2003, 2.3L Misfire Under Certain Conditions

[65Sprint]

I have a 2003 XLT, 2.3L with 5sp that has a miss or stumble between 1500 and 2000 rpm under light throttle application and light engine load. The engine is fine under medium or heavy load and at any throttle opening larger than a fast idle. There is no CEL nor any pending codes. It is quite noticeable in 4th gear between 35 and 40 mph. I have done the following which have resulted in very little to no improvement.

-searched for vacuum leaks; replaced two hoses from IMRC solenoid to manifold and actuator
-removed intake manifold, checked for cracks or signs of leaks around manifold seals at head, none found.
-cleaned MAF, IAC and MAP sensors and throttle body.
-checked the intake manifold runner (flaps) for freedom of movement and cleaned them
-replaced plugs with Autolite double platinum
-replaced plug wires
-replaced coil pack as a shot in the dark
-replaced TPS as the original lost its spring tension although resistance values look fine with no dead spots in the POT

It looks like this issue has been discussed in other threads and on other sites but I have not been able to find much of a conclusion. In a few cases a replacement coil pack was the solution but not for everyone and definitely not for me. Can any one provide additional guidance? TIA

 

[RonD]

2003 Ranger would have the Duratec 2.3l 16v DOHC engine

What spark plug gap did you use?
I have seen different gaps mentioned, 0.054 seems to be spec for the Ranger

 

[65Sprint]

Ron, Thanks for the reply. I run the stock 0.044" gap on the plugs which is what I have used for the last 12 years. The symptom is actually engine roughness/stumble rather than a miss on a cylinder or two. This only occurs when the throttle is opened (cracked) just off-idle, within a very narrow engine speed range, and only with a very light engine load. I'll get a definite roughness at about 1600 rpm (35mph) in fourth gear but it will be fine at 1700-1800. The same roughness will occur at 1900 rpm in fifth gear and clear at 2000-2100. I rechecked for vacuum leaks by disconnecting the IAC, which brings the idle down to 500 rpm, and squirting water from a garden hose at various spots on the intake manifold side of the engine. There was absolutely no change in idle speed so I think vacuum leaks are not a likely problem. I am again looking at the intake manifold runner mechanism as a potential problem.

 

[RonD]

Good tests.

Do the symptoms occur when engine is cold or just after warm up?

If you have an EGR system it should be disabled until engine warms up, but if there was a problem with the Valve or solenoid(modulator) you could be getting exhaust gas added at the wrong times causing diluted fuel mix so rough running at certain RPMs.

You can remove EGR Valve's Vacuum hose and plug it, then see if problem gets better, this tests solenoid, not the valve though.
You will get CEL since DPFE sensor won't detect EGR valve opening when it should, but that will be a test of DPFE sensor

 

[65Sprint]

The symptom is the same regardless of engine temperature. I'll test the EGR solenoid, provided I can find the vacuum hose and report back. Thanks for the suggestion.

 

[65Sprint]

The EGR valve and solenoid appear to be buried between the firewall and the head on this truck. At the least, the intake manifold looks like it needs to come off. I would certainly like to hear I am wrong. More than I want to do tonight but this remains on the table as a possible problem source.

 

[Fleetspell]

Did you ever find the problem with yours? I also have posted a very similar question on here. I've done all of the same checks as you have. Lol. Also Ron d has been replying to my posts. Lol. I was just browsing this column of posts and came across this.