97 Suburban Loses Power Tried Everything

[Mo Dinger]

1997 Suburban, 5.7L Vortec, automatic, 4x4, 235K miles.

Vehicle looses power when in drive, when warm, and under load, typically, at around 1800-2000 rpm. May be near a shift point, but I am not sure. No trouble codes are being set. Check engine light does not come on, however it does illuminate during startup. Professional service has replaced crankshaft position sensor, throttle position sensor, distributor cap and rotor, ignition module, fuel pump, in-tank fuel screen, and fuel filter. All were either visibly bad, fouled, or tested as borderline (fuel pressure). Vehicle has been run with MAF connected and disconnected with no change in symptoms. Transmission shop has run some diagnostics through test port that apparently show transmission operation is normal. Generally power seems to recover, if shifted manually to 3rd gear. I need help diagnosing a solution as I am running out of ideas and money. All help is appreciated.

 

[shamrock246]

Its fine when cold? original Cats? Was a compression check done?

 

[BoxerPitMix]

What about the spider injector assembly?

 

[Mo Dinger]

I purchased the vehicle last fall and don't have any history on the maintenance or changes that have been done. However, it appeared to have regular maintenance. Oil was clear and clean. Transmission fluid was clean. Engine compartment was clean but a bit dusty that indicated to me that it wasn't just cleaned up to sell.
Yes, generally needs to be at least warm. I don't think the shop did a compression check.
The cat isn't making any noise and the engine has plenty of power otherwise, except for the speed and RPM range where the failure occurs. Sometimes during failure mode the engine simply acts like it is starving for fuel, but sometimes it will surge momentarily then lose power again and repeat several times. Again, it isn't giving a code to help diagnose. Boxer - I don't know about the spider injector assembly. Would that cause these symptoms?

 

[kennythewelder]

This could be several different things. If you cats a plugged up, it can cause this, so can low fuel pressure, so can bad or clogged up spider injector system, or even clogged up fuel filter. If it was a sensor, then that should give you a CEL. but if there is no CEL., then your sensors should be ok. If it was me the things I would do is check my cats by pulling out my o2 sensors and see if the exhaust pressure is the same coming out of the front o2 sensor hole as it is coming out of the rear o2 sensor hole. If your getting as much exhaust flow from the rear sensor hole as your are from the front o2 sensor hole, then the cats aren't plugged up, but this needs to checked while someone is reving up the engine, and it will give you a CEL. If the pressure is the same on both drivers and passengers sides, then replace the o2 sensors, and reset the ECM. Then test the fuel pressure, or do this first, ether way. You need 60+ PSI at idle. If there is less that 60+ PSI it can cause your issues. After you do all this, then I would look into the spider. Also the fuel pressure regulator is on the spider.

 

[Mo Dinger]

Kenny, I will check the cat and the fuel pressure as you describe. However new fuel pump and fuel filter just installed and exact same symptoms exist as before that work was done.

Could one of the electronic shift solenoids in the tranny (either 1-2 or 2-3) be causing the drop in RPM if they are late or not functioning?

 

[kennythewelder]

I wouldn't think the trans solenoid would be the issue. Without being able to drive the truck it makes it hard to say for sure what the issue is. It is easy to check the fuel pressure, and a faulty pump, or a week pump, or the wrong pump will cause the symptoms you describe. I went through somthing simulator on my 97 5.7L and it was the fuel pump. It was starving for fuel. A simple fuel pressure test will eliminate this, and remove any dought. If that is not the issue, then you will know to go onto somthing else. I removed the throttle body on mine, and jumped the fuel pump relay, and looked inside the intake manifold with a mirror and flash light, so I could see the spider and look for fuel leaks. I had none. A leaking spider can cause. a starving fuel issue. Pluged cats will also rod power, giving week acceleration especially at higer RPMs under a load, not so much when in park, or in nutral.

 

[TruckinAin4Sissies]

What about the spider injector assembly?

This would be my guess, for sure.

TA4S