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Old 09-24-2009, 12:31 PM   #1
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Default PCM possibly bad, anything else to check?

1999 F150 5.4L V8, 4x2, 178,000 miles, K&N air filter

Symptoms:
Truck feels like it is misfiring, engine shakes. Started doing this in a consistent pattern – drove fine to work in morning, drove fine to lunch, began misfiring as soon as was restarted after lunch, drove home after work fine. It did this for several days. Would only act up as soon as it was turned off and re-cranked after warm at mid-day. Now does it almost every time truck is cranked, cold or hot.
Idles very rough about 400rpm until engines dies. Apply throttle – hesitates and surges. Occasional banging noise internal to engine at throttle – possibly detonation at wrong timing?
No codes thrown or pending.

Troubleshooting:
*Replaced original spark plugs (178,000 miles!) with Motorcraft plugs.
*Replaced COPs.
*TPS checked – voltage varies with throttle change within specs.
*IAT sensor checked – resistance within specs.
*DPFE sensor checked with inconclusive results, visual inspection didn’t look good, replaced with Motorcraft sensor. No change.
*EGR valve – holds vacuum when manually applied with pump, lowers rpm when vacuum applied. Temporarily replaced with new valve – no change.
*EGR solenoid – resistance within specs and does not fix when disconnected from valve.
*PCV valve checked OK.
*No vacuum hose leaks found.
*MAF sensor cleaned.
*Fuel filter replaced.
*Fuel pressure at valve on fuel rail – 37psi, within specs, stays over 30 psi long after cutoff.
*Fuel pressure regulator – holds vacuum, fuel pressure varies with vacuum change.
*CPS checked OK – induces current pulse when metal object breaks magnetic field.
*CKP sensor check OK – induces current pulse when engine turned over with breaker bar.
*IAC valve – replaced about a year ago. Problem is at idle and when throttle applied, ruling this out as cause.
*Oxygen sensors – ruled out as problem exists when engine is cold in open loop mode.
*No coolant in oil, no oil in coolant.
*Very low vacuum (barely over 0psi) measured at intake, except surge at startup.

I can’t get it to throw codes, even when pushing truck to try to force it. I think the symptom is ignition timing or fuel injection timing being off – I checked the wiring from the CPS to PCM, found continuity on both wires. PCM pins look clean. I think the cause is control/logic related as it will act fine, then misfire after shutting down then restarting truck.

I’ve about narrowed it down to the PCM, but wanted some input before I spent that money. Anything else I should check?
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Old 09-24-2009, 01:32 PM   #2
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Default EGR Tube broken?

Looks like you've done a lot of checking. I was having the same problem - finally figured out my EGR tube was cracked (along with some other things that made it hard to pin down the problem).

Good luck - let us know what you find...
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Old 09-24-2009, 02:34 PM   #3
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Quote:
Originally Posted by FedEx View Post
Looks like you've done a lot of checking. I was having the same problem - finally figured out my EGR tube was cracked (along with some other things that made it hard to pin down the problem).

Good luck - let us know what you find...
Thanks for the reply. I'll check my tube when I get home, although I would think that would cause a constant problem, not intermittant. I thought the EGR valve might be sticking open, but have eliminated that possibility.

With the way this starts just by running the truck, cutting off and re-cranking, it seems to be something mechanical sticking or electrical logic/switch issue. And since there aren't any codes, and it feels/sounds like misfire or timing off, I'm leaning towards a PCM malfunction. I hate to have to replace it, but I'm running out of options.
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Old 09-26-2009, 03:18 PM   #4
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No cracks in EGR tube.

Tests this morning:
I ran the engine for about 10-15min sitting in the garage. I held the throttle around 2000rpm. Vacuum gauge read about 17inHg. (Vacuum is only very low when at idle, I think I left out that important hint earlier.) I could feel a slight misfire at that engine speed. If I let off the throttle, rpm and vacuum quickly dropped and stalled out within a couple of seconds.

Here's the interesting part. I ran it long enough to get up to operating temperature and monitored this info with my scanner:
Coolant temp steady at 199 degrees (normal for my truck).
Ignition timing advance around 37 degrees.
NEVER left open loop mode.
All O2 sensors never measured more than 0.02volts.
Pending codes:
P0136 O2 Sensor Circuit Malfunction Bank 1 Sensor 2
P0156 O2 Sensor Circuit Malfunction Bank 2 Sensor 2
P1131 Lack of H02S1 Switches - Lean (Bank 1 Sensor 1)
P1151 Lack of H02S1 Switches - Lean (Bank 2 Sensor 1)

Now I seriously doubt all my 02 sensors died at the same time, so I'm guessing something else is going on. I also noticed this nice black leakage out of the tailpipe:
Click the image to open in full size.

Last edited by metrius; 09-26-2009 at 03:23 PM.
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Old 09-26-2009, 06:02 PM   #5
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Suggest following the pinpoint test for those codes as outlined in the ford manual that covers emmission diognoses. Pictures shows a rich exhaust buildup, need to fine out why. You could be right about the PCM. It would be interesting to know the CO content of a smog test.
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Old 09-26-2009, 09:45 PM   #6
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Default Too Lean:

HGO sensors that read lean will cause the PCM to adjust the mixture richer to compensate and is very likely the reason for the black tail pipe residue.

As Bill suggests, a pin-point diagnostic routine needs to be run to nail down the cause(s) of the trouble.
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Old 09-28-2009, 12:54 AM   #7
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Ford manual, hmm...I've been thinking about buying a used one on eBay. Any idea how the CDs sold there compare to the official service manual?

I crawled under the truck to do some electrical checks on the O2 sensors and look what I found on the drivers side pre-cat O2 sensor harness:


Click the image to open in full size.

The outer sheath is worn completely through. I'll have to pull the sensor off and investigate if there are any shorts, but I'd say it is probably about time to retire it. I doubt this is my main problem, but possibly the reason for my low O2 values.
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Old 09-28-2009, 10:45 AM   #8
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Very low vacuum at idle. Check for a leak. The rubber fitting at the rear of the throttle body elbow that leads to the PCV valve is known to rot and leak.

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Old 09-28-2009, 12:34 PM   #9
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I've spent a good bit of time looking for leaks without avail. I replaced several vacuum hoses within the last year, including the infamous PCV elbow, which has caused me trouble in the past.
I think the low vacuum at idle is because of the low engine speed. It is only idling about 400rpm or less with a vacuum of around 3-4inHg. If I open the throttle, the vacuum jumps up.
If it was a vacuum leak, then the problem should have started when the leak developed and stayed. However, it was intermittant - engine runs fine, turn off, re-crank & problem was there. It wasn't even like it started out with a small problem that got worse; there was no problem, then problem, no problem, then problem. Very much like something mechanical sticking or a logic/switch issue. That's one of the reasons I was wondering if the PCM has been fried. I know they rarely die, but I've noticed the following:
I keep the battery terminals coated in dielectric grease. The negative cable clamp slides up and down easily on the terminal with the grease, even tightened down. Possibly, the connection might have lost enough to where the alternator fried the PCM upon losing the battery load. I've read about other people frying their electronics when disconnecting the battery with the vehicle running.

Last edited by metrius; 09-28-2009 at 12:39 PM.
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Old 10-03-2009, 02:25 AM   #10
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Tonight's discoveries:
Suspect O2 sensor harness only damaged on outer sheath, inner wires intact. Sensor bench tested sensor good - applied heat and sensor returned values within switching range.
Tested wiring between O2 sensors and PCM connector. No open circuits or shorts to ground. Found continuity from sensor harnesses to PCM on all four connectors.

So if the O2 sensors return the correct values and there are no wiring problems, but the PCM doesn't report those values correctly while monitoring with my scanner...I think I've isolated the problem.

I do have a question that I haven't been able to find a definitive answer to through my searches. If I replace the PCM, will it work with the PATS key system without a visit to a dealership?
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Old 10-03-2009, 02:25 AM
 
 
 
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