Misfire under boost

98S70T5 · Post by **98S70T5** » 16 Jul 2019, 11:23

Howdy guys. First issue with the 98 since I started driving it about 10,000 miles ago. Over the last couple weeks it's been very hot in Indiana. I noticed about a 4 mpg drop in my fuel economy average and just chalked it up to the heat and AC idle etc.

The other day the car started misfiring on all cylinders when under boost. I am guessing with my drop in fuel economy it is spark related. I haven't got around to replacing anything besides plugs as it was running fine but it does have it's original cap/rotor and plug wires.

I presume it is the cap and rotor causing a weak spark and not being able to handle the extra fuel when boost is present. Also maybe the ignition coil but how common is it for those to fail? The car runs just fine other than the boost misfire and the reduced fuel economy.

Just a quick post to see if anyone has seen the same before. Will order up a Cap and Rotor to try before diving further in but as abrupt as the change was I am guessing it may be the coil.

Just south of 179,000.

Post by **erikv11** » 16 Jul 2019, 13:05

Is the cam seal under the distributor cap still seated and sealing?

98S70T5 · Post by **98S70T5** » 16 Jul 2019, 14:03

I haven't even popped it off. Getting ready for vacation so I just got the truck out of the garage.

Post by **Clemens** » 17 Jul 2019, 15:53

yeah, your dizzy, rotor or wires are most likely junk by now. My 850ies did the same, but isuslly in humid weather. For the wires: go Bourgicord and be happy for another 20 years. Cap and rotor: Bosch

98S70T5 · Post by **98S70T5** » 01 Aug 2019, 07:52

For anyone who was interested. The cap and rotor were very nasty (original at 178,000). It ran fine and I hadn't had time or need to replace.
Installed the cap and rotor first and it ran much smoother and had a very large improvement on the under boost missfire but it was still there. Coil (Bosh OE) showed up a day later. Installed last night and its running like a top. I believe its also getting much better mpg than it has since it went back into active duty in November 2018. Will report back on the average MPG.

Just posting the solution for future reference.

Thanks,