Erratic spark with rough idle
Thread Starter
|
5th Wheeling
Joined: Apr 2008
Posts: 31
Likes: 3
From: Tucson, AZ
Erratic spark with rough idle
Hello everyone –
I believe a have a problem with my 76 F150 electronic ignition system. Before I start to throw money at it with parts it may not need I’d thought I mention it here first.
The truck starts fine and generally runs strong BUT it’s a little rough at idle and at any rpm (not quite a full-on miss, but just a little flutter – If you were not looking for it, it probably would go unnoticed). After changing the plugs (gapped at .044), wires, cap, and rotor (all motorcraft) the flutter was still there, so I then hooked up the multi-meter with strobe. All of the wires/plugs are flashing very erratically (#1 and #4 are more steady).
I then swapped in my spare coil – no change. Thinking I may have gotten a bad dist cap I changed out to another – no change. Ohm tested the distributor pick up coil – right at 700 ohms, which I think is the upper limit. I’m thinking pick up coil or ignition module is on the edge. I’m hesitant to replace either one because it's my understanding these parts – pick up coil and module, like the coil, either work or they don’t with no in between.
Thoughts?
I believe a have a problem with my 76 F150 electronic ignition system. Before I start to throw money at it with parts it may not need I’d thought I mention it here first.
The truck starts fine and generally runs strong BUT it’s a little rough at idle and at any rpm (not quite a full-on miss, but just a little flutter – If you were not looking for it, it probably would go unnoticed). After changing the plugs (gapped at .044), wires, cap, and rotor (all motorcraft) the flutter was still there, so I then hooked up the multi-meter with strobe. All of the wires/plugs are flashing very erratically (#1 and #4 are more steady).
I then swapped in my spare coil – no change. Thinking I may have gotten a bad dist cap I changed out to another – no change. Ohm tested the distributor pick up coil – right at 700 ohms, which I think is the upper limit. I’m thinking pick up coil or ignition module is on the edge. I’m hesitant to replace either one because it's my understanding these parts – pick up coil and module, like the coil, either work or they don’t with no in between.
Thoughts?
Thread Starter
|
5th Wheeling
Joined: Apr 2008
Posts: 31
Likes: 3
From: Tucson, AZ
DOH! Need to throw $ at a new multi tester! The clamp part that surrounds the spark plug wire was not contacting itself properly causing an on-again-off again strobe reading. I was able to fool with it getting the two contact points to touch properly - and wouldn't you know everything checks out. The minor flutter is something else besides ignition...for another day.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
How are you using a multimeter to check for spark at each cylinder? A multimeter can only tell you if the ignition module is switching the coil - there is no way to correlate that with an actual cylinder number. You do not want to touch a common multimeter to any part of the secondary side of the ignition system when it is live. At most, you'll only injure yourself or fuse out your meter. Are you sure what you're using is really a multimeter?
You need to use a vacuum gauge to diagnose your rough idle issue. That's the only tool that's going to yield useful information. Most likely you have a vacuum leak, or the mixture is not dialed in properly. If the engine runs fine through the RPM range, your ignition system is fine. If anything, the ignition system has to work less at idle.
You need to use a vacuum gauge to diagnose your rough idle issue. That's the only tool that's going to yield useful information. Most likely you have a vacuum leak, or the mixture is not dialed in properly. If the engine runs fine through the RPM range, your ignition system is fine. If anything, the ignition system has to work less at idle.
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
How are you using a multimeter to check for spark at each cylinder? A multimeter can only tell you if the ignition module is switching the coil - there is no way to correlate that with an actual cylinder number. You do not want to touch a common multimeter to any part of the secondary side of the ignition system when it is live. At most, you'll only injure yourself or fuse out your meter. Are you sure what you're using is really a multimeter?
Thread
Thread Starter
Forum
Replies
Last Post
fly-tying.guy
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
7
Aug 20, 2014 12:23 PM
Cdouglas18
1967 - 1972 F-100 & Larger F-Series Trucks
5
Jan 22, 2014 10:31 AM
LawF250
Small Block V8 (221, 260, 289, 5.0/302, 5.8/351W)
5
Aug 21, 2011 02:41 PM
