Hesitation at Low Loads

[EDS_5_OH]

All,

My son's truck (1990 Silverado RCSB 4x4 5.7L, stock motor) runs great with one exception. When climbing hills at part throttle (maybe 25% throttle) it will sometimes buck or hesitate for a moment, then clear up and run fine. It used to do it rarely but now it's doing it every time he drives it and goes up a slight hill. If you use 50% or more throttle, it never hesitates. It idles smooth, easily passes emissions etc.

Any advice on troubleshooting the issue?

We replaced the fuel pump and filter with a Vortec unit as the pump was getting noisy. Didn't fix this problem. We checked that the EGR was functioning correctly (it was). Cap and rotor look good with no corrosion. All the plugs look go with a nice light tan color to them. Truck runs like a champ otherwise.

Thanks,
Ed

 

[sntrym]

I would test the TPS first. It might have an incorrect reading at 1/4 throttle but nowhere else. TPS test procedures are pretty easy.

 

[chipskittles]

How did you verify EGR was working?

 

[EDS_5_OH]

http://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/stuck-open-egr-valve-tests-1

 

[EDS_5_OH]

I would test the TPS first. It might have an incorrect reading at 1/4 throttle but nowhere else. TPS test procedures are pretty easy.

Can you point me to the TPS test procedure for a TBI truck?

 

[chipskittles]

That's a good site for troubleshooting and they have a good process for testing actual EGR function. One thing I did not see though is a test of the EGR valve diaphragm. Commonly the EGR valve diaphragm will rupture and 1) create a vacuum leak as well as 2) not allowing the EGR valve to open and actually work. Easy check, hook some vacuum hose to it and see if the valve will hold vacuum. If it doesn't the valve is bad. Not all EGR valves are supposed to hold vacuum but the ones they used on our trucks were all supposed to as far as I have ever known.

 

[EDS_5_OH]

That's a good site for troubleshooting and they have a good process for testing actual EGR function. One thing I did not see though is a test of the EGR valve diaphragm. Commonly the EGR valve diaphragm will rupture and 1) create a vacuum leak as well as 2) not allowing the EGR valve to open and actually work. Easy check, hook some vacuum hose to it and see if the valve will hold vacuum. If it doesn't the valve is bad. Not all EGR valves are supposed to hold vacuum but the ones they used on our trucks were all supposed to as far as I have ever known.

I did test the EGR with a vacuum source in addition to following compressed air tests from that site as well. It did hold vacuum, and it did open when vacuum was applied.

 

[89RCLB]

http://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/tp-sensor-tests-1

 

[EDS_5_OH]

http://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/tp-sensor-tests-1

Thanks. Straight forward, just like any other TPS I've ever messed with.

Any other suggestions on what could cause the issue?

 

[chipskittles]

How old is the oxygen sensor? I realize it's probably a 1 wire sensor but it does play a part in the TBI system. Just something to check