E450 RV Stuttering when Slowing Down or Warm Idle

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Apr 4, 2026 | 10:45 PM

Fordker

Thread Starter

2nd Gear

Joined: Apr 2026

Posts: 2

Likes: 0

E450 RV Stuttering when Slowing Down or Warm Idle

Hey all — looking for help diagnosing a weird low-RPM chug/rough idle on my 1999 Ford E-450 RV (6.8L V10, gas). I’ve been chasing this for a while and have ruled out a lot. Hoping someone has seen this exact combo.

Symptoms

Cold start in Park: idles fine.
Shift into Drive with foot on brake: engine immediately starts chugging/rough idle.
If I give it a little throttle and start moving: it’s fine. Regular driving she sounds like a dream.
After driving and getting warm: any braking/deceleration / creeping / parking makes it chug consistently at low RPM.

Feels like low RPM is the trigger.

What I’ve done:

Vacuum leaks: multiple smoke tests.
- Fixed a leaks on PCV and HVAC.
- After repairs: smoke test shows no more intake/vacuum leaks.
EVAP/purge:
- Smoke-tested canister side with purge unpowered — purge valve seals, no smoke crossing to manifold side.
EGR:
- Did EGR tests (vacuum hose off/capped, hand vacuum pump) and behavior seemed normal (engine reacts when commanded, holds/release as expected).
Fuel pressure:
- Stable for idle, drive, and even when chugging.
Ignition:
- All coils, spark plugs, injector are brand new and OEM.
IAC:
- Replaced OEM
MAF & throttle body:
- Cleaned.
Fuel:
- Filter and pump changed.

At my wits end. Any thoughts? It never chugs enough to stop the engine. Drives fine otherwise.

May 18, 2026 | 08:21 PM

kevlook

Mountain Pass

Joined: Dec 2023

Posts: 163

Likes: 36

From: Tennessee

You have replaced everything in sight. maybe it is the ECU? Or a temp sensor etc? I replace the temp sensor on my 25 year old Jeep and it amazed me that something like that could make a difference.

May 18, 2026 | 08:28 PM

Fordker

Thread Starter

2nd Gear

Joined: Apr 2026

Posts: 2

Likes: 0

@kevlook interesting, I’ll have to give at least the temp sensor a try. Not sure where I’d source a decent ECU.

In the works of testing each vacuum tube separately with a hand pump, I’m curious if it’s the purge.

May 18, 2026 | 09:35 PM

Don Ridley

Fleet Mechanic

Joined: Jan 2023

Posts: 1,296

Likes: 439

Before you keep changing parts, buy a good scan tool or get Forscan to see if there are any DTCs and to read data PIDs to see what the computer thinks is happening. THE 99s don't have a lot of modules but there is still data that can be read. Even if you have to spend a little (maybe $300 for a decent Autel) you will have a tool that opens up a world of troubeshooting. A bad PCM for this model year is very uncommon.