GMC Repair: 1997 GMC Yukon Power Loss Problem, yukon 4x4, gmc yukon

Question
Hello,

I have a 1997 GMC Yukon 4x4 5.7 Liter with 127,000 Miles.  The vehicle is suffering from a severe power loss problem at WOT.  However ¼ to 1/3 throttle seems to function properly.  In fact, ¼ throttle is the fastest way to accelerate the vehicle currently.

At WOT, the engine cannot rev past 4200 RPM or climbs in RPM very, very slowly from that point on up.  It stays in first gear since I assume the trans is waiting to shift into second at a higher RPM because the throttle is fully depressed. (Because of the TPS I assume) Part throttle the vehicle functions properly.  Even in neutral at WOT, the engine revs up to about 4200 and hits a brick wall.  RPM's climb very, very slowly after that point.  However, ¼ to 1/3 throttle in neutral, the engine can rev up faster and higher in RPM – all the way up to 5500 –5700 RPM.  (I release the throttle at this point since I don't want to over rev the engine.)  If I maintain 5200 RPM in neutral at part throttle and then go to WOT, the RPM's will actually start to drop to about 4500 RPM, and then very, very, slowly, the engine will try to climb in RPM.   So essentially, part throttle function seems to operate normally.

The car is very hard to start on cold starts.  Sometimes it does not start at all and I have to use starting fluid to get it to start.  The idle is a tad rough too.  

There was never a check engine light.  I took it to a Chevy dealer to have it diagnosed.  There were no codes at all in the computer and they said it was the fuel filter.  So I installed a new fuel filter.  This did not fix the problem.  I changed the cap, rotor, plugs, and plug wires about a year ago.  (Before this problem even existed.)  So these items are fresh.  I check the spark with an old spark plug with the #1 spark plug wire connected and the plug grounded to the block.  It sparked fine with the engine idling.  While cranking, it sparked as well, but wasn't the bluest spark in the world.  But I would say it is strong enough to start the car.  (I got jolted several times in the process and seems to sparking with a strong current)

To rule out the fuel pump, I hooked up a fuel pressure gauge.  (I did NOT remove any vacuum lines from the fuel pressure regulator or anywhere)  At idle the fuel pressure was about 42 PSI.  When driving the car at WOT, the pressure rose to about 53 PSI and maintained the whole way thru.  Did not drop at all as the car went through the gears.  The previous owner at about 79,000 miles changed the fuel pump.  So it is not the original.  Recently I installed a new EGR.  Also recently installed was a vacuum booster for the brakes.

I had both cats checked out by a Meinekee muffler.  They drilled a hole in front of each cat and hooked up some sort of a pressure meter.  They started the car and hit the accelerator.  Both cats checked out ok.  They welded the holes shut.  I installed a new air filter too.

What can be causing this?  If it's a faulty injector, I assume I would get some sort of engine code or engine light.  (Lean or rich) If it was a cam sensor or crank sensor, I assume a code would be thrown as well?  But the car hasn't thrown a code at all.

My wife drives a 1997 Tahoe with the same specs as my Yukon.  I plan on switching the MAF sensor to see it this is the problem.  But somehow I feel the MAF is not going to be the problem.  (Again, if the MAF is bad, I should get an engine light, right?)

Please help!  I am very frustrated and bewildered in diagnosing this.  

Thanks,

Anthony


Answer
The first thing I would look at is the VOLUME output of the fuel pump. Just because it has PRESSURE does not mean it has sufficient VOLUME.

Think about your kitchen sink. Let's say the water pressure is 40 lbs at the faucet. If you turn ON the faucet either at a small amount of water coming out or at full capacity output you will still have the same pressure.

1. Perform a FUEL VOLUME test.
2. Check the fuel pressure regulator.
3. Check for a restricted RETURN line from the fuel rail back to the gas tank.

4. I have seen intermittent faulty TPS's that did NOT illuminate the CHECK ENGINE LIGHT.

5. Meinekee muffler was testing for BACK PRESSURE in the exhaust system due to a possible catalytic converter restriction and or muffler restriction.

Did they do the test when the engine and exhaust system was COLD and retested after it warmed up to normal operating temperature ?.

6. You could also have some partially restricted fuel injectors.

7. You could have a weak coil.

8. Could be an ignition module failure of which I have also seen and the CHECK ENGINE LIGHT did NOT illuminate.

The possibilities are endless and will require hands on diagnostics by a reputable knowledgeable facility.


Without having personal access to the vehicle and performing some diagnostics, There is no way that I can give you an EXACT cause.

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