fuel pump and the sending unit. The blader can rupture because of the ethanol used in todays gas. The replacement is made out of a new material that can with stand the ethanol in fuel today. I can't say for sure which part failed but if I where to do it again I would replace everything at once with GM parts. Thanks to everyone who shared their ideas with me.