99 4wd Ls Blazer - Still Think I Have A Fueling Problem

dougg01 · August 21, 2009

But, it might not be so. Please let me know what you think is the root cause....

Symptoms:

will not start on first try after sitting overnight. Takes 2 to 3 tries to start.

Hot, warm, or cold it sometimes will drop rpm or have difficulty gaining rpm on acceleration. Sometimes you can feel the hesitation without noting a drop in rpm. When it's pronounced the rpm can drop by as much as 700rpm or so....quickly recovers and then drops down again. It occurs more at part throttle. Giving it more throttle (closer to WOT) it seems to just fail to gain rpm quickly. The more load (uphill) the more noticeable it is.

This rarely occurs at cruise speed, but it does happen.

FP is:

key on = ~60psi

idle = ~53 psi

When the 'condition' is present the FP does not fluctuate as far as my gauge sees it. I ran the gauge taped to the windshield on the way home the other day. :smile:

full tune-up, new FPR, fuel filter, clean TB, etc. No codes....ever.

It seems to do fine a good portion of the time and runs-out great.

I don't want to just throw parts at it just to not fix the problem.

bill190 · August 21, 2009

VERY good with the fuel pressure gauge on the window! That's my kind of testing, you can see what is going on, learn from it, then search for the problem elsewhere...

Anyway, in addition to the fuel pump supplying fuel, there are electrical sensors which tell the engine computer to give the engine more fuel. Might want to monitor these with a multimeter similar to the way to you monitored the fuel pressure...

Coolant temperature sensor - Lean/rich

Oxygen sensor - Lean/rich

Atmospheric (barometric) pressure - Lean/rich

Manifold Absolute Pressure - More fuel

(Mass Airflow Sensor)

Throttle Position Sensor - More fuel

The above sensors are for vehicles "in general". The actual sensors your vehicle has, descriptions of operation, and testing instructions would be in a GM factory service manual set for your year/model vehicle. Order from helminc.com or a GM dealer.

Here is a neat tutorial on the various engine sensors I found recently...

http://www.aa1car.com/library/1999/cm69910.htm