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73 Ford truck idling poorly

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 ?

You using gasoline that contains ethanol?


There is an EGR valve built into a spacer under the carburetor and the gaskets are notorious to fail and quite often the spacer itself will crack. That contraption can cause vacuum leaks that are annoying and difficult to find. Spray any aerosol lubricant around the carburetor base and listen for a change in the idle.

I think you should look and see if the choke is opening and closing per specs.

@Barkydog … The choke plate is closed at first and opens fully like it should as the engine warms up over the course of 5 minutes of idling. It is closed off fully at first when cold, then cracks open a notch after the engine starts, like it should. And the fast idle cam gets engaged at first, then releases after the engine is warm. So the choke, choke pull-off, and de-choke functions all appear to be working properly.

@“Rod Knox” … EGR … If nothing before that works and I have to remove the carb I’ll also remove and check that EGR plate and replace the EGR valve. Last time – 15 years ago probably – I removed the carb that plate was a mess of carbon deposits. I had to really work on it to get the passages open again. Yesterday I banged on the EGR valve with a screwdriver handle to see if it was stuck partially open, but saw no movement and no change to the poor idling. It looks like it is closed like it should be from what I can see of the pintle shaft. Even if it was open I don’t think there’d be any flow b/c the EGR related passages are probably all carboned up by now. Of course with all the vacuum functions disconnected for test purposes, the EGR valve isn’t functioning at the moment anyway. The spray idea, I’ll try that. Good idea.

@Tester … Ethanol in the gasoline? Not that I know of. I’ve used the same gas station for the past three years for gassing up the truck and never had a problem before. It is possible they changed the formula I guess.

I looked through the maintenance and repair file to see if I’ve ever had this kind of poor idle that comes on quickly before. It’s happened on three occasions.

  1. Split vacuum hose t-connector
  2. Split diaphragm in the air cleaner vacuum motor
  3. That choke gadget that sits on top of the carb had to be replaced. It’s that triangle shaped thing on the upper left of this photo.$_3.JPG?set_id=2

I’ve eliminated 1 & 2, but 3 is still an open issue.

Question to Rod, are these gaskets subject to degradation due to ethanol?
If so, perhaps the carb needs to be totally rebuilt using a kit with elastomerics not sensitive to same.

Just a thought. I know little about this vehicle.

To the OP: rebuilding the carb (last item on your list) sounds like an excellent plan. The float in old carbs is usually either made of thin stamped metal or a molded material that over time will begin to deteriorate and leak (the stamped metal ones) or absorb gas. Either condition can cause the float to not float so well anymore. If that happens, it no longer properly operates the needle valve that regulates the flow of gas into the bowl, causing metering problems… causing erratic operation.

These are just general thoughts. I’ve rebuilt carbs before many years ago, but never one from this engine, nor do I have any hands-on experience with this engine.

Might not hurt to check the fuel pump pressure before focusing on the carb. Unless I miss my guess, the fuel pump on these is a diaphragmatic pump, mechanical with a lever pumping a diaphragm. While these usually fail catastrophically (the diaphragm ruptures), the diaphragms can develop leaks. If the test shows a bad pump, you cold save yourself the work of rebuilding the carb for nothing. If it tests good, you can move to the carb with confidence that your efforts won’t be wasted.

The only other thought I have is to check the distributor shaft for play axially and laterally. A wandering shaft can cause instability in the idle and surging. Seen it.

Does the distributor have a vacuum advance? I would check that too.

@edb1961 … yes, it has both vacuum advance and vacuum retard. Both work correctly. Both are currently disconnected from their vacuum source so they aren’t doing anything at the moment. Idle timing remains spot on.

@“the same mountainbike” … good idea to check the fuel pressure. I replaced the pump 5 years ago after it started leaking gasoline. Yes, it is the diaphragm type knocked the side of the head by the cam shaft as the pumping action. The reason I haven’t suspected that is b/c the engine performs correctly at higher rpms and loads, which you’d think would be inconsistent with a fuel pump failure. But checking the fuel pressure before rebuilding the carb makes complete sense.

I think Testert is right obout the ethanol. I have an 82 dodge pickup with a 318 My mechanic told me to try non ethanol to see what happens. It made a big difference now runs like it should

Well, they are switching to the summer formula right about now. Maybe something to that. But I’ve never had any problems before with either the winter or summer formulas.


Answer a simple question!

Are you using ethanol gas?

Yes or No?


Not sure. I’m not intending to. There’s no label on the gas pump which says what is in the gasoline.

Open the hood at night and see if sparks are flying everywhere from the ignition wires. In the daytime, look for cracked boots. If the plug boots are black, change the wires if the boots have hardened. Removing the plugs may help with diagnosis.

There are a number of things that can cause a rough idle. It can be something as simple as a vacuum leak or carb problem or more major such as a compression issue or valve spring problem. I tend to dismiss the major stuff at this point.

What would I do? My first step would be to connect a vacuum gauge. That’s cheap, easy, and can tell you a lot about what’s going on as to whether there’s a vacuum leak, ignition miss, etc, etc.

Offhand, I might suspect some moisture in the float bowl of the carb or a piece of grit in an air bleed or emulsion tube. You could try removing the mixture screws from the carb and liberally spray some aerosol carb cleaner in to the holes. Ideally, this should be followed up with compressed air.

Just a short tale about air bleeds. Back in the late 80 or early 90s I overhauled an EEC controlled carburetor on a Subaru for a guy. The air filter was absolutely filthy and he flat refused to pay for a new one. He said that he would pick one up and do it himself. I then told him no warranty on the work.

Two weeks later this car came back to me on a wrecker after dying suddenly. Slightly pissed off, I looked and sure enough, that old air filter was still in there.I then took the air gun and forced air through the primary air bleeds. The car started right up and ran fine. Apparently a piece of sand or grit had dislodged from the air filter and it just happened to hit the air bleed while the engine was at idle. If it had happened at elevated RPMs the junk would have gone down the primary bore and never been a problem.

First, be sure all scheduled maintenance is up to date. (That’s an old favorite, right George?)

Ethanol gasoline or E10 is nearly unavoidable in the southwest. In the 1990s I drove a 1973 Grand Torino, I had to replace the accelerator pump every 18 months because the ethanol would harden the pump seal.

On your truck I suspect the power valve diaphragm has ruptured and unmetered fuel is dripping into the manifold.

:tongue: :trollface:

Someone who has a comment on every post made here can’t solve their own problem ?

OP does not know what kind of fuel ( Ethanol or not ) when pumps are required to have stickers on them ?

The term Creditability Gape comes to mind.

Volvo, that’s uncalled for.
The term “comes to criticize” comes to mind.

I think @Rod Knox and @ok4450 have the right idea. There is a vacuum leak in some gasket somewhere. Whether it is ethanol induced or not, it needs to be identified. As others have pointed out, ethanol needs to be dealt with. Fuel pumps, lines, carb gaskets ect, are all affected and will likely eventually succumb to ethanol.

Might be a good time to swap those points out for a modern electronic ignition, too.

The EGR spacer and gaskets were a problem before ethanol was mandated. And finding non-ethanol gasoline can be a pain and when found it is 50c higher than the ‘10% ethanol stuff.’