Sorry if I'm posting this in the wrong section.... I've been reading posts on this forum for quite a while now, ever since I got into my current Truck last year...a 78 ford with a 400 engine 54k original miles, c6 tranny, 2150 motorcraft carb, all original all the way around(even original paint)...well almost.

This is the first time I couldn't find the advice that I was looking for by simply browsing or searching through here, maybe I overlooked it?


my 400 runs bad when the engine hasn't come up to operating temperature. Bad to a point where it will stall if I start her up and try to drive straight off, however its not just a quick stall, it chugs, misses, throws off a small amount of black smoke from unburned fuel, etc...

When its hot, it runs like a champ, no chugging, no missing, no smoke from exhaust, the throttle is quick and responsive, there is nothing that I would describe as less than awesome, well except the noticeably high RPM

I suppose some may think my carb isn't set up right, like not properly setting fast idle cam, and not properly adjusting clearance on my choke plate or something else that might be simple, carb related and often forgotten. However this is not the case here. I rebuilt the carb a few months ago due to a worn out/leaking accelerator pump, set my timing by ear, ended up nicely at 10btdc(-4 for retarded cam) when checked with a light, after getting my fuel/air nice at about 2 1/4 turns. I also did a simple tune-up at the time, with a few extras along the way. Did cap, rotor, plugs & wires, battery, alternator and regulator, quite a few new vac hoses.

And after having done what I just said, it ran perfect for a month, it also didn't do this before... and then over the course of another two months it slowly got worse, to where it is now... it was a slow change, so i guess it was the type of problem you tend to cover up with a simple fix and ignore until it turns into a major issue... i'm nearing the major issue phase, and haven't come up with a permanent fix so i'm starting to worry.

The simple fix at first was turning the idle speed screw in a clockwise fashion to compensate for the rough idle... well to make a long story short, when i fire her up now, without the fast idle kicking in i'm looking at like 600 rpm 800-900 with the fast idle on, which drop down to nothing when i put it into gear, 450rpm or lower is usually where it starts wanting to cut off, takes a few seconds to get there, but it always does. And while these numbers sound reasonable and you are probably wondering what the problem is, well, all this while my engine is going "chug, chug, chug" and it sounds like an extremely old worn out Diesel engine.

When the engine is HOT(takes about 10 to 15 minutes) i'm staring at 1100rpm curb idle, 1600 with the fast idle cam, and about 800 in gear, runs like a champ at this point, haha. And i'm not kidding, it runs unbelievably well when its warmed up, this 34 year old engine gives off less vibration and noise when its warm than the 2009 dodge charger I recently got rid of.

Before i started having problems, without the tweaking, i was looking at a nice 500-550 when i started her up on a cold morning, 800 with the fast idle on, the best part was I could drive her like this without the RPM dropping below 500 when putting it into gear, it was a beautiful thing and with the engine all warmed up it was doing 650rpm, and 550-600 in gear.


A bit more; a friend told me to check if its firing on all cylinders when it is cold, he told me his “quick” check was to just pull off the spark plug wire 1 plug at a time, if rpm stayed the same, that one wasn't firing, if it dropped it was working... well, 1 plug at a time I had to watch my car stall brutally 8 times in a row... not a pleasant experience... according to him this ruled out my ignition system(doubtful).


ofcourse all this is only when the engine isn't warmed up yet... when warm, all is well. I can even undo most of the havoc I wreaked on my carb to get my idle speeds that high and it runs fine, but it won't even start when its cold with those settings.


The problem here only exists when the engine is cold, no other time, no other issues, and its not something that can be described as normal, and me just having to wait for the car to warm up for 15 minutes every-time I need to go somewhere.




So has anyone ever experienced something like this? If so, what did YOU do? and what would you recommend I try?

 

Try checking your EGR valve and all other emissions vacuum lines. Look to be sure everything is hooked up where they belong and working normally. I just did this today on my 400 and found (After a fresh new engine install) a leak from a valve that wasn't hooked up properly. That changed my mixture and they way my truck ran quite dramatically, now I will reset the timing and mixture in the morning.

 

If it is missing or running rough try looking at wires or ignition components. If you have original emissions stuff and vacuum hose it might be good to check the operation of those components as they are dependent on engine temperature.

I just battled this same problem and it was ignition related. I had a bad distributor cap and rotor. I scraped and cleaned the cap enough times it was just time for a new one. It ran fine once warmed up but when cold it would miss. I took all the emission crap off mine because it was in poor condition and wasn't working right. A new cap and rotor solved the cold miss.

 

Blocked off the EGR right after I bought this beast, because it wasn't working(always open). I too removed all of my emissions crap, I did find a guide to do exactly that though... so I even managed to label the stuff I took off, which is now in a box in my storage shed.

About it being ignition related.. the cap, rotor, plugs and wires are all only 3 months old... and I just pulled and checked those a week ago to re-gap them, they actually held up. Not sure if the Coil going bad would cause these symptoms, I can't imagine it would, i've been told its more like it either works or it doesn't. Ignition Control Module maybe? but that too would just cause your engine to cut off rather than it not working when the engine is cold and then perfectly fine when its warm... at-least that's my understanding of those components.(keep in mind I'm a newbie when it comes to working on cars, hell I wouldn't even call myself a hobbyist, though i doubt anyone would.)

 

My situation was weak spark, when the engine warms up it doesn't need as much spark energy to ignite the fuel mixture. So if you have a situation where a weak spark exists it is possible for that to only show up when the engine is cold.

 

to me this is a flag of a odd vacume leak somewhere. just fixed a similar situation yesterday on a little ranger pickup 2.8 with 2 barrel carb. the egr plate was the problem. was suckin in air and doing bad things. got sealed up and now my air bleeds are around just under 2 turns.

 

sorry for not checking in sooner, I didn't have time to mess with it till last weekend, and yeah "1bad79xlt" was right, I had weak spark.

I checked the ignition coil with my meter and it appeared to be not functioning at all, which made no sense since there was still spark to the plugs. Put in a new ignition coil... the BW one. And it ran perfect.

In case you're wondering the old coil is still functioning as well... it just so happens that the poles were covered in so much dirt/grime/corrosion that I couldnt even get a read on its resistance with my cheapo meter... I'm guessing all that gunk on there is what caused it to run like crap(weak spark). after some scraping it had 1.1-1.2 and 10.4k.. the new one has 1.3 and 9.58k if I'm remembering correctly.

So if anyone else has issues with bad cold starting, and intermittent misfires while the engine is cold... clean your connectors!!! you just might not be gettin enough juice to your plugs.

 

You might check the coil. It can cause the symptoms your describing.

Edit: lol i guess i should have read everything before posting.

 

When you block off the EGR you throw the fuel mixture out of balance and the timing will be much too advanced and advanced to early in the power curve.
This causes the engine to run lean and have high cyl. temps causing damage to rings and valves.(this does not show up as coolant overheating.
To fix this the carb must be re-jetted and the distributor re-worked.