99 Yukon intermittent loss of power usually on freeway or grade

[99YukonAZ]

I'm actually trying to solve a problem on my mother's 2002 Express van, but it has the same drivetrain as my 99 Yukon, so I thought I would post here. It will occasionally lose power (when more than 1/4 throttle), starts coughing and just won't go. If I pull over and turn it off and then start it again, it drives normally until it starts acting up again. Often I can drive it for several hours without any problems. It had a new GM fuel pump 4 years ago, and recently has had a fuel pressure test, new fuel filter (2 times), fuel injection system flush, new spark plugs and ignition wires, new ignition coil. She has taken it to 2 separate GM dealers and 2 independent shops, and nobody can seem to diagnose it, they all just want to throw parts at it.

I drove it last week to try to reproduce the problem, and it did it after an hour of driving at 75 and going up a fairly steep grade. I plugged in my code reader and there were no pending fault codes, but when I put on the real time fuel trim widgets, it pegged them out at 54.7% when this happened. The timing advance was consistent, it wasn't jumping around. I pulled over, shut it off, and immediately restarted it, and it drove fine for another 20 miles or so and then it did it again. The latest mechanic wanted to install a MAF sensor, but it has never logged a fault code or turned on the CEL.

I suggested she drive it until it starts failing and disconnect the MAF sensor to see if that remedies the problem, but she has't been able to reproduce the issue.

I would appreciate any suggestions, I'm out of ideas and apparently the 4 mechanics she has taken it to don't have a clue.

 

[Kelsda]

Had a vehicle with a wire harness rubbing on firewall drove me insane did same thing. Intermittent issues with power. Found this out after going to dealer,they couldn’t figure it. Then went to local wrench and he discovered wire bouncing around sometimes grounding out on firewall

 

[99YukonAZ]

I guess that's possible, but the thing that's really confusing to me is that it will be fine every time after just shutting it off and starting it again. It seems to me that a bad ground or short wouldn't stop causing the problem until the wire moved back where it belonged.

 

[TigerEyz3]

Sounds like the fuel filter needs to be replaced (should be replaced annually). They're cheap and easy to replace. Bad, dirty fuel is common. Replacing the filter is cheap insurance. Since you typically observe the problem under load (grade/freeway), I'm inclined to go with this being the culprit.

Wiring gremlins are a pain in the butt and could definitely be the source of the problem as well.

 

[99YukonAZ]

Sounds like the fuel filter needs to be replaced (should be replaced annually). They're cheap and easy to replace. Bad, dirty fuel is common. Replacing the filter is cheap insurance. Since you typically observe the problem under load (grade/freeway), I'm inclined to go with this being the culprit.

Wiring gremlins are a pain in the butt and could definitely be the source of the problem as well.

Thanks for the response. The fuel filter was replaced twice since it began having the problem, so I'm inclined to rule that out. It has since been to another mechanic who wanted to replace the MAF sensor, but I decided to disconnect it the next time the problem occurs and see if that changes the situation before throwing more money at it.

 

[TigerEyz3]

Could be the TPS (throttle position sensor) going out. I've had to replace mine in the past.

 

[99YukonAZ]

Thanks for the suggestion. It works on a bench test (backprobing for smooth voltage increase while gradually opening throttle with the key on). Have you had issues with them when they passed that test? I'm asking because I think something is failing intermittently, and wondering if TPS can fail like that.

 

[PG01]

Theres not a lot of room under the hood in these full size vans, u drive it with engine cover off? Seems to me something is getting hot causing it to lose power.... steep grade? How steep? Enough to make wires move slightly into exhaust manifolds? Enough to slosh fuel in tank to one side? Maybe theres crud in the tank and on that steep grade its picking up crap, starving for fuel, coughing and sputtering and shuts off, stops sucking, crud drops and it starts again...ideas