Engine "missing" - spark or fuel?
 

Here's the situation, My 1985 Carrera 3.2 is starting to have an issue. It has 97,000 miles on the odometer. I purchased about a year ago with 93k. Previous owner only drove it about 500 per year. He took very good care of it. The P.O had a major service in 2012 (valves, fuel filter, etc.). About 6 months ago I noticed a very slight, almost unnoticeable. "miss" or power loss in the mid range under hard acceleration. Over the past 3 weeks or so it has gotten considerably worse. She starts and idles just fine and it is not present under normal driving, just when I get hard on the gas. It starts at about 3500 rpm and higher, and it is on and off miss not a gradual loss of power, but eventually catches up and runs good. It's just in the mid-range.

So is it a spark or fuel issue?

yes.
i can also add an air leak and even the crank position sensor.

Anyone else?

Althought it could be a number of things I'd start with a couple of fuel filters. Cheap and easy enough to do.

an on and off miss is most likely ignition. if ignition, most likely the coil.

I've had coils that ran perfectly at idle and easy acceleration. get on it harder and they'd miss.

Agree that I would start with a new fuel filter. Also plug wires can get old and cause issues. Clewett makes a nice set that's less expensive than stock.

Hot engine miss under load points toward secondary ignition. Try spraying spark plug wires (spark plug cavities) with a pump bottle to eliminate HT wires. Spark plugs should be renewed too.

missing under load could definitely be plugs . You said it was done but start with the basics, dont take any ones word for anything.

Also remember under full load the fuel pump could be considered a possibility also. Personally i would:

1. Replace plugs
2. Run at night and peer in to see if i have any spark leak
3. Swap coil ( only real way to tell )
4. Fuel Filter
5. Verify CHT is two wire sensor and replace it if it isnt
6. Put in your spare DME relay just to elimintate


a good start i would think ..

And don't discount the speed sensor - even though it usually is an RPM range more so than a load range where it fails intermittently.

Thanks for the suggestions. Plugs picked up today and the hunt begins...

Check the air flow meter contacts inside the box. At high mileage the copper arm almost wears out the contact strip. You can reposition it to have proper resistance throughout the range .

It could be as simple as a plug connector that came loose. Might be worth checking.

So I thought I post a follow-up message. Well after purchasing a new CHT sensor and Oxygen sensor I was in the garage preparing to install and my Dad happened by. He says "why don't you start simple and try some fuel injector cleaner?" Ha! I scoffed and said it couldn't be that easy. But with certainly a few hours of work in front of me I decided to try it. I bought Lucas injector cleaner and Viola' problem solved. It literally only took about a 3 blocks of driving and all surging/missing ceased. Moral of the story...start simple.