stumbling - intermittent idle?!

Sunroof · 12-15-2008, 04:28 AM

This came on suddenly in my 1973.5T (CIS) this weekend.

The car starts up well, warms up, accelerates, decelerates and drives smoothly through all gears and runs well up in speed.

At a stop; however, at idle, the engine will begin to stumble. The engine will not die, but the stumbling sounds like its running on half its cylinders. My first instinct is fuel starvation so I will start with a new fuel filter.

Any suggestions?

Thanks

Bob
1973.5T

stlrj · 04:43 AM

If it's been raining, I'd check your distributor cap for moisture. That's something that can come on suddenly.

Sunroof · 12-15-2008, 04:52 AM

No direct rain.......but it sat in the garage (rear jacked up) for a month while I repaired my oil tank. We did have very heavy rain here last week, so I will check the cap.

Thanks
Bob

Walter_Middie · Walter_Middie's Garage

If it's time for a new fuel filter, that would be a good idea. However, I have found that fuel problems are most likely to show up at higher RPM and under load. Idle problems may point to ignition, where higher RPMs mask a weak spark. I'd start at the basics - plugs, points, cap, and rotor. I recently put new plugs in my car, and was pleasantly surprised with the improved drivability and performance. This, after no issues to make me think I needed plugs - it was just time for new ones.

Paulporsche · 12-15-2008, 05:20 AM

As said above, start w/ ignition items first. Make sure the wires are in good shape and check your point gap.
That moisture can get into everything, not just the cap.

psalt · 12-15-2008, 05:21 AM

Bob,

If it accelerates OK when fuel demand is highest, it isn't a fuel delivery problem. I'd start with ignition, vacuum leaks, and mixture adjustment. Small vacuum leaks have the most effect at idle and can upset mixture in a random way.

Sunroof · 12-15-2008, 05:38 AM

I replaced the plugs, spark plug wires, cap and rotor recently. I did not change the points though as they appeared okay. The mixture is set and after experiencing the stumbling I took off the injection boot for inspection and cleaned the sensor plate. No change from that.

If I had an ignition point problem (worn points) wouldn't I experience stumbling through acceleration? Erratic tachometer?

I recall going through this when I first bought the car about nine years ago. I wound up replacing every CIS component including the air-box, because I thought I had a crack in it!!

Bob

psalt · 12-15-2008, 05:50 AM

Bob,

If you symptom is really just a lousy hot idle, vacuum leaks and mixture are on the top of the list. Does the idle speed increase when the engine gets hotter ? Try checking for leaks with an unlit propane torch at the injectors, boots, throttle shaft, a change in idle speed will tip you off. Points can do all sorts of odd things, since the breaker plate moves, you can get a bad connection at different times.

kodioneill · kodioneill's Garage

Quote:

Originally Posted by psalt

Bob,

If you symptom is really just a lousy hot idle, vacuum leaks and mixture are on the top of the list. Does the idle speed increase when the engine gets hotter ? Try checking for leaks with an unlit propane torch at the injectors, boots, throttle shaft, a change in idle speed will tip you off. Points can do all sorts of odd things, since the breaker plate moves, you can get a bad connection at different times.

right on right on right on!

Sunroof · 12-15-2008, 07:03 AM

Thanks...........

I have a new set of points on hand, so I will install those as too the fuel filter. Injectors are new. In fact everything has been replaced but the fuel distributor! Its the intermittent stumbling behavior that came on quick and has been baffling.
Since the rear was jacked up for awhile (month and more) to access the oil tank, I thought some gunk might have settled in the fuel tank forward and impacted the filter. The idle stumbling event worsened over the weekend the more I drove.

I had the fuel tank refurbished last year and coated inside with "redkote". I have the retrofitted smaller fuel filter that was installed about 6K ago. I am always concerned about crap breaking loose inside the tank and partially clogging the filter. Too much focus on the filter!!

Appreciate the responses.

Bob