98 F-150 has Mystery Miss

Hi All - My '98 F-150 has 88k miles and has always been a great truck that had regular service. After an oil change 3 mo. ago it developed a “hairball”…a random miss that travels around the cylinders and keeps setting false code. My ASE Certified Mechanic is beat - for the first time in 25 years, no solution.

We have troubleshot this to DEATH and have replaced a fair amount of parts to no avail. Am at wits end, this truck should have at least another 50 to 80k left in it.

I offered this problem up for the show, but Bob & Tom weren’t interested.

Is there someone out there who has an answer???

Here is what has been done to date:

Truck Came in with DCT P1000

Engine has definite miss

OBD II Drive Cycle completed

Engine inspected for Vacuum leaks and wire chafe

KOEO test performed, no codes

KOER Test performed DTC P0302 misfire cyl 2

DTC P0302 misfire cyl 2 corrected , coil & plug replaced

Codes Cleared

DTC P0300 appeared 1 time immediately after DTC P0302 corrected before second drive cycle completed

second drive cycle completed

Random misfire occurring under light load that seems to disappear under WOT

No pending codes, not memory codes after second drive cycle complete

EGR removed and manually inspected

All plugs replaced.

MAF, ECT, DPFE, IAT, O2, all verified by signal and data.

Fuel injectors verified by Noid, no pulse width measured

No balance test was able to be performed.

Hoping for a miracle! Steve Hunter

Boy! I’d be scratching my head too!

At this point, you have to start thinking of something maybe mechanically wrong with the engine. For example, a problem with the timing chain? I know it only has 88k miles, but ya gotta eliminate the possibilty.

Or a problem with the crankshaft position sensor or it’s signal rings on the crankshaft? This is where a misfire is detected. When there’s a misfire, this causes the crankshaft to slow down if only for a millisecond. The computer detects this slow down of the crankshaft via the crankshaft position sensor and the windows in the signal rings. So the computer is able to determine when crankshaft slowed down and what cylinder was suppose to fire before the slow down, and sets a misfire code for that cylinder.

That’s all I can think of at this point.


I agree,get a scope pattern of the crankshaft position sensor. The O-scope should have been brought into this problem at the beginning. It seems using a O-scope is something only driveability techs use. Is your ASE guy a driveability specialist? if not you need to get your truck to one.

Have your Mechanic check out this site for lab scope training. They are off for summer but DVD’s are available,everybody needs continuing training and there is plenty of Web Based training out there. This site is just one example.


The crankshaft position sensor is in a place, on the front of the engine, where it’s electrical connector can collect a lot of gunk. Use a cleaning spray to remove the gunk. Let dry (or, blow dry), erase codes, and fire up the engine. If no change, ohm and voltage check the wires to the sensor, and to the engine computer (just like the Haynes repair manual says).

If this was an '02 to '05 Thunderbird I’d replace all the coils on the plugs. The coils, and perhaps the boots between the coil and plug break down over time. I think it is heat related in the Thunderbird in that the coils are covered and have no access to fresh air. The T’birds don’t throw a code immediately, you have a misfire under light load that gets progressively worse.

The coils aren’t cheap but on your truck they are easily accessible. It might be good to replace the plugs too. I’ll give it an 80% chance that new coils and plugs will result in a smooth running engine again.

I have to ring in here. My name is Bryon Best, and Steve hunter has been a Good customer of mine for over 25 years, I won?t say how long I have been in this business, but its been a while. Steven didn’t Mention but I am a Master tech. Not that it makes much difference.

Steve?s particular problem is outlined fairly well here, I just have a few things to add.

In response to “Tester” I have run signal output tests on the CMP that showed no gross abnormalities. The other thing that I might mention here is that since the second drive cycle was completed, The only code that will set now regardless of what is done or how many times the ECM is cleared, or distance driven is DTC P0300 (checked using OS on Snap-On Modis) Not the greatest but what I have.

Several ghost codes had set prior to the completion of the second drive cycle, including a P0402. During the diagnostic on this code I came across something that I could not explain but has since disappeared.

While testing voltage for the transducer I saw dropouts that seemed to coincide with the engine miss. On further check I saw that the reference voltage did a considering drop out as well, My conclusion was that the ECM was bad, but I hesitated at replacing it until I could Talk to Mr. Hunter. He is a fairly knowledgeable person and I like to keep My customers in the loop. When He returned from vacation and I brought Him and his truck into the shop for a re evaluation and console, the miss still existed, but the drop out on the EGR transducer and corresponding reference voltage drop had disappeared completely.

I know I haven?t covered everything here that has been done to diagnose this truck, but as I think of them again, I will post them. Steve was correct when he spoke about my being stumped. I may not be the fastest anymore at getting the job done, but I have always prided myself on the fact that when the vehicle leaves my shop its fixed and fixed right, a customer of mine never pays twice. I am not one to just throw parts at a job in hopes of it being right. I can only remember one other time in my history as a Tech that I was unable to come up with the winning solution. This one has broken my record and my back. I feel bad for Steve, and am personally expending every effort to find the right solution for this job.

I feel that replacing the ECM is the correct move, but until I can verify that diagnosis in some manner I am reluctant to do so for a host of reason

Adding an edit note in regard to ?UncleTurbo?, I did not separate the CKP connector but will do so and check the full run to the ECM upon my next round with this truck, Thanks. I did think of one more thing to add, it has been mentioned before, but something to keep in mind when considering a solution, is that the miss disappears under heavy load, or WOT, and the TPS checks good.

That code P0402 is interesting. Exhaust Gas Recirculation Flow Excessive.

This code can be caused from the orifice inside the orifice tube to the EGR valve being burned out. The EGR system on this engine is backpressure induced type EGR system. Because this EGR system relies on exhaust backpressure to aide in opening the EGR valve. If this orifice is burned out of the orifice tube, it allows too much exhaust backpressure to be applied to the EGR valve. This can cause the EGR valve to open slightly, allowing EGR gasses into the engine when it shouldn’t. And this will cause multiple misfires.


I knew I would leave things out, The EGR Valve was replaced in the Dignostic process Because of some concerns I had about it and BP checked. Thats why I went straight to the transducer(DPFE) when the code appeared. But it was a genuine good shot. You know your diagnostic.

And as I ponder the significant nature of the P0402 I have to think, that if anything were actually wrong with the EGR system, it should be at some point reproducible given that the run problem still exists, but so far I haven’t been able to reproduce the code.

I have done several driving diagnostic runs attempting to do a freeze frame capture of any data that might be out of whack at the moment the ECM registers a DTC. With no data being blatantly or even subtly out of line, the only code that ever sets now is the P0300 random misfire code

I keep adding stuff, food for thought. As I mentioned the only code is the P0300 now, but it never sets under observation, meaning with the scanner connected to the DLC, only when no scan or test is in progress. Never during the koeo or koer tests either


Well? If it were me, I’d pull the CKP and check out the reluctor wheel on the crankshaft. This is one thing that wasn’t done. And it’s one of the steps in diagnosing this code as mentioned here http://www.bba-reman.com/content.aspx?content=NEXT_DTC_P0300_2