inconsistant missfire?? Help!

[liltuber]

Im not exactly sure what it is or why its happening. Im not even sure if it is a missfire.

Ok, let me explain it the best I can. The car is a 71 240z.

With no consistant pattern the engine will suddenly drop 300 to 500 rpm and then catch itself and return to idle. Almost like I turned the key off and then back on again quickly. That is at idle. When going down the road the car does the same thing except when i try to give it gas it just makes a sucking sound because theres no combustion(im guessing), then the car will catch itself, but because the car is in gear it will learch(quite embarrasing). Because of this I have not been driving it much and just been tinkering with everything possible to try and fix it when I can. Please help me so I can get my baby back on the road.

Thanks,

Adam

[Force Fed]

Weak fuel pump maybe?

[liltuber]

Weak fuel pump maybe?

New fuel pump and it runs a little rich most the time

[liltuber]

Come on, anybody?

[Curtis240Z]

dirty fuel filter? Sound fuel related because you say the motor only slows down, not momentarily shuts off.

If it is an electrical misfire then it would be jumping around in the distributor cap (carbon tracking) or it would be jumping across spark plup wires (which you could see in the dark and usually hear the spark jumping).

[liltuber]

Thanks Curtis. There are no signs of carbon tracking so i'm going to pick up a new fuel filter today. I've never changed it since I've owned the car so it might be overdue.

[Mr Camouflage]

What condition are the plugs in?

I'd start with a basic tune up, plugs & points gaps and check plug leads.

You could try the "pull one lead off at a time" test. The revs should drop, sound different. If you pull one lead off and nothing changes, then there is a problem with the plug or the lead.

[JimmyZ]

What Mr Camo said plus....

Check for excessive runout/play in the distributor shaft. This could cause some erratic motion in the points.

A dwell meter could also be used to see if your points gap jumping all over the place. (From excessive runout)

A timing light will tell you if a plug or coil wire isn't firing/grounding. Try using the TL on plug wires and coil wire. If it flashes erratically then you have a fouled plug or some other ignition issue. At least this will help rule out fuel issues.

Wiggle the plug on your ignition switch a little. Sometimes the switch goes bad and can intermittently fail. (The TL would show this as missed firing also. )

2c

Jim

Edited August 8, 2008 by JimmyZ

[ozconnection]

I had a problem like this once.

I found that the wires that connected to the ballast resistor were a little corroded and loose. Cleaned them all up and crimped the insulated end of the wire a little to improve the 'clamp' on the balast resistor electrical posts. The problem never returned. I guess the electrical connection was poor and caused the intermittent misfire that I was experiencing.

Good luck, let us know how you get on mate.

[peterc]

From the sounds of things, you're randomly "missing" fire rather than "misfiring", so it sounds more like an electrical issue rather than fuel related. A new fuel filter should confirm that. That being the case, as mentioned in previous posts, the most obvious culprits would be a bad connection(s) or worn distributor.

[AggieZ]

This problem sounds a lot like a problem I had years ago with my first 240. There were multiple problems. There are 2 caps on your distributor.... one inside the distributor cap, and one outside. You should replace them both. Check the connections on both also. Second, replace the ignition coil. Check the inside of your distributor cap. If it has oil or water film inside, then you have another problem. Either your cap is cracked or deformed, letting moister in, and/or your distributor has a bigger problem letting oil into the points from below.

Hope this helps.

[ozconnection]

This problem sounds a lot like a problem I had years ago with my first 240. There were multiple problems. There are 2 caps on your distributor.... one inside the distributor cap, and one outside. You should replace them both. Check the connections on both also. Second, replace the ignition coil. Check the inside of your distributor cap. If it has oil or water film inside, then you have another problem. Either your cap is cracked or deformed, letting moister in, and/or your distributor has a bigger problem letting oil into the points from below.

Hope this helps.

I'm not sure what you're talking about here in regards to your distributor cap?

Two caps on your distributor? What does that mean? Do you have a picture or something that will help to explain what you're on about?!