Intermittent Misfire / Start Failure - Ford Powerstroke Diesel Forum
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Old 06-28-2013, 05:09 AM
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Intermittent Misfire / Start Failure

I'll try to keep this short, yet complete (not all that easy).

1. My '95 F250 7.3 occasionally misfires when accelerating gently from a stop or slow coasting. It feels like just one or two misfires, after which it runs normally. This occurs an average of maybe once per month (sometimes 2-3 times in a day; sometimes not for a few months). It has occurred since I bought the truck in 2000 with 75K miles on it. It now has 204K.

2. A couple of months ago, when starting the truck, the "Wait to Start" light didn't come on, and it wouldn't fire. Only after turning the ignition off and on again did the "Wait to Start" light come on. After it extinguished, the truck started and ran fine. This has happened 4-5 times since then.

3. New today (2013.06.28): on a 40-mile round trip, the engine cut out three times. There was no response from the accelerator for a few seconds, but the engine was still turning (I didn't lose power steering), so it was either idling or being turned over by the force from the rear wheels through the transmission. After a couple of seconds (with no intervention from me), the engine resumed running normally. In one instance, the "Wait to Start" light came on for a second, maybe less.

Other observations:

1. At 127K miles, it began running horribly, and I replaced the cam angle sensor. That fixed the really bad behavior, but didn't affect the occasional misfire.

2. At 180K miles, I replaced the stop / tail / turn lamps, hi-mount brake lamp, and front parking / turn lamps with LEDs and the flasher with an electronic unit (no load resistors). I noticed (after much frustration) that the hi-mount brake light was dimly illuminated when the truck was running, that the transmission wouldn't lock up in overdrive, and the cruise control wouldn't engage. The cause was reverse-leakage from the engine control unit. Without the incandescent lamp load, the back-voltage rose high enough to give the indication that the brake was being applied. I installed a relay that clamped the brake light line to ground when the brake was off, which solved the problem. But it seems like that's a lot of reverse leakage. Could this be indicative of a failing "chip"?

Some or all of this stuff could be related, or they could all be independent, exasperating issues. Any thoughts from the community would be really welcome.

Thanks.

Last edited by John Miller; 06-28-2013 at 10:58 AM. Reason: A third symptom
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  #2  
Old 07-02-2013, 07:53 AM
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When it cuts out do the gauges go back to rest position?
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Old 07-02-2013, 10:52 AM
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The gauges don't drop back when it cuts out (but the tach, understandably, takes a dive - but not to zero). But here's the latest:

When I turn on the ignition and the "Wait to Start" light doesn't come on, the "Check Engine" light illuminates. If the "Wait to Start" light does come on, the "Check Engine" light stays out, as it should.

I monitored the PCM and IDM relays, and found that when the "Wait to Start" light fails to come on when the ignition is switched on, power is applied through the PCM relay to the IDM relay, as it should be. But the PCM doesn't complete the IDM relay coil circuit, so the IDM doesn't get energized. So I guess the next step is to take it in to get the diagnostic codes read out.
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Old 07-02-2013, 11:00 AM
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I think you are going the same direction I am with this. Looking for an intermittent power or communication failure between the two. Also don't forget the simple, pull the PCM wiring connector and look for water/rust/burned/backed out/bent pin/s. Same with the IDM. When was the last time the IDM was opened to check for water? Are you sure that your chip is seated properly?
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Old 07-03-2013, 04:26 AM
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I pulled the PCM & IDM connectors. They're squeaky-clean, no dirt, no water. It appears that either the PCM is intermittent or it's being told by one of the sensor inputs to cut the engine. My question is, which input(s) would direct it to do that? Seems like the only way you'd want the engine to be killed is to turn off the ignition, yet the IDM relay is also controlled by the PCM. Anyone know which input(s) can direct the PCM to cut the engine?
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Old 07-03-2013, 07:46 AM
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The temp sensor in a auto tranny (would put the engine and tranny in limp) and the CPS (generally kills the engine dead). _Might_ also look at the Idle validation switch and Pedal Position Switch under your right foot.

Last edited by BigFuel; 07-03-2013 at 07:53 AM.
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Old 07-29-2013, 12:30 PM
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The truck ran fine for a day, but on my last trip, it stalled every few hundred yards. Fortunately, I was always able to re-start it, often having to flip the ignition on and off several times. I made it back to a local shop, where the mechanic was unable to establish communication with the PCM. Although I couldn't say with absolute certainty that the PCM was at fault, everything seemed to point to it, so I ordered a replacement. Got the PCM today. They're not easy to come by, I've found. As luck would have it, I got mine on eBay, and it included a DP Tuner 60 hp Econotune. After installing it, the truck fired up and idled smoothly for 15 minutes or so. Took it on a short trip, and it ran flawlessly. I'm still holding my breath, since the problem was, indeed, intermittent, but so far, it looks as though the PCM was the source of all my ills.
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Old 07-29-2013, 01:05 PM
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Not a cheap fix but good luck. Do you know the PCM code for the one you took out?
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Old 07-29-2013, 01:36 PM
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@BigFuel - yup. P/N F5TF-12A650-BJF; module code DPC-202; flash code TEE5.
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Old 07-29-2013, 01:51 PM
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Do you have to send it back as a core? I've always wanted to take one apart and a busted one would do just fine.
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