AFM problem ?
 

1984 N/A, stock engine, catless with Flowmaster muffler: DE car

I'm running into a weird and hard to duplicate problem. Earlier this year I upgraded to the '86 DME & AFM and added a 4^ advance cam key. Engine feels great and I like the smoother idle and improved lower end torque.

I tested the AFM prior to installing using the Clarks garage and The944.com procedures. Bench performance was spotless.

In August on the way to a DE I noticed that during long highway driving at steady throttle the engine would occasionally bog and the AFR would go into the 17 to 18:1 range. WOT throttle would clear this up instantly and it would last for anywhere from 20 minutes to several hours. I cleaned the AFM connector and had no more trouble during the DE, nor the ride home.

This Friday I had another DE. I had the bog & AFR problem one time during the 3 hour trip there. I cleaned the AFM connector again but it was spotless anyway. During my first two sessions the car seemed okay but lap times were 2-3 seconds off. Then I started to pay attention the AFR. At idle the AFR would slowly work its way from 14.6 to 17.1. Snap the throttle and the process would repeat. On track WOT AFR readings varied from 13.1 to 10.8. This was worst on the last session if the day. The power was flat and the car slow.

During the 3 hour trip home the bog came a couple of times but the car is very drivable and reliable otherwise.

Before I spend hours chasing gremlins, have any of you experienced these symptoms?

I'd look at a loose AFM connector (or bad connections), vacuum lines connected to the FPR or FPD, also consider another easy component, the Fuel Filter or loose electrical connection to the fuel pump. While on that tact, possibly the DME relay could be going. They're only about $20 to replace so I'd consider that as well (I recommend always carrying a spare in the glove box).

+1. If AFM bench tested OK (and you are confident with your test procedures), you have a fuel delivery problem. I'm leaning toward the pump or pump inlet screen.

Thank you for replying guys. I'll take your advice.

I did replace all vacuum hoses and the fuel filter within the last 1,000miles. The part that confuses me is that I've got a lean condition at idle/steady mid-throttle, and a rich condition at full throttle. To add insult to injury it is not an every time, every condition. However it does seem to be getting worse.

In my mind the AFM, or AFM wiring would definitely cause this. On the other hand do either of you think the FPR/FPD would cause this? Maybe the throttle position sensor?

The reason I ask is the problem's sensitivity to throttle and the wide range of indicated, real time AFR data. I don't know...just thinking out loud...