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Rough Idle when Cold


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#1 Rocko1

 
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Posted 09 March 2010 - 04:41 PM

My 78' F250 with a 400 engine idles roughly and bucks slightly when cold. It has had a fresh tune up(wires cap rotor plugs) less than 3K miles ago. After 5 minutes or so it runs much smoother. Here is a video of the vacuum gauge while cold and warm. The vacuum bounces and is much lower when cold. I have checked for an intake gasket leak and found nothing. Any ideas on what to do? Thanks.



#2 Karrpilot

 
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Posted 09 March 2010 - 10:03 PM

You could play a little with the choke and choke pull off. Maybe the choke is coming off too soon, or the choke pull off is opening up too far.

Ignition timing and advance working correctly? If you have the stock 2 barrel carb on it, the Motorcraft 2150, those are usually pretty easy to play around with.

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#3 Rocko1

 
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Posted 10 March 2010 - 06:05 PM

You could play a little with the choke and choke pull off. Maybe the choke is coming off too soon, or the choke pull off is opening up too far.

Ignition timing and advance working correctly? If you have the stock 2 barrel carb on it, the Motorcraft 2150, those are usually pretty easy to play around with.



Timing is right on. I have adjusted the choke and pulloff and it made no difference. It is the stock 2150 carb. How can I check the vacuum advance? Thanks.

#4 Karrpilot

 
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Posted 10 March 2010 - 07:18 PM

To check the vacuum advance, put a vacuum hand pump on the advance unit. While checking the timing, with no vacuum applied, slowly apply vacuum. The timing should start to advance while watching the timing light. And the vacuum amount should hold on the hand pump.

If the timing does not advance, but the vacuum holds, sometimes the advance unit may still be good, but the mechanizum it attaches to can be frozen.

If the vacuum bleeds off and or will not hold, the vacuum advance diaphram is bad.

Also, check the vacuum line to the distributor. And where said line hooks up to. Sometimes there are what is call TVS switches. Which stands for Thermal Vacuum Switch. Thease things do not let vacuum go thru until the engine reaches a set tempeture. Vacuum in but never out no matter what the temp of the engine means it has failed.

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#5 Rocko1

 
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Posted 10 March 2010 - 07:30 PM

To check the vacuum advance, put a vacuum hand pump on the advance unit. While checking the timing, with no vacuum applied, slowly apply vacuum. The timing should start to advance while watching the timing light. And the vacuum amount should hold on the hand pump.

If the timing does not advance, but the vacuum holds, sometimes the advance unit may still be good, but the mechanizum it attaches to can be frozen.

If the vacuum bleeds off and or will not hold, the vacuum advance diaphram is bad.

Also, check the vacuum line to the distributor. And where said line hooks up to. Sometimes there are what is call TVS switches. Which stands for Thermal Vacuum Switch. Thease things do not let vacuum go thru until the engine reaches a set tempeture. Vacuum in but never out no matter what the temp of the engine means it has failed.


I will check it out. Mine does have the TVS switch. Thank again.

#6 Jim Warman

 
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Posted 13 March 2010 - 03:04 AM

Timing is right on. I have adjusted the choke and pulloff and it made no difference. It is the stock 2150 carb. How can I check the vacuum advance? Thanks.


"Adjusted the choke" has me wondering... If you are referring to the bakelight cap, you are generally further ahead leaving that where it was indexed originally and giving extra attention to the pull off.

It is vitally important that the pull off be checked for leak down. Even the smallest vacuum leak can become a real concern when the engine is cold. When testing for vacuum integrity, pay close attention to the presence of any vacuum bleeds in the pull off. Many had them and, when testing and/or adjusting, it was prudent to block them off...

Pay close attention to the choke cap.... some of these with electric heaters also contained thermostatic contacts. The choke was left with warm air from the choke stove to bring the cap to a set temperature before the electrical contacts closed and the choke opening rate would increase.

Try this... with the engine running poorly when cold, manipulate the choke plate with your finger... OK.. I missed stuff... you are going to make sure that the fast idle cam is on the proper step... you are going to make sure you didn't skip any steps in choke initial adjustment (like pull down and unloader and anything I might forget after all these years).

With the engine idling (and, in all honesty I'd be surprised to see any engine with the emissions controls vacuum hoses connected properly after this many years) at cold start, feel the EGR valve with your hand. If, for some reason, you have EGR flow, the valve will heat. A cold motor started and left to idle should have a cold EGR valve.

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#7 Warren Johnson

 
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Posted 13 March 2010 - 01:06 PM

Boy does this thread bring back memories...
I agree with Jim make sure you have the choke cap at the index mark. Make sure the sove pipe is not rotted off or missing. Check power to the heater. If playing with the choke flap does make it run better you may just have a warn out spring in the choke cap.
You vacuum reading look good I do not see a problem if you had timming chain issues the bouncing needle would be a lot more.. right now it is just showing it is running rough.
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#8 Rocko1

 
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Posted 15 March 2010 - 12:25 PM

"Adjusted the choke" has me wondering... If you are referring to the bakelight cap, you are generally further ahead leaving that where it was indexed originally and giving extra attention to the pull off.

It is vitally important that the pull off be checked for leak down. Even the smallest vacuum leak can become a real concern when the engine is cold. When testing for vacuum integrity, pay close attention to the presence of any vacuum bleeds in the pull off. Many had them and, when testing and/or adjusting, it was prudent to block them off...

Pay close attention to the choke cap.... some of these with electric heaters also contained thermostatic contacts. The choke was left with warm air from the choke stove to bring the cap to a set temperature before the electrical contacts closed and the choke opening rate would increase.

Try this... with the engine running poorly when cold, manipulate the choke plate with your finger... OK.. I missed stuff... you are going to make sure that the fast idle cam is on the proper step... you are going to make sure you didn't skip any steps in choke initial adjustment (like pull down and unloader and anything I might forget after all these years).

With the engine idling (and, in all honesty I'd be surprised to see any engine with the emissions controls vacuum hoses connected properly after this many years) at cold start, feel the EGR valve with your hand. If, for some reason, you have EGR flow, the valve will heat. A cold motor started and left to idle should have a cold EGR valve.



I will check these items out. I am getting the vacuum pump today. Believe it or not, all the hoses and emissions components(all original)(87,000 miles) are connected. The truck has been well taken care of. Stove pipe and all vacuum hoses were checked, no cracks, etc. How long would it take the EGR to heat up after it's started?


Started it this weekend without depressing the gas as I usually do and idled fine for 15 seconds then started the rough idling and almost loping. This goes away after 2-5 minutes.

Just a thought, is there a change the catalytic converter is clogged? It's a CA truck.

#9 Warren Johnson

 
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Posted 15 March 2010 - 02:25 PM

Clogged cat = very low vacuum. You vacuum is fine. The egr will heat up in about 1 to 2 min if it is leaking
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#10 Rocko1

 
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Posted 15 March 2010 - 03:29 PM

Clogged cat = very low vacuum. You vacuum is fine. The egr will heat up in about 1 to 2 min if it is leaking


Thanks Warren. Is it typical to get a lower vacuum reading when the engine is cold?

#11 Warren Johnson

 
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Posted 16 March 2010 - 06:45 AM

Yes
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#12 Rocko1

 
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Posted 16 March 2010 - 11:12 AM

Yes



Couldn't get a vacuum gauge as they were out at two places.

I did check the EGR. Cold after 5 minutes of running. I also checked the idle fuel mixture screws-which the limiter caps are off-and they were fully open-about 6 turns outward. I closed them then backed them off 2 turns. The sticker on the engine says Choke - Index for the setting. I would assume this is dead center of the markings? Set that to center it was towards the Richer setting. It is electric, forgot to check for voltage. Seemed to run a bit better at idle.

#13 Warren Johnson

 
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Posted 16 March 2010 - 11:33 AM

6 turns out on the carb is way to rich 2 to 2 1/2 is a good starting point.
One other thing on that carb. The float may be starting to sink. If it is the origonal black float over time they start to get "water logged" with fuel and will sink just a bit causing a rich mixture.
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#14 Rocko1

 
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Posted 16 March 2010 - 12:55 PM

6 turns out on the carb is way to rich 2 to 2 1/2 is a good starting point.
One other thing on that carb. The float may be starting to sink. If it is the origonal black float over time they start to get "water logged" with fuel and will sink just a bit causing a rich mixture.



Interesting. Would the sinking float cause a harder start when hot? Seems gas is leaking into the intake and mimics a bit of flooding.

The carb needs to be rebuilt I am sure.

Thanks again for all your help. It is appreciated.

#15 Rocko1

 
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Posted 26 March 2010 - 10:24 AM

Well dug into this more the last week. Found a very loose vacuum tree and the choke pull off was shot. Replaced the pull off, tightened the tree, replaced a few vacuum lines and it idles 10x better. Not 100% smooth, but very acceptable for a 32 year old motor.

Thanks for all your help, it is appreciated.




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