7/98 build date. '98 f-150 4.2 v6 5 speed 166k. off and on for the past year have gotten an excessive egr flow code. sometimes I cleared it myself, sometimes it cleared on its own. no other problems until recently. now get a p0303 misfire cyl 3 after a short heat soak. doesn't miss from cold. just after you stop and has a short heat soak cycle. when missing, strong spark when #3 wire pulled from coil pack. autolite plugs 10k ago, wires and coil pack more recently. egr valve seems to work properly; vacuum applied and idle stumbles. am I looking at pulling the upper intake and clogged egr ports, or something else? I see insufficient egr flow problem when searched, but not much on excessive egr flow problems. looking for some ideas here, you guys are the best.

 

Two different issues.
The excessive flow is not after the EGR valve but before it if the EGR holds vacuum ok on a test.
Check out the hoses for a break and connections to the DPFE sensor and EGR tube. It could be a faulty DPFE sensor.
Check the Vacuum Regulator that gates vacuum to the EGR for sticking open.

The 303 code can be a coil tower crack, carbon tracking, plug wire, plug porcelain or cylinder mechanics.
Good luck.

 

thank you, sir.

 

well, I've changed coil pack, plug wire, spark plug on #3 and get same results. no miss when cold, dead miss after warm up. stumped. guess I'm pulling upper intake to check egr ports.

 

Only time EGR is operational is in OD above 45 mph and light throttle.
If the cylinder misses any other time, it's not a blocked EGR port
If port 3 was blocked it would not cause a misfire. Note your test of the EGR resulted in stumble indicating the ports are open.
EGR returns exhaust gas into the intake ports and would cause misfire "at the wrong time" because exhaust does not burn the second time and all conditions are not present for 'proper' EGR operation.
They are.... EGR valve opens, fuel injection is reduced and ignition timing goes advanced by program design, for all cylinders.
All these conditions makes the EGR operation transparent to the driver. (no missing, no hesitation, no loss of power).
Test compression as the next test item..
Good luck.

 

compression test returns 180 psi on cyl 3. only miss happens after a heat soak cycle i,e. 10 minutes after a run. doesn't matter if you run 10 miles or 50 miles. compression test run when cold and #3 was only plug removed; and coil disconnected.

 

perhaps a fuel injector not operating when hot?

 

The 303 code is set when the Pcm detects slow rotation time as compared to the average of all 6 cylinders with no faults. It detects the elapsed time from the crank sensor signal measuring crank rotation time.
After doing all the parts replacements it's time to look with more in depth diagnostics.
A Scope looking at all cylinders ignition to see if it is a spark issue can be done.
Something is being missed. Now you have to see it in test equipment to prove it.
At cold start the fuel is rich until the Ox sensors take over fuel control; but that happens about a minute after starting as the Ox sensors heat and coolant, and air temp sense rises.
I would test the spark plug lead with an Ohm meter right after it starts missing.
Another point is the cylinders are paired in a V6. When a cylinder's plug is fired it's mate also fires during that cylinder's exhaust stroke. One side is positive the other side is negative so you have two leads and plugs involved at any given time.
. To be honest, the plugs are actually speced different on each pair. Only difference is the electrode wear over time if they are both the same due to the difference in voltage polarity.
A carbon filled lead can cause a miss.
Quite often a low rpm miss shows up when setting at a light or stop sign on a uphill grade.
The truck will tend to jerk a bit from the misfire jn gear through the converter.
Once you find the problem it will likely turn out to be simple but over looked.
Good luck.

 

 

waiting now for fuel injectors. replacing all of them while I'm in there. wasn't thinking egr port on #3 was clogged; was thinking all the others were; or partly so. would this not cause cyl 3 to be leaner than the others? and more prone to a misfire? this ONLY happens after a heat soak cycle. never while running.

 

There is no EGR function except in OD or gear movijng over 45-55 mph and 'light throttle'. application. This is a specific requirement.
No other time.
You can see this with a Scanner looking at ignition timing.
It will advance to well over 35 degrees when EGR functions under the above driving conditions.
If you can associate this with a misfire, you have a lead.
If not it's not EGR operation caused unless the EGR valve opens for some fault reason outside the above normal operation.

 

thank you, sir...I understand.