Slight Misfire

[jfrawley]

'82 FI Spider, 98,000 miles. While driving and decelerating through, or holding revs near 3,000, I'm getting a slight misfire. It's not happening in any other rev range. I used Brad Artigue's FI manual to trouble shoot all the FI components and they check out fine. New plugs, rotor, and dizzy cap. No issue when accelerating through 3,000. Any thoughts?

[manoa matt]

If you got the factory manual, do all the ignition system checks that are outlined for the electronic distributors.

If you have a spare ignition module I'd try that too. How old are the plug wires and the dizzy pickup wires?

[jfrawley]

I have the manual and will look at those items. No idea about the plug wires and pick up wires but will replace and report back. Thanks.

[rlux4]

John, here's another indespensible diagnostic manual to download in addition to Brad's:
http://www.angelfire.com/wa2/fits/index.html#ljetguide
It doesn't cover your particular problem, but it's great for outlining steps to check the FI components.
Any chance you feel any hesitation as you increase the pressure on the gas pedal? A while back someone had a similar issue and it turned out to be a bad spot on the throttle position sensor.
Is the flap in the air flow meter moving smoothly throughout the entire range of its movement?
Ron

[jfrawley]

Thanks for the manual. No hesitation at all and the AFM seems to move freely. It'll turn out to be something small. I'll find it eventually.

[So Cal Mark]

how's the resistance of the plug wires?
What is the spark plug gap on each cyl?
Have the injectors ever been cleaned?
what is the fuel mixture set at?

[jfrawley]

NGK's gap'd at .029.
Plug wire resistance is 800 ohms (if I'm reading it right).
Other than running some injector cleaner through it periodically, they have not been cleaned that I know of.
And, on FI cars I didn't think you could adjust the fuel/air mixture.

Ron, the throttle position sensor is an interesting idea. It was a bit sticky when I went to adjust it.

[So Cal Mark]

the air mixture is adjustable, the screw is on the afm. Put a voltmeter on the oxygen sensor and read the voltage at your misfire rpm. There's a good chance you have an injector that needs cleaning. Adding treatment to the tank isn't a strong enough concentration to really clean an injector, it's to maintain currently clean injectors

[jfrawley]

Replaced the throttle position sensor last night and ran it around the neighborhood a little before it started to rain. Seems to have fixed the misfire, but I'll run it a lot this weekend and really see. Thanks all.

[rlux4]

I have my fingers crossed for you John.
(Try typing like that!)
Ron

[jfrawley]

Thanks Ron, but I ran it about 275 miles on the trip down and back to the customer appreciation day at IAP today, and it's still doing it. It's very slight so it doesn't stop me from driving it, but I would like to find it. I'll try a few more things before ripping the whole FI system apart.