MAS, TPS, MAF, O2, EGR? RICH CONDITION..killing me.

Hi ALL,
First post, awesome forum setup. Hope ya'll can help.
1997 Tahoe 5.7L Vortec - 150K miles
Here goes:
1. Died backing out of driveway - no start
2. Swapped out fuel fillter - no start
3. Towed to shop, determined 10psi fuel pressure, replaced pump (now 63psi, yippie) BUT there's a problem.
4. Now I'm smokin rich, pulled up P1133 and P1153 codes (pre-cat 02 switching), swapped out the two front O2 sensors (voltage readings looked good for all 4). Still same...
5. It will clean up and run ok above 1500 rpm but I'm getting terrible mileage (like 7-9mpg). Then, pulling up to intersection or stop will idle for couple seconds (intermittant) then bog down to 400 and sputter.
6. Pulled the EGR and cleaned it up a bit - same condition
7. New rotor, cap, wires, and plugs - SAME CONDITION!
8. Pulled MAF plug to see if it would lean out...acts the same as it has been, shot a mist of some carb cleaner through the MAF, nice and clean now but, No change
9. I have an electric coolant temp sensor in my hand, thinking about replacing- YES?
10. Was told by guy at parts store possibly Throttle Position Sensor or Manifold Absolute Sensor....(another $100 worth of sensors?)... : (

I'm stumped.... got over $600 into parts in this thing...and yes..it needed some of them but how many sensors am I going to end up replacing before I find which one is causing this?

Oh ya....and I still got the P1133/1153 codes on the "service engine soon" light...^&*(#%!

Hope someone can help before I plug up my cat, or go broke feeding this thing....$45 for a 125 miles is a ripoff...heh...

That is all for now....anyone got my answer? I stumped three guys at the shop (and a father-in-law mechanic, why I have the coolant temp sensor).

Any help would be appriciated!!
Thanks!
CrazyHerb

Of all the sensors you listed, the MAF and O2s can cause a rich condition. The EGR should not cause a rich condition. TPS also affects shifting so if the tranny is shifting normal, I doubt that is it.

A leaky injector (or injectors) can cause it to run rich. A bad FPR (fuel pressure regulator) can also cause it to run rich and that is what I think your problem is. But before we throw more parts at it, we should verify that the FPR is doing it's job.

Hook up a fuel pressure gauge and start the engine. While you are watching the pressure, turn off the engine. The pressure should hold. It will lose pressure over time but should not drop. If the pressure is 30 PSI within a minute, the FPR is leaking gas back into the intake.

The bad news is that the FPR is under the top half the intake manifold.

Do the fuel pressure test and post the results. Note the pressure at 30 secs, 1 minute, 2 minutes, 5 minutes or until the pressure drops to zero.

//2000CAYukon

Good job pointing me in the right direction...I was sick of "skirting" around the fuel delivery problem by letting people make me think it was sensor/ignition related.

Went back to shop today and plugged in the fuel pressure guage...

Turned on ignition - pump came up to 63psi
Turned off ignition (leaving gauge hooked) - saw the inevitable...a slow drop in pressure!! By 2 minutes into it was at 30psi.. (thinking bad pump)
Started the truck with guage hooked...pressure jumped to 73psi!!! ACK!!
No wonder it's RICH...I hope I haven't damaged Fuel Pressure Regulator or Injectors...or clogged my CAT...

Guys at the shop are replacing it Thursday (parts were guarenteed). That's good.

Many thanks, I'll post follow-up and let ya'll know how it all goes after pump replacement.

(sigh of relief)....for now

CrazyHerb

Good job pointing me in the right direction...I was sick of "skirting" around the fuel delivery problem by letting people make me think it was sensor/ignition related.

Went back to shop today and plugged in the fuel pressure guage...

Turned on ignition - pump came up to 63psi
Turned off ignition (leaving gauge hooked) - saw the inevitable...a slow drop in pressure!! By 2 minutes into it was at 30psi.. (thinking bad pump)
Started the truck with guage hooked...pressure jumped to 73psi!!! ACK!!
No wonder it's RICH...I hope I haven't damaged Fuel Pressure Regulator or Injectors...or clogged my CAT...

Guys at the shop are replacing it Thursday (parts were guarenteed). That's good.

Many thanks, I'll post follow-up and let ya'll know how it all goes after pump replacement.

(sigh of relief)....for now

CrazyHerb

I hope you are having the fuel pressure regulator replaced because that is your problem not the pump.

The fact that the pressure dropped to 30 PSI in 2 minutes indicates that the diaphram of the regulator is broken dumping fuel into the intake.

//2000CAYukon

CrazyHerb -

I am experiencing a similiar problem with my 95 Tahoe sputtering at low RPMs... did your vehicle drop 100-200 rpms and then return to normal rpms when traveling at speeds greater than 25 mphs?

I am troubleshooting my truck and I think this post has given me a great starting point... I also have replaced plugs, fuel filter, etc. trying to troubleshoot this problem...

Thanks -

David

2000CAYukon,
First, thank you immensly for you help. I agree that next point was going to be the Fuel Pressure Regulator (FPR)...However, the guys at shop offered free replacement of the pump because it was under a parts guarentee. The new pump now holds pressure and only primes up to 62psi, no over-pressure conditions. I was worried the last pump might have caused some damage. But I think we're good.

My Tahoe is now running even better than it was before (probably due to ignition components being replaced).


David,
Are you smoking at idle as well? My vehicle would sputter/smoke for a while then sometimes return to a (semi-decent) idle if I just left it alone.....but as soon as I touched the throttle would sputter and load up...till I got it above 1500rpm.....

There is an idle mix sensor just behind the throttle position sensor that someone recommended I replace (along with the others..MAF, MAP)...I'd start just as 2000CAYukon suggested. Plug in a fuel pressure guage and watch it for at least 5 min. Make sure your getting real close to 62psi (ignition key turned, not started). And that it holds that pressure.

Good luck, and thanks again CAYukon!
CrazyHerb

David,
Are you smoking at idle as well? My vehicle would sputter/smoke for a while then sometimes return to a (semi-decent) idle if I just left it alone.....but as soon as I touched the throttle would sputter and load up...till I got it above 1500rpm.....


No. The car doesn't smoke at all... but your symptoms (minus the smoke) seem identical. This seems to be the most promising lead yet!

Thanks Crazyherb! Good luck with your vehicle! I will post back here when I find something out...

David