Odd misfire/electrical problem

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Oct 17, 2011 | 08:19 AM

benspeedbarn

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Odd misfire/electrical problem

2000 F350 5.4l Automatic

This one has me stumped so far. Truck has newer coils and plugs. It has a hesitation at idle if you blip the throttle, and sometimes under load. It seems to get better once it warms up, but that may just be a coincidence. Here is the part I don't understand:

Whenever it starts misfiring there is a clicking coming from under the dash(not sure if it's the pcm or gauge cluster), and the fuel/oil/water gauges drop down to nothing and the oil pressure/fuel light come on, then go off and the gauges come back up to normal. I checked the voltage at the battery, and it's steady throughout, so it seems to me that the sensors are all loosing power. I don't think the gauge cluster is because the low readings cause the warning lights to come on. Only code is for random multi cylinder misfire.

What are your thoughts? I'm leaning towards either a bad PCM, or something causing the pcm to loose power under engine load? If that's even possible.

Oct 20, 2011 | 07:42 AM

benspeedbarn

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Anyone have an idea or experience this before?

Oct 20, 2011 | 05:02 PM

BareBones

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From: Burnsville, MN

Step number 1: Check the grounds.

Step number 2: Check the grounds.

Step number 3: Check the grounds.

You get the idea. Look for corrosion, loose terminals, etc. For instance, if you checked the battery between the negative and positive terminals, its a waste of time for a dynamic problem.

Oct 20, 2011 | 05:09 PM

benspeedbarn

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Got it. Do you have any tips of where the grounds are located? I know the one strap between frame and body under passenger door is gone. Ill replace that one. Grounds coming off of the battery are good. I'll check around for some more this weekend when I go in.

Thanks for the reply.

Oct 20, 2011 | 06:27 PM

BareBones

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From: Burnsville, MN

You'll recognize them. They're short, and the ends are bare. Loosen any fasteners, wire brush /scrape and refasten.

The thing is, systemic intermittent problems as you describe are pretty difficult to occur. Once you're positive you don't have a grounding problem, you should load test the battery. Then if that checks out, maybe consider the ignition switch.

Sure, its probably possible that the pcm could be the problem. Maybe its overheating, shutting down, and restarting, but that's a real long shot in my opinion.

Oct 20, 2011 | 06:44 PM

BareBones

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From: Burnsville, MN

Another thing you might try for diagnostic purposes would be to run the vehicle with the alternator disconnected. If your battery has a good charge you can run for quite a while without an alternator (half hour?). You haven't indicated any starting problem, which suggests you probably don't have a charging problem, but it might be possible that the alternator is throwing out a bunch of electrical noise that's confusing the pcm. Operating without the alternator connected would test that theory.

And what is your battery voltage when operating.... 14 something?

Oct 21, 2011 | 08:10 AM

benspeedbarn

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Battery load tests good. Just cleaned the terminals last weekend as they were getting pretty crusty. Alternator is less than a year old, and a higher amp version. It's putting out a steady 14.x volts throughout the rpm range. I'm going in Sunday, and will replace the one ground, and check out the rest of the grounds i can find. I'll also try running it without the alt hooked up to see if that changes anything. I'll also check around the ignition switch, and maybe try swapping the ignition relay to see if maybe it's going bad.

It's hard since I only get to see the truck once a week to really get a feel for all of the scenarios that cause the issue. Kind of like the chicken and the egg, are the misfires causing the electrical issues, or the electrical issues causing the misfires. If I can't get any closer to an answer I'll probably send it to a 'real' mechanic with better diagnostic equipment next week.