Suspect ignition coil - help please
 

I drove to town, and shut engine down for 45 minute.
Started engine and idle was fine. Started to go and engine started to miss fire. Rpm does not matter, but the more load you put on the engine the worse it got. On a free rev at WOT it acted like the key was turned off - no fire at all until throttle was closed. Stopped and checked fuel pressure (EFI) and it was on the money. Looked everything over could not find anything obvious. It started running better. Closed the hood and headed out of town.

Stopped at several lights and missfires got worse. As I got out of town and picked up speed it started running normal.

Let it set a few hours and did a repeat test. Ran fine cold. After setting for 25 minutes hot and restarting, I got the same missfire. Once is was up to highway speed, it started running ok.

Seems like something is breaking down when hot. My first suspect is the coil.

5.0 based 347 engine. It has Mallory distributor EFI computer controlled advance. The coil is Mallory as well. Coil looks like a Mallory Promaster E Series. Where its mounted I could not see any markings other than Mallory.

I measured 0.5 ohms across the primary and 8740 ohms from secondary to either primary. Engine is cold. Coil does not appear to be grounded. I read open from either primary to engine block and open from secondary to engine block. Coil is mounted to fiberglass fender well. I do not see any gound wires. I was a little surprised that I get the same reading from secondary to either primary terminal, but sense it is a coil, I guess it would be a continuous wire on each side.

Am I measuring this correctly?
What all should I check or better yet how should I procede from here?

Your coil resistance is dead on indicating it is not the coil WHEN you test it, as cheap as they are the easiest may be to just swap. I had a coil pack do this 10 years ago, when it got hot it would open and short. Tested good when cold.

I wish I could give better help, sounds like you know what you are doing, you will be the one to solve it.

I pulled the control modual off the distributor and will have the auto parts store test it in the morning. If it tests good, is it really good? I hate throughing parts at a problem, but that is one part that might be handy to have a spare on hand at all times.

Two other things came to mind:
1. check the obvious battery connections. If you have a loose positive it will cause your ignition system to default.

2. Do you have a MSD 6AL ignition? If it's located in the engine bay area and overheats it will cause the same problem. Second, check the hot wire to your ignition. I experienced a similar issue with my MSD 6AL and thought I had fried the system until I discovered a loose positive battery contact which in effect was delivering low power to the system causing it to flicker into the default mode.

The control modual tested good. I had them run it 5 time, which got it pretty darn hot. It tested good all 5 times.

My ignition is Mallory distributor (that says direct OEM replacement) and a Mallory Promaster E Series Coil. Nothing else in the system.

Since it only happens when it is hot under the hood, I have assumed something electrical is breaking down. A local guy, who does a lot of auto electrical work, suggested putting the coil in an oven for a half hour and then testing it with an ohm meter.

I had a car that acted similar to what you have mentioned. The dizzy cap was old and had a ton of small tracks inside causing misfires only when it got hot. There was a lot of moisture in the cap and dizzy housing.

Dried it out, replaced the cap and rotor and problem solved. Take a real good look at the inside of the cap.

Pull the codes from the EEC.

Make sure the coil wire is not to close to the spark plug wires coming out of the distributor.

If you have a new tank, or an old fuel cell, I bet your problem is a dirty fuel filter. Low fuel pressure problems typically do not show up at idle...That is one of the reason I prefer to have a fuel pressure gauge that I can read while I am driving.

Distributor cap looks like new and is dry.

The only code is the left O2 sensor not switching. I had this code a couple weeks ago and replaced both O2 sensors. I never cleared the codes, and I have only ran it a few times since replacing them. The new O2 were Bosh for a 5.0. They were not exactly like what came out, but no one could match the part number up. Since it is a Ford EEC-4, I figured they should be OK.

I drove it to a car show yesterday and it ran fine. It only seems to do this if you get it hot and then shut it off for about a half hour. Which is when the under hood temps would be the worst. Then when you start it, it misses under load. With the hood open and vacuum line off the fuel pressure regulator, I pulled the throttle and gunned it hard to make it missfire. When it missed the fuel pressure held steady at 45 psi. Form this I do not think it is a fuel pressure drop issue.

I think my next move is to try to get it hot and make it go into a miss again. Then I'll check the coil with an ohm meter. I'm also going to try to borrow a coil and swap it while it is hot and missing. I wonder if my 98 Ford Ranger (Mazda) coil would work?

When you changed the O2 sensors did you disconnect the battery for about 30 mins. so the computer re-calibrates?

Also do you have the old sensors to try putting back in?

Pull the plugs and look at them, may be a lean condition occuring.

Are you still getting the O2 reporting error after resetting the EEC?

No I did not reset the computer after changing the O2.

Yes I do have the old O2.

I'm feeling under the weather. It may be a couple days until I can try some of this.

Thanks for the input.

Here's a great book to read while you're resting up:

Ford Fuel Injection and Electronic Engine Control

http://ecx.images-amazon.com/images/...500_AA280_.jpg


Get better soon.

Well I drove to Cobra to work and back, which includes 15 minutes of city driving. I left the fan off and let it run up to 205 deg (it was only a 75 deg day). When I got home, I left the fan off and held it at 2000 RPM until it hit 205 F. Shut it off and I let it sit about 30 minutes. Started it and it ran fine. So again I held it to 2000 RPM until 205 and shut it down. Let it sit another 30 minutes. And you guessed it. it still ran perfectly. It was a 95 deg day, when it did it last week. I guess I'm stuck with waiting until it screws up on its own.

I looked the book up. $60-90 used. The reviews had several comments that it is excellent for understanding EFI and troubleshooting, but it is not helpful for tuning modified engines. In fact they say, the author tells you not to modify the engine much at all. I kept following the leads from the reviews and other purchases and they all seem to be useless for tuners.

What are your thoughts?