5.4L V8 - RPMs drop to ~500 when stopping and fluctuates up to 650 and back down again. No check engine light.

When very hot outside, while driving - engine will lose all power, RPMs drop down like it is going to stall then surge back up again. Foot on accelerator the whole time. Made it home, put it into Park, and RPMs fluctuated between 500 and 1000-1200. Very random.

Had fuel pump replaced a while ago to address random starting problems. Before that I replaced the PCM relay and fuel pump relay.

Recently took to the shop and they replaced IAC, cleaned fuel injectors, replaced air filter and cleaned air intake. They also suggested replacing plugs and coils since the truck has 206,000 miles at a cost of $1,300.

Seems to me that if it were a coil going bad when hot that it would throw a code, run rough / misfire vs. losing all power (?)

I have had vacuum leak issues in the past, where the A/C airflow will go from the front vents to the defrost vent while driving higher RPM up a hill, then return to vents when slowing down. Shop replaced a cracked hose or two which made this issue better but it still has happened a couple of times since over the years.

Thoughts?

 

The IAC controls idle in a fault free engine.
If it is sticky it will not be a smooth function.
If there is an intermittent air leak that is equal to or greater than the air bypassed past the IAC, it loses it's control of the idle.
You need to advance to a Scanner to look at live data to see what is happening.
Look at the Long term Fuel Trim tables for both banks.
You have to go past just replacing parts and diagnose the issue.
The Idle hunting is the computer trying to regain control of the Idle.
It does this by looking at the crank sensor for RPM and compare that signal to a software table, then tries to modulate the IAC to regain that control to table specs.
Also check electrical plug at the IAC.
In some cases the IAT being out of tolerance on the 5.4 can result in a loss of power when the ambient temps are high but not affect Idle.
Let the rest alone.

 

Thanks for the quick response, Bluegrass 7. I totally agree with not continuing to throw parts at it, which is what I've read many people have gone through with their mechanics.

A couple of quick questions:

- When you say "air leak", do you mean vacuum leak?

- In order to see what's going on using a live scanner, I assume the problem would need to be occurring at that time? It's so intermittent I'm not sure that a shop would be able to recreate it

- This issue is definitely temperature related. If the IAT were bad wouldn't I get a check engine light, or no? And it's more than a gradual loss of power when the problem occurs - it's like a total loss of power for a couple seconds, then it comes back and surges.

 

Air leak - Vacuum leak for this purpose doesn't make much difference how you call it.
It's air getting into the system that is at a negative pressure below normal atmospheric pressure so air forces it's way in.
You have to take a look with a Scanner to see any indications.
The Long Term Fuel Trims do not change at a fast rate like the short Term Trims do but may be shifted as an indication there is an air leak.
When they shift more than about 25% a code is set..
Short term Trims may move in a way that is not normal once you have the motor running and observe any associations going on. These tables do not set codes but still can be useful in troubleshooting..
Many parameters can be seen for evidence of malfunction.
Other losses of power can be fuel pump/fuel pressure going low. Look at the system pressure with a gage.
Good luck.

 

This is exactly what our truck is doing! Same year. So far we have replaced plugs and boots, air filter, gas filter, changed the oil. Had a p0171 system lean bank code...took it back in and replaced an O2 sensor...back down to the heat and it's surging again. We had the fuel pressure and fuel injectors checked...all ok. It runs great in the cool coastal weather but when we take it inland into the high heat it burps and farts and throws a fit. It's a crazy maker...if you find out what it is, please post!

 

Well you see, there is an example.
You do NOT replace an OX sensor for a 171 code.
That is not what a 171 code tells you to do.
Good luck.

 

Yes, I know that now after giving the mechanic another $195!
Will be searching for an airleak when the truck is home again...thank you!

 

I took it into the shop today. We'll see if they can figure anything out, and I will update here.

 

Fuel pressure was around 20psi. Replaced the fuel filter, pressure increased and the problem hasn't recurred yet. The shop said what came out of the old filter was very dirty. This makes sense to me since the fuel pump was replaced in February - I'm guessing that stirred up the gunk in the tank.

 

Sounds good so far.
You need to see about 35 psi when all said and done.
To low a pressure and you begin to run out of enough fuel under acceleration.
Good luck.

 

Thanks. They said it was in the 30's with the new filter. It seems to idle smoother as well, but that could be in my head.

I asked why they didn't replace the filter when they replaced the pump, and they said it was because they had replaced it a year earlier. $18 total.