89 F150 - 4.9L IL6 105K
I have recently developed a problem with the ignition system, (sort of like you develop a problem with the flu), anyway when the truck is idling, the tach jumps around wildly between nomal idle and 1500 RPM. The engine does not rev however, remaining at an idle speed. If I raise the idle speed, the tach will jumpfrom say 1200 RPM to over 2000 RPM, while the engine remains steady. When I drive down the road, it will intermit. buck and miss and this scenario will follw the tach jumping---- and sometimes not. The tach will bounce around, and the engine will run smooth-then- the tach will stay relatively steady and the engine will miss badly. Hot or cold makes no difference, neither does RPM although it seems the heavier the load the worse the prob. which lead me to the ignition area. Sometimes it is barely driveable. It will then clear up for a couple of minutes and start the cycle again.When the engine bucks the tach dives, as I would expect this.
OK , I have changed the plugs, rotor, cap, wires, and coil. I measured the TPS voltage and it is solid as a rock (on a Fluke 75 DVM) changing linearly as the throttle is pressed. I measured the grounds, cleaned the grounds as well as every connector in the vicinity I could get to. I put a scope on the black lead of the ignition module and instead of a symet. square wave, I am getting a wave with a collapsing leading edge which looks like it is doubling the output freq. intermittently. The overall freq increases with the engine RPM.
Do you guys think the ignition module is shot or should I be looking somewhere else like a PIP coil or the like ???????? I am out of ideas and am getting dangerously low on patience.
BTW I have already cleaned the EGR and currently have it disc. to rule this out. (the vacuum line that is) I have also pulled on every wire harness there is both under the dash and at the firewall. It did nothing, good or bad - no change.

 

I would seriously look into the ignition module. I've seen ignition modules do some crazy things to engines. Engine throwing any codes?

 

Have you checked the stator? Or the module as low92 said?

 

The Module

 

I had almost the same problem it ended up being a ground wire that was not attached to the computer in the fender well. Connected the ground wire and it ran great.

 

I spent the best part of this week working on this problem, which by the way cropped up overnight. You guys are good --- it was the iginition module, AND the PIP coil and one other thing...... After they were both changed it worked fine. If it ever happens to you, when you change the PIP it will run better, for a little while, but it will still buck when it warms up. (At least that is what it did for me). I had to take it to our company garage to have the gear pulled of the dist to change the PIP. That will cost me some pizza, and besides, I am now an expert at removing distribs on an I6, as I had mine out at least 6 times. The final time in our company parking lot during lunch. I wondered how I was going to get home.
Here's the good part. After everything was changed and running well, as an extra meas of insurance I decided to change the plugs again. What I found amazed me. The plugs on 2,3,4, and 6 were so loose I did not need a wrench to take them out. I think the only reason they did not unscrew the rest of the way is because the wires kept them from rotating any further. First, I cannot believe I missed this. Second, I am furious at the fool that put them in too loose. (me). Third, I am now wondering if the intermittent ground connection of the plugs started the whole failure sequence by punishing the module into failure. (In my defense, when I changed the plugs the last time I could not get them out, so apparently in my mind I must of relaxed on the torque to prevent a replay of this.)
This time I put a little anti seize on the threads and torqued them down well - within reason.
Perhaps if anything can be learned from this fiasco, is to never ever overlook obvious things, even if you just worked on them, and second always post problems here, maybe, someone can give me tips on how to use a spark plug wrench.
Thanks for the replies guys !!!

Frank

 

FTE is the place!!!