TJCOS

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?

Jackedup00

Can't really help on your issue, but those guys are trying to rob you at $1300 for coils and plugs! Doing plugs also, its an hour job, doing just coils its about a 20-30 minute job, if you decide to do them, and at that mileage I would, do it yourself.

rcairbear

Still sounds like you have a vacuum leak somewhere-

TJCOS

Thanks for your responses. I had the plugs replaced at 133K miles, but not the coils.

It's the heat issue the baffles me. Only happens when it's 90+ degrees out, after a fair amount of driving, and the engine compartment is obviously very hot. Runs fine otherwise. Makes me think it's a sensor of some sort that is failing under the heat.

On another forum, it was suggested that I need to get it to somebody that can actually diagnose the root cause with an analyzer vs. continuing to throw parts at it.

w0lvez

Disable the Air Metering mode by disconnecting the MAF electrical connector. Start the engine and check if still having the issues. If engine runs better while MAF is unplug. It's either a vacuum leak or faulty MAF sensor.

Ford Fan#1

yeah... This is exactly what you don't want to do well unless you want to take a chance at melting your catalytic converters

w0lvez

Failure Mode Effects Management (FMEM)
is an alternate system strategy in the PCM designed to maintain engine operation if one or more sensor inputs fail. When a sensor input is determined to be out-of-limits by the PCM, an alternative strategy is initiated. The PCM substitutes a fixed value for the incorrect input and continues to monitor the suspect sensor input. If the suspect sensor begins to operate within limits, the PCM returns to the normal engine operational strategy. All FMEM sensors display a sequence error message on the diagnostic tool.

TJCOS

Well, I replaced the MAF sensor since it was easy and relatively cheap. It didn't fix the problem. I read somewhere that it doesn't make much sense to replace all of the coils unless I'm getting a code. So far, no codes.

This never happened before I had the fuel pump replaced for a different issue. Now I'm wondering if it could be related to that. But, what on the fuel system would start failing after it gets very hot?

rcairbear

Still sounds like you have a vacuum leak somewhere.

Jbrew

Sounds like a bad converter IMO. On that's getting plugged up. They can flow test at the dealer.

Could also be a sticky EGR valve. It's going to be hard to assist you with an old truck that you don't work on yourself.

But, -I would have those things looked at specifically.