Drivability issues. Would love some help.

2003 3.4 automatic 4wd

No codes at all.

parts replaced (all OE) :
MAF Sensor
Throttle body ( includes TPS Sensor)
Fuel Filter
Fuel pressure regulator

Proper fuel pressure has been verified according to FSM test methods. Has not been verified during road operation.

Problem: at about 4000 rpm anything over about 1/4 throttle and the engine just falls down. No longer accelerates. Pull all the way out of the throttle and slowly accelerate again and it runs fine. Until more than 1/4 throttle.

Unique conditions: It only happens after a few days of off-roading. It’s happened 3 times in Baja and once in Nevada this year. It’s not the quality of the gas as everyone in the group has gotten gas at the same places.
Now the wierd part. It usually goes away after several hundred miles of highway driving. However, this ‘cure’ has taken more miles each time.

Note: The engine always runs smoothly. Never any misfire.

Has anybody ever experienced this condition? If so, did you find the cause?

 

You really should do a live data acquisition with a scanner when this is happening, especially Oxygen sensor 1 on both banks. If they go lean, then investigate the fuel delivery issue.

 

I hate when no codes pop

an inline fuel pressure gauge under hood would rule out or confirm FP if condition occurs again

 

I'd test the crank position sensor to see if it is in spec. This information is in the FSM.

 

Wouldn’t it be either in spec or not? How would it sometimes be in spec and other times not? Is that possible?

 

Yes, it would be in spec or not. That's why I said to perform the test, so you don't blindly replace a good sensor.

 

Temp and vibration can do a lot of things to sensors.

 

No codes does not mean that there isnt a pending code. Each code has a certain set of parameters the must be met before the CEL illuminates. In addition when using a scan tool there is generic OBDII scan tool and year, make and model specific scan tool. Some can do both some cant.

 

No codes at all. No stored or pending.

 

It is the off roading thing that is the head scratcher part… My first thought was a partially plugged exhaust but it sure sounds fuel related. If it were me I would pull the fuel pump and check the filter sock and look at the hose that connects the pump to the outlet tube for a split/swelling. At this point in diagnosing you have to start somewhere and I will do the hardest thing first just to get it off my mind. I know that when you finally find it you are going to tell yourself that that makes sense, why didnt I check that first.

 

It’s a fuel cell with a Holley Hydramat. And I’ve checked that the pick up is clear.
And the off-road part baffles me as well. I drove CA to TX to CA in June with no issue.

 

Duct tape a fuel pressure gauge to the windshield and go drive it. You need to see what that pressure is when the problem happens

 

I was hoping it wouldn’t have to come to that.

 

^^^yeah
fitting/monitoring a FP gauge WOULD be much easier than FP replacement
i got a FP off of a member here.....its on my bench
but will not install until that new FP gauge of mine starts showing 30lbs or so(currently 45lbs so im good.....for now)

 

I agree that a pressure gauge would be helpful to confirm a pressure drop. But that still wouldn’t unveil the cause of the pressure drop. It’s not the pump. I’ve tried 3. And why would a pump drop pressure/ volume at a particular RPM/ throttle position/ vacuum point? That’s what I need.


But in the other hand, it could be an ignition timing fault? At the critical point, the timing gets retarded somehow.

 

That's exactly what I thought, hence my recommendation to check the crank position sensor.
At low rpm's the crank position sensor signal is less critical, but as you get to higher rpm's it better be correct.

 

‘Ahhh now we are getting some where!’

multiple FP’s!

Im off fuel issue/delivery

I know there is a way to bench test a sensor.
Of course, chargedshot’s recommendation of ‘real data log’ totally applies here
Besides the fact that my 96 would not communicate with a shop friend’s SnapOn computer, I’m old.
Rather bench test but as chargedshot said…..might work/pass….then fails when ‘conditions’ make the sensor threw incorrect signals

how much (ballpark) is this sensor?

 

$131. I’m tempted to just replace it since it has almost 400k miles on it. And do the cam sensor at the same time.

 

A pressure gauge would tell you a lot. If it drops pressure, look into the fuel pump/delivery stuff

If it doesn't drop pressure then you can check that off the list and look elsewhere

You've thrown a bunch of parts at it already, without doing some testing all we can do is guess

 

Hmm. Never disconnected the knock sensor before and driven around… worth a try plus its free.