Just three weeks ago this truck – 302 V8 Ford – was running and idling perfectly. I usually only drive it a couple times per week, but I had to drive the truck every day and for quite a few miles each day for several weeks the last couple of months b/c my Corolla was off the road being prepared for emissions testing. And the truck was running fine the whole time. Then about three weeks ago it started to idle rough, and over the next week or two started to increasingly sputter and cough at idle, stalling occasionally. Warm or cold engine, no matter. Although it seems a little more noticeable when the engine is cold. The symptom, whatever the cause, is progressively getting worse. When I feel (with the back of my hand) and listen to the exhaust stream at idle, there’s a definite sign of engine missing. It’s not the normal rhythmic “put-put-put”, but more like put-put — sigh – put-put-put – sigh … well you get the idea. Something’s definitely not right. It seems to me like the air/fuel mixture is way too lean when this is happening.
The biggest clue I’ve got is that this problem goes completely away with the throttle even slightly above idle. Idle rpm is about 750, and if the rpm is over 1000 by pressing on the gas pedal, much improved. And acceleration performance from 15 mph to 60 mph remains perfect, a lot of ummmph, no missing at all. Under 15 mph, very noticeable rough running and missing.
Here’s what I’ve done
- Plugged all the vacuum sources, both intake manifold and carb. So no vacuum leaks from any of those are possible. No difference.
- Filled the tank with fresh gasoline, thinking it might have been a case of bad gas. No difference.
- Idle timing is spot on
- Timing advances with rpm like it should so distributor centrifugal weights are working.
- Dwell is spot on
- Everything inside distributor looks like new, no signs of carbon tracking, points in good shape
- Idle rpm is correct on average, but unstable. Surging is noticed if I increase the idle rpm beyond spec.
- Driving at 20 mph, no missing, but surging causing the speed to increase and decrease slightly remains.
- No change with or without air cleaner installed.
- I did discover a couple of problems with some vacuum operated gadgets, the air cleaner vacuum motor and a temperature controlled vacuum switch, but those are now completely disconnected and no change.
Next up on my plan
- Remove the vacuum operated gadget on the carb that controls the choke, see if the diaphragm has a leak in it. That’s the only obvious place a vacuum leak could be happening now.
- Replace the fuel filter.
- Replace points/condenser, spark plugs. This hasn’t been done in a number of years, but less than 30 K miles ago.
- Remove the carb and take it apart, look for something plugged up, rebuild if necessary.
Beyond all that, any ideas? Is the clue where the problem goes completely away just off-idle important ?