1991 Chevy Blazer S-10 Blazer fuel problem

Tiny
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  • MEMBER
  • 1991 CHEVROLET BLAZER
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 12,000 MILES
Engine lost power in the midst of trying to identify what was causing a sag during the 1-2 upshift. Previous to the loss, engine ran quite well, and especially well in open loop. Here is a list of what has recently been replaced for various other reasons :
Fuel pump(tank like new inside)
Wires, dist cap and rotor
fuel filter/ with new pump
AC O2 sensor
rebuilt injectors(cleaned and flow tested)
air filter
AC plugs
dist P/U coil and ignition module, Ignition coil
MAP sensor
Temp sensor
Here are the conditions and symptoms:
Exh pressure at o2 tap before cat, converter <1/2 at idle, <1 at 2000 RPM and ~3 PSI under heavy load (brake torque at 2000 RPM)
Base timing set at 0 deg. Temp: 190 deg. F smooth idle, fuel pressure 11 psi, Oxygen sensor voltage bouncing between 800 and 200 MV at idle.

During very moderate acceleration, O2 drops to near zero, BLM maxed out at 160 (lean) per WINALDL and my S10 Blazer falls flat on its face. There are no backfires. Fuel pressure during this whole time is 11 PSI before a new regulator. Normal O2 voltage recovers each time at idle always. A known good ECM does exactly the same thing. Both injector spray patterns look good and flow increases when revving in neutral. Neutral revs seem normal. TPS and MAP sensors are in range and voltages are smooth.
ECM grounds read zero from battery and 12 and + 5 volts are OK. EGR is closed at idle and motor displays the same symptoms with EGR hose plugged. Manifold vacuum is 20" at idle and 20" at 2500 RPM in neutral and jumps up briefly when throttle snaps shut.
Open loop ( O2 sensor disconnected) operates equally bad. Vacuum tube thru center of TB and hose to MAP sensor is clear. Gasoline in tank is the same fuel as when truck performed well.
I am out of things to check and could use some help.
Regards,
jack
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Monday, June 14th, 2010 AT 9:00 PM

6 Replies

Tiny
DOCFIXIT
  • EXPERT
Hi
Just came across your post and it has weird and challenging all over. Hook up timing light point at injector spray pattern is it 360 deg? Assume you have ohm checked O2 wire to ECM and no codes. On scanner does it switch from closed to open loop during mod. Accel? BLM sure indicates LEAN. Have you tried some gas in squirt bottle and add to see if it accel?
Will give this some investigation
Thanks for donate
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Wednesday, June 16th, 2010 AT 8:56 AM
Tiny
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Hi,
Both injector patterns are a nice 360 deg. Of equal angles. O2 wire reads 0 ohms. All grounds at ECM
read 0 ohms, except A7 (A/T TCC) which is switched in ECM and I believe should be N.O. New genuine Delco O2 sensor generates voltage and engine idles nicely with O2 bouncing normally. Now comes the problem. While lightly accelerating in gear in closed loop O2 voltage drops steadily towards zero within seconds AND STAYS THERE and BLM rises to 162. If I release the throttle, normal closed loop operation returns while coasting.
When I run in open loop (disconnect O2 sensor signal),
idle is fine at a fixed.455 Volts generated by ECM, but again, if I lightly accelerate in gear, BLM goes from 128 to 162 and engine loses all power. I would like to leave in open loop for troubleshooting purposes as this is my vehicle and I know how it responds in that mode.
Is it possible to have a steady 11 PSI of fuel pressure (range is 9-13) and still have a fuel delivery problem, if the injectors are receiving the correct pulses?
Thanks,
jack
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Wednesday, June 16th, 2010 AT 8:20 PM
Tiny
DOCFIXIT
  • EXPERT
Fuel pressure is what I suspect delivery rate need 1/2 pint in 15 sec. Could have faulty relief regulator dumping fuel back to tank.
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Thursday, June 17th, 2010 AT 3:06 PM
Tiny
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Hello all,
I reported a lean condition per the ECM/ ALDL (low O2 volts and high BLM) but believe is was actually a rich condition because the plugs, O2 senssor and exh. Are all getting black. I think a very rich mix was quenching the O2 sensor.
I checked fuel volume with engine running thru a Tee past a schreader valve and is around 4 oz per 15 sec. Seems trouble started when I pulled a functioning Bosch O2 sen and replaced it with a Delco. Injector screens are clean and spray seems OK although by now I am getting delusional and see droplets at certain angles, however the 2 bores seem to be "sweating" equally. I will reinstall the Bosch O2 sensor and replace the fuel filter, road test each change and let you know.
Regards,
jack
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Saturday, June 19th, 2010 AT 7:45 AM
Tiny
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Dear Doc,
Thanks for your reply. Your input was logical for the symptoms I gave, however,
I pulled the fuel filter and the pint of fuel draining in a pan had about two tablespoons of water in it Fuel filter was clear but had "rusty" gas in it Dropped the tank and swished it out and replaced filter.
Ran well for around 1/2 hour, then back to running like crap. Checked manifold vacuum again but this time vacuum decreased with engine RPMs. Checked exhaust back pressure and it was NOT less than 1.5 psi at 2500 Rs as it was the last 5 times I checked recently. I let off on the gas before I pegged my sensitive gage. I substituted a higher pressure gage and the rubber hose blew off at around 25 psi. The converter was plugged. Changed the BLACK engine oil way before due and runs great now. Now back to my elusive quest, 14.7: 1 across the board.
Thanks again Doc and sorry for the curve balls.
Regards,
jack
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Sunday, June 20th, 2010 AT 8:39 PM
Tiny
DOCFIXIT
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Hi jack
Great news and good diagnostics on your part. The manifold vacuum dropping on RPm increase is the test for exhaust blockage.
Thanks for feedback
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Monday, June 21st, 2010 AT 8:12 AM

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